Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

119 files changed, 31336 insertions, 0 deletions

diff --git a/xpcom/ds/ArenaAllocator.h b/xpcom/ds/ArenaAllocator.h
new file mode 100644
index 0000000000..76498986ce
--- /dev/null
+++ b/xpcom/ds/ArenaAllocator.h
@@ -0,0 +1,214 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ArenaAllocator_h
+#define mozilla_ArenaAllocator_h
+
+#include <algorithm>
+#include <cstdint>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/fallible.h"
+#include "mozilla/Likely.h"
+#include "mozilla/MemoryChecking.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/Poison.h"
+#include "mozilla/TemplateLib.h"
+#include "nsDebug.h"
+
+namespace mozilla {
+
+/**
+ * A very simple arena allocator based on NSPR's PLArena.
+ *
+ * The arena allocator only provides for allocation, all memory is retained
+ * until the allocator is destroyed. It's useful for situations where a large
+ * amount of small transient allocations are expected.
+ *
+ * Example usage:
+ *
+ * // Define an allocator that is page sized and returns allocations that are
+ * // 8-byte aligned.
+ * ArenaAllocator<4096, 8> a;
+ * for (int i = 0; i < 1000; i++) {
+ *   DoSomething(a.Allocate(i));
+ * }
+ */
+template <size_t ArenaSize, size_t Alignment = 1>
+class ArenaAllocator {
+ public:
+  constexpr ArenaAllocator() : mHead(), mCurrent(nullptr) {
+    static_assert(mozilla::tl::FloorLog2<Alignment>::value ==
+                      mozilla::tl::CeilingLog2<Alignment>::value,
+                  "ArenaAllocator alignment must be a power of two");
+  }
+
+  ArenaAllocator(const ArenaAllocator&) = delete;
+  ArenaAllocator& operator=(const ArenaAllocator&) = delete;
+
+  /**
+   * Frees all internal arenas but does not call destructors for objects
+   * allocated out of the arena.
+   */
+  ~ArenaAllocator() { Clear(); }
+
+  /**
+   * Fallibly allocates a chunk of memory with the given size from the internal
+   * arenas. If the allocation size is larger than the chosen arena a size an
+   * entire arena is allocated and used.
+   */
+  MOZ_ALWAYS_INLINE void* Allocate(size_t aSize, const fallible_t&) {
+    MOZ_RELEASE_ASSERT(aSize, "Allocation size must be non-zero");
+    return InternalAllocate(AlignedSize(aSize));
+  }
+
+  void* Allocate(size_t aSize) {
+    void* p = Allocate(aSize, fallible);
+    if (MOZ_UNLIKELY(!p)) {
+      NS_ABORT_OOM(std::max(aSize, ArenaSize));
+    }
+
+    return p;
+  }
+
+  /**
+   * Frees all entries. The allocator can be reused after this is called.
+   *
+   * NB: This will not run destructors of any objects that were allocated from
+   * the arena.
+   */
+  void Clear() {
+    // Free all chunks.
+    auto a = mHead.next;
+    while (a) {
+      auto tmp = a;
+      a = a->next;
+      free(tmp);
+    }
+
+    // Reset the list head.
+    mHead.next = nullptr;
+    mCurrent = nullptr;
+  }
+
+  /**
+   * Adjusts the given size to the required alignment.
+   */
+  static constexpr size_t AlignedSize(size_t aSize) {
+    return (aSize + (Alignment - 1)) & ~(Alignment - 1);
+  }
+
+  size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    size_t s = 0;
+    for (auto arena = mHead.next; arena; arena = arena->next) {
+      s += aMallocSizeOf(arena);
+    }
+
+    return s;
+  }
+
+  void Check() {
+    if (mCurrent) {
+      mCurrent->canary.Check();
+    }
+  }
+
+ private:
+  struct ArenaHeader {
+    /**
+     * The location in memory of the data portion of the arena.
+     */
+    uintptr_t offset;
+    /**
+     * The location in memory of the end of the data portion of the arena.
+     */
+    uintptr_t tail;
+  };
+
+  struct ArenaChunk {
+    constexpr ArenaChunk() : header{0, 0}, next(nullptr) {}
+
+    explicit ArenaChunk(size_t aSize)
+        : header{AlignedSize(uintptr_t(this + 1)), uintptr_t(this) + aSize},
+          next(nullptr) {}
+
+    CorruptionCanary canary;
+    ArenaHeader header;
+    ArenaChunk* next;
+
+    /**
+     * Allocates a chunk of memory out of the arena and advances the offset.
+     */
+    void* Allocate(size_t aSize) {
+      MOZ_ASSERT(aSize <= Available());
+      char* p = reinterpret_cast<char*>(header.offset);
+      MOZ_RELEASE_ASSERT(p);
+      header.offset += aSize;
+      canary.Check();
+      MOZ_MAKE_MEM_UNDEFINED(p, aSize);
+      return p;
+    }
+
+    /**
+     * Calculates the amount of space available for allocation in this chunk.
+     */
+    size_t Available() const { return header.tail - header.offset; }
+  };
+
+  /**
+   * Allocates an arena chunk of the given size and initializes its header.
+   */
+  ArenaChunk* AllocateChunk(size_t aSize) {
+    static const size_t kOffset = AlignedSize(sizeof(ArenaChunk));
+    MOZ_ASSERT(kOffset < aSize);
+
+    const size_t chunkSize = aSize + kOffset;
+    void* p = malloc(chunkSize);
+    if (!p) {
+      return nullptr;
+    }
+
+    ArenaChunk* arena = new (KnownNotNull, p) ArenaChunk(chunkSize);
+    MOZ_MAKE_MEM_NOACCESS((void*)arena->header.offset,
+                          arena->header.tail - arena->header.offset);
+
+    // Insert into the head of the list.
+    arena->next = mHead.next;
+    mHead.next = arena;
+
+    // Only update |mCurrent| if this is a standard allocation, large
+    // allocations will always end up full so there's no point in updating
+    // |mCurrent| in that case.
+    if (aSize == ArenaSize - kOffset) {
+      mCurrent = arena;
+    }
+
+    return arena;
+  }
+
+  MOZ_ALWAYS_INLINE void* InternalAllocate(size_t aSize) {
+    static_assert(ArenaSize > AlignedSize(sizeof(ArenaChunk)),
+                  "Arena size must be greater than the header size");
+
+    static const size_t kMaxArenaCapacity =
+        ArenaSize - AlignedSize(sizeof(ArenaChunk));
+
+    if (mCurrent && aSize <= mCurrent->Available()) {
+      return mCurrent->Allocate(aSize);
+    }
+
+    ArenaChunk* arena = AllocateChunk(std::max(kMaxArenaCapacity, aSize));
+    return arena ? arena->Allocate(aSize) : nullptr;
+  }
+
+  ArenaChunk mHead;
+  ArenaChunk* mCurrent;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ArenaAllocator_h
diff --git a/xpcom/ds/ArenaAllocatorExtensions.h b/xpcom/ds/ArenaAllocatorExtensions.h
new file mode 100644
index 0000000000..60819381d5
--- /dev/null
+++ b/xpcom/ds/ArenaAllocatorExtensions.h
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_ArenaAllocatorExtensions_h
+#define mozilla_ArenaAllocatorExtensions_h
+
+#include "mozilla/ArenaAllocator.h"
+#include "mozilla/CheckedInt.h"
+#include "nsAString.h"
+
+/**
+ * Extensions to the ArenaAllocator class.
+ */
+namespace mozilla {
+
+namespace detail {
+
+template <typename T, size_t ArenaSize, size_t Alignment>
+T* DuplicateString(const T* aSrc, const CheckedInt<size_t>& aLen,
+                   ArenaAllocator<ArenaSize, Alignment>& aArena);
+
+}  // namespace detail
+
+/**
+ * Makes an arena allocated null-terminated copy of the source string. The
+ * source string must be null-terminated.
+ *
+ * @param aSrc String to copy.
+ * @param aArena The arena to allocate the string copy out of.
+ * @return An arena allocated null-terminated string.
+ */
+template <typename T, size_t ArenaSize, size_t Alignment>
+T* ArenaStrdup(const T* aStr, ArenaAllocator<ArenaSize, Alignment>& aArena) {
+  return detail::DuplicateString(aStr, nsCharTraits<T>::length(aStr), aArena);
+}
+
+/**
+ * Makes an arena allocated null-terminated copy of the source string.
+ *
+ * @param aSrc String to copy.
+ * @param aArena The arena to allocate the string copy out of.
+ * @return An arena allocated null-terminated string.
+ */
+template <typename T, size_t ArenaSize, size_t Alignment>
+T* ArenaStrdup(const detail::nsTStringRepr<T>& aStr,
+               ArenaAllocator<ArenaSize, Alignment>& aArena) {
+  return detail::DuplicateString(aStr.BeginReading(), aStr.Length(), aArena);
+}
+
+/**
+ * Copies the source string and adds a null terminator. Source string does not
+ * have to be null terminated.
+ */
+template <typename T, size_t ArenaSize, size_t Alignment>
+T* detail::DuplicateString(const T* aSrc, const CheckedInt<size_t>& aLen,
+                           ArenaAllocator<ArenaSize, Alignment>& aArena) {
+  const auto byteLen = (aLen + 1) * sizeof(T);
+  if (!byteLen.isValid()) {
+    return nullptr;
+  }
+
+  T* p = static_cast<T*>(aArena.Allocate(byteLen.value(), mozilla::fallible));
+  if (p) {
+    memcpy(p, aSrc, byteLen.value() - sizeof(T));
+    p[aLen.value()] = T(0);
+  }
+
+  return p;
+}
+
+}  // namespace mozilla
+
+#endif  // mozilla_ArenaAllocatorExtensions_h
diff --git a/xpcom/ds/ArrayAlgorithm.h b/xpcom/ds/ArrayAlgorithm.h
new file mode 100644
index 0000000000..2556eef465
--- /dev/null
+++ b/xpcom/ds/ArrayAlgorithm.h
@@ -0,0 +1,109 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef ArrayAlgorithm_h___
+#define ArrayAlgorithm_h___
+
+#include "nsTArray.h"
+
+#include "mozilla/Algorithm.h"
+#include "mozilla/ResultExtensions.h"
+
+namespace mozilla {
+
+// An algorithm similar to TransformAbortOnErr, which creates a new nsTArray
+// rather than inserting into an output iterator. The capacity of the result
+// array is set to the number of elements that will be inserted if all
+// transformations are successful. This variant is fallible, i.e. if setting the
+// capacity fails, NS_ERROR_OUT_OF_MEMORY is returned as an error value. This
+// variant only works with nsresult errors.
+template <
+    typename SrcIter, typename Transform,
+    typename = std::enable_if_t<std::is_same_v<
+        typename detail::TransformTraits<Transform, SrcIter>::result_err_type,
+        nsresult>>>
+Result<nsTArray<typename detail::TransformTraits<Transform,
+                                                 SrcIter>::result_ok_type>,
+       nsresult>
+TransformIntoNewArrayAbortOnErr(SrcIter aIter, SrcIter aEnd,
+                                Transform aTransform, fallible_t) {
+  nsTArray<typename detail::TransformTraits<Transform, SrcIter>::result_ok_type>
+      res;
+  if (!res.SetCapacity(std::distance(aIter, aEnd), fallible)) {
+    return Err(NS_ERROR_OUT_OF_MEMORY);
+  }
+
+  auto transformRes = TransformAbortOnErr(aIter, aEnd, MakeBackInserter(res),
+                                          std::move(aTransform));
+  if (NS_WARN_IF(transformRes.isErr())) {
+    return Err(transformRes.unwrapErr());
+  }
+
+  return res;
+}
+
+template <typename SrcRange, typename Transform>
+auto TransformIntoNewArrayAbortOnErr(SrcRange& aRange, Transform aTransform,
+                                     fallible_t) {
+  using std::begin;
+  using std::end;
+  return TransformIntoNewArrayAbortOnErr(begin(aRange), end(aRange), aTransform,
+                                         fallible);
+}
+
+// An algorithm similar to std::transform, which creates a new nsTArray
+// rather than inserting into an output iterator. The capacity of the result
+// array is set to the number of elements that will be inserted. This variant is
+// fallible, i.e. if setting the capacity fails, NS_ERROR_OUT_OF_MEMORY is
+// returned as an error value. This variant only works with nsresult errors.
+template <typename SrcIter, typename Transform>
+Result<nsTArray<detail::ArrayElementTransformType<Transform, SrcIter>>,
+       nsresult>
+TransformIntoNewArray(SrcIter aIter, SrcIter aEnd, Transform aTransform,
+                      fallible_t) {
+  nsTArray<detail::ArrayElementTransformType<Transform, SrcIter>> res;
+  if (!res.SetCapacity(std::distance(aIter, aEnd), fallible)) {
+    return Err(NS_ERROR_OUT_OF_MEMORY);
+  }
+
+  std::transform(aIter, aEnd, MakeBackInserter(res), std::move(aTransform));
+
+  return res;
+}
+
+template <typename SrcRange, typename Transform>
+auto TransformIntoNewArray(SrcRange& aRange, Transform aTransform, fallible_t) {
+  using std::begin;
+  using std::end;
+  return TransformIntoNewArray(begin(aRange), end(aRange), aTransform,
+                               fallible);
+}
+
+// An algorithm similar to std::transform, which creates a new nsTArray
+// rather than inserting into an output iterator. The capacity of the result
+// array is set to the number of elements that will be inserted. This variant is
+// infallible, i.e. if setting the capacity fails, the process is aborted.
+template <typename SrcIter, typename Transform>
+nsTArray<detail::ArrayElementTransformType<Transform, SrcIter>>
+TransformIntoNewArray(SrcIter aIter, SrcIter aEnd, Transform aTransform) {
+  nsTArray<detail::ArrayElementTransformType<Transform, SrcIter>> res;
+  res.SetCapacity(std::distance(aIter, aEnd));
+
+  std::transform(aIter, aEnd, MakeBackInserter(res), std::move(aTransform));
+
+  return res;
+}
+
+template <typename SrcRange, typename Transform>
+auto TransformIntoNewArray(SrcRange& aRange, Transform aTransform) {
+  using std::begin;
+  using std::end;
+  return TransformIntoNewArray(begin(aRange), end(aRange), aTransform);
+}
+
+}  // namespace mozilla
+
+#endif  // !defined(ArrayAlgorithm_h___)
diff --git a/xpcom/ds/ArrayIterator.h b/xpcom/ds/ArrayIterator.h
new file mode 100644
index 0000000000..cc6c8d1cb4
--- /dev/null
+++ b/xpcom/ds/ArrayIterator.h
@@ -0,0 +1,165 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Common iterator implementation for array classes e.g. nsTArray.
+
+#ifndef mozilla_ArrayIterator_h
+#define mozilla_ArrayIterator_h
+
+#include <iterator>
+#include <type_traits>
+
+namespace mozilla {
+
+namespace detail {
+template <typename T>
+struct AddInnerConst;
+
+template <typename T>
+struct AddInnerConst<T&> {
+  using Type = const T&;
+};
+
+template <typename T>
+struct AddInnerConst<T*> {
+  using Type = const T*;
+};
+
+template <typename T>
+using AddInnerConstT = typename AddInnerConst<T>::Type;
+}  // namespace detail
+
+// We have implemented a custom iterator class for array rather than using
+// raw pointers into the backing storage to improve the safety of C++11-style
+// range based iteration in the presence of array mutation, or script execution
+// (bug 1299489).
+//
+// Mutating an array which is being iterated is still wrong, and will either
+// cause elements to be missed or firefox to crash, but will not trigger memory
+// safety problems due to the release-mode bounds checking found in ElementAt.
+//
+// Dereferencing this iterator returns type Element. When Element is a reference
+// type, this iterator implements the full standard random access iterator spec,
+// and can be treated in many ways as though it is a pointer. Otherwise, it is
+// just enough to be used in range-based for loop.
+template <class Element, class ArrayType>
+class ArrayIterator {
+ public:
+  typedef ArrayType array_type;
+  typedef ArrayIterator<Element, ArrayType> iterator_type;
+  typedef typename array_type::index_type index_type;
+  typedef std::remove_reference_t<Element> value_type;
+  typedef ptrdiff_t difference_type;
+  typedef value_type* pointer;
+  typedef value_type& reference;
+  typedef std::random_access_iterator_tag iterator_category;
+  typedef ArrayIterator<detail::AddInnerConstT<Element>, ArrayType>
+      const_iterator_type;
+
+ private:
+  const array_type* mArray;
+  index_type mIndex;
+
+ public:
+  ArrayIterator() : mArray(nullptr), mIndex(0) {}
+  ArrayIterator(const iterator_type& aOther)
+      : mArray(aOther.mArray), mIndex(aOther.mIndex) {}
+  ArrayIterator(const array_type& aArray, index_type aIndex)
+      : mArray(&aArray), mIndex(aIndex) {}
+
+  iterator_type& operator=(const iterator_type& aOther) {
+    mArray = aOther.mArray;
+    mIndex = aOther.mIndex;
+    return *this;
+  }
+
+  constexpr operator const_iterator_type() const {
+    return mArray ? const_iterator_type{*mArray, mIndex}
+                  : const_iterator_type{};
+  }
+
+  bool operator==(const iterator_type& aRhs) const {
+    return mIndex == aRhs.mIndex;
+  }
+  bool operator!=(const iterator_type& aRhs) const { return !(*this == aRhs); }
+  bool operator<(const iterator_type& aRhs) const {
+    return mIndex < aRhs.mIndex;
+  }
+  bool operator>(const iterator_type& aRhs) const {
+    return mIndex > aRhs.mIndex;
+  }
+  bool operator<=(const iterator_type& aRhs) const {
+    return mIndex <= aRhs.mIndex;
+  }
+  bool operator>=(const iterator_type& aRhs) const {
+    return mIndex >= aRhs.mIndex;
+  }
+
+  // These operators depend on the release mode bounds checks in
+  // ArrayIterator::ElementAt for safety.
+  value_type* operator->() const {
+    return const_cast<value_type*>(&mArray->ElementAt(mIndex));
+  }
+  Element operator*() const {
+    return const_cast<Element>(mArray->ElementAt(mIndex));
+  }
+
+  iterator_type& operator++() {
+    ++mIndex;
+    return *this;
+  }
+  iterator_type operator++(int) {
+    iterator_type it = *this;
+    ++*this;
+    return it;
+  }
+  iterator_type& operator--() {
+    --mIndex;
+    return *this;
+  }
+  iterator_type operator--(int) {
+    iterator_type it = *this;
+    --*this;
+    return it;
+  }
+
+  iterator_type& operator+=(difference_type aDiff) {
+    mIndex += aDiff;
+    return *this;
+  }
+  iterator_type& operator-=(difference_type aDiff) {
+    mIndex -= aDiff;
+    return *this;
+  }
+
+  iterator_type operator+(difference_type aDiff) const {
+    iterator_type it = *this;
+    it += aDiff;
+    return it;
+  }
+  iterator_type operator-(difference_type aDiff) const {
+    iterator_type it = *this;
+    it -= aDiff;
+    return it;
+  }
+
+  difference_type operator-(const iterator_type& aOther) const {
+    return static_cast<difference_type>(mIndex) -
+           static_cast<difference_type>(aOther.mIndex);
+  }
+
+  Element operator[](difference_type aIndex) const {
+    return *this->operator+(aIndex);
+  }
+
+  constexpr const array_type* GetArray() const { return mArray; }
+
+  constexpr index_type GetIndex() const { return mIndex; }
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ArrayIterator_h
diff --git a/xpcom/ds/Atom.py b/xpcom/ds/Atom.py
new file mode 100644
index 0000000000..1399ba840a
--- /dev/null
+++ b/xpcom/ds/Atom.py
@@ -0,0 +1,64 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+
+class Atom:
+    def __init__(self, ident, string, ty="nsStaticAtom"):
+        self.ident = ident
+        self.string = string
+        self.ty = ty
+        self.atom_type = self.__class__.__name__
+        self.hash = hash_string(string)
+        self.is_ascii_lowercase = is_ascii_lowercase(string)
+
+
+class PseudoElementAtom(Atom):
+    def __init__(self, ident, string):
+        Atom.__init__(self, ident, string, ty="nsCSSPseudoElementStaticAtom")
+
+
+class AnonBoxAtom(Atom):
+    def __init__(self, ident, string):
+        Atom.__init__(self, ident, string, ty="nsCSSAnonBoxPseudoStaticAtom")
+
+
+class NonInheritingAnonBoxAtom(AnonBoxAtom):
+    def __init__(self, ident, string):
+        AnonBoxAtom.__init__(self, ident, string)
+
+
+class InheritingAnonBoxAtom(AnonBoxAtom):
+    def __init__(self, ident, string):
+        AnonBoxAtom.__init__(self, ident, string)
+
+
+GOLDEN_RATIO_U32 = 0x9E3779B9
+
+
+def rotate_left_5(value):
+    return ((value << 5) | (value >> 27)) & 0xFFFFFFFF
+
+
+def wrapping_multiply(x, y):
+    return (x * y) & 0xFFFFFFFF
+
+
+# Calculate the precomputed hash of the static atom. This is a port of
+# mozilla::HashString(const char16_t*), which is what we use for atomizing
+# strings. An assertion in nsAtomTable::RegisterStaticAtoms ensures that
+# the value we compute here matches what HashString() would produce.
+def hash_string(s):
+    h = 0
+    for c in s:
+        h = wrapping_multiply(GOLDEN_RATIO_U32, rotate_left_5(h) ^ ord(c))
+    return h
+
+
+# Returns true if lowercasing this string in an ascii-case-insensitive way
+# would leave the string unchanged.
+def is_ascii_lowercase(s):
+    for c in s:
+        if c >= "A" and c <= "Z":
+            return False
+    return True
diff --git a/xpcom/ds/AtomArray.h b/xpcom/ds/AtomArray.h
new file mode 100644
index 0000000000..5b1e0a0eae
--- /dev/null
+++ b/xpcom/ds/AtomArray.h
@@ -0,0 +1,19 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_AtomArray_h
+#define mozilla_AtomArray_h
+
+#include "mozilla/RefPtr.h"
+#include "nsTArray.h"
+
+class nsAtom;
+
+namespace mozilla {
+typedef AutoTArray<RefPtr<nsAtom>, 4> AtomArray;
+}
+
+#endif  // mozilla_AtomArray_h
diff --git a/xpcom/ds/Dafsa.cpp b/xpcom/ds/Dafsa.cpp
new file mode 100644
index 0000000000..8854e0ff1d
--- /dev/null
+++ b/xpcom/ds/Dafsa.cpp
@@ -0,0 +1,153 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "Dafsa.h"
+
+#include "mozilla/Assertions.h"
+#include "nsAString.h"
+
+const int mozilla::Dafsa::kKeyNotFound = -1;
+
+// Note the DAFSA implementation was lifted from eTLD code in Chromium that was
+// originally lifted from Firefox.
+
+// Read next offset from pos.
+// Returns true if an offset could be read, false otherwise.
+static bool GetNextOffset(const unsigned char** pos, const unsigned char* end,
+                          const unsigned char** offset) {
+  if (*pos == end) return false;
+
+  // When reading an offset the byte array must always contain at least
+  // three more bytes to consume. First the offset to read, then a node
+  // to skip over and finally a destination node. No object can be smaller
+  // than one byte.
+  MOZ_ASSERT(*pos + 2 < end);
+  size_t bytes_consumed;
+  switch (**pos & 0x60) {
+    case 0x60:  // Read three byte offset
+      *offset += (((*pos)[0] & 0x1F) << 16) | ((*pos)[1] << 8) | (*pos)[2];
+      bytes_consumed = 3;
+      break;
+    case 0x40:  // Read two byte offset
+      *offset += (((*pos)[0] & 0x1F) << 8) | (*pos)[1];
+      bytes_consumed = 2;
+      break;
+    default:
+      *offset += (*pos)[0] & 0x3F;
+      bytes_consumed = 1;
+  }
+  if ((**pos & 0x80) != 0) {
+    *pos = end;
+  } else {
+    *pos += bytes_consumed;
+  }
+  return true;
+}
+
+// Check if byte at offset is last in label.
+static bool IsEOL(const unsigned char* offset, const unsigned char* end) {
+  MOZ_ASSERT(offset < end);
+  return (*offset & 0x80) != 0;
+}
+
+// Check if byte at offset matches first character in key.
+// This version matches characters not last in label.
+static bool IsMatch(const unsigned char* offset, const unsigned char* end,
+                    const char* key) {
+  MOZ_ASSERT(offset < end);
+  return *offset == *key;
+}
+
+// Check if byte at offset matches first character in key.
+// This version matches characters last in label.
+static bool IsEndCharMatch(const unsigned char* offset,
+                           const unsigned char* end, const char* key) {
+  MOZ_ASSERT(offset < end);
+  return *offset == (*key | 0x80);
+}
+
+// Read return value at offset.
+// Returns true if a return value could be read, false otherwise.
+static bool GetReturnValue(const unsigned char* offset,
+                           const unsigned char* end, int* return_value) {
+  MOZ_ASSERT(offset < end);
+  if ((*offset & 0xE0) == 0x80) {
+    *return_value = *offset & 0x0F;
+    return true;
+  }
+  return false;
+}
+
+// Lookup a domain key in a byte array generated by make_dafsa.py.
+// The rule type is returned if key is found, otherwise kKeyNotFound is
+// returned.
+static int LookupString(const unsigned char* graph, size_t length,
+                        const char* key, size_t key_length) {
+  const unsigned char* pos = graph;
+  const unsigned char* end = graph + length;
+  const unsigned char* offset = pos;
+  const char* key_end = key + key_length;
+  while (GetNextOffset(&pos, end, &offset)) {
+    //   char <char>+ end_char offsets
+    //   char <char>+ return value
+    //   char end_char offsets
+    //   char return value
+    //   end_char offsets
+    //   return_value
+    bool did_consume = false;
+    if (key != key_end && !IsEOL(offset, end)) {
+      // Leading <char> is not a match. Don't dive into this child
+      if (!IsMatch(offset, end, key)) continue;
+      did_consume = true;
+      ++offset;
+      ++key;
+      // Possible matches at this point:
+      // <char>+ end_char offsets
+      // <char>+ return value
+      // end_char offsets
+      // return value
+      // Remove all remaining <char> nodes possible
+      while (!IsEOL(offset, end) && key != key_end) {
+        if (!IsMatch(offset, end, key)) return mozilla::Dafsa::kKeyNotFound;
+        ++key;
+        ++offset;
+      }
+    }
+    // Possible matches at this point:
+    // end_char offsets
+    // return_value
+    // If one or more <char> elements were consumed, a failure
+    // to match is terminal. Otherwise, try the next node.
+    if (key == key_end) {
+      int return_value;
+      if (GetReturnValue(offset, end, &return_value)) return return_value;
+      // The DAFSA guarantees that if the first char is a match, all
+      // remaining char elements MUST match if the key is truly present.
+      if (did_consume) return mozilla::Dafsa::kKeyNotFound;
+      continue;
+    }
+    if (!IsEndCharMatch(offset, end, key)) {
+      if (did_consume) return mozilla::Dafsa::kKeyNotFound;  // Unexpected
+      continue;
+    }
+    ++key;
+    pos = ++offset;  // Dive into child
+  }
+  return mozilla::Dafsa::kKeyNotFound;  // No match
+}
+
+namespace mozilla {
+
+int Dafsa::Lookup(const nsACString& aKey) const {
+  return LookupString(mData.Elements(), mData.Length(), aKey.BeginReading(),
+                      aKey.Length());
+}
+
+}  // namespace mozilla
diff --git a/xpcom/ds/Dafsa.h b/xpcom/ds/Dafsa.h
new file mode 100644
index 0000000000..1a5584f632
--- /dev/null
+++ b/xpcom/ds/Dafsa.h
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_Dafsa_h
+#define mozilla_Dafsa_h
+
+#include "stdint.h"
+
+#include "mozilla/Span.h"
+#include "nsStringFwd.h"
+
+namespace mozilla {
+
+/**
+ * A deterministic acyclic finite state automaton suitable for storing static
+ * dictionaries of tagged ascii strings. Consider using this if you have a very
+ * large set of strings that need an associated enum value.
+ *
+ * Currently the string tag is limited by `make_dafsa.py` to a value of [0-4].
+ * In theory [0-15] can easily be supported.
+ *
+ * See `make_dafsa.py` for more details.
+ */
+class Dafsa {
+ public:
+  using Graph = Span<const uint8_t>;
+
+  /**
+   * Initializes the DAFSA with a binary encoding generated by `make_dafsa.py`.
+   */
+  explicit Dafsa(const Graph& aData) : mData(aData) {}
+
+  ~Dafsa() = default;
+
+  /**
+   * Searches for the given string in the DAFSA.
+   *
+   * @param aKey The string to search for.
+   * @returns kKeyNotFound if not found, otherwise the associated tag.
+   */
+  int Lookup(const nsACString& aKey) const;
+
+  static const int kKeyNotFound;
+
+ private:
+  const Graph mData;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_Dafsa_h
diff --git a/xpcom/ds/HTMLAtoms.py b/xpcom/ds/HTMLAtoms.py
new file mode 100644
index 0000000000..ae4df76cc3
--- /dev/null
+++ b/xpcom/ds/HTMLAtoms.py
@@ -0,0 +1,257 @@
+# THIS FILE IS GENERATED BY THE HTML PARSER TRANSLATOR AND WILL BE OVERWRITTEN!
+from Atom import Atom
+
+HTML_PARSER_ATOMS = [
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink", "xlink"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xml_space", "xml:space"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xml_lang", "xml:lang"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("aria_grab", "aria-grab"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("aria_channel", "aria-channel"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("aria_secret", "aria-secret"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("aria_templateid", "aria-templateid"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("aria_datatype", "aria-datatype"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("local", "local"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xchannelselector", "xchannelselector"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("ychannelselector", "ychannelselector"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("enable_background", "enable-background"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("calcmode", "calcmode"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("specularexponent", "specularexponent"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("specularconstant", "specularconstant"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("gradienttransform", "gradienttransform"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("gradientunits", "gradientunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("rendering_intent", "rendering-intent"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("stddeviation", "stddeviation"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("basefrequency", "basefrequency"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("baseprofile", "baseprofile"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("baseProfile", "baseProfile"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("edgemode", "edgemode"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("repeatcount", "repeatcount"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("repeatdur", "repeatdur"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("spreadmethod", "spreadmethod"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("diffuseconstant", "diffuseconstant"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("surfacescale", "surfacescale"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("lengthadjust", "lengthadjust"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("origin", "origin"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("targetx", "targetx"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("targety", "targety"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("pathlength", "pathlength"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("definitionurl", "definitionurl"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("limitingconeangle", "limitingconeangle"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("markerheight", "markerheight"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("markerwidth", "markerwidth"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("maskunits", "maskunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("markerunits", "markerunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("maskcontentunits", "maskcontentunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("tablevalues", "tablevalues"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("primitiveunits", "primitiveunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("zoomandpan", "zoomandpan"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("kernelmatrix", "kernelmatrix"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("kerning", "kerning"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("kernelunitlength", "kernelunitlength"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("pointsatx", "pointsatx"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("pointsaty", "pointsaty"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("pointsatz", "pointsatz"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_href", "xlink:href"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_title", "xlink:title"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_role", "xlink:role"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_arcrole", "xlink:arcrole"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("arcrole", "arcrole"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xmlns_xlink", "xmlns:xlink"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_type", "xlink:type"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_show", "xlink:show"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("xlink_actuate", "xlink:actuate"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("color_rendering", "color-rendering"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("numoctaves", "numoctaves"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("onmousewheel", "onmousewheel"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("clippathunits", "clippathunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("glyph_orientation_vertical", "glyph-orientation-vertical"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("glyph_orientation_horizontal", "glyph-orientation-horizontal"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("glyphref", "glyphref"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("keypoints", "keypoints"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("attributename", "attributename"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("attributetype", "attributetype"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("startoffset", "startoffset"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("keysplines", "keysplines"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("preservealpha", "preservealpha"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("preserveaspectratio", "preserveaspectratio"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("alttext", "alttext"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("filterunits", "filterunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("keytimes", "keytimes"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("patterntransform", "patterntransform"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("patternunits", "patternunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("patterncontentunits", "patterncontentunits"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("stitchtiles", "stitchtiles"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("systemlanguage", "systemlanguage"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("textlength", "textlength"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("requiredfeatures", "requiredfeatures"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("requiredextensions", "requiredextensions"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("viewtarget", "viewtarget"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("viewbox", "viewbox"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("refx", "refx"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("refy", "refy"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fefunca", "fefunca"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fefuncb", "fefuncb"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feblend", "feblend"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feflood", "feflood"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feturbulence", "feturbulence"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("femergenode", "femergenode"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feimage", "feimage"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("femerge", "femerge"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fetile", "fetile"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fecomposite", "fecomposite"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altglyphdef", "altglyphdef"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altGlyphDef", "altGlyphDef"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fefuncg", "fefuncg"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fediffuselighting", "fediffuselighting"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fespecularlighting", "fespecularlighting"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altglyph", "altglyph"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altGlyph", "altGlyph"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("clippath", "clippath"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("textpath", "textpath"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altglyphitem", "altglyphitem"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("altGlyphItem", "altGlyphItem"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("animatetransform", "animatetransform"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("animatemotion", "animatemotion"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fedisplacementmap", "fedisplacementmap"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("animatecolor", "animatecolor"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fefuncr", "fefuncr"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fecomponenttransfer", "fecomponenttransfer"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fegaussianblur", "fegaussianblur"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("foreignobject", "foreignobject"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feoffset", "feoffset"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fespotlight", "fespotlight"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fepointlight", "fepointlight"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fedistantlight", "fedistantlight"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("lineargradient", "lineargradient"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("radialgradient", "radialgradient"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fedropshadow", "fedropshadow"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("fecolormatrix", "fecolormatrix"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("feconvolvematrix", "feconvolvematrix"),
+    # ATOM GENERATED BY HTML PARSER TRANSLATOR (WILL BE AUTOMATICALLY OVERWRITTEN):
+    Atom("femorphology", "femorphology"),
+]
diff --git a/xpcom/ds/IncrementalTokenizer.cpp b/xpcom/ds/IncrementalTokenizer.cpp
new file mode 100644
index 0000000000..db6fddb82b
--- /dev/null
+++ b/xpcom/ds/IncrementalTokenizer.cpp
@@ -0,0 +1,190 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/IncrementalTokenizer.h"
+
+#include "mozilla/AutoRestore.h"
+
+#include "nsIInputStream.h"
+#include "IncrementalTokenizer.h"
+#include <algorithm>
+
+namespace mozilla {
+
+IncrementalTokenizer::IncrementalTokenizer(Consumer&& aConsumer,
+                                           const char* aWhitespaces,
+                                           const char* aAdditionalWordChars,
+                                           uint32_t aRawMinBuffered)
+    : TokenizerBase(aWhitespaces, aAdditionalWordChars)
+#ifdef DEBUG
+      ,
+      mConsuming(false)
+#endif
+      ,
+      mNeedMoreInput(false),
+      mRollback(false),
+      mInputCursor(0),
+      mConsumer(std::move(aConsumer)) {
+  mInputFinished = false;
+  mMinRawDelivery = aRawMinBuffered;
+}
+
+nsresult IncrementalTokenizer::FeedInput(const nsACString& aInput) {
+  NS_ENSURE_TRUE(mConsumer, NS_ERROR_NOT_INITIALIZED);
+  MOZ_ASSERT(!mInputFinished);
+
+  mInput.Cut(0, mInputCursor);
+  mInputCursor = 0;
+
+  mInput.Append(aInput);
+
+  return Process();
+}
+
+nsresult IncrementalTokenizer::FeedInput(nsIInputStream* aInput,
+                                         uint32_t aCount) {
+  NS_ENSURE_TRUE(mConsumer, NS_ERROR_NOT_INITIALIZED);
+  MOZ_ASSERT(!mInputFinished);
+  MOZ_ASSERT(!mConsuming);
+
+  mInput.Cut(0, mInputCursor);
+  mInputCursor = 0;
+
+  nsresult rv = NS_OK;
+  while (NS_SUCCEEDED(rv) && aCount) {
+    nsCString::index_type remainder = mInput.Length();
+    nsCString::index_type load =
+        std::min<nsCString::index_type>(aCount, PR_UINT32_MAX - remainder);
+
+    if (!load) {
+      // To keep the API simple, we fail if the input data buffer if filled.
+      // It's highly unlikely there will ever be such amout of data cumulated
+      // unless a logic fault in the consumer code.
+      NS_ERROR("IncrementalTokenizer consumer not reading data?");
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    if (!mInput.SetLength(remainder + load, fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    auto buffer = mInput.BeginWriting() + remainder;
+
+    uint32_t read;
+    rv = aInput->Read(buffer, load, &read);
+    if (NS_SUCCEEDED(rv)) {
+      // remainder + load fits the uint32_t size, so must remainder + read.
+      mInput.SetLength(remainder + read);
+      aCount -= read;
+
+      rv = Process();
+    }
+  }
+
+  return rv;
+}
+
+nsresult IncrementalTokenizer::FinishInput() {
+  NS_ENSURE_TRUE(mConsumer, NS_ERROR_NOT_INITIALIZED);
+  MOZ_ASSERT(!mInputFinished);
+  MOZ_ASSERT(!mConsuming);
+
+  mInput.Cut(0, mInputCursor);
+  mInputCursor = 0;
+
+  mInputFinished = true;
+  nsresult rv = Process();
+  mConsumer = nullptr;
+  return rv;
+}
+
+bool IncrementalTokenizer::Next(Token& aToken) {
+  // Assert we are called only from the consumer callback
+  MOZ_ASSERT(mConsuming);
+
+  if (mPastEof) {
+    return false;
+  }
+
+  nsACString::const_char_iterator next = Parse(aToken);
+  mPastEof = aToken.Type() == TOKEN_EOF;
+  if (next == mCursor && !mPastEof) {
+    // Not enough input to make a deterministic decision.
+    return false;
+  }
+
+  AssignFragment(aToken, mCursor, next);
+  mCursor = next;
+  return true;
+}
+
+void IncrementalTokenizer::NeedMoreInput() {
+  // Assert we are called only from the consumer callback
+  MOZ_ASSERT(mConsuming);
+
+  // When the input has been finished, we can't set the flag to prevent
+  // indefinite wait for more input (that will never come)
+  mNeedMoreInput = !mInputFinished;
+}
+
+void IncrementalTokenizer::Rollback() {
+  // Assert we are called only from the consumer callback
+  MOZ_ASSERT(mConsuming);
+
+  mRollback = true;
+}
+
+nsresult IncrementalTokenizer::Process() {
+#ifdef DEBUG
+  // Assert we are not re-entered
+  MOZ_ASSERT(!mConsuming);
+
+  AutoRestore<bool> consuming(mConsuming);
+  mConsuming = true;
+#endif
+
+  MOZ_ASSERT(!mPastEof);
+
+  nsresult rv = NS_OK;
+
+  mInput.BeginReading(mCursor);
+  mCursor += mInputCursor;
+  mInput.EndReading(mEnd);
+
+  while (NS_SUCCEEDED(rv) && !mPastEof) {
+    Token token;
+    nsACString::const_char_iterator next = Parse(token);
+    mPastEof = token.Type() == TOKEN_EOF;
+    if (next == mCursor && !mPastEof) {
+      // Not enough input to make a deterministic decision.
+      break;
+    }
+
+    AssignFragment(token, mCursor, next);
+
+    nsACString::const_char_iterator rollback = mCursor;
+    mCursor = next;
+
+    mNeedMoreInput = mRollback = false;
+
+    rv = mConsumer(token, *this);
+    if (NS_FAILED(rv)) {
+      break;
+    }
+    if (mNeedMoreInput || mRollback) {
+      mCursor = rollback;
+      mPastEof = false;
+      if (mNeedMoreInput) {
+        break;
+      }
+    }
+  }
+
+  mInputCursor = mCursor - mInput.BeginReading();
+  return rv;
+}
+
+}  // namespace mozilla
diff --git a/xpcom/ds/IncrementalTokenizer.h b/xpcom/ds/IncrementalTokenizer.h
new file mode 100644
index 0000000000..c2647052c9
--- /dev/null
+++ b/xpcom/ds/IncrementalTokenizer.h
@@ -0,0 +1,125 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef INCREMENTAL_TOKENIZER_H__
+#define INCREMENTAL_TOKENIZER_H__
+
+#include "mozilla/Tokenizer.h"
+
+#include "nsError.h"
+#include <functional>
+
+class nsIInputStream;
+
+namespace mozilla {
+
+class IncrementalTokenizer : public TokenizerBase<char> {
+ public:
+  /**
+   * The consumer callback.  The function is called for every single token
+   * as found in the input.  Failure result returned by this callback stops
+   * the tokenization immediately and bubbles to result of Feed/FinishInput.
+   *
+   * Fragment()s of consumed tokens are ensured to remain valid until next call
+   * to Feed/FinishInput and are pointing to a single linear buffer.  Hence,
+   * those can be safely used to accumulate the data for processing after
+   * Feed/FinishInput returned.
+   */
+  typedef std::function<nsresult(Token const&, IncrementalTokenizer& i)>
+      Consumer;
+
+  /**
+   * For aWhitespaces and aAdditionalWordChars arguments see TokenizerBase.
+   *
+   * @param aConsumer
+   *    A mandatory non-null argument, a function that consumes the tokens as
+   * they come when the tokenizer is fed.
+   * @param aRawMinBuffered
+   *    When we have buffered at least aRawMinBuffered data, but there was no
+   * custom token found so far because of too small incremental feed chunks,
+   * deliver the raw data to preserve streaming and to save memory.  This only
+   * has effect in OnlyCustomTokenizing mode.
+   */
+  explicit IncrementalTokenizer(Consumer&& aConsumer,
+                                const char* aWhitespaces = nullptr,
+                                const char* aAdditionalWordChars = nullptr,
+                                uint32_t aRawMinBuffered = 1024);
+
+  /**
+   * Pushes the input to be tokenized.  These directly call the Consumer
+   * callback on every found token.  Result of the Consumer callback is returned
+   * here.
+   *
+   * The tokenizer must be initialized with a valid consumer prior call to these
+   * methods.  It's not allowed to call Feed/FinishInput from inside the
+   * Consumer callback.
+   */
+  nsresult FeedInput(const nsACString& aInput);
+  nsresult FeedInput(nsIInputStream* aInput, uint32_t aCount);
+  nsresult FinishInput();
+
+  /**
+   * Can only be called from inside the consumer callback.
+   *
+   * When there is still anything to read from the input, tokenize it, store
+   * the token type and value to aToken result and shift the cursor past this
+   * just parsed token.  Each call to Next() reads another token from
+   * the input and shifts the cursor.
+   *
+   * Returns false if there is not enough data to deterministically recognize
+   * tokens or when the last returned token was EOF.
+   */
+  [[nodiscard]] bool Next(Token& aToken);
+
+  /**
+   * Can only be called from inside the consumer callback.
+   *
+   * Tells the tokenizer to revert the cursor and stop the async parsing until
+   * next feed of the input.  This is useful when more than one token is needed
+   * to decide on the syntax but there is not enough input to get a next token
+   * (Next() returned false.)
+   */
+  void NeedMoreInput();
+
+  /**
+   * Can only be called from inside the consumer callback.
+   *
+   * This makes the consumer callback be called again while parsing
+   * the input at the previous cursor position again.  This is useful when
+   * the tokenizer state (custom tokens, tokenization mode) has changed and
+   * we want to re-parse the input again.
+   */
+  void Rollback();
+
+ private:
+  // Loops over the input with TokenizerBase::Parse and calls the Consumer
+  // callback.
+  nsresult Process();
+
+#ifdef DEBUG
+  // True when inside the consumer callback, used only for assertions.
+  bool mConsuming;
+#endif  // DEBUG
+  // Modifyable only from the Consumer callback, tells the parser to break,
+  // rollback and wait for more input.
+  bool mNeedMoreInput;
+  // Modifyable only from the Consumer callback, tells the parser to rollback
+  // and parse the input again, with (if modified) new settings of the
+  // tokenizer.
+  bool mRollback;
+  // The input buffer.  Updated with each call to Feed/FinishInput.
+  nsCString mInput;
+  // Numerical index pointing at the current cursor position.  We don't keep
+  // direct reference to the string buffer since the buffer gets often
+  // reallocated.
+  nsCString::index_type mInputCursor;
+  // Refernce to the consumer function.
+  Consumer mConsumer;
+};
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/ds/MruCache.h b/xpcom/ds/MruCache.h
new file mode 100644
index 0000000000..35c86bf19f
--- /dev/null
+++ b/xpcom/ds/MruCache.h
@@ -0,0 +1,166 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_MruCache_h
+#define mozilla_MruCache_h
+
+#include <cstdint>
+#include <type_traits>
+#include <utility>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/HashFunctions.h"
+
+namespace mozilla {
+
+namespace detail {
+
+// Helper struct for checking if a value is empty.
+//
+// `IsNotEmpty` will return true if `Value` is not a pointer type or if the
+// pointer value is not null.
+template <typename Value, bool IsPtr = std::is_pointer<Value>::value>
+struct EmptyChecker {
+  static bool IsNotEmpty(const Value&) { return true; }
+};
+// Template specialization for the `IsPtr == true` case.
+template <typename Value>
+struct EmptyChecker<Value, true> {
+  static bool IsNotEmpty(const Value& aVal) { return aVal != nullptr; }
+};
+
+}  // namespace detail
+
+// Provides a most recently used cache that can be used as a layer on top of
+// a larger container where lookups can be expensive. The default size is 31,
+// which as a prime number provides a better distrubution of cached entries.
+//
+// Users are expected to provide a `Cache` class that defines two required
+// methods:
+//   - A method for providing the hash of a key:
+//
+//     static HashNumber Hash(const KeyType& aKey)
+//
+//   - A method for matching a key to a value, for pointer types the value
+//     is guaranteed not to be null.
+//
+//     static bool Match(const KeyType& aKey, const ValueType& aVal)
+//
+// For example:
+//    class MruExample : public MruCache<void*, PtrInfo*, MruExample>
+//    {
+//      static HashNumber Hash(const KeyType& aKey)
+//      {
+//        return HashGeneric(aKey);
+//      }
+//      static Match(const KeyType& aKey, const ValueType& aVal)
+//      {
+//        return aVal->mPtr == aKey;
+//      }
+//    };
+template <class Key, class Value, class Cache, size_t Size = 31>
+class MruCache {
+  // Best distribution is achieved with a prime number. Ideally the closest
+  // to a power of two will be the most efficient use of memory. This
+  // assertion is pretty weak, but should catch the common inclination to
+  // use a power-of-two.
+  static_assert(Size % 2 != 0, "Use a prime number");
+
+  // This is a stronger assertion but significantly limits the values to just
+  // those close to a power-of-two value.
+  // static_assert(Size == 7 || Size == 13 || Size == 31 || Size == 61 ||
+  //              Size == 127 || Size == 251 || Size == 509 || Size == 1021,
+  //              "Use a prime number less than 1024");
+
+ public:
+  using KeyType = Key;
+  using ValueType = Value;
+
+  MruCache() = default;
+  MruCache(const MruCache&) = delete;
+  MruCache(const MruCache&&) = delete;
+
+  // Inserts the given value into the cache. Potentially overwrites an
+  // existing entry.
+  template <typename U>
+  void Put(const KeyType& aKey, U&& aVal) {
+    *RawEntry(aKey) = std::forward<U>(aVal);
+  }
+
+  // Removes the given entry if it is in the cache.
+  void Remove(const KeyType& aKey) { Lookup(aKey).Remove(); }
+
+  // Clears all cached entries and resets them to a default value.
+  void Clear() {
+    for (ValueType& val : mCache) {
+      val = ValueType{};
+    }
+  }
+
+  // Helper that holds an entry that matched a lookup key. Usage:
+  //
+  //    auto p = mCache.Lookup(aKey);
+  //    if (p) {
+  //      return p.Data();
+  //    }
+  //
+  //    auto foo = new Foo();
+  //    mTable.Insert(aKey, foo);
+  //    p.Set(foo);
+  //    return foo;
+  class Entry {
+   public:
+    Entry(ValueType* aEntry, bool aMatch) : mEntry(aEntry), mMatch(aMatch) {
+      MOZ_ASSERT(mEntry);
+    }
+
+    explicit operator bool() const { return mMatch; }
+
+    ValueType& Data() const {
+      MOZ_ASSERT(mMatch);
+      return *mEntry;
+    }
+
+    template <typename U>
+    void Set(U&& aValue) {
+      mMatch = true;
+      Data() = std::forward<U>(aValue);
+    }
+
+    void Remove() {
+      if (mMatch) {
+        Data() = ValueType{};
+        mMatch = false;
+      }
+    }
+
+   private:
+    ValueType* mEntry;  // Location of the entry in the cache.
+    bool mMatch;        // Whether the value matched.
+  };
+
+  // Retrieves an entry from the cache. Can be used to test if an entry is
+  // present, update the entry to a new value, or remove the entry if one was
+  // matched.
+  Entry Lookup(const KeyType& aKey) {
+    using EmptyChecker = detail::EmptyChecker<ValueType>;
+
+    auto entry = RawEntry(aKey);
+    bool match = EmptyChecker::IsNotEmpty(*entry) && Cache::Match(aKey, *entry);
+    return Entry(entry, match);
+  }
+
+ private:
+  MOZ_ALWAYS_INLINE ValueType* RawEntry(const KeyType& aKey) {
+    return &mCache[Cache::Hash(aKey) % Size];
+  }
+
+  ValueType mCache[Size] = {};
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_mrucache_h
diff --git a/xpcom/ds/Observer.h b/xpcom/ds/Observer.h
new file mode 100644
index 0000000000..3d189603eb
--- /dev/null
+++ b/xpcom/ds/Observer.h
@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_Observer_h
+#define mozilla_Observer_h
+
+#include "nsTObserverArray.h"
+
+namespace mozilla {
+
+/**
+ * Observer<T> provides a way for a class to observe something.
+ * When an event has to be broadcasted to all Observer<T>, Notify() method
+ * is called.
+ * T represents the type of the object passed in argument to Notify().
+ *
+ * @see ObserverList.
+ */
+template <class T>
+class Observer {
+ public:
+  virtual ~Observer() = default;
+  virtual void Notify(const T& aParam) = 0;
+};
+
+/**
+ * ObserverList<T> tracks Observer<T> and can notify them when Broadcast() is
+ * called.
+ * T represents the type of the object passed in argument to Broadcast() and
+ * sent to Observer<T> objects through Notify().
+ *
+ * @see Observer.
+ */
+template <class T>
+class ObserverList {
+ public:
+  /**
+   * Note: When calling AddObserver, it's up to the caller to make sure the
+   * object isn't going to be release as long as RemoveObserver hasn't been
+   * called.
+   *
+   * @see RemoveObserver()
+   */
+  void AddObserver(Observer<T>* aObserver) {
+    mObservers.AppendElementUnlessExists(aObserver);
+  }
+
+  /**
+   * Remove the observer from the observer list.
+   * @return Whether the observer has been found in the list.
+   */
+  bool RemoveObserver(Observer<T>* aObserver) {
+    return mObservers.RemoveElement(aObserver);
+  }
+
+  uint32_t Length() { return mObservers.Length(); }
+
+  /**
+   * Call Notify() on each item in the list.
+   */
+  void Broadcast(const T& aParam) {
+    for (Observer<T>* obs : mObservers.ForwardRange()) {
+      obs->Notify(aParam);
+    }
+  }
+
+ protected:
+  nsTObserverArray<Observer<T>*> mObservers;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_Observer_h
diff --git a/xpcom/ds/PLDHashTable.cpp b/xpcom/ds/PLDHashTable.cpp
new file mode 100644
index 0000000000..2814813f68
--- /dev/null
+++ b/xpcom/ds/PLDHashTable.cpp
@@ -0,0 +1,870 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <new>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "PLDHashTable.h"
+#include "nsDebug.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/ScopeExit.h"
+#include "nsAlgorithm.h"
+#include "nsPointerHashKeys.h"
+#include "mozilla/Likely.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ChaosMode.h"
+
+using namespace mozilla;
+
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+
+class AutoReadOp {
+  Checker& mChk;
+
+ public:
+  explicit AutoReadOp(Checker& aChk) : mChk(aChk) { mChk.StartReadOp(); }
+  ~AutoReadOp() { mChk.EndReadOp(); }
+};
+
+class AutoWriteOp {
+  Checker& mChk;
+
+ public:
+  explicit AutoWriteOp(Checker& aChk) : mChk(aChk) { mChk.StartWriteOp(); }
+  ~AutoWriteOp() { mChk.EndWriteOp(); }
+};
+
+class AutoIteratorRemovalOp {
+  Checker& mChk;
+
+ public:
+  explicit AutoIteratorRemovalOp(Checker& aChk) : mChk(aChk) {
+    mChk.StartIteratorRemovalOp();
+  }
+  ~AutoIteratorRemovalOp() { mChk.EndIteratorRemovalOp(); }
+};
+
+class AutoDestructorOp {
+  Checker& mChk;
+
+ public:
+  explicit AutoDestructorOp(Checker& aChk) : mChk(aChk) {
+    mChk.StartDestructorOp();
+  }
+  ~AutoDestructorOp() { mChk.EndDestructorOp(); }
+};
+
+#endif
+
+/* static */
+PLDHashNumber PLDHashTable::HashStringKey(const void* aKey) {
+  return HashString(static_cast<const char*>(aKey));
+}
+
+/* static */
+PLDHashNumber PLDHashTable::HashVoidPtrKeyStub(const void* aKey) {
+  return nsPtrHashKey<void>::HashKey(aKey);
+}
+
+/* static */
+bool PLDHashTable::MatchEntryStub(const PLDHashEntryHdr* aEntry,
+                                  const void* aKey) {
+  const PLDHashEntryStub* stub = (const PLDHashEntryStub*)aEntry;
+
+  return stub->key == aKey;
+}
+
+/* static */
+bool PLDHashTable::MatchStringKey(const PLDHashEntryHdr* aEntry,
+                                  const void* aKey) {
+  const PLDHashEntryStub* stub = (const PLDHashEntryStub*)aEntry;
+
+  // XXX tolerate null keys on account of sloppy Mozilla callers.
+  return stub->key == aKey ||
+         (stub->key && aKey &&
+          strcmp((const char*)stub->key, (const char*)aKey) == 0);
+}
+
+/* static */
+void PLDHashTable::MoveEntryStub(PLDHashTable* aTable,
+                                 const PLDHashEntryHdr* aFrom,
+                                 PLDHashEntryHdr* aTo) {
+  memcpy(aTo, aFrom, aTable->mEntrySize);
+}
+
+/* static */
+void PLDHashTable::ClearEntryStub(PLDHashTable* aTable,
+                                  PLDHashEntryHdr* aEntry) {
+  memset(aEntry, 0, aTable->mEntrySize);
+}
+
+static const PLDHashTableOps gStubOps = {
+    PLDHashTable::HashVoidPtrKeyStub, PLDHashTable::MatchEntryStub,
+    PLDHashTable::MoveEntryStub, PLDHashTable::ClearEntryStub, nullptr};
+
+/* static */ const PLDHashTableOps* PLDHashTable::StubOps() {
+  return &gStubOps;
+}
+
+static bool SizeOfEntryStore(uint32_t aCapacity, uint32_t aEntrySize,
+                             uint32_t* aNbytes) {
+  uint32_t slotSize = aEntrySize + sizeof(PLDHashNumber);
+  uint64_t nbytes64 = uint64_t(aCapacity) * uint64_t(slotSize);
+  *aNbytes = aCapacity * slotSize;
+  return uint64_t(*aNbytes) == nbytes64;  // returns false on overflow
+}
+
+// Compute max and min load numbers (entry counts). We have a secondary max
+// that allows us to overload a table reasonably if it cannot be grown further
+// (i.e. if ChangeTable() fails). The table slows down drastically if the
+// secondary max is too close to 1, but 0.96875 gives only a slight slowdown
+// while allowing 1.3x more elements.
+static inline uint32_t MaxLoad(uint32_t aCapacity) {
+  return aCapacity - (aCapacity >> 2);  // == aCapacity * 0.75
+}
+static inline uint32_t MaxLoadOnGrowthFailure(uint32_t aCapacity) {
+  return aCapacity - (aCapacity >> 5);  // == aCapacity * 0.96875
+}
+static inline uint32_t MinLoad(uint32_t aCapacity) {
+  return aCapacity >> 2;  // == aCapacity * 0.25
+}
+
+// Compute the minimum capacity (and the Log2 of that capacity) for a table
+// containing |aLength| elements while respecting the following contraints:
+// - table must be at most 75% full;
+// - capacity must be a power of two;
+// - capacity cannot be too small.
+static inline void BestCapacity(uint32_t aLength, uint32_t* aCapacityOut,
+                                uint32_t* aLog2CapacityOut) {
+  // Callers should ensure this is true.
+  MOZ_ASSERT(aLength <= PLDHashTable::kMaxInitialLength);
+
+  // Compute the smallest capacity allowing |aLength| elements to be inserted
+  // without rehashing.
+  uint32_t capacity = (aLength * 4 + (3 - 1)) / 3;  // == ceil(aLength * 4 / 3)
+  if (capacity < PLDHashTable::kMinCapacity) {
+    capacity = PLDHashTable::kMinCapacity;
+  }
+
+  // Round up capacity to next power-of-two.
+  uint32_t log2 = CeilingLog2(capacity);
+  capacity = 1u << log2;
+  MOZ_ASSERT(capacity <= PLDHashTable::kMaxCapacity);
+
+  *aCapacityOut = capacity;
+  *aLog2CapacityOut = log2;
+}
+
+/* static */ MOZ_ALWAYS_INLINE uint32_t
+PLDHashTable::HashShift(uint32_t aEntrySize, uint32_t aLength) {
+  if (aLength > kMaxInitialLength) {
+    MOZ_CRASH("Initial length is too large");
+  }
+
+  uint32_t capacity, log2;
+  BestCapacity(aLength, &capacity, &log2);
+
+  uint32_t nbytes;
+  if (!SizeOfEntryStore(capacity, aEntrySize, &nbytes)) {
+    MOZ_CRASH("Initial entry store size is too large");
+  }
+
+  // Compute the hashShift value.
+  return kPLDHashNumberBits - log2;
+}
+
+PLDHashTable::PLDHashTable(const PLDHashTableOps* aOps, uint32_t aEntrySize,
+                           uint32_t aLength)
+    : mOps(aOps),
+      mEntryStore(),
+      mGeneration(0),
+      mHashShift(HashShift(aEntrySize, aLength)),
+      mEntrySize(aEntrySize),
+      mEntryCount(0),
+      mRemovedCount(0) {
+  // An entry size greater than 0xff is unlikely, but let's check anyway. If
+  // you hit this, your hashtable would waste lots of space for unused entries
+  // and you should change your hash table's entries to pointers.
+  if (aEntrySize != uint32_t(mEntrySize)) {
+    MOZ_CRASH("Entry size is too large");
+  }
+}
+
+PLDHashTable& PLDHashTable::operator=(PLDHashTable&& aOther) {
+  if (this == &aOther) {
+    return *this;
+  }
+
+  // |mOps| and |mEntrySize| are required to stay the same, they're
+  // conceptually part of the type -- indeed, if PLDHashTable was a templated
+  // type like nsTHashtable, they *would* be part of the type -- so it only
+  // makes sense to assign in cases where they match.
+  MOZ_RELEASE_ASSERT(mOps == aOther.mOps || !mOps);
+  MOZ_RELEASE_ASSERT(mEntrySize == aOther.mEntrySize || !mEntrySize);
+
+  // Reconstruct |this|.
+  const PLDHashTableOps* ops = aOther.mOps;
+  this->~PLDHashTable();
+  new (KnownNotNull, this) PLDHashTable(ops, aOther.mEntrySize, 0);
+
+  // Move non-const pieces over.
+  mHashShift = std::move(aOther.mHashShift);
+  mEntryCount = std::move(aOther.mEntryCount);
+  mRemovedCount = std::move(aOther.mRemovedCount);
+  mEntryStore.Set(aOther.mEntryStore.Get(), &mGeneration);
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mChecker = std::move(aOther.mChecker);
+#endif
+
+  // Clear up |aOther| so its destruction will be a no-op and it reports being
+  // empty.
+  {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+    AutoDestructorOp op(mChecker);
+#endif
+    aOther.mEntryCount = 0;
+    aOther.mEntryStore.Set(nullptr, &aOther.mGeneration);
+  }
+
+  return *this;
+}
+
+PLDHashNumber PLDHashTable::Hash1(PLDHashNumber aHash0) const {
+  return aHash0 >> mHashShift;
+}
+
+void PLDHashTable::Hash2(PLDHashNumber aHash0, uint32_t& aHash2Out,
+                         uint32_t& aSizeMaskOut) const {
+  uint32_t sizeLog2 = kPLDHashNumberBits - mHashShift;
+  uint32_t sizeMask = (PLDHashNumber(1) << sizeLog2) - 1;
+  aSizeMaskOut = sizeMask;
+
+  // The incoming aHash0 always has the low bit unset (since we leave it
+  // free for the collision flag), and should have reasonably random
+  // data in the other 31 bits.  We used the high bits of aHash0 for
+  // Hash1, so we use the low bits here.  If the table size is large,
+  // the bits we use may overlap, but that's still more random than
+  // filling with 0s.
+  //
+  // Double hashing needs the second hash code to be relatively prime to table
+  // size, so we simply make hash2 odd.
+  //
+  // This also conveniently covers up the fact that we have the low bit
+  // unset since aHash0 has the low bit unset.
+  aHash2Out = (aHash0 & sizeMask) | 1;
+}
+
+// Reserve mKeyHash 0 for free entries and 1 for removed-entry sentinels. Note
+// that a removed-entry sentinel need be stored only if the removed entry had
+// a colliding entry added after it. Therefore we can use 1 as the collision
+// flag in addition to the removed-entry sentinel value. Multiplicative hash
+// uses the high order bits of mKeyHash, so this least-significant reservation
+// should not hurt the hash function's effectiveness much.
+
+// Match an entry's mKeyHash against an unstored one computed from a key.
+/* static */
+bool PLDHashTable::MatchSlotKeyhash(Slot& aSlot, const PLDHashNumber aKeyHash) {
+  return (aSlot.KeyHash() & ~kCollisionFlag) == aKeyHash;
+}
+
+// Compute the address of the indexed entry in table.
+auto PLDHashTable::SlotForIndex(uint32_t aIndex) const -> Slot {
+  return mEntryStore.SlotForIndex(aIndex, mEntrySize, CapacityFromHashShift());
+}
+
+PLDHashTable::~PLDHashTable() {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  AutoDestructorOp op(mChecker);
+#endif
+
+  if (!mEntryStore.IsAllocated()) {
+    return;
+  }
+
+  // Clear any remaining live entries (if not trivially destructible).
+  if (mOps->clearEntry) {
+    mEntryStore.ForEachSlot(Capacity(), mEntrySize, [&](const Slot& aSlot) {
+      if (aSlot.IsLive()) {
+        mOps->clearEntry(this, aSlot.ToEntry());
+      }
+    });
+  }
+
+  // Entry storage is freed last, by ~EntryStore().
+}
+
+void PLDHashTable::ClearAndPrepareForLength(uint32_t aLength) {
+  // Get these values before the destructor clobbers them.
+  const PLDHashTableOps* ops = mOps;
+  uint32_t entrySize = mEntrySize;
+
+  this->~PLDHashTable();
+  new (KnownNotNull, this) PLDHashTable(ops, entrySize, aLength);
+}
+
+void PLDHashTable::Clear() { ClearAndPrepareForLength(kDefaultInitialLength); }
+
+// If |Reason| is |ForAdd|, the return value is always non-null and it may be
+// a previously-removed entry. If |Reason| is |ForSearchOrRemove|, the return
+// value is null on a miss, and will never be a previously-removed entry on a
+// hit. This distinction is a bit grotty but this function is hot enough that
+// these differences are worthwhile. (It's also hot enough that
+// MOZ_ALWAYS_INLINE makes a significant difference.)
+template <PLDHashTable::SearchReason Reason, typename Success, typename Failure>
+MOZ_ALWAYS_INLINE auto PLDHashTable::SearchTable(const void* aKey,
+                                                 PLDHashNumber aKeyHash,
+                                                 Success&& aSuccess,
+                                                 Failure&& aFailure) const {
+  MOZ_ASSERT(mEntryStore.IsAllocated());
+  NS_ASSERTION(!(aKeyHash & kCollisionFlag), "!(aKeyHash & kCollisionFlag)");
+
+  // Compute the primary hash address.
+  PLDHashNumber hash1 = Hash1(aKeyHash);
+  Slot slot = SlotForIndex(hash1);
+
+  // Miss: return space for a new entry.
+  if (slot.IsFree()) {
+    return (Reason == ForAdd) ? aSuccess(slot) : aFailure();
+  }
+
+  // Hit: return entry.
+  PLDHashMatchEntry matchEntry = mOps->matchEntry;
+  if (MatchSlotKeyhash(slot, aKeyHash)) {
+    PLDHashEntryHdr* e = slot.ToEntry();
+    if (matchEntry(e, aKey)) {
+      return aSuccess(slot);
+    }
+  }
+
+  // Collision: double hash.
+  PLDHashNumber hash2;
+  uint32_t sizeMask;
+  Hash2(aKeyHash, hash2, sizeMask);
+
+  // Save the first removed entry slot so Add() can recycle it. (Only used
+  // if Reason==ForAdd.)
+  Maybe<Slot> firstRemoved;
+
+  for (;;) {
+    if (Reason == ForAdd && !firstRemoved) {
+      if (MOZ_UNLIKELY(slot.IsRemoved())) {
+        firstRemoved.emplace(slot);
+      } else {
+        slot.MarkColliding();
+      }
+    }
+
+    hash1 -= hash2;
+    hash1 &= sizeMask;
+
+    slot = SlotForIndex(hash1);
+    if (slot.IsFree()) {
+      if (Reason != ForAdd) {
+        return aFailure();
+      }
+      return aSuccess(firstRemoved.refOr(slot));
+    }
+
+    if (MatchSlotKeyhash(slot, aKeyHash)) {
+      PLDHashEntryHdr* e = slot.ToEntry();
+      if (matchEntry(e, aKey)) {
+        return aSuccess(slot);
+      }
+    }
+  }
+
+  // NOTREACHED
+  return aFailure();
+}
+
+// This is a copy of SearchTable(), used by ChangeTable(), hardcoded to
+//   1. assume |Reason| is |ForAdd|,
+//   2. assume that |aKey| will never match an existing entry, and
+//   3. assume that no entries have been removed from the current table
+//      structure.
+// Avoiding the need for |aKey| means we can avoid needing a way to map entries
+// to keys, which means callers can use complex key types more easily.
+MOZ_ALWAYS_INLINE auto PLDHashTable::FindFreeSlot(PLDHashNumber aKeyHash) const
+    -> Slot {
+  MOZ_ASSERT(mEntryStore.IsAllocated());
+  NS_ASSERTION(!(aKeyHash & kCollisionFlag), "!(aKeyHash & kCollisionFlag)");
+
+  // Compute the primary hash address.
+  PLDHashNumber hash1 = Hash1(aKeyHash);
+  Slot slot = SlotForIndex(hash1);
+
+  // Miss: return space for a new entry.
+  if (slot.IsFree()) {
+    return slot;
+  }
+
+  // Collision: double hash.
+  PLDHashNumber hash2;
+  uint32_t sizeMask;
+  Hash2(aKeyHash, hash2, sizeMask);
+
+  for (;;) {
+    MOZ_ASSERT(!slot.IsRemoved());
+    slot.MarkColliding();
+
+    hash1 -= hash2;
+    hash1 &= sizeMask;
+
+    slot = SlotForIndex(hash1);
+    if (slot.IsFree()) {
+      return slot;
+    }
+  }
+
+  // NOTREACHED
+}
+
+bool PLDHashTable::ChangeTable(int32_t aDeltaLog2) {
+  MOZ_ASSERT(mEntryStore.IsAllocated());
+
+  // Look, but don't touch, until we succeed in getting new entry store.
+  int32_t oldLog2 = kPLDHashNumberBits - mHashShift;
+  int32_t newLog2 = oldLog2 + aDeltaLog2;
+  uint32_t newCapacity = 1u << newLog2;
+  if (newCapacity > kMaxCapacity) {
+    return false;
+  }
+
+  uint32_t nbytes;
+  if (!SizeOfEntryStore(newCapacity, mEntrySize, &nbytes)) {
+    return false;  // overflowed
+  }
+
+  char* newEntryStore = (char*)calloc(1, nbytes);
+  if (!newEntryStore) {
+    return false;
+  }
+
+  // We can't fail from here on, so update table parameters.
+  mHashShift = kPLDHashNumberBits - newLog2;
+  mRemovedCount = 0;
+
+  // Assign the new entry store to table.
+  char* oldEntryStore = mEntryStore.Get();
+  mEntryStore.Set(newEntryStore, &mGeneration);
+  PLDHashMoveEntry moveEntry = mOps->moveEntry;
+
+  // Copy only live entries, leaving removed ones behind.
+  uint32_t oldCapacity = 1u << oldLog2;
+  EntryStore::ForEachSlot(
+      oldEntryStore, oldCapacity, mEntrySize, [&](const Slot& slot) {
+        if (slot.IsLive()) {
+          const PLDHashNumber key = slot.KeyHash() & ~kCollisionFlag;
+          Slot newSlot = FindFreeSlot(key);
+          MOZ_ASSERT(newSlot.IsFree());
+          moveEntry(this, slot.ToEntry(), newSlot.ToEntry());
+          newSlot.SetKeyHash(key);
+        }
+      });
+
+  free(oldEntryStore);
+  return true;
+}
+
+MOZ_ALWAYS_INLINE PLDHashNumber
+PLDHashTable::ComputeKeyHash(const void* aKey) const {
+  MOZ_ASSERT(mEntryStore.IsAllocated());
+
+  PLDHashNumber keyHash = mozilla::ScrambleHashCode(mOps->hashKey(aKey));
+
+  // Avoid 0 and 1 hash codes, they indicate free and removed entries.
+  if (keyHash < 2) {
+    keyHash -= 2;
+  }
+  keyHash &= ~kCollisionFlag;
+
+  return keyHash;
+}
+
+PLDHashEntryHdr* PLDHashTable::Search(const void* aKey) const {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  AutoReadOp op(mChecker);
+#endif
+
+  if (!mEntryStore.IsAllocated()) {
+    return nullptr;
+  }
+
+  return SearchTable<ForSearchOrRemove>(
+      aKey, ComputeKeyHash(aKey),
+      [&](Slot& slot) -> PLDHashEntryHdr* { return slot.ToEntry(); },
+      [&]() -> PLDHashEntryHdr* { return nullptr; });
+}
+
+PLDHashEntryHdr* PLDHashTable::Add(const void* aKey,
+                                   const mozilla::fallible_t& aFallible) {
+  auto maybeEntryHandle = MakeEntryHandle(aKey, aFallible);
+  if (!maybeEntryHandle) {
+    return nullptr;
+  }
+  return maybeEntryHandle->OrInsert([&aKey, this](PLDHashEntryHdr* entry) {
+    if (mOps->initEntry) {
+      mOps->initEntry(entry, aKey);
+    }
+  });
+}
+
+PLDHashEntryHdr* PLDHashTable::Add(const void* aKey) {
+  return MakeEntryHandle(aKey).OrInsert([&aKey, this](PLDHashEntryHdr* entry) {
+    if (mOps->initEntry) {
+      mOps->initEntry(entry, aKey);
+    }
+  });
+}
+
+void PLDHashTable::Remove(const void* aKey) {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  AutoWriteOp op(mChecker);
+#endif
+
+  if (!mEntryStore.IsAllocated()) {
+    return;
+  }
+
+  PLDHashNumber keyHash = ComputeKeyHash(aKey);
+  SearchTable<ForSearchOrRemove>(
+      aKey, keyHash,
+      [&](Slot& slot) {
+        RawRemove(slot);
+        ShrinkIfAppropriate();
+      },
+      [&]() {
+        // Do nothing.
+      });
+}
+
+void PLDHashTable::RemoveEntry(PLDHashEntryHdr* aEntry) {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  AutoWriteOp op(mChecker);
+#endif
+
+  RawRemove(aEntry);
+  ShrinkIfAppropriate();
+}
+
+void PLDHashTable::RawRemove(PLDHashEntryHdr* aEntry) {
+  Slot slot(mEntryStore.SlotForPLDHashEntry(aEntry, Capacity(), mEntrySize));
+  RawRemove(slot);
+}
+
+void PLDHashTable::RawRemove(Slot& aSlot) {
+  // Unfortunately, we can only do weak checking here. That's because
+  // RawRemove() can be called legitimately while an Enumerate() call is
+  // active, which doesn't fit well into how Checker's mState variable works.
+  MOZ_ASSERT(mChecker.IsWritable());
+
+  MOZ_ASSERT(mEntryStore.IsAllocated());
+
+  MOZ_ASSERT(aSlot.IsLive());
+
+  // Load keyHash first in case clearEntry() goofs it.
+  PLDHashNumber keyHash = aSlot.KeyHash();
+  if (mOps->clearEntry) {
+    PLDHashEntryHdr* entry = aSlot.ToEntry();
+    mOps->clearEntry(this, entry);
+  }
+  if (keyHash & kCollisionFlag) {
+    aSlot.MarkRemoved();
+    mRemovedCount++;
+  } else {
+    aSlot.MarkFree();
+  }
+  mEntryCount--;
+}
+
+// Shrink or compress if a quarter or more of all entries are removed, or if the
+// table is underloaded according to the minimum alpha, and is not minimal-size
+// already.
+void PLDHashTable::ShrinkIfAppropriate() {
+  uint32_t capacity = Capacity();
+  if (mRemovedCount >= capacity >> 2 ||
+      (capacity > kMinCapacity && mEntryCount <= MinLoad(capacity))) {
+    uint32_t log2;
+    BestCapacity(mEntryCount, &capacity, &log2);
+
+    int32_t deltaLog2 = log2 - (kPLDHashNumberBits - mHashShift);
+    MOZ_ASSERT(deltaLog2 <= 0);
+
+    (void)ChangeTable(deltaLog2);
+  }
+}
+
+size_t PLDHashTable::ShallowSizeOfExcludingThis(
+    MallocSizeOf aMallocSizeOf) const {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  AutoReadOp op(mChecker);
+#endif
+
+  return aMallocSizeOf(mEntryStore.Get());
+}
+
+size_t PLDHashTable::ShallowSizeOfIncludingThis(
+    MallocSizeOf aMallocSizeOf) const {
+  return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+}
+
+mozilla::Maybe<PLDHashTable::EntryHandle> PLDHashTable::MakeEntryHandle(
+    const void* aKey, const mozilla::fallible_t&) {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mChecker.StartWriteOp();
+  auto endWriteOp = MakeScopeExit([&] { mChecker.EndWriteOp(); });
+#endif
+
+  // Allocate the entry storage if it hasn't already been allocated.
+  if (!mEntryStore.IsAllocated()) {
+    uint32_t nbytes;
+    // We already checked this in the constructor, so it must still be true.
+    MOZ_RELEASE_ASSERT(
+        SizeOfEntryStore(CapacityFromHashShift(), mEntrySize, &nbytes));
+    mEntryStore.Set((char*)calloc(1, nbytes), &mGeneration);
+    if (!mEntryStore.IsAllocated()) {
+      return Nothing();
+    }
+  }
+
+  // If alpha is >= .75, grow or compress the table. If aKey is already in the
+  // table, we may grow once more than necessary, but only if we are on the
+  // edge of being overloaded.
+  uint32_t capacity = Capacity();
+  if (mEntryCount + mRemovedCount >= MaxLoad(capacity)) {
+    // Compress if a quarter or more of all entries are removed.
+    int deltaLog2 = 1;
+    if (mRemovedCount >= capacity >> 2) {
+      deltaLog2 = 0;
+    }
+
+    // Grow or compress the table. If ChangeTable() fails, allow overloading up
+    // to the secondary max. Once we hit the secondary max, return null.
+    if (!ChangeTable(deltaLog2) &&
+        mEntryCount + mRemovedCount >= MaxLoadOnGrowthFailure(capacity)) {
+      return Nothing();
+    }
+  }
+
+  // Look for entry after possibly growing, so we don't have to add it,
+  // then skip it while growing the table and re-add it after.
+  PLDHashNumber keyHash = ComputeKeyHash(aKey);
+  Slot slot = SearchTable<ForAdd>(
+      aKey, keyHash, [](Slot& found) -> Slot { return found; },
+      []() -> Slot {
+        MOZ_CRASH("Nope");
+        return Slot(nullptr, nullptr);
+      });
+
+  // The `EntryHandle` will handle ending the write op when it is destroyed.
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  endWriteOp.release();
+#endif
+
+  return Some(EntryHandle{this, keyHash, slot});
+}
+
+PLDHashTable::EntryHandle PLDHashTable::MakeEntryHandle(const void* aKey) {
+  auto res = MakeEntryHandle(aKey, fallible);
+  if (!res) {
+    if (!mEntryStore.IsAllocated()) {
+      // We OOM'd while allocating the initial entry storage.
+      uint32_t nbytes;
+      (void)SizeOfEntryStore(CapacityFromHashShift(), mEntrySize, &nbytes);
+      NS_ABORT_OOM(nbytes);
+    } else {
+      // We failed to resize the existing entry storage, either due to OOM or
+      // because we exceeded the maximum table capacity or size; report it as
+      // an OOM. The multiplication by 2 gets us the size we tried to allocate,
+      // which is double the current size.
+      NS_ABORT_OOM(2 * EntrySize() * EntryCount());
+    }
+  }
+  return res.extract();
+}
+
+PLDHashTable::EntryHandle::EntryHandle(PLDHashTable* aTable,
+                                       PLDHashNumber aKeyHash, Slot aSlot)
+    : mTable(aTable), mKeyHash(aKeyHash), mSlot(aSlot) {}
+
+PLDHashTable::EntryHandle::EntryHandle(EntryHandle&& aOther) noexcept
+    : mTable(std::exchange(aOther.mTable, nullptr)),
+      mKeyHash(aOther.mKeyHash),
+      mSlot(aOther.mSlot) {}
+
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+PLDHashTable::EntryHandle::~EntryHandle() {
+  if (!mTable) {
+    return;
+  }
+
+  mTable->mChecker.EndWriteOp();
+}
+#endif
+
+void PLDHashTable::EntryHandle::Remove() {
+  MOZ_ASSERT(HasEntry());
+
+  mTable->RawRemove(mSlot);
+}
+
+void PLDHashTable::EntryHandle::OrRemove() {
+  if (HasEntry()) {
+    Remove();
+  }
+}
+
+void PLDHashTable::EntryHandle::OccupySlot() {
+  MOZ_ASSERT(!HasEntry());
+
+  PLDHashNumber keyHash = mKeyHash;
+  if (mSlot.IsRemoved()) {
+    mTable->mRemovedCount--;
+    keyHash |= kCollisionFlag;
+  }
+  mSlot.SetKeyHash(keyHash);
+  mTable->mEntryCount++;
+}
+
+PLDHashTable::Iterator::Iterator(Iterator&& aOther)
+    : mTable(aOther.mTable),
+      mCurrent(aOther.mCurrent),
+      mNexts(aOther.mNexts),
+      mNextsLimit(aOther.mNextsLimit),
+      mHaveRemoved(aOther.mHaveRemoved),
+      mEntrySize(aOther.mEntrySize) {
+  // No need to change |mChecker| here.
+  aOther.mTable = nullptr;
+  // We don't really have the concept of a null slot, so leave mCurrent.
+  aOther.mNexts = 0;
+  aOther.mNextsLimit = 0;
+  aOther.mHaveRemoved = false;
+  aOther.mEntrySize = 0;
+}
+
+PLDHashTable::Iterator::Iterator(PLDHashTable* aTable)
+    : mTable(aTable),
+      mCurrent(mTable->mEntryStore.SlotForIndex(0, mTable->mEntrySize,
+                                                mTable->Capacity())),
+      mNexts(0),
+      mNextsLimit(mTable->EntryCount()),
+      mHaveRemoved(false),
+      mEntrySize(aTable->mEntrySize) {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mTable->mChecker.StartReadOp();
+#endif
+
+  if (ChaosMode::isActive(ChaosFeature::HashTableIteration) &&
+      mTable->Capacity() > 0) {
+    // Start iterating at a random entry. It would be even more chaotic to
+    // iterate in fully random order, but that's harder.
+    uint32_t capacity = mTable->CapacityFromHashShift();
+    uint32_t i = ChaosMode::randomUint32LessThan(capacity);
+    mCurrent =
+        mTable->mEntryStore.SlotForIndex(i, mTable->mEntrySize, capacity);
+  }
+
+  // Advance to the first live entry, if there is one.
+  if (!Done() && IsOnNonLiveEntry()) {
+    MoveToNextLiveEntry();
+  }
+}
+
+PLDHashTable::Iterator::Iterator(PLDHashTable* aTable, EndIteratorTag aTag)
+    : mTable(aTable),
+      mCurrent(mTable->mEntryStore.SlotForIndex(0, mTable->mEntrySize,
+                                                mTable->Capacity())),
+      mNexts(mTable->EntryCount()),
+      mNextsLimit(mTable->EntryCount()),
+      mHaveRemoved(false),
+      mEntrySize(aTable->mEntrySize) {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mTable->mChecker.StartReadOp();
+#endif
+
+  MOZ_ASSERT(Done());
+}
+
+PLDHashTable::Iterator::Iterator(const Iterator& aOther)
+    : mTable(aOther.mTable),
+      mCurrent(aOther.mCurrent),
+      mNexts(aOther.mNexts),
+      mNextsLimit(aOther.mNextsLimit),
+      mHaveRemoved(aOther.mHaveRemoved),
+      mEntrySize(aOther.mEntrySize) {
+  // TODO: Is this necessary?
+  MOZ_ASSERT(!mHaveRemoved);
+
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mTable->mChecker.StartReadOp();
+#endif
+}
+
+PLDHashTable::Iterator::~Iterator() {
+  if (mTable) {
+    if (mHaveRemoved) {
+      mTable->ShrinkIfAppropriate();
+    }
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+    mTable->mChecker.EndReadOp();
+#endif
+  }
+}
+
+MOZ_ALWAYS_INLINE bool PLDHashTable::Iterator::IsOnNonLiveEntry() const {
+  MOZ_ASSERT(!Done());
+  return !mCurrent.IsLive();
+}
+
+void PLDHashTable::Iterator::Next() {
+  MOZ_ASSERT(!Done());
+
+  mNexts++;
+
+  // Advance to the next live entry, if there is one.
+  if (!Done()) {
+    MoveToNextLiveEntry();
+  }
+}
+
+MOZ_ALWAYS_INLINE void PLDHashTable::Iterator::MoveToNextLiveEntry() {
+  // Chaos mode requires wraparound to cover all possible entries, so we can't
+  // simply move to the next live entry and stop when we hit the end of the
+  // entry store. But we don't want to introduce extra branches into our inner
+  // loop. So we are going to exploit the structure of the entry store in this
+  // method to implement an efficient inner loop.
+  //
+  // The idea is that since we are really only iterating through the stored
+  // hashes and because we know that there are a power-of-two number of
+  // hashes, we can use masking to implement the wraparound for us. This
+  // method does have the downside of needing to recalculate where the
+  // associated entry is once we've found it, but that seems OK.
+
+  // Our current slot and its associated hash.
+  Slot slot = mCurrent;
+  PLDHashNumber* p = slot.HashPtr();
+  const uint32_t capacity = mTable->CapacityFromHashShift();
+  const uint32_t mask = capacity - 1;
+  auto hashes = reinterpret_cast<PLDHashNumber*>(mTable->mEntryStore.Get());
+  uint32_t slotIndex = p - hashes;
+
+  do {
+    slotIndex = (slotIndex + 1) & mask;
+  } while (!Slot::IsLiveHash(hashes[slotIndex]));
+
+  // slotIndex now indicates where a live slot is. Rematerialize the entry
+  // and the slot.
+  mCurrent = mTable->mEntryStore.SlotForIndex(slotIndex, mEntrySize, capacity);
+}
+
+void PLDHashTable::Iterator::Remove() {
+  mTable->RawRemove(mCurrent);
+  mHaveRemoved = true;
+}
diff --git a/xpcom/ds/PLDHashTable.h b/xpcom/ds/PLDHashTable.h
new file mode 100644
index 0000000000..af17ee6fd6
--- /dev/null
+++ b/xpcom/ds/PLDHashTable.h
@@ -0,0 +1,805 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// See the comment at the top of mfbt/HashTable.h for a comparison between
+// PLDHashTable and mozilla::HashTable.
+
+#ifndef PLDHashTable_h
+#define PLDHashTable_h
+
+#include <utility>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/fallible.h"
+#include "nscore.h"
+
+using PLDHashNumber = mozilla::HashNumber;
+static const uint32_t kPLDHashNumberBits = mozilla::kHashNumberBits;
+
+#if defined(DEBUG) || defined(FUZZING)
+#  define MOZ_HASH_TABLE_CHECKS_ENABLED 1
+#endif
+
+class PLDHashTable;
+struct PLDHashTableOps;
+
+// Table entry header structure.
+//
+// In order to allow in-line allocation of key and value, we do not declare
+// either here. Instead, the API uses const void *key as a formal parameter.
+// The key need not be stored in the entry; it may be part of the value, but
+// need not be stored at all.
+//
+// Callback types are defined below and grouped into the PLDHashTableOps
+// structure, for single static initialization per hash table sub-type.
+//
+// Each hash table sub-type should make its entry type a subclass of
+// PLDHashEntryHdr. PLDHashEntryHdr is merely a common superclass to present a
+// uniform interface to PLDHashTable clients. The zero-sized base class
+// optimization, employed by all of our supported C++ compilers, will ensure
+// that this abstraction does not make objects needlessly larger.
+struct PLDHashEntryHdr {
+  PLDHashEntryHdr() = default;
+  PLDHashEntryHdr(const PLDHashEntryHdr&) = delete;
+  PLDHashEntryHdr& operator=(const PLDHashEntryHdr&) = delete;
+  PLDHashEntryHdr(PLDHashEntryHdr&&) = default;
+  PLDHashEntryHdr& operator=(PLDHashEntryHdr&&) = default;
+
+ private:
+  friend class PLDHashTable;
+};
+
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+
+// This class does three kinds of checking:
+//
+// - that calls to one of |mOps| or to an enumerator do not cause re-entry into
+//   the table in an unsafe way;
+//
+// - that multiple threads do not access the table in an unsafe way;
+//
+// - that a table marked as immutable is not modified.
+//
+// "Safe" here means that multiple concurrent read operations are ok, but a
+// write operation (i.e. one that can cause the entry storage to be reallocated
+// or destroyed) cannot safely run concurrently with another read or write
+// operation. This meaning of "safe" is only partial; for example, it does not
+// cover whether a single entry in the table is modified by two separate
+// threads. (Doing such checking would be much harder.)
+//
+// It does this with two variables:
+//
+// - mState, which embodies a tri-stage tagged union with the following
+//   variants:
+//   - Idle
+//   - Read(n), where 'n' is the number of concurrent read operations
+//   - Write
+//
+// - mIsWritable, which indicates if the table is mutable.
+//
+class Checker {
+ public:
+  constexpr Checker() : mState(kIdle), mIsWritable(true) {}
+
+  Checker& operator=(Checker&& aOther) {
+    // Atomic<> doesn't have an |operator=(Atomic<>&&)|.
+    mState = uint32_t(aOther.mState);
+    mIsWritable = bool(aOther.mIsWritable);
+
+    aOther.mState = kIdle;
+    // XXX Shouldn't we set mIsWritable to true here for consistency?
+
+    return *this;
+  }
+
+  static bool IsIdle(uint32_t aState) { return aState == kIdle; }
+  static bool IsRead(uint32_t aState) {
+    return kRead1 <= aState && aState <= kReadMax;
+  }
+  static bool IsRead1(uint32_t aState) { return aState == kRead1; }
+  static bool IsWrite(uint32_t aState) { return aState == kWrite; }
+
+  bool IsIdle() const { return mState == kIdle; }
+
+  bool IsWritable() const { return mIsWritable; }
+
+  void SetNonWritable() { mIsWritable = false; }
+
+  // NOTE: the obvious way to implement these functions is to (a) check
+  // |mState| is reasonable, and then (b) update |mState|. But the lack of
+  // atomicity in such an implementation can cause problems if we get unlucky
+  // thread interleaving between (a) and (b).
+  //
+  // So instead for |mState| we are careful to (a) first get |mState|'s old
+  // value and assign it a new value in single atomic operation, and only then
+  // (b) check the old value was reasonable. This ensures we don't have
+  // interleaving problems.
+  //
+  // For |mIsWritable| we don't need to be as careful because it can only in
+  // transition in one direction (from writable to non-writable).
+
+  void StartReadOp() {
+    uint32_t oldState = mState++;  // this is an atomic increment
+    MOZ_RELEASE_ASSERT(IsIdle(oldState) || IsRead(oldState));
+    MOZ_RELEASE_ASSERT(oldState < kReadMax);  // check for overflow
+  }
+
+  void EndReadOp() {
+    uint32_t oldState = mState--;  // this is an atomic decrement
+    MOZ_RELEASE_ASSERT(IsRead(oldState));
+  }
+
+  void StartWriteOp() {
+    MOZ_RELEASE_ASSERT(IsWritable());
+    uint32_t oldState = mState.exchange(kWrite);
+    MOZ_RELEASE_ASSERT(IsIdle(oldState));
+  }
+
+  void EndWriteOp() {
+    // Check again that the table is writable, in case it was marked as
+    // non-writable just after the IsWritable() assertion in StartWriteOp()
+    // occurred.
+    MOZ_RELEASE_ASSERT(IsWritable());
+    uint32_t oldState = mState.exchange(kIdle);
+    MOZ_RELEASE_ASSERT(IsWrite(oldState));
+  }
+
+  void StartIteratorRemovalOp() {
+    // When doing removals at the end of iteration, we go from Read1 state to
+    // Write and then back.
+    MOZ_RELEASE_ASSERT(IsWritable());
+    uint32_t oldState = mState.exchange(kWrite);
+    MOZ_RELEASE_ASSERT(IsRead1(oldState));
+  }
+
+  void EndIteratorRemovalOp() {
+    // Check again that the table is writable, in case it was marked as
+    // non-writable just after the IsWritable() assertion in
+    // StartIteratorRemovalOp() occurred.
+    MOZ_RELEASE_ASSERT(IsWritable());
+    uint32_t oldState = mState.exchange(kRead1);
+    MOZ_RELEASE_ASSERT(IsWrite(oldState));
+  }
+
+  void StartDestructorOp() {
+    // A destructor op is like a write, but the table doesn't need to be
+    // writable.
+    uint32_t oldState = mState.exchange(kWrite);
+    MOZ_RELEASE_ASSERT(IsIdle(oldState));
+  }
+
+  void EndDestructorOp() {
+    uint32_t oldState = mState.exchange(kIdle);
+    MOZ_RELEASE_ASSERT(IsWrite(oldState));
+  }
+
+ private:
+  // Things of note about the representation of |mState|.
+  // - The values between kRead1..kReadMax represent valid Read(n) values.
+  // - kIdle and kRead1 are deliberately chosen so that incrementing the -
+  //   former gives the latter.
+  // - 9999 concurrent readers should be enough for anybody.
+  static const uint32_t kIdle = 0;
+  static const uint32_t kRead1 = 1;
+  static const uint32_t kReadMax = 9999;
+  static const uint32_t kWrite = 10000;
+
+  mozilla::Atomic<uint32_t, mozilla::SequentiallyConsistent> mState;
+  mozilla::Atomic<bool, mozilla::SequentiallyConsistent> mIsWritable;
+};
+#endif
+
+// A PLDHashTable may be allocated on the stack or within another structure or
+// class. No entry storage is allocated until the first element is added. This
+// means that empty hash tables are cheap, which is good because they are
+// common.
+//
+// There used to be a long, math-heavy comment here about the merits of
+// double hashing vs. chaining; it was removed in bug 1058335. In short, double
+// hashing is more space-efficient unless the element size gets large (in which
+// case you should keep using double hashing but switch to using pointer
+// elements). Also, with double hashing, you can't safely hold an entry pointer
+// and use it after an add or remove operation, unless you sample Generation()
+// before adding or removing, and compare the sample after, dereferencing the
+// entry pointer only if Generation() has not changed.
+class PLDHashTable {
+ private:
+  // A slot represents a cached hash value and its associated entry stored in
+  // the hash table. The hash value and the entry are not stored contiguously.
+  struct Slot {
+    Slot(PLDHashEntryHdr* aEntry, PLDHashNumber* aKeyHash)
+        : mEntry(aEntry), mKeyHash(aKeyHash) {}
+
+    Slot(const Slot&) = default;
+    Slot(Slot&& aOther) = default;
+
+    Slot& operator=(Slot&& aOther) = default;
+
+    bool operator==(const Slot& aOther) { return mEntry == aOther.mEntry; }
+
+    PLDHashNumber KeyHash() const { return *HashPtr(); }
+    void SetKeyHash(PLDHashNumber aHash) { *HashPtr() = aHash; }
+
+    PLDHashEntryHdr* ToEntry() const { return mEntry; }
+
+    bool IsFree() const { return KeyHash() == 0; }
+    bool IsRemoved() const { return KeyHash() == 1; }
+    bool IsLive() const { return IsLiveHash(KeyHash()); }
+    static bool IsLiveHash(uint32_t aHash) { return aHash >= 2; }
+
+    void MarkFree() { *HashPtr() = 0; }
+    void MarkRemoved() { *HashPtr() = 1; }
+    void MarkColliding() { *HashPtr() |= kCollisionFlag; }
+
+    void Next(uint32_t aEntrySize) {
+      char* p = reinterpret_cast<char*>(mEntry);
+      p += aEntrySize;
+      mEntry = reinterpret_cast<PLDHashEntryHdr*>(p);
+      mKeyHash++;
+    }
+    PLDHashNumber* HashPtr() const { return mKeyHash; }
+
+   private:
+    PLDHashEntryHdr* mEntry;
+    PLDHashNumber* mKeyHash;
+  };
+
+  // This class maintains the invariant that every time the entry store is
+  // changed, the generation is updated.
+  //
+  // The data layout separates the cached hashes of entries and the entries
+  // themselves to save space. We could store the entries thusly:
+  //
+  // +--------+--------+---------+
+  // | entry0 | entry1 | ...     |
+  // +--------+--------+---------+
+  //
+  // where the entries themselves contain the cached hash stored as their
+  // first member. PLDHashTable did this for a long time, with entries looking
+  // like:
+  //
+  // class PLDHashEntryHdr
+  // {
+  //   PLDHashNumber mKeyHash;
+  // };
+  //
+  // class MyEntry : public PLDHashEntryHdr
+  // {
+  //   ...
+  // };
+  //
+  // The problem with this setup is that, depending on the layout of
+  // `MyEntry`, there may be platform ABI-mandated padding between `mKeyHash`
+  // and the first member of `MyEntry`. This ABI-mandated padding is wasted
+  // space, and was surprisingly common, e.g. when MyEntry contained a single
+  // pointer on 64-bit platforms.
+  //
+  // As previously alluded to, the current setup stores things thusly:
+  //
+  // +-------+-------+-------+-------+--------+--------+---------+
+  // | hash0 | hash1 | ..... | hashN | entry0 | entry1 | ...     |
+  // +-------+-------+-------+-------+--------+--------+---------+
+  //
+  // which contains no wasted space between the hashes themselves, and no
+  // wasted space between the entries themselves. malloc is guaranteed to
+  // return blocks of memory with at least word alignment on all of our major
+  // platforms. PLDHashTable mandates that the size of the hash table is
+  // always a power of two, so the alignment of the memory containing the
+  // first entry is always at least the alignment of the entire entry store.
+  // That means the alignment of `entry0` should be its natural alignment.
+  // Entries may have problems if they contain over-aligned members such as
+  // SIMD vector types, but this has not been a problem in practice.
+  //
+  // Note: It would be natural to store the generation within this class, but
+  // we can't do that without bloating sizeof(PLDHashTable) on 64-bit machines.
+  // So instead we store it outside this class, and Set() takes a pointer to it
+  // and ensures it is updated as necessary.
+  class EntryStore {
+   private:
+    char* mEntryStore;
+
+    static char* Entries(char* aStore, uint32_t aCapacity) {
+      return aStore + aCapacity * sizeof(PLDHashNumber);
+    }
+
+    char* Entries(uint32_t aCapacity) const {
+      return Entries(Get(), aCapacity);
+    }
+
+   public:
+    EntryStore() : mEntryStore(nullptr) {}
+
+    ~EntryStore() {
+      free(mEntryStore);
+      mEntryStore = nullptr;
+    }
+
+    char* Get() const { return mEntryStore; }
+    bool IsAllocated() const { return !!mEntryStore; }
+
+    Slot SlotForIndex(uint32_t aIndex, uint32_t aEntrySize,
+                      uint32_t aCapacity) const {
+      char* entries = Entries(aCapacity);
+      auto entry =
+          reinterpret_cast<PLDHashEntryHdr*>(entries + aIndex * aEntrySize);
+      auto hashes = reinterpret_cast<PLDHashNumber*>(Get());
+      return Slot(entry, &hashes[aIndex]);
+    }
+
+    Slot SlotForPLDHashEntry(PLDHashEntryHdr* aEntry, uint32_t aCapacity,
+                             uint32_t aEntrySize) {
+      char* entries = Entries(aCapacity);
+      char* entry = reinterpret_cast<char*>(aEntry);
+      uint32_t entryOffset = entry - entries;
+      uint32_t slotIndex = entryOffset / aEntrySize;
+      return SlotForIndex(slotIndex, aEntrySize, aCapacity);
+    }
+
+    template <typename F>
+    void ForEachSlot(uint32_t aCapacity, uint32_t aEntrySize, F&& aFunc) {
+      ForEachSlot(Get(), aCapacity, aEntrySize, std::move(aFunc));
+    }
+
+    template <typename F>
+    static void ForEachSlot(char* aStore, uint32_t aCapacity,
+                            uint32_t aEntrySize, F&& aFunc) {
+      char* entries = Entries(aStore, aCapacity);
+      Slot slot(reinterpret_cast<PLDHashEntryHdr*>(entries),
+                reinterpret_cast<PLDHashNumber*>(aStore));
+      for (size_t i = 0; i < aCapacity; ++i) {
+        aFunc(slot);
+        slot.Next(aEntrySize);
+      }
+    }
+
+    void Set(char* aEntryStore, uint16_t* aGeneration) {
+      mEntryStore = aEntryStore;
+      *aGeneration += 1;
+    }
+  };
+
+  // These fields are packed carefully. On 32-bit platforms,
+  // sizeof(PLDHashTable) is 20. On 64-bit platforms, sizeof(PLDHashTable) is
+  // 32; 28 bytes of data followed by 4 bytes of padding for alignment.
+  const PLDHashTableOps* const mOps;  // Virtual operations; see below.
+  EntryStore mEntryStore;             // (Lazy) entry storage and generation.
+  uint16_t mGeneration;               // The storage generation.
+  uint8_t mHashShift;                 // Multiplicative hash shift.
+  const uint8_t mEntrySize;           // Number of bytes in an entry.
+  uint32_t mEntryCount;               // Number of entries in table.
+  uint32_t mRemovedCount;             // Removed entry sentinels in table.
+
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+  mutable Checker mChecker;
+#endif
+
+ public:
+  // Table capacity limit; do not exceed. The max capacity used to be 1<<23 but
+  // that occasionally that wasn't enough. Making it much bigger than 1<<26
+  // probably isn't worthwhile -- tables that big are kind of ridiculous.
+  // Also, the growth operation will (deliberately) fail if |capacity *
+  // mEntrySize| overflows a uint32_t, and mEntrySize is always at least 8
+  // bytes.
+  static const uint32_t kMaxCapacity = ((uint32_t)1 << 26);
+
+  static const uint32_t kMinCapacity = 8;
+
+  // Making this half of kMaxCapacity ensures it'll fit. Nobody should need an
+  // initial length anywhere nearly this large, anyway.
+  static const uint32_t kMaxInitialLength = kMaxCapacity / 2;
+
+  // This gives a default initial capacity of 8.
+  static const uint32_t kDefaultInitialLength = 4;
+
+  // Initialize the table with |aOps| and |aEntrySize|. The table's initial
+  // capacity is chosen such that |aLength| elements can be inserted without
+  // rehashing; if |aLength| is a power-of-two, this capacity will be
+  // |2*length|. However, because entry storage is allocated lazily, this
+  // initial capacity won't be relevant until the first element is added; prior
+  // to that the capacity will be zero.
+  //
+  // This will crash if |aEntrySize| and/or |aLength| are too large.
+  PLDHashTable(const PLDHashTableOps* aOps, uint32_t aEntrySize,
+               uint32_t aLength = kDefaultInitialLength);
+
+  PLDHashTable(PLDHashTable&& aOther)
+      // Initialize fields which are checked by the move assignment operator
+      // and the destructor (which the move assignment operator calls).
+      : mOps(nullptr), mEntryStore(), mGeneration(0), mEntrySize(0) {
+    *this = std::move(aOther);
+  }
+
+  PLDHashTable& operator=(PLDHashTable&& aOther);
+
+  ~PLDHashTable();
+
+  // This should be used rarely.
+  const PLDHashTableOps* Ops() const { return mOps; }
+
+  // Size in entries (gross, not net of free and removed sentinels) for table.
+  // This can be zero if no elements have been added yet, in which case the
+  // entry storage will not have yet been allocated.
+  uint32_t Capacity() const {
+    return mEntryStore.IsAllocated() ? CapacityFromHashShift() : 0;
+  }
+
+  uint32_t EntrySize() const { return mEntrySize; }
+  uint32_t EntryCount() const { return mEntryCount; }
+  uint32_t Generation() const { return mGeneration; }
+
+  // To search for a |key| in |table|, call:
+  //
+  //   entry = table.Search(key);
+  //
+  // If |entry| is non-null, |key| was found. If |entry| is null, key was not
+  // found.
+  PLDHashEntryHdr* Search(const void* aKey) const;
+
+  // To add an entry identified by |key| to table, call:
+  //
+  //   entry = table.Add(key, mozilla::fallible);
+  //
+  // If |entry| is null upon return, then the table is severely overloaded and
+  // memory can't be allocated for entry storage.
+  //
+  // Otherwise, if the initEntry hook was provided, |entry| will be
+  // initialized.  If the initEntry hook was not provided, the caller
+  // should initialize |entry| as appropriate.
+  PLDHashEntryHdr* Add(const void* aKey, const mozilla::fallible_t&);
+
+  // This is like the other Add() function, but infallible, and so never
+  // returns null.
+  PLDHashEntryHdr* Add(const void* aKey);
+
+  // To remove an entry identified by |key| from table, call:
+  //
+  //   table.Remove(key);
+  //
+  // If |key|'s entry is found, it is cleared (via table->mOps->clearEntry).
+  // The table's capacity may be reduced afterwards.
+  void Remove(const void* aKey);
+
+  // To remove an entry found by a prior search, call:
+  //
+  //   table.RemoveEntry(entry);
+  //
+  // The entry, which must be present and in use, is cleared (via
+  // table->mOps->clearEntry). The table's capacity may be reduced afterwards.
+  void RemoveEntry(PLDHashEntryHdr* aEntry);
+
+  // Remove an entry already accessed via Search() or Add().
+  //
+  // NB: this is a "raw" or low-level method. It does not shrink the table if
+  // it is underloaded. Don't use it unless necessary and you know what you are
+  // doing, and if so, please explain in a comment why it is necessary instead
+  // of RemoveEntry().
+  void RawRemove(PLDHashEntryHdr* aEntry);
+
+  // This function is equivalent to
+  // ClearAndPrepareForLength(kDefaultInitialLength).
+  void Clear();
+
+  // This function clears the table's contents and frees its entry storage,
+  // leaving it in a empty state ready to be used again. Afterwards, when the
+  // first element is added the entry storage that gets allocated will have a
+  // capacity large enough to fit |aLength| elements without rehashing.
+  //
+  // It's conceptually the same as calling the destructor and then re-calling
+  // the constructor with the original |aOps| and |aEntrySize| arguments, and
+  // a new |aLength| argument.
+  void ClearAndPrepareForLength(uint32_t aLength);
+
+  // Measure the size of the table's entry storage. If the entries contain
+  // pointers to other heap blocks, you have to iterate over the table and
+  // measure those separately; hence the "Shallow" prefix.
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  // Like ShallowSizeOfExcludingThis(), but includes sizeof(*this).
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  // Mark a table as immutable for the remainder of its lifetime. This
+  // changes the implementation from asserting one set of invariants to
+  // asserting a different set.
+  void MarkImmutable() {
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+    mChecker.SetNonWritable();
+#endif
+  }
+
+  // If you use PLDHashEntryStub or a subclass of it as your entry struct, and
+  // if your entries move via memcpy and clear via memset(0), you can use these
+  // stub operations.
+  static const PLDHashTableOps* StubOps();
+
+  // The individual stub operations in StubOps().
+  static PLDHashNumber HashVoidPtrKeyStub(const void* aKey);
+  static bool MatchEntryStub(const PLDHashEntryHdr* aEntry, const void* aKey);
+  static void MoveEntryStub(PLDHashTable* aTable, const PLDHashEntryHdr* aFrom,
+                            PLDHashEntryHdr* aTo);
+  static void ClearEntryStub(PLDHashTable* aTable, PLDHashEntryHdr* aEntry);
+
+  // Hash/match operations for tables holding C strings.
+  static PLDHashNumber HashStringKey(const void* aKey);
+  static bool MatchStringKey(const PLDHashEntryHdr* aEntry, const void* aKey);
+
+  class EntryHandle {
+   public:
+    EntryHandle(EntryHandle&& aOther) noexcept;
+#ifdef MOZ_HASH_TABLE_CHECKS_ENABLED
+    ~EntryHandle();
+#endif
+
+    EntryHandle(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&) = delete;
+    EntryHandle& operator=(EntryHandle&& aOther) = delete;
+
+    // Is this slot currently occupied?
+    bool HasEntry() const { return mSlot.IsLive(); }
+
+    explicit operator bool() const { return HasEntry(); }
+
+    // Get the entry stored in this slot. May not be called unless the slot is
+    // currently occupied.
+    PLDHashEntryHdr* Entry() {
+      MOZ_ASSERT(HasEntry());
+      return mSlot.ToEntry();
+    }
+
+    template <class F>
+    void Insert(F&& aInitEntry) {
+      MOZ_ASSERT(!HasEntry());
+      OccupySlot();
+      std::forward<F>(aInitEntry)(Entry());
+    }
+
+    // If the slot is currently vacant, the slot is occupied and `initEntry` is
+    // invoked to initialize the entry. Returns the entry stored in now-occupied
+    // slot.
+    template <class F>
+    PLDHashEntryHdr* OrInsert(F&& aInitEntry) {
+      if (!HasEntry()) {
+        Insert(std::forward<F>(aInitEntry));
+      }
+      return Entry();
+    }
+
+    /** Removes the entry. Note that the table won't shrink on destruction of
+     * the EntryHandle.
+     *
+     * \pre HasEntry()
+     * \post !HasEntry()
+     */
+    void Remove();
+
+    /** Removes the entry, if it exists. Note that the table won't shrink on
+     * destruction of the EntryHandle.
+     *
+     * \post !HasEntry()
+     */
+    void OrRemove();
+
+   private:
+    friend class PLDHashTable;
+
+    EntryHandle(PLDHashTable* aTable, PLDHashNumber aKeyHash, Slot aSlot);
+
+    void OccupySlot();
+
+    PLDHashTable* mTable;
+    PLDHashNumber mKeyHash;
+    Slot mSlot;
+  };
+
+  template <class F>
+  auto WithEntryHandle(const void* aKey, F&& aFunc)
+      -> std::invoke_result_t<F, EntryHandle&&> {
+    return std::forward<F>(aFunc)(MakeEntryHandle(aKey));
+  }
+
+  template <class F>
+  auto WithEntryHandle(const void* aKey, const mozilla::fallible_t& aFallible,
+                       F&& aFunc)
+      -> std::invoke_result_t<F, mozilla::Maybe<EntryHandle>&&> {
+    return std::forward<F>(aFunc)(MakeEntryHandle(aKey, aFallible));
+  }
+
+  // This is an iterator for PLDHashtable. Assertions will detect some, but not
+  // all, mid-iteration table modifications that might invalidate (e.g.
+  // reallocate) the entry storage.
+  //
+  // Any element can be removed during iteration using Remove(). If any
+  // elements are removed, the table may be resized once iteration ends.
+  //
+  // Example usage:
+  //
+  //   for (auto iter = table.Iter(); !iter.Done(); iter.Next()) {
+  //     auto entry = static_cast<FooEntry*>(iter.Get());
+  //     // ... do stuff with |entry| ...
+  //     // ... possibly call iter.Remove() once ...
+  //   }
+  //
+  // or:
+  //
+  //   for (PLDHashTable::Iterator iter(&table); !iter.Done(); iter.Next()) {
+  //     auto entry = static_cast<FooEntry*>(iter.Get());
+  //     // ... do stuff with |entry| ...
+  //     // ... possibly call iter.Remove() once ...
+  //   }
+  //
+  // The latter form is more verbose but is easier to work with when
+  // making subclasses of Iterator.
+  //
+  class Iterator {
+   public:
+    explicit Iterator(PLDHashTable* aTable);
+    struct EndIteratorTag {};
+    Iterator(PLDHashTable* aTable, EndIteratorTag aTag);
+    Iterator(Iterator&& aOther);
+    ~Iterator();
+
+    // Have we finished?
+    bool Done() const { return mNexts == mNextsLimit; }
+
+    // Get the current entry.
+    PLDHashEntryHdr* Get() const {
+      MOZ_ASSERT(!Done());
+      MOZ_ASSERT(mCurrent.IsLive());
+      return mCurrent.ToEntry();
+    }
+
+    // Advance to the next entry.
+    void Next();
+
+    // Remove the current entry. Must only be called once per entry, and Get()
+    // must not be called on that entry afterwards.
+    void Remove();
+
+    bool operator==(const Iterator& aOther) const {
+      MOZ_ASSERT(mTable == aOther.mTable);
+      return mNexts == aOther.mNexts;
+    }
+
+    Iterator Clone() const { return {*this}; }
+
+   protected:
+    PLDHashTable* mTable;  // Main table pointer.
+
+   private:
+    Slot mCurrent;         // Pointer to the current entry.
+    uint32_t mNexts;       // Number of Next() calls.
+    uint32_t mNextsLimit;  // Next() call limit.
+
+    bool mHaveRemoved;   // Have any elements been removed?
+    uint8_t mEntrySize;  // Size of entries.
+
+    bool IsOnNonLiveEntry() const;
+
+    void MoveToNextLiveEntry();
+
+    Iterator() = delete;
+    Iterator(const Iterator&);
+    Iterator& operator=(const Iterator&) = delete;
+    Iterator& operator=(const Iterator&&) = delete;
+  };
+
+  Iterator Iter() { return Iterator(this); }
+
+  // Use this if you need to initialize an Iterator in a const method. If you
+  // use this case, you should not call Remove() on the iterator.
+  Iterator ConstIter() const {
+    return Iterator(const_cast<PLDHashTable*>(this));
+  }
+
+ private:
+  static uint32_t HashShift(uint32_t aEntrySize, uint32_t aLength);
+
+  static const PLDHashNumber kCollisionFlag = 1;
+
+  PLDHashNumber Hash1(PLDHashNumber aHash0) const;
+  void Hash2(PLDHashNumber aHash, uint32_t& aHash2Out,
+             uint32_t& aSizeMaskOut) const;
+
+  static bool MatchSlotKeyhash(Slot& aSlot, const PLDHashNumber aHash);
+  Slot SlotForIndex(uint32_t aIndex) const;
+
+  // We store mHashShift rather than sizeLog2 to optimize the collision-free
+  // case in SearchTable.
+  uint32_t CapacityFromHashShift() const {
+    return ((uint32_t)1 << (kPLDHashNumberBits - mHashShift));
+  }
+
+  PLDHashNumber ComputeKeyHash(const void* aKey) const;
+
+  enum SearchReason { ForSearchOrRemove, ForAdd };
+
+  // Avoid using bare `Success` and `Failure`, as those names are commonly
+  // defined as macros.
+  template <SearchReason Reason, typename PLDSuccess, typename PLDFailure>
+  auto SearchTable(const void* aKey, PLDHashNumber aKeyHash,
+                   PLDSuccess&& aSucess, PLDFailure&& aFailure) const;
+
+  Slot FindFreeSlot(PLDHashNumber aKeyHash) const;
+
+  bool ChangeTable(int aDeltaLog2);
+
+  void RawRemove(Slot& aSlot);
+  void ShrinkIfAppropriate();
+
+  mozilla::Maybe<EntryHandle> MakeEntryHandle(const void* aKey,
+                                              const mozilla::fallible_t&);
+
+  EntryHandle MakeEntryHandle(const void* aKey);
+
+  PLDHashTable(const PLDHashTable& aOther) = delete;
+  PLDHashTable& operator=(const PLDHashTable& aOther) = delete;
+};
+
+// Compute the hash code for a given key to be looked up, added, or removed.
+// A hash code may have any PLDHashNumber value.
+typedef PLDHashNumber (*PLDHashHashKey)(const void* aKey);
+
+// Compare the key identifying aEntry with the provided key parameter. Return
+// true if keys match, false otherwise.
+typedef bool (*PLDHashMatchEntry)(const PLDHashEntryHdr* aEntry,
+                                  const void* aKey);
+
+// Copy the data starting at aFrom to the new entry storage at aTo. Do not add
+// reference counts for any strong references in the entry, however, as this
+// is a "move" operation: the old entry storage at from will be freed without
+// any reference-decrementing callback shortly.
+typedef void (*PLDHashMoveEntry)(PLDHashTable* aTable,
+                                 const PLDHashEntryHdr* aFrom,
+                                 PLDHashEntryHdr* aTo);
+
+// Clear the entry and drop any strong references it holds. This callback is
+// invoked by Remove(), but only if the given key is found in the table.
+typedef void (*PLDHashClearEntry)(PLDHashTable* aTable,
+                                  PLDHashEntryHdr* aEntry);
+
+// Initialize a new entry. This function is called when
+// Add() finds no existing entry for the given key, and must add a new one.
+typedef void (*PLDHashInitEntry)(PLDHashEntryHdr* aEntry, const void* aKey);
+
+// Finally, the "vtable" structure for PLDHashTable. The first four hooks
+// must be provided by implementations; they're called unconditionally by the
+// generic PLDHashTable.cpp code. Hooks after these may be null.
+//
+// Summary of allocation-related hook usage with C++ placement new emphasis:
+//  initEntry           Call placement new using default key-based ctor.
+//  moveEntry           Call placement new using copy ctor, run dtor on old
+//                      entry storage.
+//  clearEntry          Run dtor on entry.
+//
+// Note the reason why initEntry is optional: the default hooks (stubs) clear
+// entry storage. On a successful Add(tbl, key), the returned entry pointer
+// addresses an entry struct whose entry members are still clear (null). Add()
+// callers can test such members to see whether the entry was newly created by
+// the Add() call that just succeeded. If placement new or similar
+// initialization is required, define an |initEntry| hook. Of course, the
+// |clearEntry| hook must zero or null appropriately.
+//
+// XXX assumes 0 is null for pointer types.
+struct PLDHashTableOps {
+  // Mandatory hooks. All implementations must provide these.
+  PLDHashHashKey hashKey;
+  PLDHashMatchEntry matchEntry;
+  PLDHashMoveEntry moveEntry;
+
+  // Optional hooks start here. If null, these are not called.
+  PLDHashClearEntry clearEntry;
+  PLDHashInitEntry initEntry;
+};
+
+// A minimal entry is a subclass of PLDHashEntryHdr and has a void* key pointer.
+struct PLDHashEntryStub : public PLDHashEntryHdr {
+  const void* key;
+};
+
+#endif /* PLDHashTable_h */
diff --git a/xpcom/ds/PerfectHash.h b/xpcom/ds/PerfectHash.h
new file mode 100644
index 0000000000..1e75855462
--- /dev/null
+++ b/xpcom/ds/PerfectHash.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Helper routines for perfecthash.py.  Not to be used directly. */
+
+#ifndef mozilla_PerfectHash_h
+#define mozilla_PerfectHash_h
+
+#include <type_traits>
+
+namespace mozilla {
+namespace perfecthash {
+
+// 32-bit FNV offset basis and prime value.
+// NOTE: Must match values in |perfecthash.py|
+constexpr uint32_t FNV_OFFSET_BASIS = 0x811C9DC5;
+constexpr uint32_t FNV_PRIME = 16777619;
+
+/**
+ * Basic FNV hasher function used by perfecthash. Generic over the unit type.
+ */
+template <typename C>
+inline uint32_t Hash(uint32_t aBasis, const C* aKey, size_t aLen) {
+  for (size_t i = 0; i < aLen; ++i) {
+    aBasis =
+        (aBasis ^ static_cast<std::make_unsigned_t<C>>(aKey[i])) * FNV_PRIME;
+  }
+  return aBasis;
+}
+
+/**
+ * Helper method for getting the index from a perfect hash.
+ * Called by code generated from |perfecthash.py|.
+ */
+template <typename C, typename Base, size_t NBases, typename Entry,
+          size_t NEntries>
+inline const Entry& Lookup(const C* aKey, size_t aLen,
+                           const Base (&aTable)[NBases],
+                           const Entry (&aEntries)[NEntries]) {
+  uint32_t basis = aTable[Hash(FNV_OFFSET_BASIS, aKey, aLen) % NBases];
+  return aEntries[Hash(basis, aKey, aLen) % NEntries];
+}
+
+}  // namespace perfecthash
+}  // namespace mozilla
+
+#endif  // !defined(mozilla_PerfectHash_h)
diff --git a/xpcom/ds/SimpleEnumerator.h b/xpcom/ds/SimpleEnumerator.h
new file mode 100644
index 0000000000..463b9ecaed
--- /dev/null
+++ b/xpcom/ds/SimpleEnumerator.h
@@ -0,0 +1,73 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_SimpleEnumerator_h
+#define mozilla_SimpleEnumerator_h
+
+#include "nsCOMPtr.h"
+#include "nsISimpleEnumerator.h"
+
+namespace mozilla {
+
+/**
+ * A wrapper class around nsISimpleEnumerator to support ranged iteration. This
+ * requires every element in the enumeration to implement the same interface, T.
+ * If any element does not implement this interface, the enumeration ends at
+ * that element, and triggers an assertion in debug builds.
+ *
+ * Typical usage looks something like:
+ *
+ *   for (auto& docShell : SimpleEnumerator<nsIDocShell>(docShellEnum)) {
+ *     docShell.LoadURI(...);
+ *   }
+ */
+
+template <typename T>
+class SimpleEnumerator final {
+ public:
+  explicit SimpleEnumerator(nsISimpleEnumerator* aEnum) : mEnum(aEnum) {}
+
+  class Entry {
+   public:
+    explicit Entry(T* aPtr) : mPtr(aPtr) {}
+
+    explicit Entry(nsISimpleEnumerator& aEnum) : mEnum(&aEnum) { ++*this; }
+
+    const nsCOMPtr<T>& operator*() {
+      MOZ_ASSERT(mPtr);
+      return mPtr;
+    }
+
+    Entry& operator++() {
+      MOZ_ASSERT(mEnum);
+      nsCOMPtr<nsISupports> next;
+      if (NS_SUCCEEDED(mEnum->GetNext(getter_AddRefs(next)))) {
+        mPtr = do_QueryInterface(next);
+        MOZ_ASSERT(mPtr);
+      } else {
+        mPtr = nullptr;
+      }
+      return *this;
+    }
+
+    bool operator!=(const Entry& aOther) const { return mPtr != aOther.mPtr; }
+
+   private:
+    nsCOMPtr<T> mPtr;
+    nsCOMPtr<nsISimpleEnumerator> mEnum;
+  };
+
+  Entry begin() { return Entry(*mEnum); }
+
+  Entry end() { return Entry(nullptr); }
+
+ private:
+  nsCOMPtr<nsISimpleEnumerator> mEnum;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SimpleEnumerator_h
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
new file mode 100644
index 0000000000..1b49ddc99b
--- /dev/null
+++ b/xpcom/ds/StaticAtoms.py
@@ -0,0 +1,2643 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# flake8: noqa
+
+import sys
+
+from Atom import (
+    Atom,
+    InheritingAnonBoxAtom,
+    NonInheritingAnonBoxAtom,
+    PseudoElementAtom,
+)
+from HTMLAtoms import HTML_PARSER_ATOMS
+
+# Static atom definitions, used to generate nsGkAtomList.h.
+#
+# Each atom is defined by a call to Atom, PseudoElementAtom,
+# NonInheritingAnonBoxAtom or InheritingAnonBoxAtom.
+#
+# The first argument is the atom's identifier.
+# The second argument is the atom's string value.
+#
+# Please keep the Atom() definitions on one line as this is parsed by the
+#   htmlparser: parser/html/java/htmlparser
+# Please keep "START ATOMS" and "END ATOMS" comments as the parser uses them.
+#
+# It is not possible to conditionally define static atoms with #ifdef etc.
+# fmt: off
+STATIC_ATOMS = [
+    # START ATOMS
+    # --------------------------------------------------------------------------
+    # Generic atoms
+    # --------------------------------------------------------------------------
+    Atom("SystemPrincipal", "[System Principal]"),
+    Atom("_empty", ""),
+    Atom("_0", "0"),
+    Atom("_1", "1"),
+    Atom("mozframetype", "mozframetype"),
+    Atom("_moz_abspos", "_moz_abspos"),
+    Atom("_moz_activated", "_moz_activated"),
+    Atom("_moz_anonclass", "_moz_anonclass"),
+    Atom("_moz_resizing", "_moz_resizing"),
+    Atom("moztype", "_moz-type"),
+    Atom("mozdirty", "_moz_dirty"),
+    Atom("mozdisallowselectionprint", "mozdisallowselectionprint"),
+    Atom("mozdonotsend", "moz-do-not-send"),
+    Atom("mozfwcontainer", "moz-forward-container"),  # Used by MailNews.
+    Atom("mozgeneratedcontentbefore", "_moz_generated_content_before"),
+    Atom("mozgeneratedcontentafter", "_moz_generated_content_after"),
+    Atom("mozgeneratedcontentmarker", "_moz_generated_content_marker"),
+    Atom("mozgeneratedcontentimage", "_moz_generated_content_image"),
+    Atom("mozquote", "_moz_quote"),
+    Atom("mozsignature", "moz-signature"),  # Used by MailNews.
+    Atom("_moz_bullet_font", "-moz-bullet-font"),
+    Atom("_moz_is_glyph", "-moz-is-glyph"),
+    Atom("_moz_original_size", "_moz_original_size"),
+    Atom("_moz_print_preview", "-moz-print-preview"),
+    Atom("_moz_non_native_content_theme", "-moz-non-native-content-theme"),
+    Atom("menuactive", "_moz-menuactive"),
+    Atom("_poundDefault", "#default"),
+    Atom("_asterisk", "*"),
+    Atom("a", "a"),
+    Atom("abbr", "abbr"),
+    Atom("abort", "abort"),
+    Atom("above", "above"),
+    Atom("acceltext", "acceltext"),
+    Atom("accept", "accept"),
+    Atom("acceptcharset", "accept-charset"),
+    Atom("accessiblenode", "accessible-node"),
+    Atom("accesskey", "accesskey"),
+    Atom("acronym", "acronym"),
+    Atom("action", "action"),
+    Atom("active", "active"),
+    Atom("activateontab", "activateontab"),
+    Atom("actuate", "actuate"),
+    Atom("address", "address"),
+    Atom("adoptedsheetclones", "adoptedsheetclones"),
+    Atom("after", "after"),
+    Atom("align", "align"),
+    Atom("alink", "alink"),
+    Atom("all", "all"),
+    Atom("allow", "allow"),
+    Atom("allowdownloads", "allow-downloads"),
+    Atom("allowevents", "allowevents"),
+    Atom("allowforms", "allow-forms"),
+    Atom("allowfullscreen", "allowfullscreen"),
+    Atom("allowmodals", "allow-modals"),
+    Atom("alloworientationlock", "allow-orientation-lock"),
+    Atom("allowpointerlock", "allow-pointer-lock"),
+    Atom("allowpopupstoescapesandbox", "allow-popups-to-escape-sandbox"),
+    Atom("allowpopups", "allow-popups"),
+    Atom("allowpresentation", "allow-presentation"),
+    Atom("allowstorageaccessbyuseractivatetion", "allow-storage-access-by-user-activation"),
+    Atom("allowsameorigin", "allow-same-origin"),
+    Atom("allowscripts", "allow-scripts"),
+    Atom("allowscriptstoclose", "allowscriptstoclose"),
+    Atom("allowtopnavigation", "allow-top-navigation"),
+    Atom("allowtopnavigationbyuseractivation", "allow-top-navigation-by-user-activation"),
+    Atom("allowtopnavigationcustomprotocols", "allow-top-navigation-to-custom-protocols"),
+    Atom("allowuntrusted", "allowuntrusted"),
+    Atom("alt", "alt"),
+    Atom("alternate", "alternate"),
+    Atom("always", "always"),
+    Atom("ancestor", "ancestor"),
+    Atom("ancestorOrSelf", "ancestor-or-self"),
+    Atom("anchor", "anchor"),
+    Atom("_and", "and"),
+    Atom("animations", "animations"),
+    Atom("anonid", "anonid"),
+    Atom("anonlocation", "anonlocation"),
+    Atom("any", "any"),
+    Atom("any_hover", "any-hover"),
+    Atom("any_pointer", "any-pointer"),
+    Atom("applet", "applet"),
+    Atom("applyImports", "apply-imports"),
+    Atom("applyTemplates", "apply-templates"),
+    Atom("archive", "archive"),
+    Atom("area", "area"),
+    Atom("aria", "aria"),
+    Atom("aria_activedescendant", "aria-activedescendant"),
+    Atom("aria_atomic", "aria-atomic"),
+    Atom("aria_autocomplete", "aria-autocomplete"),
+    Atom("aria_busy", "aria-busy"),
+    Atom("aria_checked", "aria-checked"),
+    Atom("aria_controls", "aria-controls"),
+    Atom("aria_current", "aria-current"),
+    Atom("aria_describedby", "aria-describedby"),
+    Atom("aria_description", "aria-description"),
+    Atom("aria_disabled", "aria-disabled"),
+    Atom("aria_dropeffect", "aria-dropeffect"),
+    Atom("aria_expanded", "aria-expanded"),
+    Atom("aria_flowto", "aria-flowto"),
+    Atom("aria_haspopup", "aria-haspopup"),
+    Atom("aria_hidden", "aria-hidden"),
+    Atom("aria_invalid", "aria-invalid"),
+    Atom("aria_labelledby", "aria-labelledby"),
+    Atom("aria_level", "aria-level"),
+    Atom("aria_live", "aria-live"),
+    Atom("aria_multiline", "aria-multiline"),
+    Atom("aria_multiselectable", "aria-multiselectable"),
+    Atom("aria_owns", "aria-owns"),
+    Atom("aria_posinset", "aria-posinset"),
+    Atom("aria_pressed", "aria-pressed"),
+    Atom("aria_readonly", "aria-readonly"),
+    Atom("aria_relevant", "aria-relevant"),
+    Atom("aria_required", "aria-required"),
+    Atom("aria_selected", "aria-selected"),
+    Atom("aria_setsize", "aria-setsize"),
+    Atom("aria_sort", "aria-sort"),
+    Atom("aria_valuemax", "aria-valuemax"),
+    Atom("aria_valuemin", "aria-valuemin"),
+    Atom("aria_valuenow", "aria-valuenow"),
+    Atom("arrow", "arrow"),
+    Atom("article", "article"),
+    Atom("as", "as"),
+    Atom("ascending", "ascending"),
+    Atom("aside", "aside"),
+    Atom("aspectRatio", "aspect-ratio"),
+    Atom("async", "async"),
+    Atom("attribute", "attribute"),
+    Atom("attributes", "attributes"),
+    Atom("attributeSet", "attribute-set"),
+    Atom("_auto", "auto"),
+    Atom("autocapitalize", "autocapitalize"),
+    Atom("autocheck", "autocheck"),
+    Atom("autocomplete", "autocomplete"),
+    Atom("autocomplete_richlistbox", "autocomplete-richlistbox"),
+    Atom("autofocus", "autofocus"),
+    Atom("autoplay", "autoplay"),
+    Atom("axis", "axis"),
+    Atom("b", "b"),
+    Atom("background", "background"),
+    Atom("bar", "bar"),
+    Atom("base", "base"),
+    Atom("basefont", "basefont"),
+    Atom("baseline", "baseline"),
+    Atom("bdi", "bdi"),
+    Atom("bdo", "bdo"),
+    Atom("before", "before"),
+    Atom("behavior", "behavior"),
+    Atom("below", "below"),
+    Atom("bgcolor", "bgcolor"),
+    Atom("bgsound", "bgsound"),
+    Atom("big", "big"),
+    Atom("binding", "binding"),
+    Atom("bindings", "bindings"),
+    Atom("bindToUntrustedContent", "bindToUntrustedContent"),
+    Atom("black", "black"),
+    Atom("block", "block"),
+    Atom("block_size", "block-size"),
+    Atom("blockquote", "blockquote"),
+    Atom("blur", "blur"),
+    Atom("body", "body"),
+    Atom("boolean", "boolean"),
+    Atom("border", "border"),
+    Atom("bordercolor", "bordercolor"),
+    Atom("both", "both"),
+    Atom("bottom", "bottom"),
+    Atom("bottomend", "bottomend"),
+    Atom("bottomstart", "bottomstart"),
+    Atom("bottomleft", "bottomleft"),
+    Atom("bottommargin", "bottommargin"),
+    Atom("bottomright", "bottomright"),
+    Atom("box", "box"),
+    Atom("br", "br"),
+    Atom("browser", "browser"),
+    Atom("mozbrowser", "mozbrowser"),
+    Atom("button", "button"),
+    Atom("callTemplate", "call-template"),
+    Atom("canvas", "canvas"),
+    Atom("caption", "caption"),
+    Atom("captionBox", "caption-box"),
+    Atom("capture", "capture"),
+    Atom("caseOrder", "case-order"),
+    Atom("cdataSectionElements", "cdata-section-elements"),
+    Atom("ceiling", "ceiling"),
+    Atom("cell", "cell"),
+    Atom("cellpadding", "cellpadding"),
+    Atom("cellspacing", "cellspacing"),
+    Atom("center", "center"),
+    Atom("change", "change"),
+    Atom("_char", "char"),
+    Atom("characterData", "characterData"),
+    Atom("charcode", "charcode"),
+    Atom("charoff", "charoff"),
+    Atom("charset", "charset"),
+    Atom("checkbox", "checkbox"),
+    Atom("checkboxLabel", "checkbox-label"),
+    Atom("checked", "checked"),
+    Atom("child", "child"),
+    Atom("children", "children"),
+    Atom("childList", "childList"),
+    Atom("child_item_count", "child-item-count"),
+    Atom("choose", "choose"),
+    Atom("chromemargin", "chromemargin"),
+    Atom("exposeToUntrustedContent", "exposeToUntrustedContent"),
+    Atom("circ", "circ"),
+    Atom("circle", "circle"),
+    Atom("cite", "cite"),
+    Atom("_class", "class"),
+    Atom("classid", "classid"),
+    Atom("clear", "clear"),
+    Atom("click", "click"),
+    Atom("clickcount", "clickcount"),
+    Atom("clickthrough", "clickthrough"),
+    Atom("movetoclick", "movetoclick"),
+    Atom("clip", "clip"),
+    Atom("close", "close"),
+    Atom("closed", "closed"),
+    Atom("closemenu", "closemenu"),
+    Atom("code", "code"),
+    Atom("codebase", "codebase"),
+    Atom("codetype", "codetype"),
+    Atom("col", "col"),
+    Atom("colgroup", "colgroup"),
+    Atom("collapse", "collapse"),
+    Atom("collapsed", "collapsed"),
+    Atom("color", "color"),
+    Atom("color_gamut", "color-gamut"),
+    Atom("color_index", "color-index"),
+    Atom("color_scheme", "color-scheme"),
+    Atom("cols", "cols"),
+    Atom("colspan", "colspan"),
+    Atom("combobox", "combobox"),
+    Atom("command", "command"),
+    Atom("commandupdater", "commandupdater"),
+    Atom("comment", "comment"),
+    Atom("compact", "compact"),
+    Atom("concat", "concat"),
+    Atom("constructor", "constructor"),
+    Atom("consumeoutsideclicks", "consumeoutsideclicks"),
+    Atom("container", "container"),
+    Atom("contains", "contains"),
+    Atom("content", "content"),
+    Atom("contenteditable", "contenteditable"),
+    Atom("headerContentDisposition", "content-disposition"),
+    Atom("headerContentLanguage", "content-language"),
+    Atom("contentLocation", "content-location"),
+    Atom("headerContentScriptType", "content-script-type"),
+    Atom("headerContentStyleType", "content-style-type"),
+    Atom("headerContentType", "content-type"),
+    Atom("consumeanchor", "consumeanchor"),
+    Atom("context", "context"),
+    Atom("contextmenu", "contextmenu"),
+    Atom("control", "control"),
+    Atom("controls", "controls"),
+    Atom("coords", "coords"),
+    Atom("copy", "copy"),
+    Atom("copyOf", "copy-of"),
+    Atom("count", "count"),
+    Atom("crop", "crop"),
+    Atom("crossorigin", "crossorigin"),
+    Atom("curpos", "curpos"),
+    Atom("current", "current"),
+    Atom("cutoutregion", "cutoutregion"),
+    Atom("cycler", "cycler"),
+    Atom("dashed", "dashed"),
+    Atom("data", "data"),
+    Atom("dataAtShortcutkeys", "data-at-shortcutkeys"),
+    Atom("datalist", "datalist"),
+    Atom("datal10nid", "data-l10n-id"),
+    Atom("datal10nargs", "data-l10n-args"),
+    Atom("datal10nattrs", "data-l10n-attrs"),
+    Atom("datal10nname", "data-l10n-name"),
+    Atom("datal10nsync", "data-l10n-sync"),
+    Atom("dataType", "data-type"),
+    Atom("dateTime", "date-time"),
+    Atom("date", "date"),
+    Atom("datetime", "datetime"),
+    Atom("datetime_local", "datetime-local"),
+    Atom("datetimeInputBoxWrapper", "datetime-input-box-wrapper"),
+    Atom("dd", "dd"),
+    Atom("decimal", "decimal"),
+    Atom("decimalFormat", "decimal-format"),
+    Atom("decimalSeparator", "decimal-separator"),
+    Atom("declare", "declare"),
+    Atom("decoderDoctor", "decoder-doctor"),
+    Atom("decoding", "decoding"),
+    Atom("decrement", "decrement"),
+    Atom("_default", "default"),
+    Atom("headerDefaultStyle", "default-style"),
+    Atom("defer", "defer"),
+    Atom("del", "del"),
+    Atom("delegatesanchor", "delegatesanchor"),
+    Atom("deletion", "deletion"),
+    Atom("deprecation", "deprecation"),
+    Atom("descendant", "descendant"),
+    Atom("descendantOrSelf", "descendant-or-self"),
+    Atom("descending", "descending"),
+    Atom("description", "description"),
+    Atom("destructor", "destructor"),
+    Atom("details", "details"),
+    Atom("deviceAspectRatio", "device-aspect-ratio"),
+    Atom("deviceHeight", "device-height"),
+    Atom("devicePixelRatio", "device-pixel-ratio"),
+    Atom("deviceWidth", "device-width"),
+    Atom("dfn", "dfn"),
+    Atom("dialog", "dialog"),
+    Atom("difference", "difference"),
+    Atom("digit", "digit"),
+    Atom("dir", "dir"),
+    Atom("dirAutoSetBy", "dirAutoSetBy"),
+    Atom("directory", "directory"),
+    Atom("disableOutputEscaping", "disable-output-escaping"),
+    Atom("disabled", "disabled"),
+    Atom("disableglobalhistory", "disableglobalhistory"),
+    Atom("disablehistory", "disablehistory"),
+    Atom("disablefullscreen", "disablefullscreen"),
+    Atom("disclosure_closed", "disclosure-closed"),
+    Atom("disclosure_open", "disclosure-open"),
+    Atom("display", "display"),
+    Atom("displayMode", "display-mode"),
+    Atom("distinct", "distinct"),
+    Atom("div", "div"),
+    Atom("dl", "dl"),
+    Atom("docAbstract", "doc-abstract"),
+    Atom("docAcknowledgments", "doc-acknowledgments"),
+    Atom("docAfterword", "doc-afterword"),
+    Atom("docAppendix", "doc-appendix"),
+    Atom("docBacklink", "doc-backlink"),
+    Atom("docBiblioentry", "doc-biblioentry"),
+    Atom("docBibliography", "doc-bibliography"),
+    Atom("docBiblioref", "doc-biblioref"),
+    Atom("docChapter", "doc-chapter"),
+    Atom("docColophon", "doc-colophon"),
+    Atom("docConclusion", "doc-conclusion"),
+    Atom("docCover", "doc-cover"),
+    Atom("docCredit", "doc-credit"),
+    Atom("docCredits", "doc-credits"),
+    Atom("docDedication", "doc-dedication"),
+    Atom("docEndnote", "doc-endnote"),
+    Atom("docEndnotes", "doc-endnotes"),
+    Atom("docEpigraph", "doc-epigraph"),
+    Atom("docEpilogue", "doc-epilogue"),
+    Atom("docErrata", "doc-errata"),
+    Atom("docExample", "doc-example"),
+    Atom("docFootnote", "doc-footnote"),
+    Atom("docForeword", "doc-foreword"),
+    Atom("docGlossary", "doc-glossary"),
+    Atom("docGlossref", "doc-glossref"),
+    Atom("docIndex", "doc-index"),
+    Atom("docIntroduction", "doc-introduction"),
+    Atom("docNoteref", "doc-noteref"),
+    Atom("docNotice", "doc-notice"),
+    Atom("docPagebreak", "doc-pagebreak"),
+    Atom("docPagelist", "doc-pagelist"),
+    Atom("docPart", "doc-part"),
+    Atom("docPreface", "doc-preface"),
+    Atom("docPrologue", "doc-prologue"),
+    Atom("docPullquote", "doc-pullquote"),
+    Atom("docQna", "doc-qna"),
+    Atom("docSubtitle", "doc-subtitle"),
+    Atom("docTip", "doc-tip"),
+    Atom("docToc", "doc-toc"),
+    Atom("doctypePublic", "doctype-public"),
+    Atom("doctypeSystem", "doctype-system"),
+    Atom("document", "document"),
+    Atom("down", "down"),
+    Atom("download", "download"),
+    Atom("drag", "drag"),
+    Atom("draggable", "draggable"),
+    Atom("dragging", "dragging"),
+    Atom("dragSession", "dragSession"),
+    Atom("drawintitlebar", "drawintitlebar"),
+    Atom("drawtitle", "drawtitle"),
+    Atom("dropAfter", "dropAfter"),
+    Atom("dropBefore", "dropBefore"),
+    Atom("dropOn", "dropOn"),
+    Atom("dropMarker", "dropmarker"),
+    Atom("dt", "dt"),
+    Atom("e", "e"),
+    Atom("editable", "editable"),
+    Atom("editing", "editing"),
+    Atom("editor", "editor"),
+    Atom("element", "element"),
+    Atom("elementAvailable", "element-available"),
+    Atom("elements", "elements"),
+    Atom("em", "em"),
+    Atom("embed", "embed"),
+    Atom("empty", "empty"),
+    Atom("encoding", "encoding"),
+    Atom("enctype", "enctype"),
+    Atom("end", "end"),
+    Atom("endEvent", "endEvent"),
+    Atom("enterkeyhint", "enterkeyhint"),
+    Atom("error", "error"),
+    Atom("ethiopic_numeric", "ethiopic-numeric"),
+    Atom("even", "even"),
+    Atom("event", "event"),
+    Atom("events", "events"),
+    Atom("excludeResultPrefixes", "exclude-result-prefixes"),
+    Atom("exportparts", "exportparts"),
+    Atom("explicit_name", "explicit-name"),
+    Atom("extends", "extends"),
+    Atom("extensionElementPrefixes", "extension-element-prefixes"),
+    Atom("face", "face"),
+    Atom("fallback", "fallback"),
+    Atom("_false", "false"),
+    Atom("farthest", "farthest"),
+    Atom("featurePolicyViolation", "feature-policy-violation"),
+    Atom("field", "field"),
+    Atom("fieldset", "fieldset"),
+    Atom("file", "file"),
+    Atom("figcaption", "figcaption"),
+    Atom("figure", "figure"),
+    Atom("findbar", "findbar"),
+    Atom("firstInput", "first-input"),
+    Atom("fixed", "fixed"),
+    Atom("flags", "flags"),
+    Atom("flex", "flex"),
+    Atom("flip", "flip"),
+    Atom("floating", "floating"),
+    Atom("floor", "floor"),
+    Atom("flowlength", "flowlength"),
+    Atom("focus", "focus"),
+    Atom("focused", "focused"),
+    Atom("followanchor", "followanchor"),
+    Atom("following", "following"),
+    Atom("followingSibling", "following-sibling"),
+    Atom("font", "font"),
+    Atom("fontWeight", "font-weight"),
+    Atom("footer", "footer"),
+    Atom("_for", "for"),
+    Atom("forEach", "for-each"),
+    Atom("forcedColors", "forced-colors"),
+    Atom("forceOwnRefreshDriver", "forceOwnRefreshDriver"),
+    Atom("form", "form"),
+    Atom("formaction", "formaction"),
+    Atom("format", "format"),
+    Atom("formatNumber", "format-number"),
+    Atom("formenctype", "formenctype"),
+    Atom("formmethod", "formmethod"),
+    Atom("formnovalidate", "formnovalidate"),
+    Atom("formtarget", "formtarget"),
+    Atom("frame", "frame"),
+    Atom("frameborder", "frameborder"),
+    Atom("frameset", "frameset"),
+    Atom("from", "from"),
+    Atom("fullscreenchange", "fullscreenchange"),
+    Atom("fullscreenerror", "fullscreenerror"),
+    Atom("functionAvailable", "function-available"),
+    Atom("generateId", "generate-id"),
+    Atom("getter", "getter"),
+    Atom("graphicsDocument", "graphics-document"),
+    Atom("graphicsObject", "graphics-object"),
+    Atom("graphicsSymbol", "graphics-symbol"),
+    Atom("grid", "grid"),
+    Atom("group", "group"),
+    Atom("groups", "groups"),
+    Atom("groupbox", "groupbox"),
+    Atom("groupingSeparator", "grouping-separator"),
+    Atom("groupingSize", "grouping-size"),
+    Atom("grow", "grow"),
+    Atom("h1", "h1"),
+    Atom("h2", "h2"),
+    Atom("h3", "h3"),
+    Atom("h4", "h4"),
+    Atom("h5", "h5"),
+    Atom("h6", "h6"),
+    Atom("handheldFriendly", "HandheldFriendly"),
+    Atom("handler", "handler"),
+    Atom("handlers", "handlers"),
+    Atom("HARD", "HARD"),
+    Atom("hasSameNode", "has-same-node"),
+    Atom("hbox", "hbox"),
+    Atom("head", "head"),
+    Atom("header", "header"),
+    Atom("headers", "headers"),
+    Atom("hebrew", "hebrew"),
+    Atom("height", "height"),
+    Atom("hgroup", "hgroup"),
+    Atom("hidden", "hidden"),
+    Atom("hidechrome", "hidechrome"),
+    Atom("hidecolumnpicker", "hidecolumnpicker"),
+    Atom("high", "high"),
+    Atom("highest", "highest"),
+    Atom("horizontal", "horizontal"),
+    Atom("hover", "hover"),
+    Atom("hr", "hr"),
+    Atom("href", "href"),
+    Atom("hreflang", "hreflang"),
+    Atom("hsides", "hsides"),
+    Atom("hspace", "hspace"),
+    Atom("html", "html"),
+    Atom("httpEquiv", "http-equiv"),
+    Atom("i", "i"),
+    Atom("icon", "icon"),
+    Atom("id", "id"),
+    Atom("_if", "if"),
+    Atom("iframe", "iframe"),
+    Atom("ignorekeys", "ignorekeys"),
+    Atom("ignoreuserfocus", "ignoreuserfocus"),
+    Atom("image", "image"),
+    Atom("imageClickedPoint", "image-clicked-point"),
+    Atom("imagesizes", "imagesizes"),
+    Atom("imagesrcset", "imagesrcset"),
+    Atom("img", "img"),
+    Atom("implementation", "implementation"),
+    Atom("implements", "implements"),
+    Atom("import", "import"),
+    Atom("include", "include"),
+    Atom("includes", "includes"),
+    Atom("incontentshell", "incontentshell"),
+    Atom("increment", "increment"),
+    Atom("indent", "indent"),
+    Atom("indeterminate", "indeterminate"),
+    Atom("index", "index"),
+    Atom("inert", "inert"),
+    Atom("infinity", "infinity"),
+    Atom("inherits", "inherits"),
+    Atom("inheritOverflow", "inherit-overflow"),
+    Atom("inheritstyle", "inheritstyle"),
+    Atom("initial_scale", "initial-scale"),
+    Atom("input", "input"),
+    Atom("inputmode", "inputmode"),
+    Atom("ins", "ins"),
+    Atom("insertafter", "insertafter"),
+    Atom("insertbefore", "insertbefore"),
+    Atom("insertion", "insertion"),
+    Atom("integer", "integer"),
+    Atom("integrity", "integrity"),
+    Atom("internal", "internal"),
+    Atom("internals", "internals"),
+    Atom("intersection", "intersection"),
+    Atom("intersectionobserverlist", "intersectionobserverlist"),
+    Atom("is", "is"),
+    Atom("ismap", "ismap"),
+    Atom("itemid", "itemid"),
+    Atom("itemprop", "itemprop"),
+    Atom("itemref", "itemref"),
+    Atom("itemscope", "itemscope"),
+    Atom("itemtype", "itemtype"),
+    Atom("japanese_formal", "japanese-formal"),
+    Atom("japanese_informal", "japanese-informal"),
+    Atom("kbd", "kbd"),
+    Atom("keepcurrentinview", "keepcurrentinview"),
+    Atom("keepobjectsalive", "keepobjectsalive"),
+    Atom("key", "key"),
+    Atom("keycode", "keycode"),
+    Atom("keydown", "keydown"),
+    Atom("keygen", "keygen"),
+    Atom("keypress", "keypress"),
+    Atom("keyset", "keyset"),
+    Atom("keysystem", "keysystem"),
+    Atom("keyup", "keyup"),
+    Atom("kind", "kind"),
+    Atom("korean_hangul_formal", "korean-hangul-formal"),
+    Atom("korean_hanja_formal", "korean-hanja-formal"),
+    Atom("korean_hanja_informal", "korean-hanja-informal"),
+    Atom("label", "label"),
+    Atom("lang", "lang"),
+    Atom("language", "language"),
+    Atom("last", "last"),
+    Atom("layer", "layer"),
+    Atom("LayerActivity", "LayerActivity"),
+    Atom("layout_guess", "layout-guess"),
+    Atom("leading", "leading"),
+    Atom("leaf", "leaf"),
+    Atom("left", "left"),
+    Atom("leftmargin", "leftmargin"),
+    Atom("legend", "legend"),
+    Atom("length", "length"),
+    Atom("letterValue", "letter-value"),
+    Atom("level", "level"),
+    Atom("lhs", "lhs"),
+    Atom("li", "li"),
+    Atom("line", "line"),
+    Atom("link", "link"),
+    Atom("linkset", "linkset"),
+    # Atom("list", "list"),  # "list" is present below
+    Atom("listbox", "listbox"),
+    Atom("listener", "listener"),
+    Atom("listheader", "listheader"),
+    Atom("listing", "listing"),
+    Atom("listitem", "listitem"),
+    Atom("load", "load"),
+    Atom("loading", "loading"),
+    Atom("triggeringprincipal", "triggeringprincipal"),
+    Atom("localedir", "localedir"),
+    Atom("localName", "local-name"),
+    Atom("localization", "localization"),
+    Atom("longdesc", "longdesc"),
+    Atom("loop", "loop"),
+    Atom("low", "low"),
+    Atom("lowerFirst", "lower-first"),
+    Atom("lowest", "lowest"),
+    Atom("lowsrc", "lowsrc"),
+    Atom("ltr", "ltr"),
+    Atom("lwtheme", "lwtheme"),
+    Atom("main", "main"),
+    Atom("map", "map"),
+    Atom("manifest", "manifest"),
+    Atom("marginBottom", "margin-bottom"),
+    Atom("marginLeft", "margin-left"),
+    Atom("marginRight", "margin-right"),
+    Atom("marginTop", "margin-top"),
+    Atom("marginheight", "marginheight"),
+    Atom("marginwidth", "marginwidth"),
+    Atom("mark", "mark"),
+    Atom("marquee", "marquee"),
+    Atom("match", "match"),
+    Atom("max", "max"),
+    Atom("maxheight", "maxheight"),
+    Atom("maximum_scale", "maximum-scale"),
+    Atom("maxlength", "maxlength"),
+    Atom("maxpos", "maxpos"),
+    Atom("maxwidth", "maxwidth"),
+    Atom("measure", "measure"),
+    Atom("media", "media"),
+    Atom("mediaType", "media-type"),
+    Atom("menu", "menu"),
+    Atom("menubar", "menubar"),
+    Atom("menucaption", "menucaption"),
+    Atom("menugroup", "menugroup"),
+    Atom("menuitem", "menuitem"),
+    Atom("menulist", "menulist"),
+    Atom("menupopup", "menupopup"),
+    Atom("menuseparator", "menuseparator"),
+    Atom("mesh", "mesh"),
+    Atom("message", "message"),
+    Atom("meta", "meta"),
+    Atom("referrer", "referrer"),
+    Atom("referrerpolicy", "referrerpolicy"),
+    Atom("renderroot", "renderroot"),
+    Atom("headerReferrerPolicy", "referrer-policy"),
+    Atom("meter", "meter"),
+    Atom("method", "method"),
+    Atom("middle", "middle"),
+    Atom("min", "min"),
+    Atom("minheight", "minheight"),
+    Atom("minimum_scale", "minimum-scale"),
+    Atom("minlength", "minlength"),
+    Atom("minpos", "minpos"),
+    Atom("minusSign", "minus-sign"),
+    Atom("minwidth", "minwidth"),
+    Atom("mixed", "mixed"),
+    Atom("messagemanagergroup", "messagemanagergroup"),
+    Atom("mod", "mod"),
+    Atom("mode", "mode"),
+    Atom("modifiers", "modifiers"),
+    Atom("monochrome", "monochrome"),
+    Atom("mouseover", "mouseover"),
+    Atom("mozAccessiblecaret", "moz-accessiblecaret"),
+    Atom("mozCustomContentContainer", "moz-custom-content-container"),
+    Atom("mozGrabber", "mozGrabber"),
+    Atom("mozNativeAnonymous", "-moz-native-anonymous"),
+    Atom("mozprivatebrowsing", "mozprivatebrowsing"),
+    Atom("mozResizer", "mozResizer"),
+    Atom("mozResizingInfo", "mozResizingInfo"),
+    Atom("mozResizingShadow", "mozResizingShadow"),
+    Atom("mozTableAddColumnAfter", "mozTableAddColumnAfter"),
+    Atom("mozTableAddColumnBefore", "mozTableAddColumnBefore"),
+    Atom("mozTableAddRowAfter", "mozTableAddRowAfter"),
+    Atom("mozTableAddRowBefore", "mozTableAddRowBefore"),
+    Atom("mozTableRemoveRow", "mozTableRemoveRow"),
+    Atom("mozTableRemoveColumn", "mozTableRemoveColumn"),
+    Atom("moz_opaque", "moz-opaque"),
+    Atom("moz_action_hint", "mozactionhint"),
+    Atom("multicol", "multicol"),
+    Atom("multiple", "multiple"),
+    Atom("muted", "muted"),
+    Atom("name", "name"),
+    Atom("_namespace", "namespace"),
+    Atom("namespaceAlias", "namespace-alias"),
+    Atom("namespaceUri", "namespace-uri"),
+    Atom("NaN", "NaN"),
+    Atom("n", "n"),
+    Atom("nativeAnonymousChildList", "nativeAnonymousChildList"),
+    Atom("nav", "nav"),
+    Atom("ne", "ne"),
+    Atom("never", "never"),
+    Atom("_new", "new"),
+    Atom("newline", "newline"),
+    Atom("nextRemoteTabId", "nextRemoteTabId"),
+    Atom("no", "no"),
+    Atom("noautofocus", "noautofocus"),
+    Atom("noautohide", "noautohide"),
+    Atom("norolluponanchor", "norolluponanchor"),
+    Atom("noBar", "no-bar"),
+    Atom("nobr", "nobr"),
+    Atom("nodefaultsrc", "nodefaultsrc"),
+    Atom("nodeSet", "node-set"),
+    Atom("noembed", "noembed"),
+    Atom("noframes", "noframes"),
+    Atom("nohref", "nohref"),
+    Atom("nomodule", "nomodule"),
+    Atom("nonce", "nonce"),
+    Atom("none", "none"),
+    Atom("noresize", "noresize"),
+    Atom("normal", "normal"),
+    Atom("normalizeSpace", "normalize-space"),
+    Atom("noscript", "noscript"),
+    Atom("noshade", "noshade"),
+    Atom("notification", "notification"),
+    Atom("novalidate", "novalidate"),
+    Atom("_not", "not"),
+    Atom("nowrap", "nowrap"),
+    Atom("number", "number"),
+    Atom("nw", "nw"),
+    Atom("object", "object"),
+    Atom("objectType", "object-type"),
+    Atom("observes", "observes"),
+    Atom("odd", "odd"),
+    Atom("OFF", "OFF"),
+    Atom("ol", "ol"),
+    Atom("omitXmlDeclaration", "omit-xml-declaration"),
+    Atom("onabort", "onabort"),
+    Atom("onmozaccesskeynotfound", "onmozaccesskeynotfound"),
+    Atom("onactivate", "onactivate"),
+    Atom("onafterprint", "onafterprint"),
+    Atom("onafterscriptexecute", "onafterscriptexecute"),
+    Atom("onanimationcancel", "onanimationcancel"),
+    Atom("onanimationend", "onanimationend"),
+    Atom("onanimationiteration", "onanimationiteration"),
+    Atom("onanimationstart", "onanimationstart"),
+    Atom("onAppCommand", "onAppCommand"),
+    Atom("onaudioprocess", "onaudioprocess"),
+    Atom("onauxclick", "onauxclick"),
+    Atom("onbeforecopy", "onbeforecopy"),
+    Atom("onbeforecut", "onbeforecut"),
+    Atom("onbeforeinput", "onbeforeinput"),
+    Atom("onbeforepaste", "onbeforepaste"),
+    Atom("onbeforeprint", "onbeforeprint"),
+    Atom("onbeforescriptexecute", "onbeforescriptexecute"),
+    Atom("onbeforeunload", "onbeforeunload"),
+    Atom("onblocked", "onblocked"),
+    Atom("onblur", "onblur"),
+    Atom("onbounce", "onbounce"),
+    Atom("onboundschange", "onboundschange"),
+    Atom("onbroadcast", "onbroadcast"),
+    Atom("onbufferedamountlow", "onbufferedamountlow"),
+    Atom("oncached", "oncached"),
+    Atom("oncancel", "oncancel"),
+    Atom("onchange", "onchange"),
+    Atom("onchargingchange", "onchargingchange"),
+    Atom("onchargingtimechange", "onchargingtimechange"),
+    Atom("onchecking", "onchecking"),
+    Atom("onCheckboxStateChange", "onCheckboxStateChange"),
+    Atom("onCheckKeyPressEventModel", "onCheckKeyPressEventModel"),
+    Atom("onclick", "onclick"),
+    Atom("onclose", "onclose"),
+    Atom("oncommand", "oncommand"),
+    Atom("oncommandupdate", "oncommandupdate"),
+    Atom("oncomplete", "oncomplete"),
+    Atom("oncompositionend", "oncompositionend"),
+    Atom("oncompositionstart", "oncompositionstart"),
+    Atom("oncompositionupdate", "oncompositionupdate"),
+    Atom("onconnect", "onconnect"),
+    Atom("onconnectionavailable", "onconnectionavailable"),
+    Atom("oncontextmenu", "oncontextmenu"),
+    Atom("oncontextlost", "oncontextlost"),
+    Atom("oncontextrestored", "oncontextrestored"),
+    Atom("oncopy", "oncopy"),
+    Atom("oncut", "oncut"),
+    Atom("ondblclick", "ondblclick"),
+    Atom("ondischargingtimechange", "ondischargingtimechange"),
+    Atom("ondownloading", "ondownloading"),
+    Atom("onDOMActivate", "onDOMActivate"),
+    Atom("onDOMAttrModified", "onDOMAttrModified"),
+    Atom("onDOMCharacterDataModified", "onDOMCharacterDataModified"),
+    Atom("onDOMFocusIn", "onDOMFocusIn"),
+    Atom("onDOMFocusOut", "onDOMFocusOut"),
+    Atom("onDOMMouseScroll", "onDOMMouseScroll"),
+    Atom("onDOMNodeInserted", "onDOMNodeInserted"),
+    Atom("onDOMNodeInsertedIntoDocument", "onDOMNodeInsertedIntoDocument"),
+    Atom("onDOMNodeRemoved", "onDOMNodeRemoved"),
+    Atom("onDOMNodeRemovedFromDocument", "onDOMNodeRemovedFromDocument"),
+    Atom("onDOMSubtreeModified", "onDOMSubtreeModified"),
+    Atom("ondata", "ondata"),
+    Atom("ondrag", "ondrag"),
+    Atom("ondragdrop", "ondragdrop"),
+    Atom("ondragend", "ondragend"),
+    Atom("ondragenter", "ondragenter"),
+    Atom("ondragexit", "ondragexit"),
+    Atom("ondragleave", "ondragleave"),
+    Atom("ondragover", "ondragover"),
+    Atom("ondragstart", "ondragstart"),
+    Atom("ondrain", "ondrain"),
+    Atom("ondrop", "ondrop"),
+    Atom("onerror", "onerror"),
+    Atom("onfinish", "onfinish"),
+    Atom("onfocus", "onfocus"),
+    Atom("onfocusin", "onfocusin"),
+    Atom("onfocusout", "onfocusout"),
+    Atom("onfullscreenchange", "onfullscreenchange"),
+    Atom("onfullscreenerror", "onfullscreenerror"),
+    Atom("onget", "onget"),
+    Atom("onhashchange", "onhashchange"),
+    Atom("oninput", "oninput"),
+    Atom("oninputsourceschange", "oninputsourceschange"),
+    Atom("oninstall", "oninstall"),
+    Atom("oninvalid", "oninvalid"),
+    Atom("onkeydown", "onkeydown"),
+    Atom("onkeypress", "onkeypress"),
+    Atom("onkeyup", "onkeyup"),
+    Atom("onlanguagechange", "onlanguagechange"),
+    Atom("onlevelchange", "onlevelchange"),
+    Atom("onload", "onload"),
+    Atom("onloading", "onloading"),
+    Atom("onloadingdone", "onloadingdone"),
+    Atom("onloadingerror", "onloadingerror"),
+    Atom("onpopstate", "onpopstate"),
+    Atom("only", "only"),  # this one is not an event
+    Atom("onmerchantvalidation", "onmerchantvalidation"),
+    Atom("onmessage", "onmessage"),
+    Atom("onmessageerror", "onmessageerror"),
+    Atom("onmidimessage", "onmidimessage"),
+    Atom("onmousedown", "onmousedown"),
+    Atom("onmouseenter", "onmouseenter"),
+    Atom("onmouseleave", "onmouseleave"),
+    Atom("onmouselongtap", "onmouselongtap"),
+    Atom("onmousemove", "onmousemove"),
+    Atom("onmouseout", "onmouseout"),
+    Atom("onmouseover", "onmouseover"),
+    Atom("onMozMouseHittest", "onMozMouseHittest"),
+    Atom("onMozMouseExploreByTouch", "onMozMouseExploreByTouch"),
+    Atom("onmouseup", "onmouseup"),
+    Atom("onMozAfterPaint", "onMozAfterPaint"),
+    Atom("onmozfullscreenchange", "onmozfullscreenchange"),
+    Atom("onmozfullscreenerror", "onmozfullscreenerror"),
+    Atom("onmozpointerlockchange", "onmozpointerlockchange"),
+    Atom("onmozpointerlockerror", "onmozpointerlockerror"),
+    Atom("onMozMousePixelScroll", "onMozMousePixelScroll"),
+    Atom("onMozScrolledAreaChanged", "onMozScrolledAreaChanged"),
+    Atom("onmute", "onmute"),
+    Atom("onnotificationclick", "onnotificationclick"),
+    Atom("onnotificationclose", "onnotificationclose"),
+    Atom("onnoupdate", "onnoupdate"),
+    Atom("onobsolete", "onobsolete"),
+    Atom("ononline", "ononline"),
+    Atom("onoffline", "onoffline"),
+    Atom("onopen", "onopen"),
+    Atom("onorientationchange", "onorientationchange"),
+    Atom("onoverflow", "onoverflow"),
+    Atom("onpagehide", "onpagehide"),
+    Atom("onpageshow", "onpageshow"),
+    Atom("onpaste", "onpaste"),
+    Atom("onpayerdetailchange", "onpayerdetailchange"),
+    Atom("onpaymentmethodchange", "onpaymentmethodchange"),
+    Atom("onpointerlockchange", "onpointerlockchange"),
+    Atom("onpointerlockerror", "onpointerlockerror"),
+    Atom("onpopuphidden", "onpopuphidden"),
+    Atom("onpopuphiding", "onpopuphiding"),
+    Atom("onpopuppositioned", "onpopuppositioned"),
+    Atom("onpopupshowing", "onpopupshowing"),
+    Atom("onpopupshown", "onpopupshown"),
+    Atom("onprocessorerror", "onprocessorerror"),
+    Atom("onprioritychange", "onprioritychange"),
+    Atom("onpush", "onpush"),
+    Atom("onpushsubscriptionchange", "onpushsubscriptionchange"),
+    Atom("onRadioStateChange", "onRadioStateChange"),
+    Atom("onreadystatechange", "onreadystatechange"),
+    Atom("onrejectionhandled", "onrejectionhandled"),
+    Atom("onremove", "onremove"),
+    Atom("onrequestprogress", "onrequestprogress"),
+    Atom("onresourcetimingbufferfull", "onresourcetimingbufferfull"),
+    Atom("onresponseprogress", "onresponseprogress"),
+    Atom("onRequest", "onRequest"),
+    Atom("onreset", "onreset"),
+    Atom("onresize", "onresize"),
+    Atom("onscroll", "onscroll"),
+    Atom("onsecuritypolicyviolation", "onsecuritypolicyviolation"),
+    Atom("onselect", "onselect"),
+    Atom("onselectionchange", "onselectionchange"),
+    Atom("onselectend", "onselectend"),
+    Atom("onselectstart", "onselectstart"),
+    Atom("onset", "onset"),
+    Atom("onshippingaddresschange", "onshippingaddresschange"),
+    Atom("onshippingoptionchange", "onshippingoptionchange"),
+    Atom("onshow", "onshow"),
+    Atom("onslotchange", "onslotchange"),
+    Atom("onsqueeze", "onsqueeze"),
+    Atom("onsqueezeend", "onsqueezeend"),
+    Atom("onsqueezestart", "onsqueezestart"),
+    Atom("onstatechange", "onstatechange"),
+    Atom("onstorage", "onstorage"),
+    Atom("onsubmit", "onsubmit"),
+    Atom("onsuccess", "onsuccess"),
+    Atom("onsystemstatusbarclick", "onsystemstatusbarclick"),
+    Atom("ontypechange", "ontypechange"),
+    Atom("onterminate", "onterminate"),
+    Atom("ontext", "ontext"),
+    Atom("ontoggle", "ontoggle"),
+    Atom("ontonechange", "ontonechange"),
+    Atom("ontouchstart", "ontouchstart"),
+    Atom("ontouchend", "ontouchend"),
+    Atom("ontouchmove", "ontouchmove"),
+    Atom("ontouchcancel", "ontouchcancel"),
+    Atom("ontransitioncancel", "ontransitioncancel"),
+    Atom("ontransitionend", "ontransitionend"),
+    Atom("ontransitionrun", "ontransitionrun"),
+    Atom("ontransitionstart", "ontransitionstart"),
+    Atom("onuncapturederror", "onuncapturederror"),
+    Atom("onunderflow", "onunderflow"),
+    Atom("onunhandledrejection", "onunhandledrejection"),
+    Atom("onunload", "onunload"),
+    Atom("onunmute", "onunmute"),
+    Atom("onupdatefound", "onupdatefound"),
+    Atom("onupdateready", "onupdateready"),
+    Atom("onupgradeneeded", "onupgradeneeded"),
+    Atom("onversionchange", "onversionchange"),
+    Atom("onvisibilitychange", "onvisibilitychange"),
+    Atom("onvoiceschanged", "onvoiceschanged"),
+    Atom("onvrdisplayactivate", "onvrdisplayactivate"),
+    Atom("onvrdisplayconnect", "onvrdisplayconnect"),
+    Atom("onvrdisplaydeactivate", "onvrdisplaydeactivate"),
+    Atom("onvrdisplaydisconnect", "onvrdisplaydisconnect"),
+    Atom("onvrdisplaypresentchange", "onvrdisplaypresentchange"),
+    Atom("onwebkitAnimationEnd", "onwebkitAnimationEnd"),
+    Atom("onwebkitAnimationIteration", "onwebkitAnimationIteration"),
+    Atom("onwebkitAnimationStart", "onwebkitAnimationStart"),
+    Atom("onwebkitTransitionEnd", "onwebkitTransitionEnd"),
+    Atom("onwebkitanimationend", "onwebkitanimationend"),
+    Atom("onwebkitanimationiteration", "onwebkitanimationiteration"),
+    Atom("onwebkitanimationstart", "onwebkitanimationstart"),
+    Atom("onwebkittransitionend", "onwebkittransitionend"),
+    Atom("onwheel", "onwheel"),
+    Atom("open", "open"),
+    Atom("optgroup", "optgroup"),
+    Atom("optimum", "optimum"),
+    Atom("option", "option"),
+    Atom("_or", "or"),
+    Atom("order", "order"),
+    Atom("orient", "orient"),
+    Atom("orientation", "orientation"),
+    Atom("origin_trial", "origin-trial"),
+    Atom("otherwise", "otherwise"),
+    Atom("output", "output"),
+    Atom("overflow", "overflow"),
+    Atom("overflowBlock", "overflow-block"),
+    Atom("overflowInline", "overflow-inline"),
+    Atom("overlay", "overlay"),
+    Atom("p", "p"),
+    Atom("pack", "pack"),
+    Atom("page", "page"),
+    Atom("pageincrement", "pageincrement"),
+    Atom("paint", "paint"),
+    Atom("paint_order", "paint-order"),
+    Atom("panel", "panel"),
+    Atom("paragraph", "paragraph"),
+    Atom("param", "param"),
+    Atom("parameter", "parameter"),
+    Atom("parent", "parent"),
+    Atom("parentfocused", "parentfocused"),
+    Atom("parsererror", "parsererror"),
+    Atom("part", "part"),
+    Atom("password", "password"),
+    Atom("pattern", "pattern"),
+    Atom("patternSeparator", "pattern-separator"),
+    Atom("perMille", "per-mille"),
+    Atom("percent", "percent"),
+    Atom("persist", "persist"),
+    Atom("phase", "phase"),
+    Atom("picture", "picture"),
+    Atom("ping", "ping"),
+    Atom("pinned", "pinned"),
+    Atom("placeholder", "placeholder"),
+    Atom("plaintext", "plaintext"),
+    Atom("playbackrate", "playbackrate"),
+    Atom("pointSize", "point-size"),
+    Atom("poly", "poly"),
+    Atom("polygon", "polygon"),
+    Atom("popup", "popup"),
+    Atom("popupalign", "popupalign"),
+    Atom("popupanchor", "popupanchor"),
+    Atom("popupgroup", "popupgroup"),
+    Atom("popupset", "popupset"),
+    Atom("popupsinherittooltip", "popupsinherittooltip"),
+    Atom("portal", "portal"),
+    Atom("position", "position"),
+    Atom("poster", "poster"),
+    Atom("pre", "pre"),
+    Atom("preceding", "preceding"),
+    Atom("precedingSibling", "preceding-sibling"),
+    Atom("prefersReducedMotion", "prefers-reduced-motion"),
+    Atom("prefersColorScheme", "prefers-color-scheme"),
+    Atom("prefersContrast", "prefers-contrast"),
+    Atom("prefix", "prefix"),
+    Atom("prefwidth", "prefwidth"),
+    Atom("dynamicRange", "dynamic-range"),
+    Atom("videoDynamicRange", "video-dynamic-range"),
+    Atom("preload", "preload"),
+    Atom("preserve", "preserve"),
+    Atom("preserveSpace", "preserve-space"),
+    Atom("preventdefault", "preventdefault"),
+    Atom("previewDiv", "preview-div"),
+    Atom("primary", "primary"),
+    Atom("print", "print"),
+    Atom("printisfocuseddoc", "printisfocuseddoc"),
+    Atom("printselectionranges", "printselectionranges"),
+    Atom("priority", "priority"),
+    Atom("processingInstruction", "processing-instruction"),
+    Atom("profile", "profile"),
+    Atom("progress", "progress"),
+    Atom("prompt", "prompt"),
+    Atom("properties", "properties"),
+    Atom("property", "property"),
+    Atom("pubdate", "pubdate"),
+    Atom("q", "q"),
+    Atom("radio", "radio"),
+    Atom("radioLabel", "radio-label"),
+    Atom("radiogroup", "radiogroup"),
+    Atom("range", "range"),
+    Atom("readonly", "readonly"),
+    Atom("rect", "rect"),
+    Atom("rectangle", "rectangle"),
+    Atom("refresh", "refresh"),
+    Atom("rel", "rel"),
+    Atom("relativeBounds", "relative-bounds"),
+    Atom("rem", "rem"),
+    Atom("remote", "remote"),
+    Atom("removeelement", "removeelement"),
+    Atom("renderingobserverset", "renderingobserverset"),
+    Atom("repeat", "repeat"),
+    Atom("replace", "replace"),
+    Atom("requestcontextid", "requestcontextid"),
+    Atom("required", "required"),
+    Atom("reserved", "reserved"),
+    Atom("reset", "reset"),
+    Atom("resizeafter", "resizeafter"),
+    Atom("resizebefore", "resizebefore"),
+    Atom("resizer", "resizer"),
+    Atom("resolution", "resolution"),
+    Atom("resources", "resources"),
+    Atom("result", "result"),
+    Atom("resultPrefix", "result-prefix"),
+    Atom("retargetdocumentfocus", "retargetdocumentfocus"),
+    Atom("rev", "rev"),
+    Atom("reverse", "reverse"),
+    Atom("reversed", "reversed"),
+    Atom("rhs", "rhs"),
+    Atom("richlistbox", "richlistbox"),
+    Atom("richlistitem", "richlistitem"),
+    Atom("right", "right"),
+    Atom("rightmargin", "rightmargin"),
+    Atom("role", "role"),
+    Atom("rolluponmousewheel", "rolluponmousewheel"),
+    Atom("round", "round"),
+    Atom("row", "row"),
+    Atom("rows", "rows"),
+    Atom("rowspan", "rowspan"),
+    Atom("rb", "rb"),
+    Atom("rp", "rp"),
+    Atom("rt", "rt"),
+    Atom("rtc", "rtc"),
+    Atom("rtl", "rtl"),
+    Atom("ruby", "ruby"),
+    Atom("rubyBase", "ruby-base"),
+    Atom("rubyBaseContainer", "ruby-base-container"),
+    Atom("rubyText", "ruby-text"),
+    Atom("rubyTextContainer", "ruby-text-container"),
+    Atom("rules", "rules"),
+    Atom("s", "s"),
+    Atom("safe_area_inset_top", "safe-area-inset-top"),
+    Atom("safe_area_inset_bottom", "safe-area-inset-bottom"),
+    Atom("safe_area_inset_left", "safe-area-inset-left"),
+    Atom("safe_area_inset_right", "safe-area-inset-right"),
+    Atom("samp", "samp"),
+    Atom("sandbox", "sandbox"),
+    Atom("sbattr", "sbattr"),
+    Atom("scale", "scale"),
+    Atom("scan", "scan"),
+    Atom("scheme", "scheme"),
+    Atom("scope", "scope"),
+    Atom("scoped", "scoped"),
+    Atom("screen", "screen"),
+    Atom("screenX", "screenX"),
+    Atom("screenx", "screenx"),
+    Atom("screenY", "screenY"),
+    Atom("screeny", "screeny"),
+    Atom("script", "script"),
+    Atom("scrollbar", "scrollbar"),
+    Atom("scrollbarThumb", "scrollbar-thumb"),
+    Atom("scrollamount", "scrollamount"),
+    Atom("scrollbarbutton", "scrollbarbutton"),
+    Atom("scrollbarDownBottom", "scrollbar-down-bottom"),
+    Atom("scrollbarDownTop", "scrollbar-down-top"),
+    Atom("scrollbarUpBottom", "scrollbar-up-bottom"),
+    Atom("scrollbarUpTop", "scrollbar-up-top"),
+    Atom("scrollbox", "scrollbox"),
+    Atom("scrollcorner", "scrollcorner"),
+    Atom("scrolldelay", "scrolldelay"),
+    Atom("scrolling", "scrolling"),
+    Atom("scrollPosition", "scroll-position"),
+    Atom("se", "se"),
+    Atom("section", "section"),
+    Atom("select", "select"),
+    Atom("selected", "selected"),
+    Atom("selectedIndex", "selectedIndex"),
+    Atom("selectedindex", "selectedindex"),
+    Atom("selectmenu", "selectmenu"),
+    Atom("self", "self"),
+    Atom("seltype", "seltype"),
+    Atom("setcookie", "set-cookie"),
+    Atom("setter", "setter"),
+    Atom("shadow", "shadow"),
+    Atom("shape", "shape"),
+    Atom("show", "show"),
+    Atom("showcaret", "showcaret"),
+    Atom("sibling", "sibling"),
+    Atom("simple", "simple"),
+    Atom("simp_chinese_formal", "simp-chinese-formal"),
+    Atom("simp_chinese_informal", "simp-chinese-informal"),
+    Atom("single", "single"),
+    Atom("size", "size"),
+    Atom("sizes", "sizes"),
+    Atom("sizemode", "sizemode"),
+    Atom("sizetopopup", "sizetopopup"),
+    Atom("slider", "slider"),
+    Atom("small", "small"),
+    Atom("smooth", "smooth"),
+    Atom("snap", "snap"),
+    Atom("solid", "solid"),
+    Atom("sort", "sort"),
+    Atom("sortActive", "sortActive"),
+    Atom("sortDirection", "sortDirection"),
+    Atom("sorted", "sorted"),
+    Atom("sorthints", "sorthints"),
+    Atom("source", "source"),
+    Atom("sourcetext", "sourcetext"),
+    Atom("space", "space"),
+    Atom("spacer", "spacer"),
+    Atom("span", "span"),
+    Atom("spellcheck", "spellcheck"),
+    Atom("split", "split"),
+    Atom("splitter", "splitter"),
+    Atom("square", "square"),
+    Atom("src", "src"),
+    Atom("srcdoc", "srcdoc"),
+    Atom("srclang", "srclang"),
+    Atom("srcset", "srcset"),
+    Atom("standalone", "standalone"),
+    Atom("standby", "standby"),
+    Atom("start", "start"),
+    Atom("startsWith", "starts-with"),
+    Atom("state", "state"),
+    Atom("statusbar", "statusbar"),
+    Atom("step", "step"),
+    Atom("stop", "stop"),
+    Atom("stretch", "stretch"),
+    Atom("strike", "strike"),
+    Atom("string", "string"),
+    Atom("stringLength", "string-length"),
+    Atom("stripSpace", "strip-space"),
+    Atom("strong", "strong"),
+    Atom("style", "style"),
+    Atom("stylesheet", "stylesheet"),
+    Atom("stylesheetPrefix", "stylesheet-prefix"),
+    Atom("submit", "submit"),
+    Atom("substate", "substate"),
+    Atom("substring", "substring"),
+    Atom("substringAfter", "substring-after"),
+    Atom("substringBefore", "substring-before"),
+    Atom("sub", "sub"),
+    Atom("suggestion", "suggestion"),
+    Atom("sum", "sum"),
+    Atom("sup", "sup"),
+    Atom("summary", "summary"),
+    Atom("sw", "sw"),
+    # Atom("_switch", "switch"),  # "switch" is present below
+    Atom("systemProperty", "system-property"),
+    Atom("tab", "tab"),
+    Atom("tabindex", "tabindex"),
+    Atom("table", "table"),
+    Atom("tabpanel", "tabpanel"),
+    Atom("tabpanels", "tabpanels"),
+    Atom("tag", "tag"),
+    Atom("target", "target"),
+    Atom("targets", "targets"),
+    Atom("tbody", "tbody"),
+    Atom("td", "td"),
+    Atom("tel", "tel"),
+    Atom("_template", "template"),
+    Atom("text_decoration", "text-decoration"),
+    Atom("terminate", "terminate"),
+    Atom("term", "term"),
+    Atom("test", "test"),
+    Atom("text", "text"),
+    Atom("textAlign", "text-align"),
+    Atom("textarea", "textarea"),
+    Atom("textbox", "textbox"),
+    Atom("textLink", "text-link"),
+    Atom("textNodeDirectionalityMap", "textNodeDirectionalityMap"),
+    Atom("textOverlay", "text-overlay"),
+    Atom("tfoot", "tfoot"),
+    Atom("th", "th"),
+    Atom("thead", "thead"),
+    Atom("thumb", "thumb"),
+    Atom("time", "time"),
+    Atom("title", "title"),
+    Atom("titlebar", "titlebar"),
+    Atom("titletip", "titletip"),
+    Atom("token", "token"),
+    Atom("tokenize", "tokenize"),
+    Atom("toolbar", "toolbar"),
+    Atom("toolbarbutton", "toolbarbutton"),
+    Atom("toolbaritem", "toolbaritem"),
+    Atom("toolbarpaletteitem", "toolbarpaletteitem"),
+    Atom("toolbox", "toolbox"),
+    Atom("tooltip", "tooltip"),
+    Atom("tooltiptext", "tooltiptext"),
+    Atom("top", "top"),
+    Atom("topleft", "topleft"),
+    Atom("topmargin", "topmargin"),
+    Atom("topright", "topright"),
+    Atom("tr", "tr"),
+    Atom("track", "track"),
+    Atom("trad_chinese_formal", "trad-chinese-formal"),
+    Atom("trad_chinese_informal", "trad-chinese-informal"),
+    Atom("trailing", "trailing"),
+    Atom("transform", "transform"),
+    Atom("transform_3d", "transform-3d"),
+    Atom("transformiix", "transformiix"),
+    Atom("translate", "translate"),
+    Atom("transparent", "transparent"),
+    Atom("tree", "tree"),
+    Atom("treecell", "treecell"),
+    Atom("treechildren", "treechildren"),
+    Atom("treecol", "treecol"),
+    Atom("treecolpicker", "treecolpicker"),
+    Atom("treecols", "treecols"),
+    Atom("treeitem", "treeitem"),
+    Atom("treerow", "treerow"),
+    Atom("treeseparator", "treeseparator"),
+    Atom("_true", "true"),
+    Atom("truespeed", "truespeed"),
+    Atom("tt", "tt"),
+    Atom("type", "type"),
+    Atom("u", "u"),
+    Atom("ul", "ul"),
+    Atom("unparsedEntityUri", "unparsed-entity-uri"),
+    Atom("up", "up"),
+    Atom("update", "update"),
+    Atom("upperFirst", "upper-first"),
+    Atom("use", "use"),
+    Atom("useAttributeSets", "use-attribute-sets"),
+    Atom("usemap", "usemap"),
+    Atom("user_scalable", "user-scalable"),
+    Atom("validate", "validate"),
+    Atom("valign", "valign"),
+    Atom("value", "value"),
+    Atom("values", "values"),
+    Atom("valueOf", "value-of"),
+    Atom("valuetype", "valuetype"),
+    Atom("var", "var"),
+    Atom("variable", "variable"),
+    Atom("vendor", "vendor"),
+    Atom("vendorUrl", "vendor-url"),
+    Atom("version", "version"),
+    Atom("vertical", "vertical"),
+    Atom("audio", "audio"),
+    Atom("video", "video"),
+    Atom("viewport", "viewport"),
+    Atom("viewport_fit", "viewport-fit"),
+    Atom("viewport_height", "viewport-height"),
+    Atom("viewport_initial_scale", "viewport-initial-scale"),
+    Atom("viewport_maximum_scale", "viewport-maximum-scale"),
+    Atom("viewport_minimum_scale", "viewport-minimum-scale"),
+    Atom("viewport_user_scalable", "viewport-user-scalable"),
+    Atom("viewport_width", "viewport-width"),
+    Atom("visibility", "visibility"),
+    Atom("visuallyselected", "visuallyselected"),
+    Atom("vlink", "vlink"),
+    Atom("_void", "void"),
+    Atom("vsides", "vsides"),
+    Atom("vspace", "vspace"),
+    Atom("w", "w"),
+    Atom("wbr", "wbr"),
+    Atom("webkitdirectory", "webkitdirectory"),
+    Atom("when", "when"),
+    Atom("white", "white"),
+    Atom("width", "width"),
+    Atom("willChange", "will-change"),
+    Atom("window", "window"),
+    Atom("headerWindowTarget", "window-target"),
+    Atom("windowtype", "windowtype"),
+    Atom("withParam", "with-param"),
+    Atom("wrap", "wrap"),
+    Atom("headerDNSPrefetchControl", "x-dns-prefetch-control"),
+    Atom("headerCSP", "content-security-policy"),
+    Atom("headerCSPReportOnly", "content-security-policy-report-only"),
+    Atom("headerXFO", "x-frame-options"),
+    Atom("x_western", "x-western"),
+    Atom("xml", "xml"),
+    Atom("xml_stylesheet", "xml-stylesheet"),
+    Atom("xmlns", "xmlns"),
+    Atom("xmp", "xmp"),
+    Atom("xul", "xul"),
+    Atom("yes", "yes"),
+    Atom("z_index", "z-index"),
+    Atom("zeroDigit", "zero-digit"),
+    Atom("zlevel", "zlevel"),
+    Atom("percentage", "%"),
+    Atom("A", "A"),
+    Atom("alignment_baseline", "alignment-baseline"),
+    Atom("amplitude", "amplitude"),
+    Atom("animate", "animate"),
+    Atom("animateColor", "animateColor"),
+    Atom("animateMotion", "animateMotion"),
+    Atom("animateTransform", "animateTransform"),
+    Atom("arithmetic", "arithmetic"),
+    Atom("atop", "atop"),
+    Atom("azimuth", "azimuth"),
+    Atom("B", "B"),
+    Atom("backgroundColor", "background-color"),
+    Atom("background_image", "background-image"),
+    Atom("baseFrequency", "baseFrequency"),
+    Atom("baseline_shift", "baseline-shift"),
+    Atom("bias", "bias"),
+    Atom("caption_side", "caption-side"),
+    Atom("clip_path", "clip-path"),
+    Atom("clip_rule", "clip-rule"),
+    Atom("clipPath", "clipPath"),
+    Atom("clipPathUnits", "clipPathUnits"),
+    Atom("cm", "cm"),
+    Atom("colorBurn", "color-burn"),
+    Atom("colorDodge", "color-dodge"),
+    Atom("colorInterpolation", "color-interpolation"),
+    Atom("colorInterpolationFilters", "color-interpolation-filters"),
+    Atom("colorProfile", "color-profile"),
+    Atom("cursor", "cursor"),
+    Atom("cx", "cx"),
+    Atom("cy", "cy"),
+    Atom("d", "d"),
+    Atom("darken", "darken"),
+    Atom("defs", "defs"),
+    Atom("deg", "deg"),
+    Atom("desc", "desc"),
+    Atom("diffuseConstant", "diffuseConstant"),
+    Atom("dilate", "dilate"),
+    Atom("direction", "direction"),
+    Atom("disable", "disable"),
+    Atom("disc", "disc"),
+    Atom("discrete", "discrete"),
+    Atom("divisor", "divisor"),
+    Atom("dominant_baseline", "dominant-baseline"),
+    Atom("duplicate", "duplicate"),
+    Atom("dx", "dx"),
+    Atom("dy", "dy"),
+    Atom("edgeMode", "edgeMode"),
+    Atom("ellipse", "ellipse"),
+    Atom("elevation", "elevation"),
+    Atom("erode", "erode"),
+    Atom("ex", "ex"),
+    Atom("exact", "exact"),
+    Atom("exclusion", "exclusion"),
+    Atom("exponent", "exponent"),
+    Atom("feBlend", "feBlend"),
+    Atom("feColorMatrix", "feColorMatrix"),
+    Atom("feComponentTransfer", "feComponentTransfer"),
+    Atom("feComposite", "feComposite"),
+    Atom("feConvolveMatrix", "feConvolveMatrix"),
+    Atom("feDiffuseLighting", "feDiffuseLighting"),
+    Atom("feDisplacementMap", "feDisplacementMap"),
+    Atom("feDistantLight", "feDistantLight"),
+    Atom("feDropShadow", "feDropShadow"),
+    Atom("feFlood", "feFlood"),
+    Atom("feFuncA", "feFuncA"),
+    Atom("feFuncB", "feFuncB"),
+    Atom("feFuncG", "feFuncG"),
+    Atom("feFuncR", "feFuncR"),
+    Atom("feGaussianBlur", "feGaussianBlur"),
+    Atom("feImage", "feImage"),
+    Atom("feMerge", "feMerge"),
+    Atom("feMergeNode", "feMergeNode"),
+    Atom("feMorphology", "feMorphology"),
+    Atom("feOffset", "feOffset"),
+    Atom("fePointLight", "fePointLight"),
+    Atom("feSpecularLighting", "feSpecularLighting"),
+    Atom("feSpotLight", "feSpotLight"),
+    Atom("feTile", "feTile"),
+    Atom("feTurbulence", "feTurbulence"),
+    Atom("fill", "fill"),
+    Atom("fill_opacity", "fill-opacity"),
+    Atom("fill_rule", "fill-rule"),
+    Atom("filter", "filter"),
+    Atom("filterUnits", "filterUnits"),
+    Atom("_float", "float"),
+    Atom("flood_color", "flood-color"),
+    Atom("flood_opacity", "flood-opacity"),
+    Atom("font_face", "font-face"),
+    Atom("font_face_format", "font-face-format"),
+    Atom("font_face_name", "font-face-name"),
+    Atom("font_face_src", "font-face-src"),
+    Atom("font_face_uri", "font-face-uri"),
+    Atom("font_family", "font-family"),
+    Atom("font_size", "font-size"),
+    Atom("font_size_adjust", "font-size-adjust"),
+    Atom("font_stretch", "font-stretch"),
+    Atom("font_style", "font-style"),
+    Atom("font_variant", "font-variant"),
+    Atom("formatting", "formatting"),
+    Atom("foreignObject", "foreignObject"),
+    Atom("fractalNoise", "fractalNoise"),
+    Atom("fr", "fr"),
+    Atom("fx", "fx"),
+    Atom("fy", "fy"),
+    Atom("G", "G"),
+    Atom("g", "g"),
+    Atom("gamma", "gamma"),
+    Atom("glyphRef", "glyphRef"),
+    Atom("grad", "grad"),
+    Atom("gradientTransform", "gradientTransform"),
+    Atom("gradientUnits", "gradientUnits"),
+    Atom("hardLight", "hard-light"),
+    Atom("hue", "hue"),
+    Atom("hueRotate", "hueRotate"),
+    Atom("identity", "identity"),
+    Atom("image_rendering", "image-rendering"),
+    Atom("in", "in"),
+    Atom("in2", "in2"),
+    Atom("intercept", "intercept"),
+    Atom("k1", "k1"),
+    Atom("k2", "k2"),
+    Atom("k3", "k3"),
+    Atom("k4", "k4"),
+    Atom("kernelMatrix", "kernelMatrix"),
+    Atom("kernelUnitLength", "kernelUnitLength"),
+    Atom("lengthAdjust", "lengthAdjust"),
+    Atom("letter_spacing", "letter-spacing"),
+    Atom("lighten", "lighten"),
+    Atom("lighter", "lighter"),
+    Atom("lighting_color", "lighting-color"),
+    Atom("limitingConeAngle", "limitingConeAngle"),
+    Atom("linear", "linear"),
+    Atom("linearGradient", "linearGradient"),
+    Atom("list_item", "list-item"),
+    Atom("list_style_type", "list-style-type"),
+    Atom("luminanceToAlpha", "luminanceToAlpha"),
+    Atom("luminosity", "luminosity"),
+    Atom("magnify", "magnify"),
+    Atom("marker", "marker"),
+    Atom("marker_end", "marker-end"),
+    Atom("marker_mid", "marker-mid"),
+    Atom("marker_start", "marker-start"),
+    Atom("markerHeight", "markerHeight"),
+    Atom("markerUnits", "markerUnits"),
+    Atom("markerWidth", "markerWidth"),
+    Atom("mask", "mask"),
+    Atom("maskContentUnits", "maskContentUnits"),
+    Atom("mask_type", "mask-type"),
+    Atom("maskUnits", "maskUnits"),
+    Atom("matrix", "matrix"),
+    Atom("metadata", "metadata"),
+    Atom("missingGlyph", "missing-glyph"),
+    Atom("mm", "mm"),
+    Atom("mpath", "mpath"),
+    Atom("noStitch", "noStitch"),
+    Atom("numOctaves", "numOctaves"),
+    Atom("multiply", "multiply"),
+    Atom("objectBoundingBox", "objectBoundingBox"),
+    Atom("offset", "offset"),
+    Atom("onSVGLoad", "onSVGLoad"),
+    Atom("onSVGScroll", "onSVGScroll"),
+    Atom("onzoom", "onzoom"),
+    Atom("opacity", "opacity"),
+    Atom("_operator", "operator"),
+    Atom("out", "out"),
+    Atom("over", "over"),
+    Atom("overridePreserveAspectRatio", "overridePreserveAspectRatio"),
+    Atom("pad", "pad"),
+    Atom("path", "path"),
+    Atom("pathLength", "pathLength"),
+    Atom("patternContentUnits", "patternContentUnits"),
+    Atom("patternTransform", "patternTransform"),
+    Atom("patternUnits", "patternUnits"),
+    Atom("pc", "pc"),
+    Atom("pointer", "pointer"),
+    Atom("pointer_events", "pointer-events"),
+    Atom("points", "points"),
+    Atom("pointsAtX", "pointsAtX"),
+    Atom("pointsAtY", "pointsAtY"),
+    Atom("pointsAtZ", "pointsAtZ"),
+    Atom("polyline", "polyline"),
+    Atom("preserveAlpha", "preserveAlpha"),
+    Atom("preserveAspectRatio", "preserveAspectRatio"),
+    Atom("primitiveUnits", "primitiveUnits"),
+    Atom("pt", "pt"),
+    Atom("px", "px"),
+    Atom("R", "R"),
+    Atom("r", "r"),
+    Atom("rad", "rad"),
+    Atom("radialGradient", "radialGradient"),
+    Atom("radius", "radius"),
+    Atom("reflect", "reflect"),
+    Atom("refX", "refX"),
+    Atom("refY", "refY"),
+    Atom("requiredExtensions", "requiredExtensions"),
+    Atom("requiredFeatures", "requiredFeatures"),
+    Atom("rotate", "rotate"),
+    Atom("rx", "rx"),
+    Atom("ry", "ry"),
+    Atom("saturate", "saturate"),
+    Atom("saturation", "saturation"),
+    Atom("set", "set"),
+    Atom("seed", "seed"),
+    Atom("shape_rendering", "shape-rendering"),
+    Atom("simpleScopeChain", "simpleScopeChain"),
+    Atom("skewX", "skewX"),
+    Atom("skewY", "skewY"),
+    Atom("slope", "slope"),
+    Atom("slot", "slot"),
+    Atom("softLight", "soft-light"),
+    Atom("spacing", "spacing"),
+    Atom("spacingAndGlyphs", "spacingAndGlyphs"),
+    Atom("specularConstant", "specularConstant"),
+    Atom("specularExponent", "specularExponent"),
+    Atom("spreadMethod", "spreadMethod"),
+    Atom("startOffset", "startOffset"),
+    Atom("stdDeviation", "stdDeviation"),
+    Atom("stitch", "stitch"),
+    Atom("stitchTiles", "stitchTiles"),
+    Atom("stop_color", "stop-color"),
+    Atom("stop_opacity", "stop-opacity"),
+    Atom("stroke", "stroke"),
+    Atom("stroke_dasharray", "stroke-dasharray"),
+    Atom("stroke_dashoffset", "stroke-dashoffset"),
+    Atom("stroke_linecap", "stroke-linecap"),
+    Atom("stroke_linejoin", "stroke-linejoin"),
+    Atom("stroke_miterlimit", "stroke-miterlimit"),
+    Atom("stroke_opacity", "stroke-opacity"),
+    Atom("stroke_width", "stroke-width"),
+    Atom("strokeWidth", "strokeWidth"),
+    Atom("surfaceScale", "surfaceScale"),
+    Atom("svg", "svg"),
+    Atom("svgSwitch", "switch"),
+    Atom("symbol", "symbol"),
+    Atom("systemLanguage", "systemLanguage"),
+    Atom("tableValues", "tableValues"),
+    Atom("targetX", "targetX"),
+    Atom("targetY", "targetY"),
+    Atom("text_anchor", "text-anchor"),
+    Atom("text_rendering", "text-rendering"),
+    Atom("textLength", "textLength"),
+    Atom("textPath", "textPath"),
+    Atom("transform_origin", "transform-origin"),
+    Atom("tref", "tref"),
+    Atom("tspan", "tspan"),
+    Atom("turbulence", "turbulence"),
+    Atom("unicode_bidi", "unicode-bidi"),
+    Atom("userSpaceOnUse", "userSpaceOnUse"),
+    Atom("view", "view"),
+    Atom("viewBox", "viewBox"),
+    Atom("viewTarget", "viewTarget"),
+    Atom("white_space", "white-space"),
+    Atom("word_spacing", "word-spacing"),
+    Atom("writing_mode", "writing-mode"),
+    Atom("x", "x"),
+    Atom("x1", "x1"),
+    Atom("x2", "x2"),
+    Atom("xChannelSelector", "xChannelSelector"),
+    Atom("xor_", "xor"),
+    Atom("y", "y"),
+    Atom("y1", "y1"),
+    Atom("y2", "y2"),
+    Atom("yChannelSelector", "yChannelSelector"),
+    Atom("z", "z"),
+    Atom("zoomAndPan", "zoomAndPan"),
+    Atom("vector_effect", "vector-effect"),
+    Atom("vertical_align", "vertical-align"),
+    Atom("accumulate", "accumulate"),
+    Atom("additive", "additive"),
+    Atom("attributeName", "attributeName"),
+    Atom("attributeType", "attributeType"),
+    Atom("auto_reverse", "auto-reverse"),
+    Atom("begin", "begin"),
+    Atom("beginEvent", "beginEvent"),
+    Atom("by", "by"),
+    Atom("calcMode", "calcMode"),
+    Atom("dur", "dur"),
+    Atom("keyPoints", "keyPoints"),
+    Atom("keySplines", "keySplines"),
+    Atom("keyTimes", "keyTimes"),
+    Atom("mozAnimateMotionDummyAttr", "_mozAnimateMotionDummyAttr"),
+    Atom("onbegin", "onbegin"),
+    Atom("onbeginEvent", "onbeginEvent"),
+    Atom("onend", "onend"),
+    Atom("onendEvent", "onendEvent"),
+    Atom("onrepeat", "onrepeat"),
+    Atom("onrepeatEvent", "onrepeatEvent"),
+    Atom("repeatCount", "repeatCount"),
+    Atom("repeatDur", "repeatDur"),
+    Atom("repeatEvent", "repeatEvent"),
+    Atom("restart", "restart"),
+    Atom("to", "to"),
+    Atom("abs_", "abs"),
+    Atom("accent_", "accent"),
+    Atom("accentunder_", "accentunder"),
+    Atom("actiontype_", "actiontype"),
+    Atom("alignmentscope_", "alignmentscope"),
+    Atom("altimg_", "altimg"),
+    Atom("altimg_height_", "altimg-height"),
+    Atom("altimg_valign_", "altimg-valign"),
+    Atom("altimg_width_", "altimg-width"),
+    Atom("annotation_", "annotation"),
+    Atom("annotation_xml_", "annotation-xml"),
+    Atom("apply_", "apply"),
+    Atom("approx_", "approx"),
+    Atom("arccos_", "arccos"),
+    Atom("arccosh_", "arccosh"),
+    Atom("arccot_", "arccot"),
+    Atom("arccoth_", "arccoth"),
+    Atom("arccsc_", "arccsc"),
+    Atom("arccsch_", "arccsch"),
+    Atom("arcsec_", "arcsec"),
+    Atom("arcsech_", "arcsech"),
+    Atom("arcsin_", "arcsin"),
+    Atom("arcsinh_", "arcsinh"),
+    Atom("arctan_", "arctan"),
+    Atom("arctanh_", "arctanh"),
+    Atom("arg_", "arg"),
+    Atom("bevelled_", "bevelled"),
+    Atom("bind_", "bind"),
+    Atom("bvar_", "bvar"),
+    Atom("card_", "card"),
+    Atom("cartesianproduct_", "cartesianproduct"),
+    Atom("cbytes_", "cbytes"),
+    Atom("cd_", "cd"),
+    Atom("cdgroup_", "cdgroup"),
+    Atom("cerror_", "cerror"),
+    Atom("charalign_", "charalign"),
+    Atom("ci_", "ci"),
+    Atom("closure_", "closure"),
+    Atom("cn_", "cn"),
+    Atom("codomain_", "codomain"),
+    Atom("columnalign_", "columnalign"),
+    Atom("columnalignment_", "columnalignment"),
+    Atom("columnlines_", "columnlines"),
+    Atom("columnspacing_", "columnspacing"),
+    Atom("columnspan_", "columnspan"),
+    Atom("columnwidth_", "columnwidth"),
+    Atom("complexes_", "complexes"),
+    Atom("compose_", "compose"),
+    Atom("condition_", "condition"),
+    Atom("conjugate_", "conjugate"),
+    Atom("cos_", "cos"),
+    Atom("cosh_", "cosh"),
+    Atom("cot_", "cot"),
+    Atom("coth_", "coth"),
+    Atom("crossout_", "crossout"),
+    Atom("csc_", "csc"),
+    Atom("csch_", "csch"),
+    Atom("cs_", "cs"),
+    Atom("csymbol_", "csymbol"),
+    Atom("csp", "csp"),
+    Atom("curl_", "curl"),
+    Atom("decimalpoint_", "decimalpoint"),
+    Atom("definitionURL_", "definitionURL"),
+    Atom("degree_", "degree"),
+    Atom("denomalign_", "denomalign"),
+    Atom("depth_", "depth"),
+    Atom("determinant_", "determinant"),
+    Atom("diff_", "diff"),
+    Atom("displaystyle_", "displaystyle"),
+    Atom("divergence_", "divergence"),
+    Atom("divide_", "divide"),
+    Atom("domain_", "domain"),
+    Atom("domainofapplication_", "domainofapplication"),
+    Atom("edge_", "edge"),
+    Atom("el", "el"),
+    Atom("emptyset_", "emptyset"),
+    Atom("eq_", "eq"),
+    Atom("equalcolumns_", "equalcolumns"),
+    Atom("equalrows_", "equalrows"),
+    Atom("equivalent_", "equivalent"),
+    Atom("eulergamma_", "eulergamma"),
+    Atom("exists_", "exists"),
+    Atom("exp_", "exp"),
+    Atom("exponentiale_", "exponentiale"),
+    Atom("factorial_", "factorial"),
+    Atom("factorof_", "factorof"),
+    Atom("fence_", "fence"),
+    Atom("fn_", "fn"),
+    Atom("fontfamily_", "fontfamily"),
+    Atom("fontsize_", "fontsize"),
+    Atom("fontstyle_", "fontstyle"),
+    Atom("fontweight_", "fontweight"),
+    Atom("forall_", "forall"),
+    Atom("framespacing_", "framespacing"),
+    Atom("gcd_", "gcd"),
+    Atom("geq_", "geq"),
+    Atom("groupalign_", "groupalign"),
+    Atom("gt_", "gt"),
+    Atom("ident_", "ident"),
+    Atom("imaginaryi_", "imaginaryi"),
+    Atom("imaginary_", "imaginary"),
+    Atom("implies_", "implies"),
+    Atom("indentalignfirst_", "indentalignfirst"),
+    Atom("indentalign_", "indentalign"),
+    Atom("indentalignlast_", "indentalignlast"),
+    Atom("indentshiftfirst_", "indentshiftfirst"),
+    Atom("indentshift_", "indentshift"),
+    Atom("indenttarget_", "indenttarget"),
+    Atom("integers_", "integers"),
+    Atom("intersect_", "intersect"),
+    Atom("interval_", "interval"),
+    Atom("int_", "int"),
+    Atom("inverse_", "inverse"),
+    Atom("lambda_", "lambda"),
+    Atom("laplacian_", "laplacian"),
+    Atom("largeop_", "largeop"),
+    Atom("lcm_", "lcm"),
+    Atom("leq_", "leq"),
+    Atom("limit_", "limit"),
+    Atom("linebreak_", "linebreak"),
+    Atom("linebreakmultchar_", "linebreakmultchar"),
+    Atom("linebreakstyle_", "linebreakstyle"),
+    Atom("linethickness_", "linethickness"),
+    Atom("list_", "list"),
+    Atom("ln_", "ln"),
+    Atom("location_", "location"),
+    Atom("logbase_", "logbase"),
+    Atom("log_", "log"),
+    Atom("longdivstyle_", "longdivstyle"),
+    Atom("lowlimit_", "lowlimit"),
+    Atom("lquote_", "lquote"),
+    Atom("lspace_", "lspace"),
+    Atom("lt_", "lt"),
+    Atom("maction_", "maction"),
+    Atom("maligngroup_", "maligngroup"),
+    Atom("malignmark_", "malignmark"),
+    Atom("mathbackground_", "mathbackground"),
+    Atom("mathcolor_", "mathcolor"),
+    Atom("mathsize_", "mathsize"),
+    Atom("mathvariant_", "mathvariant"),
+    Atom("matrixrow_", "matrixrow"),
+    Atom("maxsize_", "maxsize"),
+    Atom("mean_", "mean"),
+    Atom("median_", "median"),
+    Atom("menclose_", "menclose"),
+    Atom("merror_", "merror"),
+    Atom("mfenced_", "mfenced"),
+    Atom("mfrac_", "mfrac"),
+    Atom("mglyph_", "mglyph"),
+    Atom("mi_", "mi"),
+    Atom("minlabelspacing_", "minlabelspacing"),
+    Atom("minsize_", "minsize"),
+    Atom("minus_", "minus"),
+    Atom("mlabeledtr_", "mlabeledtr"),
+    Atom("mlongdiv_", "mlongdiv"),
+    Atom("mmultiscripts_", "mmultiscripts"),
+    Atom("mn_", "mn"),
+    Atom("momentabout_", "momentabout"),
+    Atom("moment_", "moment"),
+    Atom("mo_", "mo"),
+    Atom("movablelimits_", "movablelimits"),
+    Atom("mover_", "mover"),
+    Atom("mpadded_", "mpadded"),
+    Atom("mphantom_", "mphantom"),
+    Atom("mprescripts_", "mprescripts"),
+    Atom("mroot_", "mroot"),
+    Atom("mrow_", "mrow"),
+    Atom("mscarries_", "mscarries"),
+    Atom("mscarry_", "mscarry"),
+    Atom("msgroup_", "msgroup"),
+    Atom("msline_", "msline"),
+    Atom("ms_", "ms"),
+    Atom("mspace_", "mspace"),
+    Atom("msqrt_", "msqrt"),
+    Atom("msrow_", "msrow"),
+    Atom("mstack_", "mstack"),
+    Atom("mstyle_", "mstyle"),
+    Atom("msub_", "msub"),
+    Atom("msubsup_", "msubsup"),
+    Atom("msup_", "msup"),
+    Atom("mtable_", "mtable"),
+    Atom("mtd_", "mtd"),
+    Atom("mtext_", "mtext"),
+    Atom("mtr_", "mtr"),
+    Atom("munder_", "munder"),
+    Atom("munderover_", "munderover"),
+    Atom("naturalnumbers_", "naturalnumbers"),
+    Atom("neq_", "neq"),
+    Atom("notanumber_", "notanumber"),
+    Atom("notation_", "notation"),
+    Atom("note_", "note"),
+    Atom("notin_", "notin"),
+    Atom("notprsubset_", "notprsubset"),
+    Atom("notsubset_", "notsubset"),
+    Atom("numalign_", "numalign"),
+    Atom("other", "other"),
+    Atom("outerproduct_", "outerproduct"),
+    Atom("partialdiff_", "partialdiff"),
+    Atom("piece_", "piece"),
+    Atom("piecewise_", "piecewise"),
+    Atom("pi_", "pi"),
+    Atom("plus_", "plus"),
+    Atom("power_", "power"),
+    Atom("primes_", "primes"),
+    Atom("product_", "product"),
+    Atom("prsubset_", "prsubset"),
+    Atom("quotient_", "quotient"),
+    Atom("rationals_", "rationals"),
+    Atom("real_", "real"),
+    Atom("reals_", "reals"),
+    Atom("reln_", "reln"),
+    Atom("root_", "root"),
+    Atom("rowalign_", "rowalign"),
+    Atom("rowlines_", "rowlines"),
+    Atom("rowspacing_", "rowspacing"),
+    Atom("rquote_", "rquote"),
+    Atom("rspace_", "rspace"),
+    Atom("scalarproduct_", "scalarproduct"),
+    Atom("schemaLocation_", "schemaLocation"),
+    Atom("scriptlevel_", "scriptlevel"),
+    Atom("scriptminsize_", "scriptminsize"),
+    Atom("scriptsizemultiplier_", "scriptsizemultiplier"),
+    Atom("scriptsize_", "scriptsize"),
+    Atom("sdev_", "sdev"),
+    Atom("sech_", "sech"),
+    Atom("sec_", "sec"),
+    Atom("selection_", "selection"),
+    Atom("selector_", "selector"),
+    Atom("semantics_", "semantics"),
+    Atom("separator_", "separator"),
+    Atom("separators_", "separators"),
+    Atom("sep_", "sep"),
+    Atom("setdiff_", "setdiff"),
+    # Atom("set_", "set"),  # "set" is present above
+    Atom("share_", "share"),
+    Atom("shift_", "shift"),
+    Atom("side_", "side"),
+    Atom("sinh_", "sinh"),
+    Atom("sin_", "sin"),
+    Atom("stackalign_", "stackalign"),
+    Atom("stretchy_", "stretchy"),
+    Atom("subscriptshift_", "subscriptshift"),
+    Atom("subset_", "subset"),
+    Atom("superscriptshift_", "superscriptshift"),
+    Atom("symmetric_", "symmetric"),
+    Atom("tanh_", "tanh"),
+    Atom("tan_", "tan"),
+    Atom("tendsto_", "tendsto"),
+    Atom("times_", "times"),
+    Atom("transpose_", "transpose"),
+    Atom("union_", "union"),
+    Atom("uplimit_", "uplimit"),
+    Atom("variance_", "variance"),
+    Atom("vectorproduct_", "vectorproduct"),
+    Atom("vector_", "vector"),
+    Atom("voffset_", "voffset"),
+    Atom("xref_", "xref"),
+    Atom("math", "math"),  # the only one without an underscore
+    Atom("booleanFromString", "boolean-from-string"),
+    Atom("countNonEmpty", "count-non-empty"),
+    Atom("daysFromDate", "days-from-date"),
+    Atom("secondsFromDateTime", "seconds-from-dateTime"),
+    Atom("tabbrowser_arrowscrollbox", "tabbrowser-arrowscrollbox"),
+    # Simple gestures support
+    Atom("onMozSwipeGestureMayStart", "onMozSwipeGestureMayStart"),
+    Atom("onMozSwipeGestureStart", "onMozSwipeGestureStart"),
+    Atom("onMozSwipeGestureUpdate", "onMozSwipeGestureUpdate"),
+    Atom("onMozSwipeGestureEnd", "onMozSwipeGestureEnd"),
+    Atom("onMozSwipeGesture", "onMozSwipeGesture"),
+    Atom("onMozMagnifyGestureStart", "onMozMagnifyGestureStart"),
+    Atom("onMozMagnifyGestureUpdate", "onMozMagnifyGestureUpdate"),
+    Atom("onMozMagnifyGesture", "onMozMagnifyGesture"),
+    Atom("onMozRotateGestureStart", "onMozRotateGestureStart"),
+    Atom("onMozRotateGestureUpdate", "onMozRotateGestureUpdate"),
+    Atom("onMozRotateGesture", "onMozRotateGesture"),
+    Atom("onMozTapGesture", "onMozTapGesture"),
+    Atom("onMozPressTapGesture", "onMozPressTapGesture"),
+    Atom("onMozEdgeUIStarted", "onMozEdgeUIStarted"),
+    Atom("onMozEdgeUICanceled", "onMozEdgeUICanceled"),
+    Atom("onMozEdgeUICompleted", "onMozEdgeUICompleted"),
+    # Pointer events
+    Atom("onpointerdown", "onpointerdown"),
+    Atom("onpointermove", "onpointermove"),
+    Atom("onpointerup", "onpointerup"),
+    Atom("onpointercancel", "onpointercancel"),
+    Atom("onpointerover", "onpointerover"),
+    Atom("onpointerout", "onpointerout"),
+    Atom("onpointerenter", "onpointerenter"),
+    Atom("onpointerleave", "onpointerleave"),
+    Atom("ongotpointercapture", "ongotpointercapture"),
+    Atom("onlostpointercapture", "onlostpointercapture"),
+    # orientation support
+    Atom("ondevicemotion", "ondevicemotion"),
+    Atom("ondeviceorientation", "ondeviceorientation"),
+    Atom("ondeviceorientationabsolute", "ondeviceorientationabsolute"),
+    Atom("onmozorientationchange", "onmozorientationchange"),
+    Atom("onuserproximity", "onuserproximity"),
+    # light sensor support
+    Atom("ondevicelight", "ondevicelight"),
+    # MediaDevices device change event
+    Atom("ondevicechange", "ondevicechange"),
+    # Internal Visual Viewport events
+    Atom("onmozvisualresize", "onmozvisualresize"),
+    Atom("onmozvisualscroll", "onmozvisualscroll"),
+    # Miscellaneous events included for memory usage optimization (see bug 1542885)
+    Atom("onDOMAutoComplete", "onDOMAutoComplete"),
+    Atom("onDOMContentLoaded", "onDOMContentLoaded"),
+    Atom("onDOMDocElementInserted", "onDOMDocElementInserted"),
+    Atom("onDOMFormBeforeSubmit", "onDOMFormBeforeSubmit"),
+    Atom("onDOMFormHasPassword", "onDOMFormHasPassword"),
+    Atom("onDOMFrameContentLoaded", "onDOMFrameContentLoaded"),
+    Atom("onDOMHeadElementParsed", "onDOMHeadElementParsed"),
+    Atom("onDOMInputPasswordAdded", "onDOMInputPasswordAdded"),
+    Atom("onDOMLinkAdded", "onDOMLinkAdded"),
+    Atom("onDOMLinkChanged", "onDOMLinkChanged"),
+    Atom("onDOMMetaAdded", "onDOMMetaAdded"),
+    Atom("onDOMMetaChanged", "onDOMMetaChanged"),
+    Atom("onDOMMetaRemoved", "onDOMMetaRemoved"),
+    Atom("onDOMPopupBlocked", "onDOMPopupBlocked"),
+    Atom("onDOMTitleChanged", "onDOMTitleChanged"),
+    Atom("onDOMWindowClose", "onDOMWindowClose"),
+    Atom("onDOMWindowCreated", "onDOMWindowCreated"),
+    Atom("onDOMWindowFocus", "onDOMWindowFocus"),
+    Atom("onFullZoomChange", "onFullZoomChange"),
+    Atom("onGloballyAutoplayBlocked", "onGloballyAutoplayBlocked"),
+    Atom("onMozApplicationManifest", "onMozApplicationManifest"),
+    Atom("onMozDOMFullscreen_Entered", "onMozDOMFullscreen:Entered"),
+    Atom("onMozDOMFullscreen_Exit", "onMozDOMFullscreen:Exit"),
+    Atom("onMozDOMFullscreen_Exited", "onMozDOMFullscreen:Exited"),
+    Atom("onMozDOMFullscreen_NewOrigin", "onMozDOMFullscreen:NewOrigin"),
+    Atom("onMozDOMFullscreen_Request", "onMozDOMFullscreen:Request"),
+    Atom("onMozDOMPointerLock_Entered", "onMozDOMPointerLock:Entered"),
+    Atom("onMozDOMPointerLock_Exited", "onMozDOMPointerLock:Exited"),
+    Atom("onMozInvalidForm", "onMozInvalidForm"),
+    Atom("onMozLocalStorageChanged", "onMozLocalStorageChanged"),
+    Atom("onMozOpenDateTimePicker", "onMozOpenDateTimePicker"),
+    Atom("onMozSessionStorageChanged", "onMozSessionStorageChanged"),
+    Atom("onMozTogglePictureInPicture", "onMozTogglePictureInPicture"),
+    Atom("onPluginCrashed", "onPluginCrashed"),
+    Atom("onPrintingError", "onPrintingError"),
+    Atom("onTextZoomChange", "onTextZoomChange"),
+    Atom("onUAWidgetSetupOrChange", "onUAWidgetSetupOrChange"),
+    Atom("onUAWidgetTeardown", "onUAWidgetTeardown"),
+    Atom("onUnselectedTabHover_Disable", "onUnselectedTabHover:Disable"),
+    Atom("onUnselectedTabHover_Enable", "onUnselectedTabHover:Enable"),
+    Atom("onmozshowdropdown", "onmozshowdropdown"),
+    Atom("onmozshowdropdown_sourcetouch", "onmozshowdropdown-sourcetouch"),
+    Atom("onprintPreviewUpdate", "onprintPreviewUpdate"),
+    Atom("onscrollend", "onscrollend"),
+    # WebExtensions
+    Atom("moz_extension", "moz-extension"),
+    Atom("all_urlsPermission", "<all_urls>"),
+    Atom("clipboardRead", "clipboardRead"),
+    Atom("clipboardWrite", "clipboardWrite"),
+    Atom("debugger", "debugger"),
+    Atom("mozillaAddons", "mozillaAddons"),
+    Atom("tabs", "tabs"),
+    Atom("webRequestBlocking", "webRequestBlocking"),
+    Atom("webRequestFilterResponse_serviceWorkerScript", "webRequestFilterResponse.serviceWorkerScript"),
+    Atom("http", "http"),
+    Atom("https", "https"),
+    Atom("ws", "ws"),
+    Atom("wss", "wss"),
+    Atom("ftp", "ftp"),
+    Atom("chrome", "chrome"),
+    Atom("moz", "moz"),
+    Atom("moz_icon", "moz-icon"),
+    Atom("moz_gio", "moz-gio"),
+    Atom("proxy", "proxy"),
+    Atom("privateBrowsingAllowedPermission", "internal:privateBrowsingAllowed"),
+    Atom("svgContextPropertiesAllowedPermission", "internal:svgContextPropertiesAllowed"),
+    Atom("theme", "theme"),
+    # CSS Counter Styles
+    Atom("decimal_leading_zero", "decimal-leading-zero"),
+    Atom("arabic_indic", "arabic-indic"),
+    Atom("armenian", "armenian"),
+    Atom("upper_armenian", "upper-armenian"),
+    Atom("lower_armenian", "lower-armenian"),
+    Atom("bengali", "bengali"),
+    Atom("cambodian", "cambodian"),
+    Atom("khmer", "khmer"),
+    Atom("cjk_decimal", "cjk-decimal"),
+    Atom("devanagari", "devanagari"),
+    Atom("georgian", "georgian"),
+    Atom("gujarati", "gujarati"),
+    Atom("gurmukhi", "gurmukhi"),
+    Atom("kannada", "kannada"),
+    Atom("lao", "lao"),
+    Atom("malayalam", "malayalam"),
+    Atom("mongolian", "mongolian"),
+    Atom("myanmar", "myanmar"),
+    Atom("oriya", "oriya"),
+    Atom("persian", "persian"),
+    Atom("lower_roman", "lower-roman"),
+    Atom("upper_roman", "upper-roman"),
+    Atom("tamil", "tamil"),
+    Atom("telugu", "telugu"),
+    Atom("thai", "thai"),
+    Atom("tibetan", "tibetan"),
+    Atom("lower_alpha", "lower-alpha"),
+    Atom("lower_latin", "lower-latin"),
+    Atom("upper_alpha", "upper-alpha"),
+    Atom("upper_latin", "upper-latin"),
+    Atom("cjk_heavenly_stem", "cjk-heavenly-stem"),
+    Atom("cjk_earthly_branch", "cjk-earthly-branch"),
+    Atom("lower_greek", "lower-greek"),
+    Atom("hiragana", "hiragana"),
+    Atom("hiragana_iroha", "hiragana-iroha"),
+    Atom("katakana", "katakana"),
+    Atom("katakana_iroha", "katakana-iroha"),
+    Atom("cjk_ideographic", "cjk-ideographic"),
+    Atom("_moz_arabic_indic", "-moz-arabic-indic"),
+    Atom("_moz_persian", "-moz-persian"),
+    Atom("_moz_urdu", "-moz-urdu"),
+    Atom("_moz_devanagari", "-moz-devanagari"),
+    Atom("_moz_bengali", "-moz-bengali"),
+    Atom("_moz_gurmukhi", "-moz-gurmukhi"),
+    Atom("_moz_gujarati", "-moz-gujarati"),
+    Atom("_moz_oriya", "-moz-oriya"),
+    Atom("_moz_tamil", "-moz-tamil"),
+    Atom("_moz_telugu", "-moz-telugu"),
+    Atom("_moz_kannada", "-moz-kannada"),
+    Atom("_moz_malayalam", "-moz-malayalam"),
+    Atom("_moz_thai", "-moz-thai"),
+    Atom("_moz_lao", "-moz-lao"),
+    Atom("_moz_myanmar", "-moz-myanmar"),
+    Atom("_moz_khmer", "-moz-khmer"),
+    Atom("_moz_cjk_heavenly_stem", "-moz-cjk-heavenly-stem"),
+    Atom("_moz_cjk_earthly_branch", "-moz-cjk-earthly-branch"),
+    Atom("_moz_hangul", "-moz-hangul"),
+    Atom("_moz_hangul_consonant", "-moz-hangul-consonant"),
+    Atom("_moz_ethiopic_halehame", "-moz-ethiopic-halehame"),
+    Atom("_moz_ethiopic_halehame_am", "-moz-ethiopic-halehame-am"),
+    Atom("_moz_ethiopic_halehame_ti_er", "-moz-ethiopic-halehame-ti-er"),
+    Atom("_moz_ethiopic_halehame_ti_et", "-moz-ethiopic-halehame-ti-et"),
+    Atom("_moz_trad_chinese_informal", "-moz-trad-chinese-informal"),
+    Atom("_moz_trad_chinese_formal", "-moz-trad-chinese-formal"),
+    Atom("_moz_simp_chinese_informal", "-moz-simp-chinese-informal"),
+    Atom("_moz_simp_chinese_formal", "-moz-simp-chinese-formal"),
+    Atom("_moz_japanese_informal", "-moz-japanese-informal"),
+    Atom("_moz_japanese_formal", "-moz-japanese-formal"),
+    Atom("_moz_ethiopic_numeric", "-moz-ethiopic-numeric"),
+    # --------------------------------------------------------------------------
+    # Special atoms
+    # --------------------------------------------------------------------------
+    # Node types
+    Atom("cdataTagName", "#cdata-section"),
+    Atom("commentTagName", "#comment"),
+    Atom("documentNodeName", "#document"),
+    Atom("documentFragmentNodeName", "#document-fragment"),
+    Atom("documentTypeNodeName", "#document-type"),
+    Atom("processingInstructionTagName", "#processing-instruction"),
+    Atom("textTagName", "#text"),
+    # Frame types
+    #
+    # TODO(emilio): Rename this? This is only used now to mark the style context of
+    # the placeholder with a dummy pseudo.
+    Atom("placeholderFrame", "PlaceholderFrame"),
+    Atom("onloadend", "onloadend"),
+    Atom("onloadstart", "onloadstart"),
+    Atom("onprogress", "onprogress"),
+    Atom("onsuspend", "onsuspend"),
+    Atom("onemptied", "onemptied"),
+    Atom("onstalled", "onstalled"),
+    Atom("onplay", "onplay"),
+    Atom("onpause", "onpause"),
+    Atom("onloadedmetadata", "onloadedmetadata"),
+    Atom("onloadeddata", "onloadeddata"),
+    Atom("onwaiting", "onwaiting"),
+    Atom("onplaying", "onplaying"),
+    Atom("oncanplay", "oncanplay"),
+    Atom("oncanplaythrough", "oncanplaythrough"),
+    Atom("onseeking", "onseeking"),
+    Atom("onseeked", "onseeked"),
+    Atom("ontimeout", "ontimeout"),
+    Atom("ontimeupdate", "ontimeupdate"),
+    Atom("onended", "onended"),
+    Atom("onformdata", "onformdata"),
+    Atom("onratechange", "onratechange"),
+    Atom("ondurationchange", "ondurationchange"),
+    Atom("onvolumechange", "onvolumechange"),
+    Atom("onaddtrack", "onaddtrack"),
+    Atom("oncontrollerchange", "oncontrollerchange"),
+    Atom("oncuechange", "oncuechange"),
+    Atom("onenter", "onenter"),
+    Atom("onexit", "onexit"),
+    Atom("onencrypted", "onencrypted"),
+    Atom("onwaitingforkey", "onwaitingforkey"),
+    Atom("onkeystatuseschange", "onkeystatuseschange"),
+    Atom("onremovetrack", "onremovetrack"),
+    Atom("loadstart", "loadstart"),
+    Atom("suspend", "suspend"),
+    Atom("emptied", "emptied"),
+    Atom("play", "play"),
+    Atom("pause", "pause"),
+    Atom("loadedmetadata", "loadedmetadata"),
+    Atom("loadeddata", "loadeddata"),
+    Atom("waiting", "waiting"),
+    Atom("playing", "playing"),
+    Atom("timeupdate", "timeupdate"),
+    Atom("canplay", "canplay"),
+    Atom("canplaythrough", "canplaythrough"),
+    Atom("ondataavailable", "ondataavailable"),
+    Atom("onwarning", "onwarning"),
+    Atom("onstart", "onstart"),
+    Atom("onstop", "onstop"),
+    Atom("onphoto", "onphoto"),
+    Atom("ongamepadbuttondown", "ongamepadbuttondown"),
+    Atom("ongamepadbuttonup", "ongamepadbuttonup"),
+    Atom("ongamepadaxismove", "ongamepadaxismove"),
+    Atom("ongamepadconnected", "ongamepadconnected"),
+    Atom("ongamepaddisconnected", "ongamepaddisconnected"),
+    Atom("onfetch", "onfetch"),
+    # Content property names
+    Atom("afterPseudoProperty", "afterPseudoProperty"),  # nsXMLElement*
+    Atom("animationsProperty", "AnimationsProperty"),  # FrameAnimations*
+    Atom("animationsOfBeforeProperty", "AnimationsOfBeforeProperty"),  # FrameAnimations*
+    Atom("animationsOfAfterProperty", "AnimationsOfAfterProperty"),  # FrameAnimations*
+    Atom("animationsOfMarkerProperty", "AnimationsOfMarkerProperty"),  # FrameAnimations*
+    Atom("animationEffectsProperty", "AnimationEffectsProperty"),  # EffectSet*
+    Atom("animationEffectsForBeforeProperty", "AnimationsEffectsForBeforeProperty"),  # EffectSet*
+    Atom("animationEffectsForAfterProperty", "AnimationsEffectsForAfterProperty"),  # EffectSet*
+    Atom("animationEffectsForMarkerProperty", "AnimationsEffectsForMarkerProperty"),  # EffectSet*
+    Atom("beforePseudoProperty", "beforePseudoProperty"),  # nsXMLElement*
+    Atom("cssPseudoElementBeforeProperty", "CSSPseudoElementBeforeProperty"),  # CSSPseudoElement*
+    Atom("cssPseudoElementAfterProperty", "CSSPseudoElementAfterProperty"),  # CSSPseudoElement*
+    Atom("cssPseudoElementMarkerProperty", "CSSPseudoElementMarkerProperty"),  # CSSPseudoElement*
+    Atom("transitionsProperty", "TransitionsProperty"),  # FrameTransitions*
+    Atom("transitionsOfBeforeProperty", "TransitionsOfBeforeProperty"),  # FrameTransitions*
+    Atom("transitionsOfAfterProperty", "TransitionsOfAfterProperty"),  # FrameTransitions*
+    Atom("transitionsOfMarkerProperty", "TransitionsOfMarkerProperty"),  # FrameTransitions*
+    Atom("genConInitializerProperty", "QuoteNodeProperty"),
+    Atom("labelMouseDownPtProperty", "LabelMouseDownPtProperty"),
+    Atom("lockedStyleStates", "lockedStyleStates"),
+    Atom("apzCallbackTransform", "apzCallbackTransform"),
+    Atom("apzDisabled", "ApzDisabled"),  # bool
+    Atom("restylableAnonymousNode", "restylableAnonymousNode"),  # bool
+    Atom("docLevelNativeAnonymousContent", "docLevelNativeAnonymousContent"),  # bool
+    Atom("paintRequestTime", "PaintRequestTime"),
+    Atom("pseudoProperty", "PseudoProperty"),  # PseudoStyleType
+    Atom("scrollTimelinesProperty", "SrollTimelinesProperty"),  # ScrollTimelineSet*
+    Atom("manualNACProperty", "ManualNACProperty"),  # ManualNAC*
+    Atom("markerPseudoProperty", "markerPseudoProperty"),  # nsXMLElement*
+    # Languages for lang-specific transforms
+    Atom("Japanese", "ja"),
+    Atom("Chinese", "zh-CN"),
+    Atom("Taiwanese", "zh-TW"),
+    Atom("HongKongChinese", "zh-HK"),
+    Atom("Unicode", "x-unicode"),
+    # language codes specifically referenced in the gfx code
+    Atom("ko", "ko"),
+    Atom("zh_cn", "zh-cn"),
+    Atom("zh_tw", "zh-tw"),
+    # additional codes used in nsUnicodeRange.cpp
+    Atom("x_cyrillic", "x-cyrillic"),
+    Atom("he", "he"),
+    Atom("ar", "ar"),
+    Atom("x_devanagari", "x-devanagari"),
+    Atom("x_tamil", "x-tamil"),
+    Atom("x_armn", "x-armn"),
+    Atom("x_beng", "x-beng"),
+    Atom("x_cans", "x-cans"),
+    Atom("x_ethi", "x-ethi"),
+    Atom("x_geor", "x-geor"),
+    Atom("x_gujr", "x-gujr"),
+    Atom("x_guru", "x-guru"),
+    Atom("x_khmr", "x-khmr"),
+    Atom("x_knda", "x-knda"),
+    Atom("x_mlym", "x-mlym"),
+    Atom("x_orya", "x-orya"),
+    Atom("x_sinh", "x-sinh"),
+    Atom("x_telu", "x-telu"),
+    Atom("x_tibt", "x-tibt"),
+    # additional languages that have special case transformations
+    Atom("az", "az"),
+    Atom("ba", "ba"),
+    Atom("crh", "crh"),
+    # Atom("el", "el"),  # "el" is present above
+    Atom("ga", "ga"),
+    # Atom("lt", "lt"),  # "lt" is present above (atom name "lt_")
+    Atom("nl", "nl"),
+    # mathematical language, used for MathML
+    Atom("x_math", "x-math"),
+    # other languages mentioned in :lang() rules in UA style sheets
+    Atom("zh", "zh"),
+    # Names for editor transactions
+    Atom("TypingTxnName", "Typing"),
+    Atom("IMETxnName", "IME"),
+    Atom("DeleteTxnName", "Deleting"),
+    # Font families
+    Atom("serif", "serif"),
+    Atom("sans_serif", "sans-serif"),
+    Atom("cursive", "cursive"),
+    Atom("fantasy", "fantasy"),
+    Atom("monospace", "monospace"),
+    Atom("mozfixed", "-moz-fixed"),
+    Atom("moz_fixed_pos_containing_block", "-moz-fixed-pos-containing-block"),
+    # Standard font-palette identifiers
+    Atom("light", "light"),
+    Atom("dark", "dark"),
+    # IPC stuff
+    # Atom("Remote", "remote"),  # "remote" is present above
+    Atom("RemoteId", "_remote_id"),
+    Atom("RemoteType", "remoteType"),
+    Atom("DisplayPort", "_displayport"),
+    Atom("DisplayPortMargins", "_displayportmargins"),
+    Atom("DisplayPortBase", "_displayportbase"),
+    Atom("MinimalDisplayPort", "_minimaldisplayport"),
+    Atom("forceMousewheelAutodir", "_force_mousewheel_autodir"),
+    Atom("forceMousewheelAutodirHonourRoot", "_force_mousewheel_autodir_honourroot"),
+    Atom("forcemessagemanager", "forcemessagemanager"),
+    Atom("initialBrowsingContextGroupId", "initialBrowsingContextGroupId"),
+    Atom("initiallyactive", "initiallyactive"),
+    # windows media query names
+    Atom("windows_win7", "windows-win7"),
+    Atom("windows_win8", "windows-win8"),
+    Atom("windows_win10", "windows-win10"),
+    # Names for system metrics.
+    Atom("_moz_scrollbar_start_backward", "-moz-scrollbar-start-backward"),
+    Atom("_moz_scrollbar_start_forward", "-moz-scrollbar-start-forward"),
+    Atom("_moz_scrollbar_end_backward", "-moz-scrollbar-end-backward"),
+    Atom("_moz_scrollbar_end_forward", "-moz-scrollbar-end-forward"),
+    Atom("_moz_overlay_scrollbars", "-moz-overlay-scrollbars"),
+    Atom("_moz_windows_accent_color_in_titlebar", "-moz-windows-accent-color-in-titlebar"),
+    Atom("_moz_windows_default_theme", "-moz-windows-default-theme"),
+    Atom("_moz_mac_graphite_theme", "-moz-mac-graphite-theme"),
+    Atom("_moz_mac_big_sur_theme", "-moz-mac-big-sur-theme"),
+    Atom("_moz_mac_rtl", "-moz-mac-rtl"),
+    Atom("_moz_platform", "-moz-platform"),
+    Atom("_moz_windows_compositor", "-moz-windows-compositor"),
+    Atom("_moz_windows_classic", "-moz-windows-classic"),
+    Atom("_moz_windows_glass", "-moz-windows-glass"),
+    Atom("_moz_windows_non_native_menus", "-moz-windows-non-native-menus"),
+    Atom("_moz_menubar_drag", "-moz-menubar-drag"),
+    Atom("_moz_device_pixel_ratio", "-moz-device-pixel-ratio"),
+    Atom("_moz_device_orientation", "-moz-device-orientation"),
+    Atom("_moz_is_resource_document", "-moz-is-resource-document"),
+    Atom("_moz_swipe_animation_enabled", "-moz-swipe-animation-enabled"),
+    Atom("_moz_gtk_csd_available", "-moz-gtk-csd-available"),
+    Atom("_moz_gtk_csd_titlebar_radius", "-moz-gtk-csd-titlebar-radius"),
+    Atom("_moz_gtk_csd_minimize_button", "-moz-gtk-csd-minimize-button"),
+    Atom("_moz_gtk_csd_minimize_button_position", "-moz-gtk-csd-minimize-button-position"),
+    Atom("_moz_gtk_csd_maximize_button", "-moz-gtk-csd-maximize-button"),
+    Atom("_moz_gtk_csd_maximize_button_position", "-moz-gtk-csd-maximize-button-position"),
+    Atom("_moz_gtk_csd_close_button", "-moz-gtk-csd-close-button"),
+    Atom("_moz_gtk_csd_close_button_position", "-moz-gtk-csd-close-button-position"),
+    Atom("_moz_gtk_csd_reversed_placement", "-moz-gtk-csd-reversed-placement"),
+    Atom("_moz_gtk_csd_menu_radius", "-moz-gtk-csd-menu-radius"),
+    Atom("_moz_content_prefers_color_scheme", "-moz-content-prefers-color-scheme"),
+    Atom("_moz_content_preferred_color_scheme", "-moz-content-preferred-color-scheme"),
+    Atom("_moz_system_dark_theme", "-moz-system-dark-theme"),
+    Atom("_moz_box_flexbox_emulation", "-moz-box-flexbox-emulation"),
+    Atom("_moz_panel_animations", "-moz-panel-animations"),
+    # application commands
+    Atom("Back", "Back"),
+    Atom("Forward", "Forward"),
+    Atom("Reload", "Reload"),
+    Atom("Stop", "Stop"),
+    Atom("Search", "Search"),
+    Atom("Bookmarks", "Bookmarks"),
+    Atom("Home", "Home"),
+    Atom("NextTrack", "NextTrack"),
+    Atom("PreviousTrack", "PreviousTrack"),
+    Atom("MediaStop", "MediaStop"),
+    Atom("PlayPause", "PlayPause"),
+    Atom("New", "New"),
+    Atom("Open", "Open"),
+    Atom("Close", "Close"),
+    Atom("Save", "Save"),
+    Atom("Find", "Find"),
+    Atom("Help", "Help"),
+    Atom("Print", "Print"),
+    Atom("SendMail", "SendMail"),
+    Atom("ForwardMail", "ForwardMail"),
+    Atom("ReplyToMail", "ReplyToMail"),
+    Atom("alert", "alert"),
+    Atom("alertdialog", "alertdialog"),
+    Atom("application", "application"),
+    Atom("aria_colcount", "aria-colcount"),
+    Atom("aria_colindex", "aria-colindex"),
+    Atom("aria_colindextext", "aria-colindextext"),
+    Atom("aria_colspan", "aria-colspan"),
+    Atom("aria_details", "aria-details"),
+    Atom("aria_errormessage", "aria-errormessage"),
+    Atom("aria_grabbed", "aria-grabbed"),
+    Atom("aria_keyshortcuts", "aria-keyshortcuts"),
+    Atom("aria_label", "aria-label"),
+    Atom("aria_modal", "aria-modal"),
+    Atom("aria_orientation", "aria-orientation"),
+    Atom("aria_placeholder", "aria-placeholder"),
+    Atom("aria_roledescription", "aria-roledescription"),
+    Atom("aria_rowcount", "aria-rowcount"),
+    Atom("aria_rowindex", "aria-rowindex"),
+    Atom("aria_rowindextext", "aria-rowindextext"),
+    Atom("aria_rowspan", "aria-rowspan"),
+    Atom("aria_valuetext", "aria-valuetext"),
+    Atom("assertive", "assertive"),
+    Atom("auto_generated", "auto-generated"),
+    Atom("banner", "banner"),
+    Atom("checkable", "checkable"),
+    Atom("columnheader", "columnheader"),
+    Atom("complementary", "complementary"),
+    Atom("containerAtomic", "container-atomic"),
+    Atom("containerBusy", "container-busy"),
+    Atom("containerLive", "container-live"),
+    Atom("containerLiveRole", "container-live-role"),
+    Atom("containerRelevant", "container-relevant"),
+    Atom("contentinfo", "contentinfo"),
+    Atom("cycles", "cycles"),
+    Atom("datatable", "datatable"),
+    Atom("eventFromInput", "event-from-input"),
+    Atom("feed", "feed"),
+    Atom("grammar", "grammar"),
+    Atom("gridcell", "gridcell"),
+    Atom("heading", "heading"),
+    Atom("inlinevalue", "inline"),
+    Atom("inline_size", "inline-size"),
+    Atom("invalid", "invalid"),
+    Atom("lineNumber", "line-number"),
+    Atom("menuitemcheckbox", "menuitemcheckbox"),
+    Atom("menuitemradio", "menuitemradio"),
+    # Atom("mixed", "mixed"),  # "mixed" is present above
+    Atom("navigation", "navigation"),
+    Atom("polite", "polite"),
+    Atom("posinset", "posinset"),
+    Atom("presentation", "presentation"),
+    Atom("progressbar", "progressbar"),
+    Atom("region", "region"),
+    Atom("rowgroup", "rowgroup"),
+    Atom("rowheader", "rowheader"),
+    Atom("search", "search"),
+    Atom("searchbox", "searchbox"),
+    Atom("setsize", "setsize"),
+    Atom("spelling", "spelling"),
+    Atom("spinbutton", "spinbutton"),
+    Atom("status", "status"),
+    Atom("tableCellIndex", "table-cell-index"),
+    Atom("tablist", "tablist"),
+    Atom("textIndent", "text-indent"),
+    Atom("textInputType", "text-input-type"),
+    Atom("textLineThroughColor", "text-line-through-color"),
+    Atom("textLineThroughStyle", "text-line-through-style"),
+    Atom("textPosition", "text-position"),
+    Atom("textUnderlineColor", "text-underline-color"),
+    Atom("textUnderlineStyle", "text-underline-style"),
+    Atom("timer", "timer"),
+    Atom("toolbarname", "toolbarname"),
+    Atom("toolbarseparator", "toolbarseparator"),
+    Atom("toolbarspacer", "toolbarspacer"),
+    Atom("toolbarspring", "toolbarspring"),
+    Atom("treegrid", "treegrid"),
+    Atom("_undefined", "undefined"),
+    Atom("xmlroles", "xml-roles"),
+    # MathML xml roles
+    Atom("close_fence", "close-fence"),
+    Atom("denominator", "denominator"),
+    Atom("numerator", "numerator"),
+    Atom("open_fence", "open-fence"),
+    Atom("overscript", "overscript"),
+    Atom("presubscript", "presubscript"),
+    Atom("presuperscript", "presuperscript"),
+    Atom("root_index", "root-index"),
+    Atom("subscript", "subscript"),
+    Atom("superscript", "superscript"),
+    Atom("underscript", "underscript"),
+    Atom("onaudiostart", "onaudiostart"),
+    Atom("onaudioend", "onaudioend"),
+    Atom("onsoundstart", "onsoundstart"),
+    Atom("onsoundend", "onsoundend"),
+    Atom("onspeechstart", "onspeechstart"),
+    Atom("onspeechend", "onspeechend"),
+    Atom("onresult", "onresult"),
+    Atom("onnomatch", "onnomatch"),
+    Atom("onresume", "onresume"),
+    Atom("onmark", "onmark"),
+    Atom("onboundary", "onboundary"),
+    # Media Controller
+    Atom("onactivated", "onactivated"),
+    Atom("ondeactivated", "ondeactivated"),
+    Atom("onmetadatachange", "onmetadatachange"),
+    Atom("onplaybackstatechange", "onplaybackstatechange"),
+    Atom("onpositionstatechange", "onpositionstatechange"),
+    Atom("onsupportedkeyschange", "onsupportedkeyschange"),
+    # media query for MathML Core's implementation of maction/semantics
+    Atom("_moz_mathml_core_maction_and_semantics", "-moz-mathml-core-maction-and-semantics"),
+    # media query for MathML Core's implementation of ms
+    Atom("_moz_mathml_core_ms", "-moz-mathml-core-ms"),
+    # Contextual Identity / Containers
+    Atom("usercontextid", "usercontextid"),
+    Atom("geckoViewSessionContextId", "geckoViewSessionContextId"),
+    # Namespaces
+    Atom("nsuri_xmlns", "http://www.w3.org/2000/xmlns/"),
+    Atom("nsuri_xml", "http://www.w3.org/XML/1998/namespace"),
+    Atom("nsuri_xhtml", "http://www.w3.org/1999/xhtml"),
+    Atom("nsuri_xlink", "http://www.w3.org/1999/xlink"),
+    Atom("nsuri_xslt", "http://www.w3.org/1999/XSL/Transform"),
+    Atom("nsuri_mathml", "http://www.w3.org/1998/Math/MathML"),
+    Atom("nsuri_rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#"),
+    Atom("nsuri_xul", "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"),
+    Atom("nsuri_svg", "http://www.w3.org/2000/svg"),
+    Atom("nsuri_parsererror", "http://www.mozilla.org/newlayout/xml/parsererror.xml"),
+    # MSE
+    Atom("onsourceopen", "onsourceopen"),
+    Atom("onsourceended", "onsourceended"),
+    Atom("onsourceclose", "onsourceclose"),
+    Atom("onupdatestart", "onupdatestart"),
+    Atom("onupdate", "onupdate"),
+    Atom("onupdateend", "onupdateend"),
+    Atom("onaddsourcebuffer", "onaddsourcebuffer"),
+    Atom("onremovesourcebuffer", "onremovesourcebuffer"),
+    # RDF (not used by mozilla-central, but still used by comm-central)
+    Atom("about", "about"),
+    Atom("ID", "ID"),
+    Atom("nodeID", "nodeID"),
+    Atom("aboutEach", "aboutEach"),
+    Atom("resource", "resource"),
+    Atom("RDF", "RDF"),
+    Atom("Description", "Description"),
+    Atom("Bag", "Bag"),
+    Atom("Seq", "Seq"),
+    Atom("Alt", "Alt"),
+    # Atom("kLiAtom", "li"),  # "li" is present above
+    # Atom("kXMLNSAtom", "xmlns"),  # "xmlns" is present above
+    Atom("parseType", "parseType"),
+    # Directory service
+    Atom("DirectoryService_CurrentProcess", "XCurProcD"),
+    Atom("DirectoryService_GRE_Directory", "GreD"),
+    Atom("DirectoryService_GRE_BinDirectory", "GreBinD"),
+    Atom("DirectoryService_OS_TemporaryDirectory", "TmpD"),
+    Atom("DirectoryService_OS_CurrentProcessDirectory", "CurProcD"),
+    Atom("DirectoryService_OS_CurrentWorkingDirectory", "CurWorkD"),
+    Atom("DirectoryService_OS_SystemConfigDir", "SysConfD"),
+    # Atom("DirectoryService_OS_HomeDirectory", "Home"),  # "Home" is present above
+    Atom("DirectoryService_OS_DesktopDirectory", "Desk"),
+    Atom("DirectoryService_InitCurrentProcess_dummy", "MozBinD"),
+    Atom("DirectoryService_SystemDirectory", "SysD"),
+    Atom("DirectoryService_UserLibDirectory", "ULibDir"),
+    Atom("DirectoryService_DefaultDownloadDirectory", "DfltDwnld"),
+    Atom("DirectoryService_LocalApplicationsDirectory", "LocApp"),
+    Atom("DirectoryService_UserPreferencesDirectory", "UsrPrfs"),
+    Atom("DirectoryService_PictureDocumentsDirectory", "Pct"),
+    Atom("DirectoryService_WindowsDirectory", "WinD"),
+    Atom("DirectoryService_WindowsProgramFiles", "ProgF"),
+    Atom("DirectoryService_Programs", "Progs"),
+    Atom("DirectoryService_Favorites", "Favs"),
+    Atom("DirectoryService_Appdata", "AppData"),
+    Atom("DirectoryService_LocalAppdata", "LocalAppData"),
+    Atom("DirectoryService_LocalAppdataLow", "LocalAppDataLow"),
+    Atom("DirectoryService_LowIntegrityTempBase", "LowTmpDBase"),
+    Atom("DirectoryService_WinCookiesDirectory", "CookD"),
+    # CSS pseudo-elements -- these must appear in the same order as
+    # in nsCSSPseudoElementList.h
+    PseudoElementAtom("PseudoElement_after", ":after"),
+    PseudoElementAtom("PseudoElement_before", ":before"),
+    PseudoElementAtom("PseudoElement_marker", ":marker"),
+    PseudoElementAtom("PseudoElement_backdrop", ":backdrop"),
+    PseudoElementAtom("PseudoElement_cue", ":cue"),
+    PseudoElementAtom("PseudoElement_firstLetter", ":first-letter"),
+    PseudoElementAtom("PseudoElement_firstLine", ":first-line"),
+    PseudoElementAtom("PseudoElement_selection", ":selection"),
+    PseudoElementAtom("PseudoElement_mozFocusInner", ":-moz-focus-inner"),
+    PseudoElementAtom("PseudoElement_mozNumberSpinBox", ":-moz-number-spin-box"),
+    PseudoElementAtom("PseudoElement_mozNumberSpinUp", ":-moz-number-spin-up"),
+    PseudoElementAtom("PseudoElement_mozNumberSpinDown", ":-moz-number-spin-down"),
+    PseudoElementAtom("PseudoElement_mozSearchClearButton", ":-moz-search-clear-button"),
+    PseudoElementAtom("PseudoElement_mozProgressBar", ":-moz-progress-bar"),
+    PseudoElementAtom("PseudoElement_mozRangeTrack", ":-moz-range-track"),
+    PseudoElementAtom("PseudoElement_mozRangeProgress", ":-moz-range-progress"),
+    PseudoElementAtom("PseudoElement_mozRangeThumb", ":-moz-range-thumb"),
+    PseudoElementAtom("PseudoElement_mozMeterBar", ":-moz-meter-bar"),
+    PseudoElementAtom("PseudoElement_placeholder", ":placeholder"),
+    PseudoElementAtom("PseudoElement_mozColorSwatch", ":-moz-color-swatch"),
+    PseudoElementAtom("PseudoElement_mozTextControlEditingRoot", ":-moz-text-control-editing-root"),
+    PseudoElementAtom("PseudoElement_mozTextControlPreview", ":-moz-text-control-preview"),
+    PseudoElementAtom("PseudoElement_mozReveal", ":-moz-reveal"),
+    PseudoElementAtom("PseudoElement_fileSelectorButton", ":file-selector-button"),
+    # CSS anonymous boxes -- these must appear in the same order as
+    # in nsCSSAnonBoxList.h
+    NonInheritingAnonBoxAtom("AnonBox_oofPlaceholder", ":-moz-oof-placeholder"),
+    NonInheritingAnonBoxAtom("AnonBox_horizontalFramesetBorder", ":-moz-hframeset-border"),
+    NonInheritingAnonBoxAtom("AnonBox_verticalFramesetBorder", ":-moz-vframeset-border"),
+    NonInheritingAnonBoxAtom("AnonBox_framesetBlank", ":-moz-frameset-blank"),
+    NonInheritingAnonBoxAtom("AnonBox_tableColGroup", ":-moz-table-column-group"),
+    NonInheritingAnonBoxAtom("AnonBox_tableCol", ":-moz-table-column"),
+    NonInheritingAnonBoxAtom("AnonBox_page", ":-moz-page"),
+    NonInheritingAnonBoxAtom("AnonBox_pageBreak", ":-moz-page-break"),
+    NonInheritingAnonBoxAtom("AnonBox_pageContent", ":-moz-page-content"),
+    NonInheritingAnonBoxAtom("AnonBox_printedSheet", ":-moz-printed-sheet"),
+    NonInheritingAnonBoxAtom("AnonBox_columnSpanWrapper", ":-moz-column-span-wrapper"),
+    InheritingAnonBoxAtom("AnonBox_mozText", ":-moz-text"),
+    InheritingAnonBoxAtom("AnonBox_firstLetterContinuation", ":-moz-first-letter-continuation"),
+    InheritingAnonBoxAtom("AnonBox_mozBlockInsideInlineWrapper", ":-moz-block-inside-inline-wrapper"),
+    InheritingAnonBoxAtom("AnonBox_mozMathMLAnonymousBlock", ":-moz-mathml-anonymous-block"),
+    InheritingAnonBoxAtom("AnonBox_mozLineFrame", ":-moz-line-frame"),
+    InheritingAnonBoxAtom("AnonBox_buttonContent", ":-moz-button-content"),
+    InheritingAnonBoxAtom("AnonBox_cellContent", ":-moz-cell-content"),
+    InheritingAnonBoxAtom("AnonBox_dropDownList", ":-moz-dropdown-list"),
+    InheritingAnonBoxAtom("AnonBox_fieldsetContent", ":-moz-fieldset-content"),
+    InheritingAnonBoxAtom("AnonBox_mozDisplayComboboxControlFrame", ":-moz-display-comboboxcontrol-frame"),
+    InheritingAnonBoxAtom("AnonBox_htmlCanvasContent", ":-moz-html-canvas-content"),
+    InheritingAnonBoxAtom("AnonBox_inlineTable", ":-moz-inline-table"),
+    InheritingAnonBoxAtom("AnonBox_table", ":-moz-table"),
+    InheritingAnonBoxAtom("AnonBox_tableCell", ":-moz-table-cell"),
+    InheritingAnonBoxAtom("AnonBox_tableWrapper", ":-moz-table-wrapper"),
+    InheritingAnonBoxAtom("AnonBox_tableRowGroup", ":-moz-table-row-group"),
+    InheritingAnonBoxAtom("AnonBox_tableRow", ":-moz-table-row"),
+    InheritingAnonBoxAtom("AnonBox_canvas", ":-moz-canvas"),
+    InheritingAnonBoxAtom("AnonBox_pageSequence", ":-moz-page-sequence"),
+    InheritingAnonBoxAtom("AnonBox_scrolledContent", ":-moz-scrolled-content"),
+    InheritingAnonBoxAtom("AnonBox_scrolledCanvas", ":-moz-scrolled-canvas"),
+    InheritingAnonBoxAtom("AnonBox_columnSet", ":-moz-column-set"),
+    InheritingAnonBoxAtom("AnonBox_columnContent", ":-moz-column-content"),
+    InheritingAnonBoxAtom("AnonBox_viewport", ":-moz-viewport"),
+    InheritingAnonBoxAtom("AnonBox_viewportScroll", ":-moz-viewport-scroll"),
+    InheritingAnonBoxAtom("AnonBox_anonymousItem", ":-moz-anonymous-item"),
+    InheritingAnonBoxAtom("AnonBox_blockRubyContent", ":-moz-block-ruby-content"),
+    InheritingAnonBoxAtom("AnonBox_ruby", ":-moz-ruby"),
+    InheritingAnonBoxAtom("AnonBox_rubyBase", ":-moz-ruby-base"),
+    InheritingAnonBoxAtom("AnonBox_rubyBaseContainer", ":-moz-ruby-base-container"),
+    InheritingAnonBoxAtom("AnonBox_rubyText", ":-moz-ruby-text"),
+    InheritingAnonBoxAtom("AnonBox_rubyTextContainer", ":-moz-ruby-text-container"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeColumn", ":-moz-tree-column"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeRow", ":-moz-tree-row"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeSeparator", ":-moz-tree-separator"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeCell", ":-moz-tree-cell"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeIndentation", ":-moz-tree-indentation"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeLine", ":-moz-tree-line"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeTwisty", ":-moz-tree-twisty"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeImage", ":-moz-tree-image"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeCellText", ":-moz-tree-cell-text"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeCheckbox", ":-moz-tree-checkbox"),
+    InheritingAnonBoxAtom("AnonBox_mozTreeDropFeedback", ":-moz-tree-drop-feedback"),
+    InheritingAnonBoxAtom("AnonBox_mozSVGMarkerAnonChild", ":-moz-svg-marker-anon-child"),
+    InheritingAnonBoxAtom("AnonBox_mozSVGOuterSVGAnonChild", ":-moz-svg-outer-svg-anon-child"),
+    InheritingAnonBoxAtom("AnonBox_mozSVGForeignContent", ":-moz-svg-foreign-content"),
+    InheritingAnonBoxAtom("AnonBox_mozSVGText", ":-moz-svg-text"),
+    # END ATOMS
+] + HTML_PARSER_ATOMS
+# fmt: on
+
+
+def verify():
+    idents = set()
+    strings = set()
+    failed = False
+    for atom in STATIC_ATOMS:
+        if atom.ident in idents:
+            print("StaticAtoms.py: duplicate static atom ident: %s" % atom.ident)
+            failed = True
+        if atom.string in strings:
+            print('StaticAtoms.py: duplicate static atom string: "%s"' % atom.string)
+            failed = True
+        idents.add(atom.ident)
+        strings.add(atom.string)
+    if failed:
+        sys.exit(1)
+
+
+def generate_nsgkatomlist_h(output, *ignore):
+    verify()
+    output.write(
+        "/* THIS FILE IS AUTOGENERATED BY StaticAtoms.py.  DO NOT EDIT */\n\n"
+        "#ifdef small\n"
+        "#undef small\n"
+        "#endif\n\n"
+        "// GK_ATOM(identifier, string, hash, is_ascii_lower, gecko_type, atom_type)\n"
+        + "".join(
+            [
+                'GK_ATOM(%s, "%s", 0x%08x, %s, %s, %s)\n'
+                % (
+                    a.ident,
+                    a.string,
+                    a.hash,
+                    str(a.is_ascii_lowercase).lower(),
+                    a.ty,
+                    a.atom_type,
+                )
+                for a in STATIC_ATOMS
+            ]
+        )
+    )
+
+
+def generate_nsgkatomconsts_h(output, *ignore):
+    pseudo_index = None
+    anon_box_index = None
+    pseudo_count = 0
+    anon_box_count = 0
+    for i, atom in enumerate(STATIC_ATOMS):
+        if atom.atom_type == "PseudoElementAtom":
+            if pseudo_index is None:
+                pseudo_index = i
+            pseudo_count += 1
+        elif (
+            atom.atom_type == "NonInheritingAnonBoxAtom"
+            or atom.atom_type == "InheritingAnonBoxAtom"
+        ):
+            if anon_box_index is None:
+                anon_box_index = i
+            anon_box_count += 1
+    output.write(
+        "/* THIS IS AN AUTOGENERATED FILE.  DO NOT EDIT */\n\n"
+        "#ifndef nsGkAtomConsts_h\n"
+        "#define nsGkAtomConsts_h\n\n"
+        "namespace mozilla {\n"
+        "  constexpr uint32_t kAtomIndex_PseudoElements = %d;\n"
+        "  constexpr uint32_t kAtomCount_PseudoElements = %d;\n"
+        "  constexpr uint32_t kAtomIndex_AnonBoxes = %d;\n"
+        "  constexpr uint32_t kAtomCount_AnonBoxes = %d;\n"
+        "}\n\n"
+        "#endif\n" % (pseudo_index, pseudo_count, anon_box_index, anon_box_count)
+    )
+
+
+if __name__ == "__main__":
+    generate_nsgkatomlist_h(sys.stdout)
diff --git a/xpcom/ds/StickyTimeDuration.h b/xpcom/ds/StickyTimeDuration.h
new file mode 100644
index 0000000000..4d888dfbcd
--- /dev/null
+++ b/xpcom/ds/StickyTimeDuration.h
@@ -0,0 +1,238 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_StickyTimeDuration_h
+#define mozilla_StickyTimeDuration_h
+
+#include "mozilla/TimeStamp.h"
+#include "mozilla/FloatingPoint.h"
+
+namespace mozilla {
+
+/**
+ * A ValueCalculator class that performs additional checks before performing
+ * arithmetic operations such that if either operand is Forever (or the
+ * negative equivalent) the result remains Forever (or the negative equivalent
+ * as appropriate).
+ *
+ * Currently this only checks if either argument to each operation is
+ * Forever/-Forever. However, it is possible that, for example,
+ * aA + aB > INT64_MAX (or < INT64_MIN).
+ *
+ * We currently don't check for that case since we don't expect that to
+ * happen often except under test conditions in which case the wrapping
+ * behavior is probably acceptable.
+ */
+class StickyTimeDurationValueCalculator {
+ public:
+  static int64_t Add(int64_t aA, int64_t aB) {
+    MOZ_ASSERT((aA != INT64_MAX || aB != INT64_MIN) &&
+                   (aA != INT64_MIN || aB != INT64_MAX),
+               "'Infinity + -Infinity' and '-Infinity + Infinity'"
+               " are undefined");
+
+    // Forever + x = Forever
+    // x + Forever = Forever
+    if (aA == INT64_MAX || aB == INT64_MAX) {
+      return INT64_MAX;
+    }
+    // -Forever + x = -Forever
+    // x + -Forever = -Forever
+    if (aA == INT64_MIN || aB == INT64_MIN) {
+      return INT64_MIN;
+    }
+
+    return aA + aB;
+  }
+
+  // Note that we can't just define Add and have BaseTimeDuration call Add with
+  // negative arguments since INT64_MAX != -INT64_MIN so the saturating logic
+  // won't work.
+  static int64_t Subtract(int64_t aA, int64_t aB) {
+    MOZ_ASSERT((aA != INT64_MAX && aA != INT64_MIN) || aA != aB,
+               "'Infinity - Infinity' and '-Infinity - -Infinity'"
+               " are undefined");
+
+    // Forever - x  = Forever
+    // x - -Forever = Forever
+    if (aA == INT64_MAX || aB == INT64_MIN) {
+      return INT64_MAX;
+    }
+    // -Forever - x = -Forever
+    // x - Forever  = -Forever
+    if (aA == INT64_MIN || aB == INT64_MAX) {
+      return INT64_MIN;
+    }
+
+    return aA - aB;
+  }
+
+  template <typename T>
+  static int64_t Multiply(int64_t aA, T aB) {
+    // Specializations for double, float, and int64_t are provided following.
+    return Multiply(aA, static_cast<int64_t>(aB));
+  }
+
+  static int64_t Divide(int64_t aA, int64_t aB) {
+    MOZ_ASSERT(aB != 0, "Division by zero");
+    MOZ_ASSERT((aA != INT64_MAX && aA != INT64_MIN) ||
+                   (aB != INT64_MAX && aB != INT64_MIN),
+               "Dividing +/-Infinity by +/-Infinity is undefined");
+
+    // Forever / +x = Forever
+    // Forever / -x = -Forever
+    // -Forever / +x = -Forever
+    // -Forever / -x = Forever
+    if (aA == INT64_MAX || aA == INT64_MIN) {
+      return (aA >= 0) ^ (aB >= 0) ? INT64_MIN : INT64_MAX;
+    }
+    // x /  Forever = 0
+    // x / -Forever = 0
+    if (aB == INT64_MAX || aB == INT64_MIN) {
+      return 0;
+    }
+
+    return aA / aB;
+  }
+
+  static double DivideDouble(int64_t aA, int64_t aB) {
+    MOZ_ASSERT(aB != 0, "Division by zero");
+    MOZ_ASSERT((aA != INT64_MAX && aA != INT64_MIN) ||
+                   (aB != INT64_MAX && aB != INT64_MIN),
+               "Dividing +/-Infinity by +/-Infinity is undefined");
+
+    // Forever / +x = Forever
+    // Forever / -x = -Forever
+    // -Forever / +x = -Forever
+    // -Forever / -x = Forever
+    if (aA == INT64_MAX || aA == INT64_MIN) {
+      return (aA >= 0) ^ (aB >= 0) ? NegativeInfinity<double>()
+                                   : PositiveInfinity<double>();
+    }
+    // x /  Forever = 0
+    // x / -Forever = 0
+    if (aB == INT64_MAX || aB == INT64_MIN) {
+      return 0.0;
+    }
+
+    return static_cast<double>(aA) / aB;
+  }
+
+  static int64_t Modulo(int64_t aA, int64_t aB) {
+    MOZ_ASSERT(aA != INT64_MAX && aA != INT64_MIN,
+               "Infinity modulo x is undefined");
+
+    return aA % aB;
+  }
+};
+
+template <>
+inline int64_t StickyTimeDurationValueCalculator::Multiply<int64_t>(
+    int64_t aA, int64_t aB) {
+  MOZ_ASSERT((aA != 0 || (aB != INT64_MIN && aB != INT64_MAX)) &&
+                 ((aA != INT64_MIN && aA != INT64_MAX) || aB != 0),
+             "Multiplication of infinity by zero");
+
+  // Forever  * +x = Forever
+  // Forever  * -x = -Forever
+  // -Forever * +x = -Forever
+  // -Forever * -x = Forever
+  //
+  // i.e. If one or more of the arguments is +/-Forever, then
+  // return -Forever if the signs differ, or +Forever otherwise.
+  if (aA == INT64_MAX || aA == INT64_MIN || aB == INT64_MAX ||
+      aB == INT64_MIN) {
+    return (aA >= 0) ^ (aB >= 0) ? INT64_MIN : INT64_MAX;
+  }
+
+  return aA * aB;
+}
+
+template <>
+inline int64_t StickyTimeDurationValueCalculator::Multiply<double>(int64_t aA,
+                                                                   double aB) {
+  MOZ_ASSERT((aA != 0 || (!IsInfinite(aB))) &&
+                 ((aA != INT64_MIN && aA != INT64_MAX) || aB != 0.0),
+             "Multiplication of infinity by zero");
+
+  // As with Multiply<int64_t>, if one or more of the arguments is
+  // +/-Forever or +/-Infinity, then return -Forever if the signs differ,
+  // or +Forever otherwise.
+  if (aA == INT64_MAX || aA == INT64_MIN || IsInfinite(aB)) {
+    return (aA >= 0) ^ (aB >= 0.0) ? INT64_MIN : INT64_MAX;
+  }
+
+  return aA * aB;
+}
+
+template <>
+inline int64_t StickyTimeDurationValueCalculator::Multiply<float>(int64_t aA,
+                                                                  float aB) {
+  MOZ_ASSERT(IsInfinite(aB) == IsInfinite(static_cast<double>(aB)),
+             "Casting to float loses infinite-ness");
+
+  return Multiply(aA, static_cast<double>(aB));
+}
+
+/**
+ * Specialization of BaseTimeDuration that uses
+ * StickyTimeDurationValueCalculator for arithmetic on the mValue member.
+ *
+ * Use this class when you need a time duration that is expected to hold values
+ * of Forever (or the negative equivalent) *and* when you expect that
+ * time duration to be used in arithmetic operations (and not just value
+ * comparisons).
+ */
+typedef BaseTimeDuration<StickyTimeDurationValueCalculator> StickyTimeDuration;
+
+// Template specializations to allow arithmetic between StickyTimeDuration
+// and TimeDuration objects by falling back to the safe behavior.
+inline StickyTimeDuration operator+(const TimeDuration& aA,
+                                    const StickyTimeDuration& aB) {
+  return StickyTimeDuration(aA) + aB;
+}
+inline StickyTimeDuration operator+(const StickyTimeDuration& aA,
+                                    const TimeDuration& aB) {
+  return aA + StickyTimeDuration(aB);
+}
+
+inline StickyTimeDuration operator-(const TimeDuration& aA,
+                                    const StickyTimeDuration& aB) {
+  return StickyTimeDuration(aA) - aB;
+}
+inline StickyTimeDuration operator-(const StickyTimeDuration& aA,
+                                    const TimeDuration& aB) {
+  return aA - StickyTimeDuration(aB);
+}
+
+inline StickyTimeDuration& operator+=(StickyTimeDuration& aA,
+                                      const TimeDuration& aB) {
+  return aA += StickyTimeDuration(aB);
+}
+inline StickyTimeDuration& operator-=(StickyTimeDuration& aA,
+                                      const TimeDuration& aB) {
+  return aA -= StickyTimeDuration(aB);
+}
+
+inline double operator/(const TimeDuration& aA, const StickyTimeDuration& aB) {
+  return StickyTimeDuration(aA) / aB;
+}
+inline double operator/(const StickyTimeDuration& aA, const TimeDuration& aB) {
+  return aA / StickyTimeDuration(aB);
+}
+
+inline StickyTimeDuration operator%(const TimeDuration& aA,
+                                    const StickyTimeDuration& aB) {
+  return StickyTimeDuration(aA) % aB;
+}
+inline StickyTimeDuration operator%(const StickyTimeDuration& aA,
+                                    const TimeDuration& aB) {
+  return aA % StickyTimeDuration(aB);
+}
+
+}  // namespace mozilla
+
+#endif /* mozilla_StickyTimeDuration_h */
diff --git a/xpcom/ds/Tokenizer.cpp b/xpcom/ds/Tokenizer.cpp
new file mode 100644
index 0000000000..3b0f6b02dd
--- /dev/null
+++ b/xpcom/ds/Tokenizer.cpp
@@ -0,0 +1,805 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "Tokenizer.h"
+
+#include "nsUnicharUtils.h"
+#include <algorithm>
+
+namespace mozilla {
+
+template <>
+char const TokenizerBase<char>::sWhitespaces[] = {' ', '\t', 0};
+template <>
+char16_t const TokenizerBase<char16_t>::sWhitespaces[3] = {' ', '\t', 0};
+
+template <typename TChar>
+static bool contains(TChar const* const list, TChar const needle) {
+  for (TChar const* c = list; *c; ++c) {
+    if (needle == *c) {
+      return true;
+    }
+  }
+  return false;
+}
+
+template <typename TChar>
+TTokenizer<TChar>::TTokenizer(const typename base::TAString& aSource,
+                              const TChar* aWhitespaces,
+                              const TChar* aAdditionalWordChars)
+    : TokenizerBase<TChar>(aWhitespaces, aAdditionalWordChars) {
+  base::mInputFinished = true;
+  aSource.BeginReading(base::mCursor);
+  mRecord = mRollback = base::mCursor;
+  aSource.EndReading(base::mEnd);
+}
+
+template <typename TChar>
+TTokenizer<TChar>::TTokenizer(const TChar* aSource, const TChar* aWhitespaces,
+                              const TChar* aAdditionalWordChars)
+    : TTokenizer(typename base::TDependentString(aSource), aWhitespaces,
+                 aAdditionalWordChars) {}
+
+template <typename TChar>
+bool TTokenizer<TChar>::Next(typename base::Token& aToken) {
+  if (!base::HasInput()) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  mRollback = base::mCursor;
+  base::mCursor = base::Parse(aToken);
+
+  base::AssignFragment(aToken, mRollback, base::mCursor);
+
+  base::mPastEof = aToken.Type() == base::TOKEN_EOF;
+  base::mHasFailed = false;
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::Check(const typename base::TokenType aTokenType,
+                              typename base::Token& aResult) {
+  if (!base::HasInput()) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  typename base::TAString::const_char_iterator next = base::Parse(aResult);
+  if (aTokenType != aResult.Type()) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  mRollback = base::mCursor;
+  base::mCursor = next;
+
+  base::AssignFragment(aResult, mRollback, base::mCursor);
+
+  base::mPastEof = aResult.Type() == base::TOKEN_EOF;
+  base::mHasFailed = false;
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::Check(const typename base::Token& aToken) {
+#ifdef DEBUG
+  base::Validate(aToken);
+#endif
+
+  if (!base::HasInput()) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  typename base::Token parsed;
+  typename base::TAString::const_char_iterator next = base::Parse(parsed);
+  if (!aToken.Equals(parsed)) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  mRollback = base::mCursor;
+  base::mCursor = next;
+  base::mPastEof = parsed.Type() == base::TOKEN_EOF;
+  base::mHasFailed = false;
+  return true;
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::SkipWhites(WhiteSkipping aIncludeNewLines) {
+  if (!CheckWhite() &&
+      (aIncludeNewLines == DONT_INCLUDE_NEW_LINE || !CheckEOL())) {
+    return;
+  }
+
+  typename base::TAString::const_char_iterator rollback = mRollback;
+  while (CheckWhite() || (aIncludeNewLines == INCLUDE_NEW_LINE && CheckEOL())) {
+  }
+
+  base::mHasFailed = false;
+  mRollback = rollback;
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::SkipUntil(typename base::Token const& aToken) {
+  typename base::TAString::const_char_iterator rollback = base::mCursor;
+  const typename base::Token eof = base::Token::EndOfFile();
+
+  typename base::Token t;
+  while (Next(t)) {
+    if (aToken.Equals(t) || eof.Equals(t)) {
+      Rollback();
+      break;
+    }
+  }
+
+  mRollback = rollback;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::CheckChar(bool (*aClassifier)(const TChar aChar)) {
+  if (!aClassifier) {
+    MOZ_ASSERT(false);
+    return false;
+  }
+
+  if (!base::HasInput() || base::mCursor == base::mEnd) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  if (!aClassifier(*base::mCursor)) {
+    base::mHasFailed = true;
+    return false;
+  }
+
+  mRollback = base::mCursor;
+  ++base::mCursor;
+  base::mHasFailed = false;
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::CheckPhrase(const typename base::TAString& aPhrase) {
+  if (!base::HasInput()) {
+    return false;
+  }
+
+  typedef typename base::TAString::const_char_iterator Cursor;
+
+  TTokenizer<TChar> pattern(aPhrase);
+  MOZ_ASSERT(!pattern.CheckEOF(),
+             "This will return true but won't shift the Tokenizer's cursor");
+
+  return [&](Cursor cursor, Cursor rollback) mutable {
+    while (true) {
+      if (pattern.CheckEOF()) {
+        base::mHasFailed = false;
+        mRollback = cursor;
+        return true;
+      }
+
+      typename base::Token t1, t2;
+      Unused << Next(t1);
+      Unused << pattern.Next(t2);
+      if (t1.Type() == t2.Type() && t1.Fragment().Equals(t2.Fragment())) {
+        continue;
+      }
+
+      break;
+    }
+
+    base::mHasFailed = true;
+    base::mPastEof = false;
+    base::mCursor = cursor;
+    mRollback = rollback;
+    return false;
+  }(base::mCursor, mRollback);
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadChar(TChar* aValue) {
+  MOZ_RELEASE_ASSERT(aValue);
+
+  typename base::Token t;
+  if (!Check(base::TOKEN_CHAR, t)) {
+    return false;
+  }
+
+  *aValue = t.AsChar();
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadChar(bool (*aClassifier)(const TChar aChar),
+                                 TChar* aValue) {
+  MOZ_RELEASE_ASSERT(aValue);
+
+  if (!CheckChar(aClassifier)) {
+    return false;
+  }
+
+  *aValue = *mRollback;
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadWord(typename base::TAString& aValue) {
+  typename base::Token t;
+  if (!Check(base::TOKEN_WORD, t)) {
+    return false;
+  }
+
+  aValue.Assign(t.AsString());
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadWord(typename base::TDependentSubstring& aValue) {
+  typename base::Token t;
+  if (!Check(base::TOKEN_WORD, t)) {
+    return false;
+  }
+
+  aValue.Rebind(t.AsString().BeginReading(), t.AsString().Length());
+  return true;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadUntil(typename base::Token const& aToken,
+                                  typename base::TAString& aResult,
+                                  ClaimInclusion aInclude) {
+  typename base::TDependentSubstring substring;
+  bool rv = ReadUntil(aToken, substring, aInclude);
+  aResult.Assign(substring);
+  return rv;
+}
+
+template <typename TChar>
+bool TTokenizer<TChar>::ReadUntil(typename base::Token const& aToken,
+                                  typename base::TDependentSubstring& aResult,
+                                  ClaimInclusion aInclude) {
+  typename base::TAString::const_char_iterator record = mRecord;
+  Record();
+  typename base::TAString::const_char_iterator rollback = mRollback =
+      base::mCursor;
+
+  bool found = false;
+  typename base::Token t;
+  while (Next(t)) {
+    if (aToken.Equals(t)) {
+      found = true;
+      break;
+    }
+    if (t.Equals(base::Token::EndOfFile())) {
+      // We don't want to eat it.
+      Rollback();
+      break;
+    }
+  }
+
+  Claim(aResult, aInclude);
+  mRollback = rollback;
+  mRecord = record;
+  return found;
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::Rollback() {
+  MOZ_ASSERT(base::mCursor > mRollback || base::mPastEof, "TODO!!!");
+
+  base::mPastEof = false;
+  base::mHasFailed = false;
+  base::mCursor = mRollback;
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::Record(ClaimInclusion aInclude) {
+  mRecord = aInclude == INCLUDE_LAST ? mRollback : base::mCursor;
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::Claim(typename base::TAString& aResult,
+                              ClaimInclusion aInclusion) {
+  typename base::TAString::const_char_iterator close =
+      aInclusion == EXCLUDE_LAST ? mRollback : base::mCursor;
+  aResult.Assign(Substring(mRecord, close));
+}
+
+template <typename TChar>
+void TTokenizer<TChar>::Claim(typename base::TDependentSubstring& aResult,
+                              ClaimInclusion aInclusion) {
+  typename base::TAString::const_char_iterator close =
+      aInclusion == EXCLUDE_LAST ? mRollback : base::mCursor;
+
+  MOZ_RELEASE_ASSERT(close >= mRecord, "Overflow!");
+  aResult.Rebind(mRecord, close - mRecord);
+}
+
+// TokenizerBase
+
+template <typename TChar>
+TokenizerBase<TChar>::TokenizerBase(const TChar* aWhitespaces,
+                                    const TChar* aAdditionalWordChars)
+    : mPastEof(false),
+      mHasFailed(false),
+      mInputFinished(true),
+      mMode(Mode::FULL),
+      mMinRawDelivery(1024),
+      mWhitespaces(aWhitespaces ? aWhitespaces : sWhitespaces),
+      mAdditionalWordChars(aAdditionalWordChars),
+      mCursor(nullptr),
+      mEnd(nullptr),
+      mNextCustomTokenID(TOKEN_CUSTOM0) {}
+
+template <typename TChar>
+auto TokenizerBase<TChar>::AddCustomToken(const TAString& aValue,
+                                          ECaseSensitivity aCaseInsensitivity,
+                                          bool aEnabled) -> Token {
+  MOZ_ASSERT(!aValue.IsEmpty());
+
+  UniquePtr<Token>& t = *mCustomTokens.AppendElement();
+  t = MakeUnique<Token>();
+
+  t->mType = static_cast<TokenType>(++mNextCustomTokenID);
+  t->mCustomCaseInsensitivity = aCaseInsensitivity;
+  t->mCustomEnabled = aEnabled;
+  t->mCustom.Assign(aValue);
+  return *t;
+}
+
+template <typename TChar>
+void TokenizerBase<TChar>::RemoveCustomToken(Token& aToken) {
+  if (aToken.mType == TOKEN_UNKNOWN) {
+    // Already removed
+    return;
+  }
+
+  for (UniquePtr<Token> const& custom : mCustomTokens) {
+    if (custom->mType == aToken.mType) {
+      mCustomTokens.RemoveElement(custom);
+      aToken.mType = TOKEN_UNKNOWN;
+      return;
+    }
+  }
+
+  MOZ_ASSERT(false, "Token to remove not found");
+}
+
+template <typename TChar>
+void TokenizerBase<TChar>::EnableCustomToken(Token const& aToken,
+                                             bool aEnabled) {
+  if (aToken.mType == TOKEN_UNKNOWN) {
+    // Already removed
+    return;
+  }
+
+  for (UniquePtr<Token> const& custom : mCustomTokens) {
+    if (custom->Type() == aToken.Type()) {
+      // This effectively destroys the token instance.
+      custom->mCustomEnabled = aEnabled;
+      return;
+    }
+  }
+
+  MOZ_ASSERT(false, "Token to change not found");
+}
+
+template <typename TChar>
+void TokenizerBase<TChar>::SetTokenizingMode(Mode aMode) {
+  mMode = aMode;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::HasFailed() const {
+  return mHasFailed;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::HasInput() const {
+  return !mPastEof;
+}
+
+template <typename TChar>
+auto TokenizerBase<TChar>::Parse(Token& aToken) const ->
+    typename TAString::const_char_iterator {
+  if (mCursor == mEnd) {
+    if (!mInputFinished) {
+      return mCursor;
+    }
+
+    aToken = Token::EndOfFile();
+    return mEnd;
+  }
+
+  MOZ_RELEASE_ASSERT(mEnd >= mCursor, "Overflow!");
+  typename TAString::size_type available = mEnd - mCursor;
+
+  uint32_t longestCustom = 0;
+  for (UniquePtr<Token> const& custom : mCustomTokens) {
+    if (IsCustom(mCursor, *custom, &longestCustom)) {
+      aToken = *custom;
+      return mCursor + custom->mCustom.Length();
+    }
+  }
+
+  if (!mInputFinished && available < longestCustom) {
+    // Not enough data to deterministically decide.
+    return mCursor;
+  }
+
+  typename TAString::const_char_iterator next = mCursor;
+
+  if (mMode == Mode::CUSTOM_ONLY) {
+    // We have to do a brute-force search for all of the enabled custom
+    // tokens.
+    while (next < mEnd) {
+      ++next;
+      for (UniquePtr<Token> const& custom : mCustomTokens) {
+        if (IsCustom(next, *custom)) {
+          aToken = Token::Raw();
+          return next;
+        }
+      }
+    }
+
+    if (mInputFinished) {
+      // End of the data reached.
+      aToken = Token::Raw();
+      return next;
+    }
+
+    if (longestCustom < available && available > mMinRawDelivery) {
+      // We can return some data w/o waiting for either a custom token
+      // or call to FinishData() when we leave the tail where all the
+      // custom tokens potentially fit, so we can't lose only partially
+      // delivered tokens.  This preserves reasonable granularity.
+      aToken = Token::Raw();
+      return mEnd - longestCustom + 1;
+    }
+
+    // Not enough data to deterministically decide.
+    return mCursor;
+  }
+
+  enum State {
+    PARSE_INTEGER,
+    PARSE_WORD,
+    PARSE_CRLF,
+    PARSE_LF,
+    PARSE_WS,
+    PARSE_CHAR,
+  } state;
+
+  if (IsWordFirst(*next)) {
+    state = PARSE_WORD;
+  } else if (IsNumber(*next)) {
+    state = PARSE_INTEGER;
+  } else if (contains(mWhitespaces, *next)) {  // not UTF-8 friendly?
+    state = PARSE_WS;
+  } else if (*next == '\r') {
+    state = PARSE_CRLF;
+  } else if (*next == '\n') {
+    state = PARSE_LF;
+  } else {
+    state = PARSE_CHAR;
+  }
+
+  mozilla::CheckedUint64 resultingNumber = 0;
+
+  while (next < mEnd) {
+    switch (state) {
+      case PARSE_INTEGER:
+        // Keep it simple for now
+        resultingNumber *= 10;
+        resultingNumber += static_cast<uint64_t>(*next - '0');
+
+        ++next;
+        if (IsPending(next)) {
+          break;
+        }
+        if (IsEnd(next) || !IsNumber(*next)) {
+          if (!resultingNumber.isValid()) {
+            aToken = Token::Error();
+          } else {
+            aToken = Token::Number(resultingNumber.value());
+          }
+          return next;
+        }
+        break;
+
+      case PARSE_WORD:
+        ++next;
+        if (IsPending(next)) {
+          break;
+        }
+        if (IsEnd(next) || !IsWord(*next)) {
+          aToken = Token::Word(Substring(mCursor, next));
+          return next;
+        }
+        break;
+
+      case PARSE_CRLF:
+        ++next;
+        if (IsPending(next)) {
+          break;
+        }
+        if (!IsEnd(next) && *next == '\n') {  // LF is optional
+          ++next;
+        }
+        aToken = Token::NewLine();
+        return next;
+
+      case PARSE_LF:
+        ++next;
+        aToken = Token::NewLine();
+        return next;
+
+      case PARSE_WS:
+        ++next;
+        aToken = Token::Whitespace();
+        return next;
+
+      case PARSE_CHAR:
+        ++next;
+        aToken = Token::Char(*mCursor);
+        return next;
+    }  // switch (state)
+  }    // while (next < end)
+
+  MOZ_ASSERT(!mInputFinished);
+  return mCursor;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsEnd(
+    const typename TAString::const_char_iterator& caret) const {
+  return caret == mEnd;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsPending(
+    const typename TAString::const_char_iterator& caret) const {
+  return IsEnd(caret) && !mInputFinished;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsWordFirst(const TChar aInput) const {
+  // TODO: make this fully work with unicode
+  return (ToLowerCase(static_cast<uint32_t>(aInput)) !=
+          ToUpperCase(static_cast<uint32_t>(aInput))) ||
+         '_' == aInput ||
+         (mAdditionalWordChars ? contains(mAdditionalWordChars, aInput)
+                               : false);
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsWord(const TChar aInput) const {
+  return IsWordFirst(aInput) || IsNumber(aInput);
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsNumber(const TChar aInput) const {
+  // TODO: are there unicode numbers?
+  return aInput >= '0' && aInput <= '9';
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::IsCustom(
+    const typename TAString::const_char_iterator& caret,
+    const Token& aCustomToken, uint32_t* aLongest) const {
+  MOZ_ASSERT(aCustomToken.mType > TOKEN_CUSTOM0);
+  if (!aCustomToken.mCustomEnabled) {
+    return false;
+  }
+
+  if (aLongest) {
+    *aLongest = std::max<uint32_t>(*aLongest, aCustomToken.mCustom.Length());
+  }
+
+  // This is not very likely to happen according to how we call this method
+  // and since it's on a hot path, it's just a diagnostic assert,
+  // not a release assert.
+  MOZ_DIAGNOSTIC_ASSERT(mEnd >= caret, "Overflow?");
+  uint32_t inputLength = mEnd - caret;
+  if (aCustomToken.mCustom.Length() > inputLength) {
+    return false;
+  }
+
+  TDependentSubstring inputFragment(caret, aCustomToken.mCustom.Length());
+  if (aCustomToken.mCustomCaseInsensitivity == CASE_INSENSITIVE) {
+    if constexpr (std::is_same_v<TChar, char>) {
+      return inputFragment.Equals(aCustomToken.mCustom,
+                                  nsCaseInsensitiveUTF8StringComparator);
+    } else {
+      return inputFragment.Equals(aCustomToken.mCustom,
+                                  nsCaseInsensitiveStringComparator);
+    }
+  }
+  return inputFragment.Equals(aCustomToken.mCustom);
+}
+
+template <typename TChar>
+void TokenizerBase<TChar>::AssignFragment(
+    Token& aToken, typename TAString::const_char_iterator begin,
+    typename TAString::const_char_iterator end) {
+  aToken.AssignFragment(begin, end);
+}
+
+#ifdef DEBUG
+
+template <typename TChar>
+void TokenizerBase<TChar>::Validate(Token const& aToken) {
+  if (aToken.Type() == TOKEN_WORD) {
+    typename TAString::const_char_iterator c = aToken.AsString().BeginReading();
+    typename TAString::const_char_iterator e = aToken.AsString().EndReading();
+
+    if (c < e) {
+      MOZ_ASSERT(IsWordFirst(*c));
+      while (++c < e) {
+        MOZ_ASSERT(IsWord(*c));
+      }
+    }
+  }
+}
+
+#endif
+
+// TokenizerBase::Token
+
+template <typename TChar>
+TokenizerBase<TChar>::Token::Token()
+    : mType(TOKEN_UNKNOWN),
+      mChar(0),
+      mInteger(0),
+      mCustomCaseInsensitivity(CASE_SENSITIVE),
+      mCustomEnabled(false) {}
+
+template <typename TChar>
+TokenizerBase<TChar>::Token::Token(const Token& aOther)
+    : mType(aOther.mType),
+      mCustom(aOther.mCustom),
+      mChar(aOther.mChar),
+      mInteger(aOther.mInteger),
+      mCustomCaseInsensitivity(aOther.mCustomCaseInsensitivity),
+      mCustomEnabled(aOther.mCustomEnabled) {
+  if (mType == TOKEN_WORD || mType > TOKEN_CUSTOM0) {
+    mWord.Rebind(aOther.mWord.BeginReading(), aOther.mWord.Length());
+  }
+}
+
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::operator=(const Token& aOther) -> Token& {
+  mType = aOther.mType;
+  mCustom = aOther.mCustom;
+  mChar = aOther.mChar;
+  mWord.Rebind(aOther.mWord.BeginReading(), aOther.mWord.Length());
+  mInteger = aOther.mInteger;
+  mCustomCaseInsensitivity = aOther.mCustomCaseInsensitivity;
+  mCustomEnabled = aOther.mCustomEnabled;
+  return *this;
+}
+
+template <typename TChar>
+void TokenizerBase<TChar>::Token::AssignFragment(
+    typename TAString::const_char_iterator begin,
+    typename TAString::const_char_iterator end) {
+  MOZ_RELEASE_ASSERT(end >= begin, "Overflow!");
+  mFragment.Rebind(begin, end - begin);
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Raw() -> Token {
+  Token t;
+  t.mType = TOKEN_RAW;
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Word(TAString const& aValue) -> Token {
+  Token t;
+  t.mType = TOKEN_WORD;
+  t.mWord.Rebind(aValue.BeginReading(), aValue.Length());
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Char(TChar const aValue) -> Token {
+  Token t;
+  t.mType = TOKEN_CHAR;
+  t.mChar = aValue;
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Number(uint64_t const aValue) -> Token {
+  Token t;
+  t.mType = TOKEN_INTEGER;
+  t.mInteger = aValue;
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Whitespace() -> Token {
+  Token t;
+  t.mType = TOKEN_WS;
+  t.mChar = '\0';
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::NewLine() -> Token {
+  Token t;
+  t.mType = TOKEN_EOL;
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::EndOfFile() -> Token {
+  Token t;
+  t.mType = TOKEN_EOF;
+  return t;
+}
+
+// static
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::Error() -> Token {
+  Token t;
+  t.mType = TOKEN_ERROR;
+  return t;
+}
+
+template <typename TChar>
+bool TokenizerBase<TChar>::Token::Equals(const Token& aOther) const {
+  if (mType != aOther.mType) {
+    return false;
+  }
+
+  switch (mType) {
+    case TOKEN_INTEGER:
+      return AsInteger() == aOther.AsInteger();
+    case TOKEN_WORD:
+      return AsString() == aOther.AsString();
+    case TOKEN_CHAR:
+      return AsChar() == aOther.AsChar();
+    default:
+      return true;
+  }
+}
+
+template <typename TChar>
+TChar TokenizerBase<TChar>::Token::AsChar() const {
+  MOZ_ASSERT(mType == TOKEN_CHAR || mType == TOKEN_WS);
+  return mChar;
+}
+
+template <typename TChar>
+auto TokenizerBase<TChar>::Token::AsString() const -> TDependentSubstring {
+  MOZ_ASSERT(mType == TOKEN_WORD);
+  return mWord;
+}
+
+template <typename TChar>
+uint64_t TokenizerBase<TChar>::Token::AsInteger() const {
+  MOZ_ASSERT(mType == TOKEN_INTEGER);
+  return mInteger;
+}
+
+template class TokenizerBase<char>;
+template class TokenizerBase<char16_t>;
+
+template class TTokenizer<char>;
+template class TTokenizer<char16_t>;
+
+}  // namespace mozilla
diff --git a/xpcom/ds/Tokenizer.h b/xpcom/ds/Tokenizer.h
new file mode 100644
index 0000000000..713b63f269
--- /dev/null
+++ b/xpcom/ds/Tokenizer.h
@@ -0,0 +1,524 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef Tokenizer_h__
+#define Tokenizer_h__
+
+#include <type_traits>
+
+#include "nsString.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/UniquePtr.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+
+template <typename TChar>
+class TokenizerBase {
+ public:
+  typedef nsTSubstring<TChar> TAString;
+  typedef nsTString<TChar> TString;
+  typedef nsTDependentString<TChar> TDependentString;
+  typedef nsTDependentSubstring<TChar> TDependentSubstring;
+
+  static TChar const sWhitespaces[];
+
+  /**
+   * The analyzer works with elements in the input cut to a sequence of token
+   * where each token has an elementary type
+   */
+  enum TokenType : uint32_t {
+    TOKEN_UNKNOWN,
+    TOKEN_RAW,
+    TOKEN_ERROR,
+    TOKEN_INTEGER,
+    TOKEN_WORD,
+    TOKEN_CHAR,
+    TOKEN_WS,
+    TOKEN_EOL,
+    TOKEN_EOF,
+    TOKEN_CUSTOM0 = 1000
+  };
+
+  enum ECaseSensitivity { CASE_SENSITIVE, CASE_INSENSITIVE };
+
+  /**
+   * Class holding the type and the value of a token.  It can be manually
+   * created to allow checks against it via methods of TTokenizer or are results
+   * of some of the TTokenizer's methods.
+   */
+  class Token {
+    TokenType mType;
+    TDependentSubstring mWord;
+    TString mCustom;
+    TChar mChar;
+    uint64_t mInteger;
+    ECaseSensitivity mCustomCaseInsensitivity;
+    bool mCustomEnabled;
+
+    // If this token is a result of the parsing process, this member is
+    // referencing a sub-string in the input buffer.  If this is externally
+    // created Token this member is left an empty string.
+    TDependentSubstring mFragment;
+
+    friend class TokenizerBase<TChar>;
+    void AssignFragment(typename TAString::const_char_iterator begin,
+                        typename TAString::const_char_iterator end);
+
+    static Token Raw();
+
+   public:
+    Token();
+    Token(const Token& aOther);
+    Token& operator=(const Token& aOther);
+
+    // Static constructors of tokens by type and value
+    static Token Word(TAString const& aWord);
+    static Token Char(TChar const aChar);
+    static Token Number(uint64_t const aNumber);
+    static Token Whitespace();
+    static Token NewLine();
+    static Token EndOfFile();
+    static Token Error();
+
+    // Compares the two tokens, type must be identical and value
+    // of one of the tokens must be 'any' or equal.
+    bool Equals(const Token& aOther) const;
+
+    TokenType Type() const { return mType; }
+    TChar AsChar() const;
+    TDependentSubstring AsString() const;
+    uint64_t AsInteger() const;
+
+    TDependentSubstring Fragment() const { return mFragment; }
+  };
+
+  /**
+   * Consumers may register a custom string that, when found in the input, is
+   * considered a token and returned by Next*() and accepted by Check*()
+   * methods. AddCustomToken() returns a reference to a token that can then be
+   * comapred using Token::Equals() againts the output from Next*() or be passed
+   * to Check*().
+   */
+  Token AddCustomToken(const TAString& aValue,
+                       ECaseSensitivity aCaseInsensitivity,
+                       bool aEnabled = true);
+  template <uint32_t N>
+  Token AddCustomToken(const TChar (&aValue)[N],
+                       ECaseSensitivity aCaseInsensitivity,
+                       bool aEnabled = true) {
+    return AddCustomToken(TDependentSubstring(aValue, N - 1),
+                          aCaseInsensitivity, aEnabled);
+  }
+  void RemoveCustomToken(Token& aToken);
+  /**
+   * Only applies to a custom type of a Token (see AddCustomToken above.)
+   * This turns on and off token recognition.  When a custom token is disabled,
+   * it's ignored as never added as a custom token.
+   */
+  void EnableCustomToken(Token const& aToken, bool aEnable);
+
+  /**
+   * Mode of tokenization.
+   * FULL tokenization, the default, recognizes built-in tokens and any custom
+   * tokens, if added. CUSTOM_ONLY will only recognize custom tokens, the rest
+   * is seen as 'raw'. This mode can be understood as a 'binary' mode.
+   */
+  enum class Mode { FULL, CUSTOM_ONLY };
+  void SetTokenizingMode(Mode aMode);
+
+  /**
+   * Return false iff the last Check*() call has returned false or when we've
+   * read past the end of the input string.
+   */
+  [[nodiscard]] bool HasFailed() const;
+
+ protected:
+  explicit TokenizerBase(const TChar* aWhitespaces = nullptr,
+                         const TChar* aAdditionalWordChars = nullptr);
+
+  // false if we have already read the EOF token.
+  bool HasInput() const;
+  // Main parsing function, it doesn't shift the read cursor, just returns the
+  // next token position.
+  typename TAString::const_char_iterator Parse(Token& aToken) const;
+  // Is read cursor at the end?
+  bool IsEnd(const typename TAString::const_char_iterator& caret) const;
+  // True, when we are at the end of the input data, but it has not been marked
+  // as complete yet.  In that case we cannot proceed with providing a
+  // multi-TChar token.
+  bool IsPending(const typename TAString::const_char_iterator& caret) const;
+  // Is read cursor on a character that is a word start?
+  bool IsWordFirst(const TChar aInput) const;
+  // Is read cursor on a character that is an in-word letter?
+  bool IsWord(const TChar aInput) const;
+  // Is read cursor on a character that is a valid number?
+  // TODO - support multiple radix
+  bool IsNumber(const TChar aInput) const;
+  // Is equal to the given custom token?
+  bool IsCustom(const typename TAString::const_char_iterator& caret,
+                const Token& aCustomToken, uint32_t* aLongest = nullptr) const;
+
+  // Friendly helper to assign a fragment on a Token
+  static void AssignFragment(Token& aToken,
+                             typename TAString::const_char_iterator begin,
+                             typename TAString::const_char_iterator end);
+
+#ifdef DEBUG
+  // This is called from inside Tokenizer methods to make sure the token is
+  // valid.
+  void Validate(Token const& aToken);
+#endif
+
+  // true iff we have already read the EOF token
+  bool mPastEof;
+  // true iff the last Check*() call has returned false, reverts to true on
+  // Rollback() call
+  bool mHasFailed;
+  // true if the input string is final (finished), false when we expect more
+  // data yet to be fed to the tokenizer (see IncrementalTokenizer derived
+  // class).
+  bool mInputFinished;
+  // custom only vs full tokenizing mode, see the Parse() method
+  Mode mMode;
+  // minimal raw data chunked delivery during incremental feed
+  uint32_t mMinRawDelivery;
+
+  // Customizable list of whitespaces
+  const TChar* mWhitespaces;
+  // Additinal custom word characters
+  const TChar* mAdditionalWordChars;
+
+  // All these point to the original buffer passed to the constructor or to the
+  // incremental buffer after FeedInput.
+  typename TAString::const_char_iterator
+      mCursor;  // Position of the current (actually next to read) token start
+  typename TAString::const_char_iterator mEnd;  // End of the input position
+
+  // This is the list of tokens user has registered with AddCustomToken()
+  nsTArray<UniquePtr<Token>> mCustomTokens;
+  uint32_t mNextCustomTokenID;
+
+ private:
+  TokenizerBase() = delete;
+  TokenizerBase(const TokenizerBase&) = delete;
+  TokenizerBase(TokenizerBase&&) = delete;
+  TokenizerBase(const TokenizerBase&&) = delete;
+  TokenizerBase& operator=(const TokenizerBase&) = delete;
+};
+
+/**
+ * This is a simple implementation of a lexical analyzer or maybe better
+ * called a tokenizer.
+ *
+ * Please use Tokenizer or Tokenizer16 classes, that are specializations
+ * of this template class.  Tokenizer is for ASCII input, Tokenizer16 may
+ * handle char16_t input, but doesn't recognize whitespaces or numbers
+ * other than standard `char` specialized Tokenizer class.
+ */
+template <typename TChar>
+class TTokenizer : public TokenizerBase<TChar> {
+ public:
+  typedef TokenizerBase<TChar> base;
+
+  /**
+   * @param aSource
+   *    The string to parse.
+   *    IMPORTANT NOTE: TTokenizer doesn't ensure the input string buffer
+   * lifetime. It's up to the consumer to make sure the string's buffer outlives
+   * the TTokenizer!
+   * @param aWhitespaces
+   *    If non-null TTokenizer will use this custom set of whitespaces for
+   * CheckWhite() and SkipWhites() calls. By default the list consists of space
+   * and tab.
+   * @param aAdditionalWordChars
+   *    If non-null it will be added to the list of characters that consist a
+   * word. This is useful when you want to accept e.g. '-' in HTTP headers. By
+   * default a word character is consider any character for which upper case
+   *    is different from lower case.
+   *
+   * If there is an overlap between aWhitespaces and aAdditionalWordChars, the
+   * check for word characters is made first.
+   */
+  explicit TTokenizer(const typename base::TAString& aSource,
+                      const TChar* aWhitespaces = nullptr,
+                      const TChar* aAdditionalWordChars = nullptr);
+  explicit TTokenizer(const TChar* aSource, const TChar* aWhitespaces = nullptr,
+                      const TChar* aAdditionalWordChars = nullptr);
+
+  /**
+   * When there is still anything to read from the input, tokenize it, store the
+   * token type and value to aToken result and shift the cursor past this just
+   * parsed token.  Each call to Next() reads another token from the input and
+   * shifts the cursor. Returns false if we have passed the end of the input.
+   */
+  [[nodiscard]] bool Next(typename base::Token& aToken);
+
+  /**
+   * Parse the token on the input read cursor position, check its type is equal
+   * to aTokenType and if so, put it into aResult, shift the cursor and return
+   * true.  Otherwise, leave the input read cursor position intact and return
+   * false.
+   */
+  [[nodiscard]] bool Check(const typename base::TokenType aTokenType,
+                           typename base::Token& aResult);
+  /**
+   * Same as above method, just compares both token type and token value passed
+   * in aToken. When both the type and the value equals, shift the cursor and
+   * return true.  Otherwise return false.
+   */
+  [[nodiscard]] bool Check(const typename base::Token& aToken);
+
+  /**
+   * SkipWhites method (below) may also skip new line characters automatically.
+   */
+  enum WhiteSkipping {
+    /**
+     * SkipWhites will only skip what is defined as a white space (default).
+     */
+    DONT_INCLUDE_NEW_LINE = 0,
+    /**
+     * SkipWhites will skip definited white spaces as well as new lines
+     * automatically.
+     */
+    INCLUDE_NEW_LINE = 1
+  };
+
+  /**
+   * Skips any occurence of whitespaces specified in mWhitespaces member,
+   * optionally skip also new lines.
+   */
+  void SkipWhites(WhiteSkipping aIncludeNewLines = DONT_INCLUDE_NEW_LINE);
+
+  /**
+   * Skips all tokens until the given one is found or EOF is hit.  The token
+   * or EOF are next to read.
+   */
+  void SkipUntil(typename base::Token const& aToken);
+
+  // These are mostly shortcuts for the Check() methods above.
+
+  /**
+   * Check whitespace character is present.
+   */
+  [[nodiscard]] bool CheckWhite() { return Check(base::Token::Whitespace()); }
+  /**
+   * Check there is a single character on the read cursor position.  If so,
+   * shift the read cursor position and return true.  Otherwise false.
+   */
+  [[nodiscard]] bool CheckChar(const TChar aChar) {
+    return Check(base::Token::Char(aChar));
+  }
+  /**
+   * This is a customizable version of CheckChar.  aClassifier is a function
+   * called with value of the character on the current input read position.  If
+   * this user function returns true, read cursor is shifted and true returned.
+   * Otherwise false. The user classifiction function is not called when we are
+   * at or past the end and false is immediately returned.
+   */
+  [[nodiscard]] bool CheckChar(bool (*aClassifier)(const TChar aChar));
+  /**
+   * Check for a whole expected word.
+   */
+  [[nodiscard]] bool CheckWord(const typename base::TAString& aWord) {
+    return Check(base::Token::Word(aWord));
+  }
+  /**
+   * Shortcut for literal const word check with compile time length calculation.
+   */
+  template <uint32_t N>
+  [[nodiscard]] bool CheckWord(const TChar (&aWord)[N]) {
+    return Check(
+        base::Token::Word(typename base::TDependentString(aWord, N - 1)));
+  }
+  /**
+   * Helper to check for a string compound of multiple tokens like "foo bar".
+   * The match is binary-exact, a white space or a delimiter character in the
+   * phrase must match exactly the characters in the input.
+   */
+  [[nodiscard]] bool CheckPhrase(const typename base::TAString& aPhrase);
+  template <uint32_t N>
+  [[nodiscard]] bool CheckPhrase(const TChar (&aPhrase)[N]) {
+    return CheckPhrase(typename base::TDependentString(aPhrase, N - 1));
+  }
+  /**
+   * Checks \r, \n or \r\n.
+   */
+  [[nodiscard]] bool CheckEOL() { return Check(base::Token::NewLine()); }
+  /**
+   * Checks we are at the end of the input string reading.  If so, shift past
+   * the end and returns true.  Otherwise does nothing and returns false.
+   */
+  [[nodiscard]] bool CheckEOF() { return Check(base::Token::EndOfFile()); }
+
+  /**
+   * These are shortcuts to obtain the value immediately when the token type
+   * matches.
+   */
+  [[nodiscard]] bool ReadChar(TChar* aValue);
+  [[nodiscard]] bool ReadChar(bool (*aClassifier)(const TChar aChar),
+                              TChar* aValue);
+  [[nodiscard]] bool ReadWord(typename base::TAString& aValue);
+  [[nodiscard]] bool ReadWord(typename base::TDependentSubstring& aValue);
+
+  /**
+   * This is an integer read helper.  It returns false and doesn't move the read
+   * cursor when any of the following happens:
+   *  - the token at the read cursor is not an integer
+   *  - the final number doesn't fit the T type
+   * Otherwise true is returned, aValue is filled with the integral number
+   * and the cursor is moved forward.
+   */
+  template <typename T>
+  [[nodiscard]] bool ReadInteger(T* aValue) {
+    MOZ_RELEASE_ASSERT(aValue);
+
+    typename base::TAString::const_char_iterator rollback = mRollback;
+    typename base::TAString::const_char_iterator cursor = base::mCursor;
+    typename base::Token t;
+    if (!Check(base::TOKEN_INTEGER, t)) {
+      return false;
+    }
+
+    mozilla::CheckedInt<T> checked(t.AsInteger());
+    if (!checked.isValid()) {
+      // Move to a state as if Check() call has failed
+      mRollback = rollback;
+      base::mCursor = cursor;
+      base::mHasFailed = true;
+      return false;
+    }
+
+    *aValue = checked.value();
+    return true;
+  }
+
+  /**
+   * Same as above, but accepts an integer with an optional minus sign.
+   */
+  template <typename T, typename V = std::enable_if_t<
+                            std::is_signed_v<std::remove_pointer_t<T>>,
+                            std::remove_pointer_t<T>>>
+  [[nodiscard]] bool ReadSignedInteger(T* aValue) {
+    MOZ_RELEASE_ASSERT(aValue);
+
+    typename base::TAString::const_char_iterator rollback = mRollback;
+    typename base::TAString::const_char_iterator cursor = base::mCursor;
+    auto revert = MakeScopeExit([&] {
+      // Move to a state as if Check() call has failed
+      mRollback = rollback;
+      base::mCursor = cursor;
+      base::mHasFailed = true;
+    });
+
+    // Using functional raw access because '-' could be part of the word set
+    // making CheckChar('-') not work.
+    bool minus = CheckChar([](const TChar aChar) { return aChar == '-'; });
+
+    typename base::Token t;
+    if (!Check(base::TOKEN_INTEGER, t)) {
+      return false;
+    }
+
+    mozilla::CheckedInt<T> checked(t.AsInteger());
+    if (minus) {
+      checked *= -1;
+    }
+
+    if (!checked.isValid()) {
+      return false;
+    }
+
+    *aValue = checked.value();
+    revert.release();
+    return true;
+  }
+
+  /**
+   * Returns the read cursor position back as it was before the last call of any
+   * parsing method of TTokenizer (Next, Check*, Skip*, Read*) so that the last
+   * operation can be repeated. Rollback cannot be used multiple times, it only
+   * reverts the last successfull parse operation.  It also cannot be used
+   * before any parsing operation has been called on the TTokenizer.
+   */
+  void Rollback();
+
+  /**
+   * Record() and Claim() are collecting the input as it is being parsed to
+   * obtain a substring between particular syntax bounderies defined by any
+   * recursive descent parser or simple parser the TTokenizer is used to read
+   * the input for. Inlucsion of a token that has just been parsed can be
+   * controlled using an arguemnt.
+   */
+  enum ClaimInclusion {
+    /**
+     * Include resulting (or passed) token of the last lexical analyzer
+     * operation in the result.
+     */
+    INCLUDE_LAST,
+    /**
+     * Do not include it.
+     */
+    EXCLUDE_LAST
+  };
+
+  /**
+   * Start the process of recording.  Based on aInclude value the begining of
+   * the recorded sub-string is at the current position (EXCLUDE_LAST) or at the
+   * position before the last parsed token (INCLUDE_LAST).
+   */
+  void Record(ClaimInclusion aInclude = EXCLUDE_LAST);
+  /**
+   * Claim result of the record started with Record() call before.  Depending on
+   * aInclude the ending of the sub-string result includes or excludes the last
+   * parsed or checked token.
+   */
+  void Claim(typename base::TAString& aResult,
+             ClaimInclusion aInclude = EXCLUDE_LAST);
+  void Claim(typename base::TDependentSubstring& aResult,
+             ClaimInclusion aInclude = EXCLUDE_LAST);
+
+  /**
+   * If aToken is found, aResult is set to the substring between the current
+   * position and the position of aToken, potentially including aToken depending
+   * on aInclude.
+   * If aToken isn't found aResult is set to the substring between the current
+   * position and the end of the string.
+   * If aToken is found, the method returns true. Otherwise it returns false.
+   *
+   * Calling Rollback() after ReadUntil() will return the read cursor to the
+   * position it had before ReadUntil was called.
+   */
+  [[nodiscard]] bool ReadUntil(typename base::Token const& aToken,
+                               typename base::TDependentSubstring& aResult,
+                               ClaimInclusion aInclude = EXCLUDE_LAST);
+  [[nodiscard]] bool ReadUntil(typename base::Token const& aToken,
+                               typename base::TAString& aResult,
+                               ClaimInclusion aInclude = EXCLUDE_LAST);
+
+ protected:
+  // All these point to the original buffer passed to the TTokenizer's
+  // constructor
+  typename base::TAString::const_char_iterator
+      mRecord;  // Position where the recorded sub-string for Claim() is
+  typename base::TAString::const_char_iterator
+      mRollback;  // Position of the previous token start
+
+ private:
+  TTokenizer() = delete;
+  TTokenizer(const TTokenizer&) = delete;
+  TTokenizer(TTokenizer&&) = delete;
+  TTokenizer(const TTokenizer&&) = delete;
+  TTokenizer& operator=(const TTokenizer&) = delete;
+};
+
+typedef TTokenizer<char> Tokenizer;
+typedef TTokenizer<char16_t> Tokenizer16;
+
+}  // namespace mozilla
+
+#endif  // Tokenizer_h__
diff --git a/xpcom/ds/components.conf b/xpcom/ds/components.conf
new file mode 100644
index 0000000000..1b600352b3
--- /dev/null
+++ b/xpcom/ds/components.conf
@@ -0,0 +1,24 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+Classes = [
+    {
+        'cid': '{35c66fd1-95e9-4e0a-80c5-c3bd2b375481}',
+        'contract_ids': ['@mozilla.org/array;1'],
+        'legacy_constructor': 'nsArrayBase::XPCOMConstructor',
+        'headers': ['nsArray.h'],
+    },
+    {
+        'name': 'Observer',
+        'js_name': 'obs',
+        'cid': '{d07f5195-e3d1-11d2-8acd-00105a1b8860}',
+        'contract_ids': ['@mozilla.org/observer-service;1'],
+        'interfaces': ['nsIObserverService'],
+        'legacy_constructor': 'nsObserverService::Create',
+        'headers': ['/xpcom/ds/nsObserverService.h'],
+        'processes': ProcessSelector.ALLOW_IN_GPU_RDD_VR_SOCKET_AND_UTILITY_PROCESS,
+    },
+]
diff --git a/xpcom/ds/moz.build b/xpcom/ds/moz.build
new file mode 100644
index 0000000000..1d0488b9b2
--- /dev/null
+++ b/xpcom/ds/moz.build
@@ -0,0 +1,157 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+XPIDL_SOURCES += [
+    "nsIArray.idl",
+    "nsIArrayExtensions.idl",
+    "nsIINIParser.idl",
+    "nsIMutableArray.idl",
+    "nsIObserver.idl",
+    "nsIObserverService.idl",
+    "nsIPersistentProperties2.idl",
+    "nsIProperties.idl",
+    "nsIProperty.idl",
+    "nsIPropertyBag.idl",
+    "nsIPropertyBag2.idl",
+    "nsISerializable.idl",
+    "nsISimpleEnumerator.idl",
+    "nsIStringEnumerator.idl",
+    "nsISupportsIterators.idl",
+    "nsISupportsPrimitives.idl",
+    "nsIVariant.idl",
+    "nsIWritablePropertyBag.idl",
+    "nsIWritablePropertyBag2.idl",
+]
+
+if CONFIG["OS_ARCH"] == "WINNT":
+    XPIDL_SOURCES += [
+        "nsIWindowsRegKey.idl",
+    ]
+    EXPORTS += ["nsWindowsRegKey.h"]
+    SOURCES += ["nsWindowsRegKey.cpp"]
+
+XPIDL_MODULE = "xpcom_ds"
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+EXPORTS += [
+    "!nsGkAtomConsts.h",
+    "!nsGkAtomList.h",
+    "nsArray.h",
+    "nsArrayEnumerator.h",
+    "nsArrayUtils.h",
+    "nsAtom.h",
+    "nsBaseHashtable.h",
+    "nsCharSeparatedTokenizer.h",
+    "nsCheapSets.h",
+    "nsClassHashtable.h",
+    "nsCOMArray.h",
+    "nsCRT.h",
+    "nsDeque.h",
+    "nsEnumeratorUtils.h",
+    "nsExpirationTracker.h",
+    "nsGkAtoms.h",
+    "nsHashKeys.h",
+    "nsHashPropertyBag.h",
+    "nsHashtablesFwd.h",
+    "nsInterfaceHashtable.h",
+    "nsMathUtils.h",
+    "nsPersistentProperties.h",
+    "nsPointerHashKeys.h",
+    "nsProperties.h",
+    "nsQuickSort.h",
+    "nsRefCountedHashtable.h",
+    "nsRefPtrHashtable.h",
+    "nsSimpleEnumerator.h",
+    "nsStaticAtomUtils.h",
+    "nsStaticNameTable.h",
+    "nsStringEnumerator.h",
+    "nsSupportsPrimitives.h",
+    "nsTArray-inl.h",
+    "nsTArray.h",
+    "nsTArrayForwardDeclare.h",
+    "nsTHashMap.h",
+    "nsTHashSet.h",
+    "nsTHashtable.h",
+    "nsTObserverArray.h",
+    "nsTPriorityQueue.h",
+    "nsVariant.h",
+    "nsWhitespaceTokenizer.h",
+    "PLDHashTable.h",
+]
+
+EXPORTS.mozilla += [
+    "ArenaAllocator.h",
+    "ArenaAllocatorExtensions.h",
+    "ArrayAlgorithm.h",
+    "ArrayIterator.h",
+    "AtomArray.h",
+    "Dafsa.h",
+    "IncrementalTokenizer.h",
+    "MruCache.h",
+    "Observer.h",
+    "PerfectHash.h",
+    "SimpleEnumerator.h",
+    "StickyTimeDuration.h",
+    "Tokenizer.h",
+]
+
+UNIFIED_SOURCES += [
+    "Dafsa.cpp",
+    "IncrementalTokenizer.cpp",
+    "nsArray.cpp",
+    "nsArrayEnumerator.cpp",
+    "nsArrayUtils.cpp",
+    "nsAtomTable.cpp",
+    "nsCharSeparatedTokenizer.cpp",
+    "nsCOMArray.cpp",
+    "nsCRT.cpp",
+    "nsDeque.cpp",
+    "nsEnumeratorUtils.cpp",
+    "nsGkAtoms.cpp",
+    "nsHashPropertyBag.cpp",
+    "nsINIParserImpl.cpp",
+    "nsObserverList.cpp",
+    "nsObserverService.cpp",
+    "nsPersistentProperties.cpp",
+    "nsProperties.cpp",
+    "nsQuickSort.cpp",
+    "nsSimpleEnumerator.cpp",
+    "nsStaticNameTable.cpp",
+    "nsStringEnumerator.cpp",
+    "nsSupportsPrimitives.cpp",
+    "nsTArray.cpp",
+    "nsTObserverArray.cpp",
+    "nsVariant.cpp",
+    "PLDHashTable.cpp",
+    "Tokenizer.cpp",
+]
+
+LOCAL_INCLUDES += [
+    "../io",
+]
+
+GeneratedFile(
+    "nsGkAtomList.h",
+    script="StaticAtoms.py",
+    entry_point="generate_nsgkatomlist_h",
+    inputs=["Atom.py", "HTMLAtoms.py"],
+)
+
+GeneratedFile(
+    "nsGkAtomConsts.h",
+    script="StaticAtoms.py",
+    entry_point="generate_nsgkatomconsts_h",
+    inputs=["Atom.py", "HTMLAtoms.py"],
+)
+
+FINAL_LIBRARY = "xul"
+
+PYTHON_UNITTEST_MANIFESTS += [
+    "test/python.ini",
+]
diff --git a/xpcom/ds/nsArray.cpp b/xpcom/ds/nsArray.cpp
new file mode 100644
index 0000000000..60758133d9
--- /dev/null
+++ b/xpcom/ds/nsArray.cpp
@@ -0,0 +1,146 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsArray.h"
+#include "nsArrayEnumerator.h"
+#include "nsThreadUtils.h"
+
+NS_INTERFACE_MAP_BEGIN(nsArray)
+  NS_INTERFACE_MAP_ENTRY(nsIArray)
+  NS_INTERFACE_MAP_ENTRY(nsIArrayExtensions)
+  NS_INTERFACE_MAP_ENTRY(nsIMutableArray)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIMutableArray)
+NS_INTERFACE_MAP_END
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsArrayCC)
+  NS_INTERFACE_MAP_ENTRY(nsIArray)
+  NS_INTERFACE_MAP_ENTRY(nsIArrayExtensions)
+  NS_INTERFACE_MAP_ENTRY(nsIMutableArray)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIMutableArray)
+NS_INTERFACE_MAP_END
+
+nsArrayBase::~nsArrayBase() { Clear(); }
+
+NS_IMPL_ADDREF(nsArray)
+NS_IMPL_RELEASE(nsArray)
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsArrayCC)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsArrayCC)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsArrayCC)
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsArrayCC)
+  tmp->Clear();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsArrayCC)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mArray)
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMETHODIMP
+nsArrayBase::GetLength(uint32_t* aLength) {
+  *aLength = mArray.Count();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsArrayBase::QueryElementAt(uint32_t aIndex, const nsIID& aIID,
+                            void** aResult) {
+  nsISupports* obj = mArray.SafeObjectAt(aIndex);
+  if (!obj) {
+    return NS_ERROR_ILLEGAL_VALUE;
+  }
+
+  // no need to worry about a leak here, because SafeObjectAt()
+  // doesn't addref its result
+  return obj->QueryInterface(aIID, aResult);
+}
+
+NS_IMETHODIMP
+nsArrayBase::IndexOf(uint32_t aStartIndex, nsISupports* aElement,
+                     uint32_t* aResult) {
+  int32_t idx = mArray.IndexOf(aElement, aStartIndex);
+  if (idx == -1) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aResult = static_cast<uint32_t>(idx);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsArrayBase::ScriptedEnumerate(const nsIID& aElemIID, uint8_t aArgc,
+                               nsISimpleEnumerator** aResult) {
+  if (aArgc > 0) {
+    return NS_NewArrayEnumerator(aResult, static_cast<nsIArray*>(this),
+                                 aElemIID);
+  }
+  return NS_NewArrayEnumerator(aResult, static_cast<nsIArray*>(this));
+}
+
+NS_IMETHODIMP
+nsArrayBase::EnumerateImpl(const nsID& aElemIID,
+                           nsISimpleEnumerator** aResult) {
+  return NS_NewArrayEnumerator(aResult, static_cast<nsIArray*>(this), aElemIID);
+}
+
+// nsIMutableArray implementation
+
+NS_IMETHODIMP
+nsArrayBase::AppendElement(nsISupports* aElement) {
+  bool result = mArray.AppendObject(aElement);
+  return result ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsArrayBase::RemoveElementAt(uint32_t aIndex) {
+  bool result = mArray.RemoveObjectAt(aIndex);
+  return result ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsArrayBase::InsertElementAt(nsISupports* aElement, uint32_t aIndex) {
+  bool result = mArray.InsertObjectAt(aElement, aIndex);
+  return result ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsArrayBase::ReplaceElementAt(nsISupports* aElement, uint32_t aIndex) {
+  mArray.ReplaceObjectAt(aElement, aIndex);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsArrayBase::Clear() {
+  mArray.Clear();
+  return NS_OK;
+}
+
+// nsIArrayExtensions implementation.
+
+NS_IMETHODIMP
+nsArrayBase::Count(uint32_t* aResult) { return GetLength(aResult); }
+
+NS_IMETHODIMP
+nsArrayBase::GetElementAt(uint32_t aIndex, nsISupports** aResult) {
+  nsCOMPtr<nsISupports> obj = mArray.SafeObjectAt(aIndex);
+  obj.forget(aResult);
+  return NS_OK;
+}
+
+nsresult nsArrayBase::XPCOMConstructor(const nsIID& aIID, void** aResult) {
+  nsCOMPtr<nsIMutableArray> inst = Create();
+  return inst->QueryInterface(aIID, aResult);
+}
+
+already_AddRefed<nsIMutableArray> nsArrayBase::Create() {
+  nsCOMPtr<nsIMutableArray> inst;
+  if (NS_IsMainThread()) {
+    inst = new nsArrayCC;
+  } else {
+    inst = new nsArray;
+  }
+  return inst.forget();
+}
diff --git a/xpcom/ds/nsArray.h b/xpcom/ds/nsArray.h
new file mode 100644
index 0000000000..49d701d466
--- /dev/null
+++ b/xpcom/ds/nsArray.h
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsArray_h__
+#define nsArray_h__
+
+#include "nsIMutableArray.h"
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsCycleCollectionParticipant.h"
+#include "mozilla/Attributes.h"
+
+// {35C66FD1-95E9-4e0a-80C5-C3BD2B375481}
+#define NS_ARRAY_CID                                 \
+  {                                                  \
+    0x35c66fd1, 0x95e9, 0x4e0a, {                    \
+      0x80, 0xc5, 0xc3, 0xbd, 0x2b, 0x37, 0x54, 0x81 \
+    }                                                \
+  }
+
+// nsArray without any refcounting declarations
+class nsArrayBase : public nsIMutableArray {
+ public:
+  NS_DECL_NSIARRAY
+  NS_DECL_NSIARRAYEXTENSIONS
+  NS_DECL_NSIMUTABLEARRAY
+
+  /* Both of these factory functions create a cycle-collectable array
+     on the main thread and a non-cycle-collectable array on other
+     threads.  */
+  static already_AddRefed<nsIMutableArray> Create();
+  /* Only for the benefit of the XPCOM module system, use Create()
+     instead.  */
+  static nsresult XPCOMConstructor(const nsIID& aIID, void** aResult);
+
+ protected:
+  nsArrayBase() = default;
+  nsArrayBase(const nsArrayBase& aOther);
+  explicit nsArrayBase(const nsCOMArray_base& aBaseArray)
+      : mArray(aBaseArray) {}
+  virtual ~nsArrayBase();
+
+  nsCOMArray_base mArray;
+};
+
+class nsArray final : public nsArrayBase {
+  friend class nsArrayBase;
+
+ public:
+  NS_DECL_ISUPPORTS
+
+ private:
+  nsArray() : nsArrayBase() {}
+  nsArray(const nsArray& aOther);
+  explicit nsArray(const nsCOMArray_base& aBaseArray)
+      : nsArrayBase(aBaseArray) {}
+  ~nsArray() = default;
+};
+
+class nsArrayCC final : public nsArrayBase {
+  friend class nsArrayBase;
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(nsArrayCC, nsIMutableArray)
+
+ private:
+  nsArrayCC() : nsArrayBase() {}
+  nsArrayCC(const nsArrayCC& aOther);
+  explicit nsArrayCC(const nsCOMArray_base& aBaseArray)
+      : nsArrayBase(aBaseArray) {}
+  ~nsArrayCC() = default;
+};
+
+#endif
diff --git a/xpcom/ds/nsArrayEnumerator.cpp b/xpcom/ds/nsArrayEnumerator.cpp
new file mode 100644
index 0000000000..ec475f9e5b
--- /dev/null
+++ b/xpcom/ds/nsArrayEnumerator.cpp
@@ -0,0 +1,213 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Attributes.h"
+
+#include "nsArrayEnumerator.h"
+
+#include "nsIArray.h"
+#include "nsSimpleEnumerator.h"
+
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/RefPtr.h"
+
+class nsSimpleArrayEnumerator final : public nsSimpleEnumerator {
+ public:
+  // nsISimpleEnumerator interface
+  NS_DECL_NSISIMPLEENUMERATOR
+
+  // nsSimpleArrayEnumerator methods
+  explicit nsSimpleArrayEnumerator(nsIArray* aValueArray, const nsID& aEntryIID)
+      : mValueArray(aValueArray), mEntryIID(aEntryIID), mIndex(0) {}
+
+  const nsID& DefaultInterface() override { return mEntryIID; }
+
+ private:
+  ~nsSimpleArrayEnumerator() override = default;
+
+ protected:
+  nsCOMPtr<nsIArray> mValueArray;
+  const nsID mEntryIID;
+  uint32_t mIndex;
+};
+
+NS_IMETHODIMP
+nsSimpleArrayEnumerator::HasMoreElements(bool* aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  if (!mValueArray) {
+    *aResult = false;
+    return NS_OK;
+  }
+
+  uint32_t cnt;
+  nsresult rv = mValueArray->GetLength(&cnt);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  *aResult = (mIndex < cnt);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSimpleArrayEnumerator::GetNext(nsISupports** aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  if (!mValueArray) {
+    *aResult = nullptr;
+    return NS_OK;
+  }
+
+  uint32_t cnt;
+  nsresult rv = mValueArray->GetLength(&cnt);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  if (mIndex >= cnt) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  return mValueArray->QueryElementAt(mIndex++, NS_GET_IID(nsISupports),
+                                     (void**)aResult);
+}
+
+nsresult NS_NewArrayEnumerator(nsISimpleEnumerator** aResult, nsIArray* aArray,
+                               const nsID& aEntryIID) {
+  RefPtr<nsSimpleArrayEnumerator> enumer =
+      new nsSimpleArrayEnumerator(aArray, aEntryIID);
+  enumer.forget(aResult);
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+// enumerator implementation for nsCOMArray
+// creates a snapshot of the array in question
+// you MUST use NS_NewArrayEnumerator to create this, so that
+// allocation is done correctly
+class nsCOMArrayEnumerator final : public nsSimpleEnumerator {
+ public:
+  // nsISimpleEnumerator interface
+  NS_DECL_NSISIMPLEENUMERATOR
+
+  // Use this instead of `new`.
+  static nsCOMArrayEnumerator* Allocate(const nsCOMArray_base& aArray,
+                                        const nsID& aEntryIID);
+
+  // specialized operator to make sure we make room for mValues
+  void operator delete(void* aPtr) { free(aPtr); }
+
+  const nsID& DefaultInterface() override { return mEntryIID; }
+
+ private:
+  // nsSimpleArrayEnumerator methods
+  explicit nsCOMArrayEnumerator(const nsID& aEntryIID)
+      : mIndex(0), mArraySize(0), mEntryIID(aEntryIID) {
+    mValueArray[0] = nullptr;
+  }
+
+  ~nsCOMArrayEnumerator(void) override;
+
+ protected:
+  uint32_t mIndex;      // current position
+  uint32_t mArraySize;  // size of the array
+
+  const nsID& mEntryIID;
+
+  // this is actually bigger
+  nsISupports* mValueArray[1];
+};
+
+nsCOMArrayEnumerator::~nsCOMArrayEnumerator() {
+  // only release the entries that we haven't visited yet
+  for (; mIndex < mArraySize; ++mIndex) {
+    NS_IF_RELEASE(mValueArray[mIndex]);
+  }
+}
+
+NS_IMETHODIMP
+nsCOMArrayEnumerator::HasMoreElements(bool* aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  *aResult = (mIndex < mArraySize);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsCOMArrayEnumerator::GetNext(nsISupports** aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  if (mIndex >= mArraySize) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // pass the ownership of the reference to the caller. Since
+  // we AddRef'ed during creation of |this|, there is no need
+  // to AddRef here
+  *aResult = mValueArray[mIndex++];
+
+  // this really isn't necessary. just pretend this happens, since
+  // we'll never visit this value again!
+  // mValueArray[(mIndex-1)] = nullptr;
+
+  return NS_OK;
+}
+
+nsCOMArrayEnumerator* nsCOMArrayEnumerator::Allocate(
+    const nsCOMArray_base& aArray, const nsID& aEntryIID) {
+  // create enough space such that mValueArray points to a large
+  // enough value. Note that the initial value of aSize gives us
+  // space for mValueArray[0], so we must subtract
+  size_t size = sizeof(nsCOMArrayEnumerator);
+  uint32_t count;
+  if (aArray.Count() > 0) {
+    count = static_cast<uint32_t>(aArray.Count());
+    size += (count - 1) * sizeof(aArray[0]);
+  } else {
+    count = 0;
+  }
+
+  // Allocate a buffer large enough to contain our object and its array.
+  void* mem = moz_xmalloc(size);
+  auto result =
+      new (mozilla::KnownNotNull, mem) nsCOMArrayEnumerator(aEntryIID);
+
+  result->mArraySize = count;
+
+  // now need to copy over the values, and addref each one
+  // now this might seem like a lot of work, but we're actually just
+  // doing all our AddRef's ahead of time since GetNext() doesn't
+  // need to AddRef() on the way out
+  for (uint32_t i = 0; i < count; ++i) {
+    result->mValueArray[i] = aArray[i];
+    NS_IF_ADDREF(result->mValueArray[i]);
+  }
+
+  return result;
+}
+
+nsresult NS_NewArrayEnumerator(nsISimpleEnumerator** aResult,
+                               const nsCOMArray_base& aArray,
+                               const nsID& aEntryIID) {
+  RefPtr<nsCOMArrayEnumerator> enumerator =
+      nsCOMArrayEnumerator::Allocate(aArray, aEntryIID);
+  enumerator.forget(aResult);
+  return NS_OK;
+}
diff --git a/xpcom/ds/nsArrayEnumerator.h b/xpcom/ds/nsArrayEnumerator.h
new file mode 100644
index 0000000000..a406a9ea66
--- /dev/null
+++ b/xpcom/ds/nsArrayEnumerator.h
@@ -0,0 +1,31 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsArrayEnumerator_h__
+#define nsArrayEnumerator_h__
+
+// enumerator implementation for nsIArray
+
+#include "nsISupports.h"
+
+class nsISimpleEnumerator;
+class nsIArray;
+class nsCOMArray_base;
+
+// Create an enumerator for an existing nsIArray implementation
+// The enumerator holds an owning reference to the array.
+nsresult NS_NewArrayEnumerator(nsISimpleEnumerator** aResult, nsIArray* aArray,
+                               const nsID& aEntryIID = NS_GET_IID(nsISupports));
+
+// create an enumerator for an existing nsCOMArray<T> implementation
+// The enumerator will hold an owning reference to each ELEMENT in
+// the array. This means that the nsCOMArray<T> can safely go away
+// without its objects going away.
+nsresult NS_NewArrayEnumerator(nsISimpleEnumerator** aResult,
+                               const nsCOMArray_base& aArray,
+                               const nsID& aEntryIID = NS_GET_IID(nsISupports));
+
+#endif
diff --git a/xpcom/ds/nsArrayUtils.cpp b/xpcom/ds/nsArrayUtils.cpp
new file mode 100644
index 0000000000..e8ea8bd0f2
--- /dev/null
+++ b/xpcom/ds/nsArrayUtils.cpp
@@ -0,0 +1,22 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsArrayUtils.h"
+
+//
+// do_QueryElementAt helper stuff
+//
+nsresult nsQueryArrayElementAt::operator()(const nsIID& aIID,
+                                           void** aResult) const {
+  nsresult status = mArray ? mArray->QueryElementAt(mIndex, aIID, aResult)
+                           : NS_ERROR_NULL_POINTER;
+
+  if (mErrorPtr) {
+    *mErrorPtr = status;
+  }
+
+  return status;
+}
diff --git a/xpcom/ds/nsArrayUtils.h b/xpcom/ds/nsArrayUtils.h
new file mode 100644
index 0000000000..6adad119c7
--- /dev/null
+++ b/xpcom/ds/nsArrayUtils.h
@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsArrayUtils_h__
+#define nsArrayUtils_h__
+
+#include "nsCOMPtr.h"
+#include "nsIArray.h"
+
+// helper class for do_QueryElementAt
+class MOZ_STACK_CLASS nsQueryArrayElementAt final : public nsCOMPtr_helper {
+ public:
+  nsQueryArrayElementAt(nsIArray* aArray, uint32_t aIndex, nsresult* aErrorPtr)
+      : mArray(aArray), mIndex(aIndex), mErrorPtr(aErrorPtr) {}
+
+  virtual nsresult NS_FASTCALL operator()(const nsIID& aIID,
+                                          void**) const override;
+
+ private:
+  nsIArray* MOZ_NON_OWNING_REF mArray;
+  uint32_t mIndex;
+  nsresult* mErrorPtr;
+};
+
+inline const nsQueryArrayElementAt do_QueryElementAt(nsIArray* aArray,
+                                                     uint32_t aIndex,
+                                                     nsresult* aErrorPtr = 0) {
+  return nsQueryArrayElementAt(aArray, aIndex, aErrorPtr);
+}
+
+#endif  // nsArrayUtils_h__
diff --git a/xpcom/ds/nsAtom.h b/xpcom/ds/nsAtom.h
new file mode 100644
index 0000000000..0e45db208f
--- /dev/null
+++ b/xpcom/ds/nsAtom.h
@@ -0,0 +1,297 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsAtom_h
+#define nsAtom_h
+
+#include <type_traits>
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Char16.h"
+#include "mozilla/MemoryReporting.h"
+#include "nsISupports.h"
+#include "nsString.h"
+
+namespace mozilla {
+struct AtomsSizes;
+}  // namespace mozilla
+
+class nsStaticAtom;
+class nsDynamicAtom;
+
+// This class encompasses both static and dynamic atoms.
+//
+// - In places where static and dynamic atoms can be used, use RefPtr<nsAtom>.
+//   This is by far the most common case.
+//
+// - In places where only static atoms can appear, use nsStaticAtom* to avoid
+//   unnecessary refcounting. This is a moderately common case.
+//
+// - In places where only dynamic atoms can appear, it doesn't matter much
+//   whether you use RefPtr<nsAtom> or RefPtr<nsDynamicAtom>. This is an
+//   extremely rare case.
+//
+class nsAtom {
+ public:
+  void AddSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf,
+                              mozilla::AtomsSizes& aSizes) const;
+
+  bool Equals(char16ptr_t aString, uint32_t aLength) const {
+    return mLength == aLength &&
+           memcmp(GetUTF16String(), aString, mLength * sizeof(char16_t)) == 0;
+  }
+
+  bool Equals(const nsAString& aString) const {
+    return Equals(aString.BeginReading(), aString.Length());
+  }
+
+  bool IsStatic() const { return mIsStatic; }
+  bool IsDynamic() const { return !IsStatic(); }
+
+  inline const nsStaticAtom* AsStatic() const;
+  inline const nsDynamicAtom* AsDynamic() const;
+  inline nsDynamicAtom* AsDynamic();
+
+  char16ptr_t GetUTF16String() const;
+
+  uint32_t GetLength() const { return mLength; }
+
+  operator mozilla::Span<const char16_t>() const {
+    // Explicitly specify template argument here to avoid instantiating
+    // Span<char16_t> first and then implicitly converting to Span<const
+    // char16_t>
+    return mozilla::Span<const char16_t>{GetUTF16String(), GetLength()};
+  }
+
+  void ToString(nsAString& aString) const;
+  void ToUTF8String(nsACString& aString) const;
+
+  // A hashcode that is better distributed than the actual atom pointer, for
+  // use in situations that need a well-distributed hashcode. It's called hash()
+  // rather than Hash() so we can use mozilla::BloomFilter<N, nsAtom>, because
+  // BloomFilter requires elements to implement a function called hash().
+  //
+  uint32_t hash() const { return mHash; }
+
+  // This function returns true if ToLowercaseASCII would return the string
+  // unchanged.
+  bool IsAsciiLowercase() const { return mIsAsciiLowercase; }
+
+  // This function returns true if this is the empty atom. This is exactly
+  // equivalent to `this == nsGkAtoms::_empty`, but it's a bit less foot-gunny,
+  // since we also have `nsGkAtoms::empty`.
+  //
+  // Defined in nsGkAtoms.h
+  inline bool IsEmpty() const;
+
+  // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
+  // of this type is special.
+  inline MozExternalRefCountType AddRef();
+  inline MozExternalRefCountType Release();
+
+  using HasThreadSafeRefCnt = std::true_type;
+
+ protected:
+  // Used by nsStaticAtom.
+  constexpr nsAtom(uint32_t aLength, uint32_t aHash, bool aIsAsciiLowercase)
+      : mLength(aLength),
+        mIsStatic(true),
+        mIsAsciiLowercase(aIsAsciiLowercase),
+        mHash(aHash) {}
+
+  // Used by nsDynamicAtom.
+  nsAtom(const nsAString& aString, uint32_t aHash, bool aIsAsciiLowercase)
+      : mLength(aString.Length()),
+        mIsStatic(false),
+        mIsAsciiLowercase(aIsAsciiLowercase),
+        mHash(aHash) {}
+
+  ~nsAtom() = default;
+
+  const uint32_t mLength : 30;
+  const uint32_t mIsStatic : 1;
+  const uint32_t mIsAsciiLowercase : 1;
+  const uint32_t mHash;
+};
+
+// This class would be |final| if it wasn't for nsCSSAnonBoxPseudoStaticAtom
+// and nsCSSPseudoElementStaticAtom, which are trivial subclasses used to
+// ensure only certain static atoms are passed to certain functions.
+class nsStaticAtom : public nsAtom {
+ public:
+  // These are deleted so it's impossible to RefPtr<nsStaticAtom>. Raw
+  // nsStaticAtom pointers should be used instead.
+  MozExternalRefCountType AddRef() = delete;
+  MozExternalRefCountType Release() = delete;
+
+  // The static atom's precomputed hash value is an argument here, but it
+  // must be the same as would be computed by mozilla::HashString(aStr),
+  // which is what we use when atomizing strings. We compute this hash in
+  // Atom.py and assert in nsAtomTable::RegisterStaticAtoms that the two
+  // hashes match.
+  constexpr nsStaticAtom(uint32_t aLength, uint32_t aHash,
+                         uint32_t aStringOffset, bool aIsAsciiLowercase)
+      : nsAtom(aLength, aHash, aIsAsciiLowercase),
+        mStringOffset(aStringOffset) {}
+
+  const char16_t* String() const {
+    return reinterpret_cast<const char16_t*>(uintptr_t(this) - mStringOffset);
+  }
+
+  already_AddRefed<nsAtom> ToAddRefed() {
+    return already_AddRefed<nsAtom>(static_cast<nsAtom*>(this));
+  }
+
+ private:
+  // This is an offset to the string chars, which must be at a lower address in
+  // memory.
+  uint32_t mStringOffset;
+};
+
+class nsDynamicAtom : public nsAtom {
+ public:
+  // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
+  // of this type is special.
+  MozExternalRefCountType AddRef() {
+    MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+    nsrefcnt count = ++mRefCnt;
+    if (count == 1) {
+      gUnusedAtomCount--;
+    }
+    return count;
+  }
+
+  MozExternalRefCountType Release() {
+#ifdef DEBUG
+    // We set a lower GC threshold for atoms in debug builds so that we exercise
+    // the GC machinery more often.
+    static const int32_t kAtomGCThreshold = 20;
+#else
+    static const int32_t kAtomGCThreshold = 10000;
+#endif
+
+    MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
+    nsrefcnt count = --mRefCnt;
+    if (count == 0) {
+      if (++gUnusedAtomCount >= kAtomGCThreshold) {
+        GCAtomTable();
+      }
+    }
+
+    return count;
+  }
+
+  const char16_t* String() const {
+    return reinterpret_cast<const char16_t*>(this + 1);
+  }
+
+  static nsDynamicAtom* FromChars(char16_t* chars) {
+    return reinterpret_cast<nsDynamicAtom*>(chars) - 1;
+  }
+
+ private:
+  friend class nsAtomTable;
+  friend class nsAtomSubTable;
+  friend int32_t NS_GetUnusedAtomCount();
+
+  static mozilla::Atomic<int32_t, mozilla::ReleaseAcquire> gUnusedAtomCount;
+  static void GCAtomTable();
+
+  // These shouldn't be used directly, even by friend classes. The
+  // Create()/Destroy() methods use them.
+  nsDynamicAtom(const nsAString& aString, uint32_t aHash,
+                bool aIsAsciiLowercase);
+  ~nsDynamicAtom() = default;
+
+  static nsDynamicAtom* Create(const nsAString& aString, uint32_t aHash);
+  static void Destroy(nsDynamicAtom* aAtom);
+
+  mozilla::ThreadSafeAutoRefCnt mRefCnt;
+
+  // The atom's chars are stored at the end of the struct.
+};
+
+const nsStaticAtom* nsAtom::AsStatic() const {
+  MOZ_ASSERT(IsStatic());
+  return static_cast<const nsStaticAtom*>(this);
+}
+
+const nsDynamicAtom* nsAtom::AsDynamic() const {
+  MOZ_ASSERT(IsDynamic());
+  return static_cast<const nsDynamicAtom*>(this);
+}
+
+nsDynamicAtom* nsAtom::AsDynamic() {
+  MOZ_ASSERT(IsDynamic());
+  return static_cast<nsDynamicAtom*>(this);
+}
+
+MozExternalRefCountType nsAtom::AddRef() {
+  return IsStatic() ? 2 : AsDynamic()->AddRef();
+}
+
+MozExternalRefCountType nsAtom::Release() {
+  return IsStatic() ? 1 : AsDynamic()->Release();
+}
+
+// The four forms of NS_Atomize (for use with |RefPtr<nsAtom>|) return the
+// atom for the string given. At any given time there will always be one atom
+// representing a given string. Atoms are intended to make string comparison
+// cheaper by simplifying it to pointer equality. A pointer to the atom that
+// does not own a reference is not guaranteed to be valid.
+
+// Find an atom that matches the given UTF-8 string. The string is assumed to
+// be zero terminated. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const char* aUTF8String);
+
+// Find an atom that matches the given UTF-8 string. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const nsACString& aUTF8String);
+
+// Find an atom that matches the given UTF-16 string. The string is assumed to
+// be zero terminated. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String);
+
+// Find an atom that matches the given UTF-16 string. Never returns null.
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String);
+
+// An optimized version of the method above for the main thread.
+already_AddRefed<nsAtom> NS_AtomizeMainThread(const nsAString& aUTF16String);
+
+// Return a count of the total number of atoms currently alive in the system.
+//
+// Note that the result is imprecise and racy if other threads are currently
+// operating on atoms. It's also slow, since it triggers a GC before counting.
+// Currently this function is only used in tests, which should probably remain
+// the case.
+nsrefcnt NS_GetNumberOfAtoms();
+
+// Return a pointer for a static atom for the string or null if there's no
+// static atom for this string.
+nsStaticAtom* NS_GetStaticAtom(const nsAString& aUTF16String);
+
+class nsAtomString : public nsString {
+ public:
+  explicit nsAtomString(const nsAtom* aAtom) { aAtom->ToString(*this); }
+};
+
+class nsAtomCString : public nsCString {
+ public:
+  explicit nsAtomCString(const nsAtom* aAtom) { aAtom->ToUTF8String(*this); }
+};
+
+class nsDependentAtomString : public nsDependentString {
+ public:
+  explicit nsDependentAtomString(const nsAtom* aAtom)
+      : nsDependentString(aAtom->GetUTF16String(), aAtom->GetLength()) {}
+};
+
+// Checks if the ascii chars in a given atom are already lowercase.
+// If they are, no-op. Otherwise, converts all the ascii uppercase
+// chars to lowercase and atomizes, storing the result in the inout
+// param.
+void ToLowerCaseASCII(RefPtr<nsAtom>& aAtom);
+
+#endif  // nsAtom_h
diff --git a/xpcom/ds/nsAtomTable.cpp b/xpcom/ds/nsAtomTable.cpp
new file mode 100644
index 0000000000..56618cd46c
--- /dev/null
+++ b/xpcom/ds/nsAtomTable.cpp
@@ -0,0 +1,670 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/MruCache.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/Unused.h"
+
+#include "nsAtom.h"
+#include "nsAtomTable.h"
+#include "nsCRT.h"
+#include "nsGkAtoms.h"
+#include "nsHashKeys.h"
+#include "nsPrintfCString.h"
+#include "nsString.h"
+#include "nsThreadUtils.h"
+#include "nsUnicharUtils.h"
+#include "PLDHashTable.h"
+#include "prenv.h"
+
+// There are two kinds of atoms handled by this module.
+//
+// - Dynamic: the atom itself is heap allocated, as is the char buffer it
+//   points to. |gAtomTable| holds weak references to dynamic atoms. When the
+//   refcount of a dynamic atom drops to zero, we increment a static counter.
+//   When that counter reaches a certain threshold, we iterate over the atom
+//   table, removing and deleting dynamic atoms with refcount zero. This allows
+//   us to avoid acquiring the atom table lock during normal refcounting.
+//
+// - Static: both the atom and its chars are statically allocated and
+//   immutable, so it ignores all AddRef/Release calls.
+//
+// Note that gAtomTable is used on multiple threads, and has internal
+// synchronization.
+
+using namespace mozilla;
+
+//----------------------------------------------------------------------
+
+enum class GCKind {
+  RegularOperation,
+  Shutdown,
+};
+
+//----------------------------------------------------------------------
+
+// gUnusedAtomCount is incremented when an atom loses its last reference
+// (and thus turned into unused state), and decremented when an unused
+// atom gets a reference again. The atom table relies on this value to
+// schedule GC. This value can temporarily go below zero when multiple
+// threads are operating the same atom, so it has to be signed so that
+// we wouldn't use overflow value for comparison.
+// See nsAtom::AddRef() and nsAtom::Release().
+// This atomic can be accessed during the GC and other places where recorded
+// events are not allowed, so its value is not preserved when recording or
+// replaying.
+Atomic<int32_t, ReleaseAcquire> nsDynamicAtom::gUnusedAtomCount;
+
+nsDynamicAtom::nsDynamicAtom(const nsAString& aString, uint32_t aHash,
+                             bool aIsAsciiLowercase)
+    : nsAtom(aString, aHash, aIsAsciiLowercase), mRefCnt(1) {}
+
+// Returns true if ToLowercaseASCII would return the string unchanged.
+static bool IsAsciiLowercase(const char16_t* aString, const uint32_t aLength) {
+  for (uint32_t i = 0; i < aLength; ++i) {
+    if (IS_ASCII_UPPER(aString[i])) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+nsDynamicAtom* nsDynamicAtom::Create(const nsAString& aString, uint32_t aHash) {
+  // We tack the chars onto the end of the nsDynamicAtom object.
+  size_t numCharBytes = (aString.Length() + 1) * sizeof(char16_t);
+  size_t numTotalBytes = sizeof(nsDynamicAtom) + numCharBytes;
+
+  bool isAsciiLower = ::IsAsciiLowercase(aString.Data(), aString.Length());
+
+  nsDynamicAtom* atom = (nsDynamicAtom*)moz_xmalloc(numTotalBytes);
+  new (atom) nsDynamicAtom(aString, aHash, isAsciiLower);
+  memcpy(const_cast<char16_t*>(atom->String()),
+         PromiseFlatString(aString).get(), numCharBytes);
+
+  MOZ_ASSERT(atom->String()[atom->GetLength()] == char16_t(0));
+  MOZ_ASSERT(atom->Equals(aString));
+  MOZ_ASSERT(atom->mHash == HashString(atom->String(), atom->GetLength()));
+  MOZ_ASSERT(atom->mIsAsciiLowercase == isAsciiLower);
+
+  return atom;
+}
+
+void nsDynamicAtom::Destroy(nsDynamicAtom* aAtom) {
+  aAtom->~nsDynamicAtom();
+  free(aAtom);
+}
+
+void nsAtom::ToString(nsAString& aString) const {
+  // See the comment on |mString|'s declaration.
+  if (IsStatic()) {
+    // AssignLiteral() lets us assign without copying. This isn't a string
+    // literal, but it's a static atom and thus has an unbounded lifetime,
+    // which is what's important.
+    aString.AssignLiteral(AsStatic()->String(), mLength);
+  } else {
+    aString.Assign(AsDynamic()->String(), mLength);
+  }
+}
+
+void nsAtom::ToUTF8String(nsACString& aBuf) const {
+  CopyUTF16toUTF8(nsDependentString(GetUTF16String(), mLength), aBuf);
+}
+
+void nsAtom::AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
+                                    AtomsSizes& aSizes) const {
+  // Static atoms are in static memory, and so are not measured here.
+  if (IsDynamic()) {
+    aSizes.mDynamicAtoms += aMallocSizeOf(this);
+  }
+}
+
+char16ptr_t nsAtom::GetUTF16String() const {
+  return IsStatic() ? AsStatic()->String() : AsDynamic()->String();
+}
+
+//----------------------------------------------------------------------
+
+struct AtomTableKey {
+  explicit AtomTableKey(const nsStaticAtom* aAtom)
+      : mUTF16String(aAtom->String()),
+        mUTF8String(nullptr),
+        mLength(aAtom->GetLength()),
+        mHash(aAtom->hash()) {
+    MOZ_ASSERT(HashString(mUTF16String, mLength) == mHash);
+  }
+
+  AtomTableKey(const char16_t* aUTF16String, uint32_t aLength)
+      : mUTF16String(aUTF16String), mUTF8String(nullptr), mLength(aLength) {
+    mHash = HashString(mUTF16String, mLength);
+  }
+
+  AtomTableKey(const char* aUTF8String, uint32_t aLength, bool* aErr)
+      : mUTF16String(nullptr), mUTF8String(aUTF8String), mLength(aLength) {
+    mHash = HashUTF8AsUTF16(mUTF8String, mLength, aErr);
+  }
+
+  const char16_t* mUTF16String;
+  const char* mUTF8String;
+  uint32_t mLength;
+  uint32_t mHash;
+};
+
+struct AtomTableEntry : public PLDHashEntryHdr {
+  // These references are either to dynamic atoms, in which case they are
+  // non-owning, or they are to static atoms, which aren't really refcounted.
+  // See the comment at the top of this file for more details.
+  nsAtom* MOZ_NON_OWNING_REF mAtom;
+};
+
+struct AtomCache : public MruCache<AtomTableKey, nsAtom*, AtomCache> {
+  static HashNumber Hash(const AtomTableKey& aKey) { return aKey.mHash; }
+  static bool Match(const AtomTableKey& aKey, const nsAtom* aVal) {
+    MOZ_ASSERT(aKey.mUTF16String);
+    return aVal->Equals(aKey.mUTF16String, aKey.mLength);
+  }
+};
+
+static AtomCache sRecentlyUsedMainThreadAtoms;
+
+// In order to reduce locking contention for concurrent atomization, we segment
+// the atom table into N subtables, each with a separate lock. If the hash
+// values we use to select the subtable are evenly distributed, this reduces the
+// probability of contention by a factor of N. See bug 1440824.
+//
+// NB: This is somewhat similar to the technique used by Java's
+// ConcurrentHashTable.
+class nsAtomSubTable {
+  friend class nsAtomTable;
+  Mutex mLock;
+  PLDHashTable mTable;
+  nsAtomSubTable();
+  void GCLocked(GCKind aKind) MOZ_REQUIRES(mLock);
+  void AddSizeOfExcludingThisLocked(MallocSizeOf aMallocSizeOf,
+                                    AtomsSizes& aSizes) MOZ_REQUIRES(mLock);
+
+  AtomTableEntry* Search(AtomTableKey& aKey) const MOZ_REQUIRES(mLock) {
+    mLock.AssertCurrentThreadOwns();
+    return static_cast<AtomTableEntry*>(mTable.Search(&aKey));
+  }
+
+  AtomTableEntry* Add(AtomTableKey& aKey) MOZ_REQUIRES(mLock) {
+    mLock.AssertCurrentThreadOwns();
+    return static_cast<AtomTableEntry*>(mTable.Add(&aKey));  // Infallible
+  }
+};
+
+// The outer atom table, which coordinates access to the inner array of
+// subtables.
+class nsAtomTable {
+ public:
+  nsAtomSubTable& SelectSubTable(AtomTableKey& aKey);
+  void AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf, AtomsSizes& aSizes);
+  void GC(GCKind aKind);
+  already_AddRefed<nsAtom> Atomize(const nsAString& aUTF16String);
+  already_AddRefed<nsAtom> Atomize(const nsACString& aUTF8String);
+  already_AddRefed<nsAtom> AtomizeMainThread(const nsAString& aUTF16String);
+  nsStaticAtom* GetStaticAtom(const nsAString& aUTF16String);
+  void RegisterStaticAtoms(const nsStaticAtom* aAtoms, size_t aAtomsLen);
+
+  // The result of this function may be imprecise if other threads are operating
+  // on atoms concurrently. It's also slow, since it triggers a GC before
+  // counting.
+  size_t RacySlowCount();
+
+  // This hash table op is a static member of this class so that it can take
+  // advantage of |friend| declarations.
+  static void AtomTableClearEntry(PLDHashTable* aTable,
+                                  PLDHashEntryHdr* aEntry);
+
+  // We achieve measurable reduction in locking contention in parallel CSS
+  // parsing by increasing the number of subtables up to 128. This has been
+  // measured to have neglible impact on the performance of initialization, GC,
+  // and shutdown.
+  //
+  // Another important consideration is memory, since we're adding fixed
+  // overhead per content process, which we try to avoid. Measuring a
+  // mostly-empty page [1] with various numbers of subtables, we get the
+  // following deep sizes for the atom table:
+  //       1 subtable:  278K
+  //       8 subtables: 279K
+  //      16 subtables: 282K
+  //      64 subtables: 286K
+  //     128 subtables: 290K
+  //
+  // So 128 subtables costs us 12K relative to a single table, and 4K relative
+  // to 64 subtables. Conversely, measuring parallel (6 thread) CSS parsing on
+  // tp6-facebook, a single table provides ~150ms of locking overhead per
+  // thread, 64 subtables provides ~2-3ms of overhead, and 128 subtables
+  // provides <1ms. And so while either 64 or 128 subtables would probably be
+  // acceptable, achieving a measurable reduction in contention for 4k of fixed
+  // memory overhead is probably worth it.
+  //
+  // [1] The numbers will look different for content processes with complex
+  // pages loaded, but in those cases the actual atoms will dominate memory
+  // usage and the overhead of extra tables will be negligible. We're mostly
+  // interested in the fixed cost for nearly-empty content processes.
+  const static size_t kNumSubTables = 128;  // Must be power of two.
+
+ private:
+  nsAtomSubTable mSubTables[kNumSubTables];
+};
+
+// Static singleton instance for the atom table.
+static nsAtomTable* gAtomTable;
+
+static PLDHashNumber AtomTableGetHash(const void* aKey) {
+  const AtomTableKey* k = static_cast<const AtomTableKey*>(aKey);
+  return k->mHash;
+}
+
+static bool AtomTableMatchKey(const PLDHashEntryHdr* aEntry, const void* aKey) {
+  const AtomTableEntry* he = static_cast<const AtomTableEntry*>(aEntry);
+  const AtomTableKey* k = static_cast<const AtomTableKey*>(aKey);
+
+  if (k->mUTF8String) {
+    bool err = false;
+    return (CompareUTF8toUTF16(nsDependentCSubstring(
+                                   k->mUTF8String, k->mUTF8String + k->mLength),
+                               nsDependentAtomString(he->mAtom), &err) == 0) &&
+           !err;
+  }
+
+  return he->mAtom->Equals(k->mUTF16String, k->mLength);
+}
+
+void nsAtomTable::AtomTableClearEntry(PLDHashTable* aTable,
+                                      PLDHashEntryHdr* aEntry) {
+  auto entry = static_cast<AtomTableEntry*>(aEntry);
+  entry->mAtom = nullptr;
+}
+
+static void AtomTableInitEntry(PLDHashEntryHdr* aEntry, const void* aKey) {
+  static_cast<AtomTableEntry*>(aEntry)->mAtom = nullptr;
+}
+
+static const PLDHashTableOps AtomTableOps = {
+    AtomTableGetHash, AtomTableMatchKey, PLDHashTable::MoveEntryStub,
+    nsAtomTable::AtomTableClearEntry, AtomTableInitEntry};
+
+// The atom table very quickly gets 10,000+ entries in it (or even 100,000+).
+// But choosing the best initial subtable length has some subtleties: we add
+// ~2700 static atoms at start-up, and then we start adding and removing
+// dynamic atoms. If we make the tables too big to start with, when the first
+// dynamic atom gets removed from a given table the load factor will be < 25%
+// and we will shrink it.
+//
+// So we first make the simplifying assumption that the atoms are more or less
+// evenly-distributed across the subtables (which is the case empirically).
+// Then, we take the total atom count when the first dynamic atom is removed
+// (~2700), divide that across the N subtables, and the largest capacity that
+// will allow each subtable to be > 25% full with that count.
+//
+// So want an initial subtable capacity less than (2700 / N) * 4 = 10800 / N.
+// Rounding down to the nearest power of two gives us 8192 / N. Since the
+// capacity is double the initial length, we end up with (4096 / N) per
+// subtable.
+#define INITIAL_SUBTABLE_LENGTH (4096 / nsAtomTable::kNumSubTables)
+
+nsAtomSubTable& nsAtomTable::SelectSubTable(AtomTableKey& aKey) {
+  // There are a few considerations around how we select subtables.
+  //
+  // First, we want entries to be evenly distributed across the subtables. This
+  // can be achieved by using any bits in the hash key, assuming the key itself
+  // is evenly-distributed. Empirical measurements indicate that this method
+  // produces a roughly-even distribution across subtables.
+  //
+  // Second, we want to use the hash bits that are least likely to influence an
+  // entry's position within the subtable. If we used the exact same bits used
+  // by the subtables, then each subtable would compute the same position for
+  // every entry it observes, leading to pessimal performance. In this case,
+  // we're using PLDHashTable, whose primary hash function uses the N leftmost
+  // bits of the hash value (where N is the log2 capacity of the table). This
+  // means we should prefer the rightmost bits here.
+  //
+  // Note that the below is equivalent to mHash % kNumSubTables, a replacement
+  // which an optimizing compiler should make, but let's avoid any doubt.
+  static_assert((kNumSubTables & (kNumSubTables - 1)) == 0,
+                "must be power of two");
+  return mSubTables[aKey.mHash & (kNumSubTables - 1)];
+}
+
+void nsAtomTable::AddSizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
+                                         AtomsSizes& aSizes) {
+  MOZ_ASSERT(NS_IsMainThread());
+  aSizes.mTable += aMallocSizeOf(this);
+  for (auto& table : mSubTables) {
+    MutexAutoLock lock(table.mLock);
+    table.AddSizeOfExcludingThisLocked(aMallocSizeOf, aSizes);
+  }
+}
+
+void nsAtomTable::GC(GCKind aKind) {
+  MOZ_ASSERT(NS_IsMainThread());
+  sRecentlyUsedMainThreadAtoms.Clear();
+
+  // Note that this is effectively an incremental GC, since only one subtable
+  // is locked at a time.
+  for (auto& table : mSubTables) {
+    MutexAutoLock lock(table.mLock);
+    table.GCLocked(aKind);
+  }
+
+  // We would like to assert that gUnusedAtomCount matches the number of atoms
+  // we found in the table which we removed. However, there are two problems
+  // with this:
+  // * We have multiple subtables, each with their own lock. For optimal
+  //   performance we only want to hold one lock at a time, but this means
+  //   that atoms can be added and removed between GC slices.
+  // * Even if we held all the locks and performed all GC slices atomically,
+  //   the locks are not acquired for AddRef() and Release() calls. This means
+  //   we might see a gUnusedAtomCount value in between, say, AddRef()
+  //   incrementing mRefCnt and it decrementing gUnusedAtomCount.
+  //
+  // So, we don't bother asserting that there are no unused atoms at the end of
+  // a regular GC. But we can (and do) assert this just after the last GC at
+  // shutdown.
+  //
+  // Note that, barring refcounting bugs, an atom can only go from a zero
+  // refcount to a non-zero refcount while the atom table lock is held, so
+  // so we won't try to resurrect a zero refcount atom while trying to delete
+  // it.
+
+  MOZ_ASSERT_IF(aKind == GCKind::Shutdown,
+                nsDynamicAtom::gUnusedAtomCount == 0);
+}
+
+size_t nsAtomTable::RacySlowCount() {
+  // Trigger a GC so that the result is deterministic modulo other threads.
+  GC(GCKind::RegularOperation);
+  size_t count = 0;
+  for (auto& table : mSubTables) {
+    MutexAutoLock lock(table.mLock);
+    count += table.mTable.EntryCount();
+  }
+
+  return count;
+}
+
+nsAtomSubTable::nsAtomSubTable()
+    : mLock("Atom Sub-Table Lock"),
+      mTable(&AtomTableOps, sizeof(AtomTableEntry), INITIAL_SUBTABLE_LENGTH) {}
+
+void nsAtomSubTable::GCLocked(GCKind aKind) {
+  MOZ_ASSERT(NS_IsMainThread());
+  mLock.AssertCurrentThreadOwns();
+
+  int32_t removedCount = 0;  // A non-atomic temporary for cheaper increments.
+  nsAutoCString nonZeroRefcountAtoms;
+  uint32_t nonZeroRefcountAtomsCount = 0;
+  for (auto i = mTable.Iter(); !i.Done(); i.Next()) {
+    auto entry = static_cast<AtomTableEntry*>(i.Get());
+    if (entry->mAtom->IsStatic()) {
+      continue;
+    }
+
+    nsAtom* atom = entry->mAtom;
+    if (atom->IsDynamic() && atom->AsDynamic()->mRefCnt == 0) {
+      i.Remove();
+      nsDynamicAtom::Destroy(atom->AsDynamic());
+      ++removedCount;
+    }
+#ifdef NS_FREE_PERMANENT_DATA
+    else if (aKind == GCKind::Shutdown && PR_GetEnv("XPCOM_MEM_BLOAT_LOG")) {
+      // Only report leaking atoms in leak-checking builds in a run where we
+      // are checking for leaks, during shutdown. If something is anomalous,
+      // then we'll assert later in this function.
+      nsAutoCString name;
+      atom->ToUTF8String(name);
+      if (nonZeroRefcountAtomsCount == 0) {
+        nonZeroRefcountAtoms = name;
+      } else if (nonZeroRefcountAtomsCount < 20) {
+        nonZeroRefcountAtoms += ","_ns + name;
+      } else if (nonZeroRefcountAtomsCount == 20) {
+        nonZeroRefcountAtoms += ",..."_ns;
+      }
+      nonZeroRefcountAtomsCount++;
+    }
+#endif
+  }
+  if (nonZeroRefcountAtomsCount) {
+    nsPrintfCString msg("%d dynamic atom(s) with non-zero refcount: %s",
+                        nonZeroRefcountAtomsCount, nonZeroRefcountAtoms.get());
+    NS_ASSERTION(nonZeroRefcountAtomsCount == 0, msg.get());
+  }
+
+  nsDynamicAtom::gUnusedAtomCount -= removedCount;
+}
+
+void nsDynamicAtom::GCAtomTable() {
+  MOZ_ASSERT(gAtomTable);
+  if (NS_IsMainThread()) {
+    gAtomTable->GC(GCKind::RegularOperation);
+  }
+}
+
+//----------------------------------------------------------------------
+
+// Have the static atoms been inserted into the table?
+static bool gStaticAtomsDone = false;
+
+void NS_InitAtomTable() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!gAtomTable);
+
+  // We register static atoms immediately so they're available for use as early
+  // as possible.
+  gAtomTable = new nsAtomTable();
+  gAtomTable->RegisterStaticAtoms(nsGkAtoms::sAtoms, nsGkAtoms::sAtomsLen);
+  gStaticAtomsDone = true;
+}
+
+void NS_ShutdownAtomTable() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(gAtomTable);
+
+#ifdef NS_FREE_PERMANENT_DATA
+  // Do a final GC to satisfy leak checking. We skip this step in release
+  // builds.
+  gAtomTable->GC(GCKind::Shutdown);
+#endif
+
+  delete gAtomTable;
+  gAtomTable = nullptr;
+}
+
+void NS_AddSizeOfAtoms(MallocSizeOf aMallocSizeOf, AtomsSizes& aSizes) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->AddSizeOfIncludingThis(aMallocSizeOf, aSizes);
+}
+
+void nsAtomSubTable::AddSizeOfExcludingThisLocked(MallocSizeOf aMallocSizeOf,
+                                                  AtomsSizes& aSizes) {
+  mLock.AssertCurrentThreadOwns();
+  aSizes.mTable += mTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  for (auto iter = mTable.Iter(); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<AtomTableEntry*>(iter.Get());
+    entry->mAtom->AddSizeOfIncludingThis(aMallocSizeOf, aSizes);
+  }
+}
+
+void nsAtomTable::RegisterStaticAtoms(const nsStaticAtom* aAtoms,
+                                      size_t aAtomsLen) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_RELEASE_ASSERT(!gStaticAtomsDone, "Static atom insertion is finished!");
+
+  for (uint32_t i = 0; i < aAtomsLen; ++i) {
+    const nsStaticAtom* atom = &aAtoms[i];
+    MOZ_ASSERT(IsAsciiNullTerminated(atom->String()));
+    MOZ_ASSERT(NS_strlen(atom->String()) == atom->GetLength());
+    MOZ_ASSERT(atom->IsAsciiLowercase() ==
+               ::IsAsciiLowercase(atom->String(), atom->GetLength()));
+
+    // This assertion ensures the static atom's precomputed hash value matches
+    // what would be computed by mozilla::HashString(aStr), which is what we use
+    // when atomizing strings. We compute this hash in Atom.py.
+    MOZ_ASSERT(HashString(atom->String()) == atom->hash());
+
+    AtomTableKey key(atom);
+    nsAtomSubTable& table = SelectSubTable(key);
+    MutexAutoLock lock(table.mLock);
+    AtomTableEntry* he = table.Add(key);
+
+    if (he->mAtom) {
+      // There are two ways we could get here.
+      // - Register two static atoms with the same string.
+      // - Create a dynamic atom and then register a static atom with the same
+      //   string while the dynamic atom is alive.
+      // Both cases can cause subtle bugs, and are disallowed. We're
+      // programming in C++ here, not Smalltalk.
+      nsAutoCString name;
+      he->mAtom->ToUTF8String(name);
+      MOZ_CRASH_UNSAFE_PRINTF("Atom for '%s' already exists", name.get());
+    }
+    he->mAtom = const_cast<nsStaticAtom*>(atom);
+  }
+}
+
+already_AddRefed<nsAtom> NS_Atomize(const char* aUTF8String) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->Atomize(nsDependentCString(aUTF8String));
+}
+
+already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsACString& aUTF8String) {
+  bool err;
+  AtomTableKey key(aUTF8String.Data(), aUTF8String.Length(), &err);
+  if (MOZ_UNLIKELY(err)) {
+    MOZ_ASSERT_UNREACHABLE("Tried to atomize invalid UTF-8.");
+    // The input was invalid UTF-8. Let's replace the errors with U+FFFD
+    // and atomize the result.
+    nsString str;
+    CopyUTF8toUTF16(aUTF8String, str);
+    return Atomize(str);
+  }
+  nsAtomSubTable& table = SelectSubTable(key);
+  MutexAutoLock lock(table.mLock);
+  AtomTableEntry* he = table.Add(key);
+
+  if (he->mAtom) {
+    RefPtr<nsAtom> atom = he->mAtom;
+    return atom.forget();
+  }
+
+  nsString str;
+  CopyUTF8toUTF16(aUTF8String, str);
+  RefPtr<nsAtom> atom = dont_AddRef(nsDynamicAtom::Create(str, key.mHash));
+
+  he->mAtom = atom;
+
+  return atom.forget();
+}
+
+already_AddRefed<nsAtom> NS_Atomize(const nsACString& aUTF8String) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->Atomize(aUTF8String);
+}
+
+already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->Atomize(nsDependentString(aUTF16String));
+}
+
+already_AddRefed<nsAtom> nsAtomTable::Atomize(const nsAString& aUTF16String) {
+  AtomTableKey key(aUTF16String.Data(), aUTF16String.Length());
+  nsAtomSubTable& table = SelectSubTable(key);
+  MutexAutoLock lock(table.mLock);
+  AtomTableEntry* he = table.Add(key);
+
+  if (he->mAtom) {
+    RefPtr<nsAtom> atom = he->mAtom;
+    return atom.forget();
+  }
+
+  RefPtr<nsAtom> atom =
+      dont_AddRef(nsDynamicAtom::Create(aUTF16String, key.mHash));
+  he->mAtom = atom;
+
+  return atom.forget();
+}
+
+already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->Atomize(aUTF16String);
+}
+
+already_AddRefed<nsAtom> nsAtomTable::AtomizeMainThread(
+    const nsAString& aUTF16String) {
+  MOZ_ASSERT(NS_IsMainThread());
+  RefPtr<nsAtom> retVal;
+  AtomTableKey key(aUTF16String.Data(), aUTF16String.Length());
+  auto p = sRecentlyUsedMainThreadAtoms.Lookup(key);
+  if (p) {
+    retVal = p.Data();
+    return retVal.forget();
+  }
+
+  nsAtomSubTable& table = SelectSubTable(key);
+  MutexAutoLock lock(table.mLock);
+  AtomTableEntry* he = table.Add(key);
+
+  if (he->mAtom) {
+    retVal = he->mAtom;
+  } else {
+    RefPtr<nsAtom> newAtom =
+        dont_AddRef(nsDynamicAtom::Create(aUTF16String, key.mHash));
+    he->mAtom = newAtom;
+    retVal = std::move(newAtom);
+  }
+
+  p.Set(retVal);
+  return retVal.forget();
+}
+
+already_AddRefed<nsAtom> NS_AtomizeMainThread(const nsAString& aUTF16String) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->AtomizeMainThread(aUTF16String);
+}
+
+nsrefcnt NS_GetNumberOfAtoms(void) {
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->RacySlowCount();
+}
+
+int32_t NS_GetUnusedAtomCount(void) { return nsDynamicAtom::gUnusedAtomCount; }
+
+nsStaticAtom* NS_GetStaticAtom(const nsAString& aUTF16String) {
+  MOZ_ASSERT(gStaticAtomsDone, "Static atom setup not yet done.");
+  MOZ_ASSERT(gAtomTable);
+  return gAtomTable->GetStaticAtom(aUTF16String);
+}
+
+nsStaticAtom* nsAtomTable::GetStaticAtom(const nsAString& aUTF16String) {
+  AtomTableKey key(aUTF16String.Data(), aUTF16String.Length());
+  nsAtomSubTable& table = SelectSubTable(key);
+  MutexAutoLock lock(table.mLock);
+  AtomTableEntry* he = table.Search(key);
+  return he && he->mAtom->IsStatic() ? static_cast<nsStaticAtom*>(he->mAtom)
+                                     : nullptr;
+}
+
+void ToLowerCaseASCII(RefPtr<nsAtom>& aAtom) {
+  // Assume the common case is that the atom is already ASCII lowercase.
+  if (aAtom->IsAsciiLowercase()) {
+    return;
+  }
+
+  nsAutoString lowercased;
+  ToLowerCaseASCII(nsDependentAtomString(aAtom), lowercased);
+  aAtom = NS_Atomize(lowercased);
+}
diff --git a/xpcom/ds/nsAtomTable.h b/xpcom/ds/nsAtomTable.h
new file mode 100644
index 0000000000..ac69daea9f
--- /dev/null
+++ b/xpcom/ds/nsAtomTable.h
@@ -0,0 +1,30 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsAtomTable_h__
+#define nsAtomTable_h__
+
+#include "mozilla/MemoryReporting.h"
+#include "nsAtom.h"
+
+#include <stddef.h>
+
+void NS_InitAtomTable();
+void NS_ShutdownAtomTable();
+
+namespace mozilla {
+struct AtomsSizes {
+  size_t mTable;
+  size_t mDynamicAtoms;
+
+  AtomsSizes() : mTable(0), mDynamicAtoms(0) {}
+};
+}  // namespace mozilla
+
+void NS_AddSizeOfAtoms(mozilla::MallocSizeOf aMallocSizeOf,
+                       mozilla::AtomsSizes& aSizes);
+
+#endif  // nsAtomTable_h__
diff --git a/xpcom/ds/nsBaseHashtable.h b/xpcom/ds/nsBaseHashtable.h
new file mode 100644
index 0000000000..7b57ef4666
--- /dev/null
+++ b/xpcom/ds/nsBaseHashtable.h
@@ -0,0 +1,1029 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsBaseHashtable_h__
+#define nsBaseHashtable_h__
+
+#include <functional>
+#include <utility>
+
+#include "mozilla/dom/SafeRefPtr.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Result.h"
+#include "mozilla/UniquePtr.h"
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsHashtablesFwd.h"
+#include "nsTHashtable.h"
+
+namespace mozilla::detail {
+
+template <typename SmartPtr>
+struct SmartPtrTraits {
+  static constexpr bool IsSmartPointer = false;
+  static constexpr bool IsRefCounted = false;
+};
+
+template <typename Pointee>
+struct SmartPtrTraits<UniquePtr<Pointee>> {
+  static constexpr bool IsSmartPointer = true;
+  static constexpr bool IsRefCounted = false;
+  using SmartPointerType = UniquePtr<Pointee>;
+  using PointeeType = Pointee;
+  using RawPointerType = Pointee*;
+  template <typename U>
+  using OtherSmartPtrType = UniquePtr<U>;
+
+  template <typename U, typename... Args>
+  static SmartPointerType NewObject(Args&&... aConstructionArgs) {
+    return mozilla::MakeUnique<U>(std::forward<Args>(aConstructionArgs)...);
+  }
+};
+
+template <typename Pointee>
+struct SmartPtrTraits<RefPtr<Pointee>> {
+  static constexpr bool IsSmartPointer = true;
+  static constexpr bool IsRefCounted = true;
+  using SmartPointerType = RefPtr<Pointee>;
+  using PointeeType = Pointee;
+  using RawPointerType = Pointee*;
+  template <typename U>
+  using OtherSmartPtrType = RefPtr<U>;
+
+  template <typename U, typename... Args>
+  static SmartPointerType NewObject(Args&&... aConstructionArgs) {
+    return MakeRefPtr<U>(std::forward<Args>(aConstructionArgs)...);
+  }
+};
+
+template <typename Pointee>
+struct SmartPtrTraits<SafeRefPtr<Pointee>> {
+  static constexpr bool IsSmartPointer = true;
+  static constexpr bool IsRefCounted = true;
+  using SmartPointerType = SafeRefPtr<Pointee>;
+  using PointeeType = Pointee;
+  using RawPointerType = Pointee*;
+  template <typename U>
+  using OtherSmartPtrType = SafeRefPtr<U>;
+
+  template <typename U, typename... Args>
+  static SmartPointerType NewObject(Args&&... aConstructionArgs) {
+    return MakeSafeRefPtr<U>(std::forward<Args>(aConstructionArgs)...);
+  }
+};
+
+template <typename Pointee>
+struct SmartPtrTraits<nsCOMPtr<Pointee>> {
+  static constexpr bool IsSmartPointer = true;
+  static constexpr bool IsRefCounted = true;
+  using SmartPointerType = nsCOMPtr<Pointee>;
+  using PointeeType = Pointee;
+  using RawPointerType = Pointee*;
+  template <typename U>
+  using OtherSmartPtrType = nsCOMPtr<U>;
+
+  template <typename U, typename... Args>
+  static SmartPointerType NewObject(Args&&... aConstructionArgs) {
+    return MakeRefPtr<U>(std::forward<Args>(aConstructionArgs)...);
+  }
+};
+
+template <class T>
+T* PtrGetWeak(T* aPtr) {
+  return aPtr;
+}
+
+template <class T>
+T* PtrGetWeak(const RefPtr<T>& aPtr) {
+  return aPtr.get();
+}
+
+template <class T>
+T* PtrGetWeak(const SafeRefPtr<T>& aPtr) {
+  return aPtr.unsafeGetRawPtr();
+}
+
+template <class T>
+T* PtrGetWeak(const nsCOMPtr<T>& aPtr) {
+  return aPtr.get();
+}
+
+template <class T>
+T* PtrGetWeak(const UniquePtr<T>& aPtr) {
+  return aPtr.get();
+}
+
+template <typename EntryType>
+class nsBaseHashtableValueIterator : public ::detail::nsTHashtableIteratorBase {
+  // friend class nsTHashtable<EntryType>;
+
+ public:
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = const std::decay_t<typename EntryType::DataType>;
+  using difference_type = int32_t;
+  using pointer = value_type*;
+  using reference = value_type&;
+
+  using iterator_type = nsBaseHashtableValueIterator;
+  using const_iterator_type = nsBaseHashtableValueIterator;
+
+  using nsTHashtableIteratorBase::nsTHashtableIteratorBase;
+
+  value_type* operator->() const {
+    return &static_cast<const EntryType*>(mIterator.Get())->GetData();
+  }
+  decltype(auto) operator*() const {
+    return static_cast<const EntryType*>(mIterator.Get())->GetData();
+  }
+
+  iterator_type& operator++() {
+    mIterator.Next();
+    return *this;
+  }
+  iterator_type operator++(int) {
+    iterator_type it = *this;
+    ++*this;
+    return it;
+  }
+};
+
+template <typename EntryType>
+class nsBaseHashtableValueRange {
+ public:
+  using IteratorType = nsBaseHashtableValueIterator<EntryType>;
+  using iterator = IteratorType;
+
+  explicit nsBaseHashtableValueRange(const PLDHashTable& aHashtable)
+      : mHashtable{aHashtable} {}
+
+  auto begin() const { return IteratorType{mHashtable}; }
+  auto end() const {
+    return IteratorType{mHashtable, typename IteratorType::EndIteratorTag{}};
+  }
+  auto cbegin() const { return begin(); }
+  auto cend() const { return end(); }
+
+  uint32_t Count() const { return mHashtable.EntryCount(); }
+
+ private:
+  const PLDHashTable& mHashtable;
+};
+
+template <typename EntryType>
+auto RangeSize(const detail::nsBaseHashtableValueRange<EntryType>& aRange) {
+  return aRange.Count();
+}
+
+}  // namespace mozilla::detail
+
+/**
+ * Data type conversion helper that is used to wrap and unwrap the specified
+ * DataType.
+ */
+template <class DataType, class UserDataType>
+class nsDefaultConverter {
+ public:
+  /**
+   * Maps the storage DataType to the exposed UserDataType.
+   */
+  static UserDataType Unwrap(DataType& src) { return UserDataType(src); }
+
+  /**
+   * Const ref variant used for example with nsCOMPtr wrappers.
+   */
+  static DataType Wrap(const UserDataType& src) { return DataType(src); }
+
+  /**
+   * Generic conversion, this is useful for things like already_AddRefed.
+   */
+  template <typename U>
+  static DataType Wrap(U&& src) {
+    return std::forward<U>(src);
+  }
+
+  template <typename U>
+  static UserDataType Unwrap(U&& src) {
+    return std::forward<U>(src);
+  }
+};
+
+/**
+ * the private nsTHashtable::EntryType class used by nsBaseHashtable
+ * @see nsTHashtable for the specification of this class
+ * @see nsBaseHashtable for template parameters
+ */
+template <class KeyClass, class TDataType>
+class nsBaseHashtableET : public KeyClass {
+ public:
+  using DataType = TDataType;
+
+  const DataType& GetData() const { return mData; }
+  DataType* GetModifiableData() { return &mData; }
+  template <typename U>
+  void SetData(U&& aData) {
+    mData = std::forward<U>(aData);
+  }
+
+  decltype(auto) GetWeak() const {
+    return mozilla::detail::PtrGetWeak(GetData());
+  }
+
+ private:
+  DataType mData;
+  friend class nsTHashtable<nsBaseHashtableET<KeyClass, DataType>>;
+  template <typename KeyClassX, typename DataTypeX, typename UserDataTypeX,
+            typename ConverterX>
+  friend class nsBaseHashtable;
+  friend class ::detail::nsTHashtableKeyIterator<
+      nsBaseHashtableET<KeyClass, DataType>>;
+
+  typedef typename KeyClass::KeyType KeyType;
+  typedef typename KeyClass::KeyTypePointer KeyTypePointer;
+
+  template <typename... Args>
+  explicit nsBaseHashtableET(KeyTypePointer aKey, Args&&... aArgs);
+  nsBaseHashtableET(nsBaseHashtableET<KeyClass, DataType>&& aToMove) = default;
+  ~nsBaseHashtableET() = default;
+};
+
+/**
+ * Templated hashtable. Usually, this isn't instantiated directly but through
+ * its sub-class templates nsInterfaceHashtable, nsClassHashtable,
+ * nsRefPtrHashtable and nsTHashMap.
+ *
+ * Originally, UserDataType used to be the only type exposed to the user in the
+ * public member function signatures (hence its name), but this has proven to
+ * inadequate over time. Now, UserDataType is only exposed in by-value
+ * getter member functions that are called *Get*. Member functions that provide
+ * access to the DataType are called Lookup rather than Get. Note that this rule
+ * does not apply to nsRefPtrHashtable and nsInterfaceHashtable, as they are
+ * provide a similar interface, but are no genuine sub-classes of
+ * nsBaseHashtable.
+ *
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param DataType the datatype stored in the hashtable,
+ *   for example, uint32_t or nsCOMPtr.
+ * @param UserDataType the datatype returned from the by-value getter member
+ *   functions (named *Get*), for example uint32_t or nsISupports*
+ * @param Converter that is used to map from DataType to UserDataType. A
+ *   default converter is provided that assumes implicit conversion is an
+ *   option.
+ */
+template <class KeyClass, class DataType, class UserDataType, class Converter>
+class nsBaseHashtable
+    : protected nsTHashtable<nsBaseHashtableET<KeyClass, DataType>> {
+  using Base = nsTHashtable<nsBaseHashtableET<KeyClass, DataType>>;
+  typedef mozilla::fallible_t fallible_t;
+
+ public:
+  typedef typename KeyClass::KeyType KeyType;
+  typedef nsBaseHashtableET<KeyClass, DataType> EntryType;
+
+  using nsTHashtable<EntryType>::Contains;
+  using nsTHashtable<EntryType>::GetGeneration;
+  using nsTHashtable<EntryType>::SizeOfExcludingThis;
+  using nsTHashtable<EntryType>::SizeOfIncludingThis;
+
+  nsBaseHashtable() = default;
+  explicit nsBaseHashtable(uint32_t aInitLength)
+      : nsTHashtable<EntryType>(aInitLength) {}
+
+  /**
+   * Return the number of entries in the table.
+   * @return    number of entries
+   */
+  [[nodiscard]] uint32_t Count() const {
+    return nsTHashtable<EntryType>::Count();
+  }
+
+  /**
+   * Return whether the table is empty.
+   * @return    whether empty
+   */
+  [[nodiscard]] bool IsEmpty() const {
+    return nsTHashtable<EntryType>::IsEmpty();
+  }
+
+  /**
+   * Get the value, returning a flag indicating the presence of the entry in
+   * the table.
+   *
+   * @param aKey the key to retrieve
+   * @param aData data associated with this key will be placed at this pointer.
+   *        If you only need to check if the key exists, aData may be null.
+   * @return true if the key exists. If key does not exist, aData is not
+   *   modified.
+   *
+   * @attention As opposed to Remove, this does not assign a value to *aData if
+   * no entry is present! (And also as opposed to the member function Get with
+   * the same signature that nsClassHashtable defines and hides this one.)
+   */
+  [[nodiscard]] bool Get(KeyType aKey, UserDataType* aData) const {
+    EntryType* ent = this->GetEntry(aKey);
+    if (!ent) {
+      return false;
+    }
+
+    if (aData) {
+      *aData = Converter::Unwrap(ent->mData);
+    }
+
+    return true;
+  }
+
+  /**
+   * Get the value, returning a zero-initialized POD or a default-initialized
+   * object if the entry is not present in the table.
+   *
+   * This overload can only be used if UserDataType is default-constructible.
+   * Use the double-argument Get or MaybeGet with non-default-constructible
+   * UserDataType.
+   *
+   * @param aKey the key to retrieve
+   * @return The found value, or UserDataType{} if no entry was found with the
+   *         given key.
+   * @note If zero/default-initialized values are stored in the table, it is
+   *       not possible to distinguish between such a value and a missing entry.
+   */
+  [[nodiscard]] UserDataType Get(KeyType aKey) const {
+    EntryType* ent = this->GetEntry(aKey);
+    if (!ent) {
+      return UserDataType{};
+    }
+
+    return Converter::Unwrap(ent->mData);
+  }
+
+  /**
+   * Get the value, returning Nothing if the entry is not present in the table.
+   *
+   * @param aKey the key to retrieve
+   * @return The found value wrapped in a Maybe, or Nothing if no entry was
+   *         found with the given key.
+   */
+  [[nodiscard]] mozilla::Maybe<UserDataType> MaybeGet(KeyType aKey) const {
+    EntryType* ent = this->GetEntry(aKey);
+    if (!ent) {
+      return mozilla::Nothing();
+    }
+
+    return mozilla::Some(Converter::Unwrap(ent->mData));
+  }
+
+  using SmartPtrTraits = mozilla::detail::SmartPtrTraits<DataType>;
+
+  /**
+   * Looks up aKey in the hash table. If it doesn't exist a new object of
+   * SmartPtrTraits::PointeeType will be created (using the arguments provided)
+   * and then returned.
+   *
+   * \note This can only be instantiated if DataType is a smart pointer.
+   */
+  template <typename... Args>
+  auto GetOrInsertNew(KeyType aKey, Args&&... aConstructionArgs) {
+    static_assert(
+        SmartPtrTraits::IsSmartPointer,
+        "GetOrInsertNew can only be used with smart pointer data types");
+    return mozilla::detail::PtrGetWeak(LookupOrInsertWith(std::move(aKey), [&] {
+      return SmartPtrTraits::template NewObject<
+          typename SmartPtrTraits::PointeeType>(
+          std::forward<Args>(aConstructionArgs)...);
+    }));
+  }
+
+  /**
+   * Add aKey to the table if not already present, and return a reference to its
+   * value.  If aKey is not already in the table then the a default-constructed
+   * or the provided value aData is used.
+   *
+   * If the arguments are non-trivial to provide, consider using
+   * LookupOrInsertWith instead.
+   */
+  template <typename... Args>
+  DataType& LookupOrInsert(const KeyType& aKey, Args&&... aArgs) {
+    return WithEntryHandle(aKey, [&](auto entryHandle) -> DataType& {
+      return entryHandle.OrInsert(std::forward<Args>(aArgs)...);
+    });
+  }
+
+  /**
+   * Add aKey to the table if not already present, and return a reference to its
+   * value.  If aKey is not already in the table then the value is
+   * constructed using the given factory.
+   */
+  template <typename F>
+  DataType& LookupOrInsertWith(const KeyType& aKey, F&& aFunc) {
+    return WithEntryHandle(aKey, [&aFunc](auto entryHandle) -> DataType& {
+      return entryHandle.OrInsertWith(std::forward<F>(aFunc));
+    });
+  }
+
+  /**
+   * Add aKey to the table if not already present, and return a reference to its
+   * value.  If aKey is not already in the table then the value is
+   * constructed using the given factory.
+   */
+  template <typename F>
+  [[nodiscard]] auto TryLookupOrInsertWith(const KeyType& aKey, F&& aFunc) {
+    return WithEntryHandle(
+        aKey,
+        [&aFunc](auto entryHandle)
+            -> mozilla::Result<std::reference_wrapper<DataType>,
+                               typename std::invoke_result_t<F>::err_type> {
+          if (entryHandle) {
+            return std::ref(entryHandle.Data());
+          }
+
+          // XXX Use MOZ_TRY after generalizing QM_TRY to mfbt.
+          auto res = std::forward<F>(aFunc)();
+          if (res.isErr()) {
+            return res.propagateErr();
+          }
+          return std::ref(entryHandle.Insert(res.unwrap()));
+        });
+  }
+
+  /**
+   * If it does not yet, inserts a new entry with the handle's key and the
+   * value passed to this function. Otherwise, it updates the entry by the
+   * value passed to this function.
+   *
+   * \tparam U DataType must be implicitly convertible (and assignable) from U
+   * \post HasEntry()
+   * \param aKey the key to put
+   * \param aData the new data
+   */
+  template <typename U>
+  DataType& InsertOrUpdate(KeyType aKey, U&& aData) {
+    return WithEntryHandle(aKey, [&aData](auto entryHandle) -> DataType& {
+      return entryHandle.InsertOrUpdate(std::forward<U>(aData));
+    });
+  }
+
+  template <typename U>
+  [[nodiscard]] bool InsertOrUpdate(KeyType aKey, U&& aData,
+                                    const fallible_t& aFallible) {
+    return WithEntryHandle(aKey, aFallible, [&aData](auto maybeEntryHandle) {
+      if (!maybeEntryHandle) {
+        return false;
+      }
+      maybeEntryHandle->InsertOrUpdate(std::forward<U>(aData));
+      return true;
+    });
+  }
+
+  /**
+   * Remove the entry associated with aKey (if any), _moving_ its current value
+   * into *aData.  Return true if found.
+   *
+   * This overload can only be used if DataType is default-constructible. Use
+   * the single-argument Remove or Extract with non-default-constructible
+   * DataType.
+   *
+   * @param aKey the key to remove from the hashtable
+   * @param aData where to move the value.  If an entry is not found, *aData
+   *              will be assigned a default-constructed value (i.e. reset to
+   *              zero or nullptr for primitive types).
+   * @return true if an entry for aKey was found (and removed)
+   */
+  // XXX This should also better be marked nodiscard, but due to
+  // nsClassHashtable not guaranteeing non-nullness of entries, it is usually
+  // only checked if aData is nullptr in such cases.
+  // [[nodiscard]]
+  bool Remove(KeyType aKey, DataType* aData) {
+    if (auto* ent = this->GetEntry(aKey)) {
+      if (aData) {
+        *aData = std::move(ent->mData);
+      }
+      this->RemoveEntry(ent);
+      return true;
+    }
+    if (aData) {
+      *aData = std::move(DataType());
+    }
+    return false;
+  }
+
+  /**
+   * Remove the entry associated with aKey (if any).  Return true if found.
+   *
+   * @param aKey the key to remove from the hashtable
+   * @return true if an entry for aKey was found (and removed)
+   */
+  bool Remove(KeyType aKey) {
+    if (auto* ent = this->GetEntry(aKey)) {
+      this->RemoveEntry(ent);
+      return true;
+    }
+
+    return false;
+  }
+
+  /**
+   * Retrieve the value for a key and remove the corresponding entry at
+   * the same time.
+   *
+   * @param aKey the key to retrieve and remove
+   * @return the found value, or Nothing if no entry was found with the
+   *   given key.
+   */
+  [[nodiscard]] mozilla::Maybe<DataType> Extract(KeyType aKey) {
+    mozilla::Maybe<DataType> value;
+    if (EntryType* ent = this->GetEntry(aKey)) {
+      value.emplace(std::move(ent->mData));
+      this->RemoveEntry(ent);
+    }
+    return value;
+  }
+
+  template <typename HashtableRef>
+  struct LookupResult {
+   private:
+    EntryType* mEntry;
+    HashtableRef mTable;
+#ifdef DEBUG
+    uint32_t mTableGeneration;
+#endif
+
+   public:
+    LookupResult(EntryType* aEntry, HashtableRef aTable)
+        : mEntry(aEntry),
+          mTable(aTable)
+#ifdef DEBUG
+          ,
+          mTableGeneration(aTable.GetGeneration())
+#endif
+    {
+    }
+
+    // Is there something stored in the table?
+    explicit operator bool() const {
+      MOZ_ASSERT(mTableGeneration == mTable.GetGeneration());
+      return mEntry;
+    }
+
+    void Remove() {
+      if (!*this) {
+        return;
+      }
+      mTable.RemoveEntry(mEntry);
+      mEntry = nullptr;
+    }
+
+    [[nodiscard]] DataType& Data() {
+      MOZ_ASSERT(!!*this, "must have an entry to access its value");
+      return mEntry->mData;
+    }
+
+    [[nodiscard]] const DataType& Data() const {
+      MOZ_ASSERT(!!*this, "must have an entry to access its value");
+      return mEntry->mData;
+    }
+
+    [[nodiscard]] DataType* DataPtrOrNull() {
+      return static_cast<bool>(*this) ? &mEntry->mData : nullptr;
+    }
+
+    [[nodiscard]] const DataType* DataPtrOrNull() const {
+      return static_cast<bool>(*this) ? &mEntry->mData : nullptr;
+    }
+
+    [[nodiscard]] DataType* operator->() { return &Data(); }
+    [[nodiscard]] const DataType* operator->() const { return &Data(); }
+
+    [[nodiscard]] DataType& operator*() { return Data(); }
+    [[nodiscard]] const DataType& operator*() const { return Data(); }
+  };
+
+  /**
+   * Removes all entries matching a predicate.
+   *
+   * The predicate must be compatible with signature bool (const Iterator &).
+   */
+  template <typename Pred>
+  void RemoveIf(Pred&& aPred) {
+    for (auto iter = Iter(); !iter.Done(); iter.Next()) {
+      if (aPred(const_cast<std::add_const_t<decltype(iter)>&>(iter))) {
+        iter.Remove();
+      }
+    }
+  }
+
+  /**
+   * Looks up aKey in the hashtable and returns an object that allows you to
+   * read/modify the value of the entry, or remove the entry (if found).
+   *
+   * A typical usage of this API looks like this:
+   *
+   *   if (auto entry = hashtable.Lookup(key)) {
+   *     DoSomething(entry.Data());
+   *     if (entry.Data() > 42) {
+   *       entry.Remove();
+   *     }
+   *   } // else - an entry with the given key doesn't exist
+   *
+   * This is useful for cases where you want to read/write the value of an entry
+   * and (optionally) remove the entry without having to do multiple hashtable
+   * lookups.  If you want to insert a new entry if one does not exist, then use
+   * WithEntryHandle instead, see below.
+   */
+  [[nodiscard]] auto Lookup(KeyType aKey) {
+    return LookupResult<nsBaseHashtable&>(this->GetEntry(aKey), *this);
+  }
+
+  [[nodiscard]] auto Lookup(KeyType aKey) const {
+    return LookupResult<const nsBaseHashtable&>(this->GetEntry(aKey), *this);
+  }
+
+  /**
+   * Used by WithEntryHandle as the argument type to its functor. It is
+   * associated with the Key passed to WithEntryHandle and manages only the
+   * potential entry with that key. Note that in case no modifying operations
+   * are called on the handle, the state of the hashtable remains unchanged,
+   * i.e. WithEntryHandle does not modify the hashtable itself.
+   *
+   * Provides query functions (Key, HasEntry/operator bool, Data) and
+   * modifying operations for inserting new entries (Insert), updating existing
+   * entries (Update) and removing existing entries (Remove). They have
+   * debug-only assertion that fail when the state of the entry doesn't match
+   * the expectation. There are variants prefixed with "Or" (OrInsert, OrUpdate,
+   * OrRemove) that are a no-op in case the entry does already exist resp. does
+   * not exist. There are also variants OrInsertWith and OrUpdateWith that don't
+   * accept a value, but a functor, which is only called if the operation takes
+   * place, which should be used if the provision of the value is not trivial
+   * (e.g. allocates a heap object). Finally, there's InsertOrUpdate that
+   * handles both existing and non-existing entries.
+   *
+   * Note that all functions of EntryHandle only deal with DataType, not with
+   * UserDataType.
+   */
+  class EntryHandle : protected nsTHashtable<EntryType>::EntryHandle {
+   public:
+    using Base = typename nsTHashtable<EntryType>::EntryHandle;
+
+    EntryHandle(EntryHandle&& aOther) = default;
+    ~EntryHandle() = default;
+
+    EntryHandle(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&&) = delete;
+
+    using Base::Key;
+
+    using Base::HasEntry;
+
+    using Base::operator bool;
+
+    using Base::Entry;
+
+    /**
+     * Inserts a new entry with the handle's key and the value passed to this
+     * function.
+     *
+     * \tparam Args DataType must be constructible from Args
+     * \pre !HasEntry()
+     * \post HasEntry()
+     */
+    template <typename... Args>
+    DataType& Insert(Args&&... aArgs) {
+      Base::InsertInternal(std::forward<Args>(aArgs)...);
+      return Data();
+    }
+
+    /**
+     * If it doesn't yet exist, inserts a new entry with the handle's key and
+     * the value passed to this function. The value is not consumed if no insert
+     * takes place.
+     *
+     * \tparam Args DataType must be constructible from Args
+     * \post HasEntry()
+     */
+    template <typename... Args>
+    DataType& OrInsert(Args&&... aArgs) {
+      if (!HasEntry()) {
+        return Insert(std::forward<Args>(aArgs)...);
+      }
+      return Data();
+    }
+
+    /**
+     * If it doesn't yet exist, inserts a new entry with the handle's key and
+     * the result of the functor passed to this function. The functor is not
+     * called if no insert takes place.
+     *
+     * \tparam F must return a value that is implicitly convertible to DataType
+     * \post HasEntry()
+     */
+    template <typename F>
+    DataType& OrInsertWith(F&& aFunc) {
+      if (!HasEntry()) {
+        return Insert(std::forward<F>(aFunc)());
+      }
+      return Data();
+    }
+
+    /**
+     * Updates the entry with the handle's key by the value passed to this
+     * function.
+     *
+     * \tparam U DataType must be assignable from U
+     * \pre HasEntry()
+     */
+    template <typename U>
+    DataType& Update(U&& aData) {
+      MOZ_RELEASE_ASSERT(HasEntry());
+      Data() = std::forward<U>(aData);
+      return Data();
+    }
+
+    /**
+     * If an entry with the handle's key already exists, updates its value by
+     * the value passed to this function. The value is not consumed if no update
+     * takes place.
+     *
+     * \tparam U DataType must be assignable from U
+     */
+    template <typename U>
+    void OrUpdate(U&& aData) {
+      if (HasEntry()) {
+        Update(std::forward<U>(aData));
+      }
+    }
+
+    /**
+     * If an entry with the handle's key already exists, updates its value by
+     * the the result of the functor passed to this function. The functor is not
+     * called if no update takes place.
+     *
+     * \tparam F must return a value that DataType is assignable from
+     */
+    template <typename F>
+    void OrUpdateWith(F&& aFunc) {
+      if (HasEntry()) {
+        Update(std::forward<F>(aFunc)());
+      }
+    }
+
+    /**
+     * If it does not yet, inserts a new entry with the handle's key and the
+     * value passed to this function. Otherwise, it updates the entry by the
+     * value passed to this function.
+     *
+     * \tparam U DataType must be implicitly convertible (and assignable) from U
+     * \post HasEntry()
+     */
+    template <typename U>
+    DataType& InsertOrUpdate(U&& aData) {
+      if (!HasEntry()) {
+        Insert(std::forward<U>(aData));
+      } else {
+        Update(std::forward<U>(aData));
+      }
+      return Data();
+    }
+
+    using Base::Remove;
+
+    using Base::OrRemove;
+
+    /**
+     * Returns a reference to the value of the entry.
+     *
+     * \pre HasEntry()
+     */
+    [[nodiscard]] DataType& Data() { return Entry()->mData; }
+
+    [[nodiscard]] DataType* DataPtrOrNull() {
+      return static_cast<bool>(*this) ? &Data() : nullptr;
+    }
+
+    [[nodiscard]] DataType* operator->() { return &Data(); }
+
+    [[nodiscard]] DataType& operator*() { return Data(); }
+
+   private:
+    friend class nsBaseHashtable;
+
+    explicit EntryHandle(Base&& aBase) : Base(std::move(aBase)) {}
+  };
+
+  /**
+   * Performs a scoped operation on the entry for aKey, which may or may not
+   * exist when the function is called. It calls aFunc with an EntryHandle. The
+   * result of aFunc is returned as the result of this function. Its return type
+   * may be void. See the documentation of EntryHandle for the query and
+   * modifying operations it offers.
+   *
+   * A simple use of this function is, e.g.,
+   *
+   *   hashtable.WithEntryHandle(key, [](auto&& entry) { entry.OrInsert(42); });
+   *
+   * \attention It is not safe to perform modifying operations on the hashtable
+   * other than through the EntryHandle within aFunc, and trying to do so will
+   * trigger debug assertions, and result in undefined behaviour otherwise.
+   */
+  template <class F>
+  [[nodiscard]] auto WithEntryHandle(KeyType aKey, F&& aFunc)
+      -> std::invoke_result_t<F, EntryHandle&&> {
+    return Base::WithEntryHandle(
+        aKey, [&aFunc](auto entryHandle) -> decltype(auto) {
+          return std::forward<F>(aFunc)(EntryHandle{std::move(entryHandle)});
+        });
+  }
+
+  /**
+   * Fallible variant of WithEntryHandle, with the following differences:
+   * - The functor aFunc must accept a Maybe<EntryHandle> (instead of an
+   *   EntryHandle).
+   * - In case allocation of the slot for the entry fails, Nothing is passed to
+   *   the functor.
+   *
+   * For more details, see the explanation on the non-fallible overload above.
+   */
+  template <class F>
+  [[nodiscard]] auto WithEntryHandle(KeyType aKey, const fallible_t& aFallible,
+                                     F&& aFunc)
+      -> std::invoke_result_t<F, mozilla::Maybe<EntryHandle>&&> {
+    return Base::WithEntryHandle(
+        aKey, aFallible, [&aFunc](auto maybeEntryHandle) {
+          return std::forward<F>(aFunc)(
+              maybeEntryHandle
+                  ? mozilla::Some(EntryHandle{maybeEntryHandle.extract()})
+                  : mozilla::Nothing());
+        });
+  }
+
+ public:
+  class ConstIterator {
+   public:
+    explicit ConstIterator(nsBaseHashtable* aTable)
+        : mBaseIterator(&aTable->mTable) {}
+    ~ConstIterator() = default;
+
+    KeyType Key() const {
+      return static_cast<EntryType*>(mBaseIterator.Get())->GetKey();
+    }
+    UserDataType UserData() const {
+      return Converter::Unwrap(
+          static_cast<EntryType*>(mBaseIterator.Get())->mData);
+    }
+    const DataType& Data() const {
+      return static_cast<EntryType*>(mBaseIterator.Get())->mData;
+    }
+
+    bool Done() const { return mBaseIterator.Done(); }
+    void Next() { mBaseIterator.Next(); }
+
+    ConstIterator() = delete;
+    ConstIterator(const ConstIterator&) = delete;
+    ConstIterator(ConstIterator&& aOther) = delete;
+    ConstIterator& operator=(const ConstIterator&) = delete;
+    ConstIterator& operator=(ConstIterator&&) = delete;
+
+   protected:
+    PLDHashTable::Iterator mBaseIterator;
+  };
+
+  // This is an iterator that also allows entry removal. Example usage:
+  //
+  //   for (auto iter = table.Iter(); !iter.Done(); iter.Next()) {
+  //     const KeyType key = iter.Key();
+  //     const UserDataType data = iter.UserData();
+  //     // or
+  //     const DataType& data = iter.Data();
+  //     // ... do stuff with |key| and/or |data| ...
+  //     // ... possibly call iter.Remove() once ...
+  //   }
+  //
+  class Iterator final : public ConstIterator {
+   public:
+    using ConstIterator::ConstIterator;
+
+    using ConstIterator::Data;
+    DataType& Data() {
+      return static_cast<EntryType*>(this->mBaseIterator.Get())->mData;
+    }
+
+    void Remove() { this->mBaseIterator.Remove(); }
+  };
+
+  Iterator Iter() { return Iterator(this); }
+
+  ConstIterator ConstIter() const {
+    return ConstIterator(const_cast<nsBaseHashtable*>(this));
+  }
+
+  using nsTHashtable<EntryType>::Remove;
+
+  /**
+   * Remove the entry associated with aIter.
+   *
+   * @param aIter the iterator pointing to the entry
+   * @pre !aIter.Done()
+   */
+  void Remove(ConstIterator& aIter) { aIter.mBaseIterator.Remove(); }
+
+  using typename nsTHashtable<EntryType>::iterator;
+  using typename nsTHashtable<EntryType>::const_iterator;
+
+  using nsTHashtable<EntryType>::begin;
+  using nsTHashtable<EntryType>::end;
+  using nsTHashtable<EntryType>::cbegin;
+  using nsTHashtable<EntryType>::cend;
+
+  using nsTHashtable<EntryType>::Keys;
+
+  /**
+   * Return a range of the values (of DataType). Note this range iterates over
+   * the values in place, so modifications to the nsTHashtable invalidate the
+   * range while it's iterated, except when calling Remove() with a value
+   * iterator derived from that range.
+   */
+  auto Values() const {
+    return mozilla::detail::nsBaseHashtableValueRange<EntryType>{this->mTable};
+  }
+
+  /**
+   * Remove an entry from a value range, specified via a value iterator, e.g.
+   *
+   * for (auto it = hash.Values().begin(), end = hash.Values().end();
+   *      it != end; * ++it) {
+   *   if (*it > 42) { hash.Remove(it); }
+   * }
+   *
+   * You might also consider using RemoveIf though.
+   */
+  void Remove(mozilla::detail::nsBaseHashtableValueIterator<EntryType>& aIter) {
+    aIter.mIterator.Remove();
+  }
+
+  /**
+   * reset the hashtable, removing all entries
+   */
+  void Clear() { nsTHashtable<EntryType>::Clear(); }
+
+  /**
+   * Measure the size of the table's entry storage. The size of things pointed
+   * to by entries must be measured separately; hence the "Shallow" prefix.
+   *
+   * @param   aMallocSizeOf the function used to measure heap-allocated blocks
+   * @return  the summed size of the table's storage
+   */
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return this->mTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /**
+   * Like ShallowSizeOfExcludingThis, but includes sizeof(*this).
+   */
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /**
+   * Swap the elements in this hashtable with the elements in aOther.
+   */
+  void SwapElements(nsBaseHashtable& aOther) {
+    nsTHashtable<EntryType>::SwapElements(aOther);
+  }
+
+  using nsTHashtable<EntryType>::MarkImmutable;
+
+  /**
+   * Makes a clone of this hashtable by copying all entries. This requires
+   * KeyType and DataType to be copy-constructible.
+   */
+  nsBaseHashtable Clone() const { return CloneAs<nsBaseHashtable>(); }
+
+ protected:
+  template <typename T>
+  T CloneAs() const {
+    static_assert(std::is_base_of_v<nsBaseHashtable, T>);
+    // XXX This can probably be optimized, see Bug 1694368.
+    T result(Count());
+    for (const auto& srcEntry : *this) {
+      result.WithEntryHandle(srcEntry.GetKey(), [&](auto&& dstEntry) {
+        dstEntry.Insert(srcEntry.GetData());
+      });
+    }
+    return result;
+  }
+};
+
+//
+// nsBaseHashtableET definitions
+//
+
+template <class KeyClass, class DataType>
+template <typename... Args>
+nsBaseHashtableET<KeyClass, DataType>::nsBaseHashtableET(KeyTypePointer aKey,
+                                                         Args&&... aArgs)
+    : KeyClass(aKey), mData(std::forward<Args>(aArgs)...) {}
+
+#endif  // nsBaseHashtable_h__
diff --git a/xpcom/ds/nsCOMArray.cpp b/xpcom/ds/nsCOMArray.cpp
new file mode 100644
index 0000000000..d388857573
--- /dev/null
+++ b/xpcom/ds/nsCOMArray.cpp
@@ -0,0 +1,252 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsCOMArray.h"
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "nsQuickSort.h"
+
+#include "nsCOMPtr.h"
+
+// This specialization is private to nsCOMArray.
+// It exists solely to automatically zero-out newly created array elements.
+template <>
+class nsTArrayElementTraits<nsISupports*> {
+  typedef nsISupports* E;
+
+ public:
+  // Zero out the value
+  static inline void Construct(E* aE) {
+    new (mozilla::KnownNotNull, static_cast<void*>(aE)) E();
+  }
+  // Invoke the copy-constructor in place.
+  template <class A>
+  static inline void Construct(E* aE, const A& aArg) {
+    new (mozilla::KnownNotNull, static_cast<void*>(aE)) E(aArg);
+  }
+  // Construct in place.
+  template <class... Args>
+  static inline void Emplace(E* aE, Args&&... aArgs) {
+    new (mozilla::KnownNotNull, static_cast<void*>(aE))
+        E(std::forward<Args>(aArgs)...);
+  }
+  // Invoke the destructor in place.
+  static inline void Destruct(E* aE) { aE->~E(); }
+};
+
+static void ReleaseObjects(nsTArray<nsISupports*>& aArray);
+
+// implementations of non-trivial methods in nsCOMArray_base
+
+nsCOMArray_base::nsCOMArray_base(const nsCOMArray_base& aOther) {
+  // make sure we do only one allocation
+  mArray.SetCapacity(aOther.Count());
+  AppendObjects(aOther);
+}
+
+nsCOMArray_base::~nsCOMArray_base() { Clear(); }
+
+int32_t nsCOMArray_base::IndexOf(nsISupports* aObject,
+                                 uint32_t aStartIndex) const {
+  return mArray.IndexOf(aObject, aStartIndex);
+}
+
+int32_t nsCOMArray_base::IndexOfObject(nsISupports* aObject) const {
+  nsCOMPtr<nsISupports> supports = do_QueryInterface(aObject);
+  if (NS_WARN_IF(!supports)) {
+    return -1;
+  }
+
+  uint32_t i, count;
+  int32_t retval = -1;
+  count = mArray.Length();
+  for (i = 0; i < count; ++i) {
+    nsCOMPtr<nsISupports> arrayItem = do_QueryInterface(mArray[i]);
+    if (arrayItem == supports) {
+      retval = i;
+      break;
+    }
+  }
+  return retval;
+}
+
+bool nsCOMArray_base::EnumerateForwards(nsBaseArrayEnumFunc aFunc,
+                                        void* aData) const {
+  for (uint32_t index = 0; index < mArray.Length(); ++index) {
+    if (!(*aFunc)(mArray[index], aData)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+bool nsCOMArray_base::EnumerateBackwards(nsBaseArrayEnumFunc aFunc,
+                                         void* aData) const {
+  for (uint32_t index = mArray.Length(); index--;) {
+    if (!(*aFunc)(mArray[index], aData)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+int nsCOMArray_base::VoidStarComparator(const void* aElement1,
+                                        const void* aElement2, void* aData) {
+  auto ctx = static_cast<nsISupportsComparatorContext*>(aData);
+  return (*ctx->mComparatorFunc)(*static_cast<nsISupports* const*>(aElement1),
+                                 *static_cast<nsISupports* const*>(aElement2),
+                                 ctx->mData);
+}
+
+void nsCOMArray_base::Sort(nsISupportsComparatorFunc aFunc, void* aData) {
+  if (mArray.Length() > 1) {
+    nsISupportsComparatorContext ctx = {aFunc, aData};
+    NS_QuickSort(mArray.Elements(), mArray.Length(), sizeof(nsISupports*),
+                 VoidStarComparator, &ctx);
+  }
+}
+
+bool nsCOMArray_base::InsertObjectAt(nsISupports* aObject, int32_t aIndex) {
+  if ((uint32_t)aIndex > mArray.Length()) {
+    return false;
+  }
+
+  mArray.InsertElementAt(aIndex, aObject);
+
+  NS_IF_ADDREF(aObject);
+  return true;
+}
+
+void nsCOMArray_base::InsertElementAt(uint32_t aIndex, nsISupports* aElement) {
+  mArray.InsertElementAt(aIndex, aElement);
+  NS_IF_ADDREF(aElement);
+}
+
+void nsCOMArray_base::InsertElementAt(uint32_t aIndex,
+                                      already_AddRefed<nsISupports> aElement) {
+  mArray.InsertElementAt(aIndex, aElement.take());
+}
+
+bool nsCOMArray_base::InsertObjectsAt(const nsCOMArray_base& aObjects,
+                                      int32_t aIndex) {
+  if ((uint32_t)aIndex > mArray.Length()) {
+    return false;
+  }
+
+  mArray.InsertElementsAt(aIndex, aObjects.mArray);
+
+  // need to addref all these
+  uint32_t count = aObjects.Length();
+  for (uint32_t i = 0; i < count; ++i) {
+    NS_IF_ADDREF(aObjects[i]);
+  }
+
+  return true;
+}
+
+void nsCOMArray_base::InsertElementsAt(uint32_t aIndex,
+                                       const nsCOMArray_base& aElements) {
+  mArray.InsertElementsAt(aIndex, aElements.mArray);
+
+  // need to addref all these
+  uint32_t count = aElements.Length();
+  for (uint32_t i = 0; i < count; ++i) {
+    NS_IF_ADDREF(aElements[i]);
+  }
+}
+
+void nsCOMArray_base::InsertElementsAt(uint32_t aIndex,
+                                       nsISupports* const* aElements,
+                                       uint32_t aCount) {
+  mArray.InsertElementsAt(aIndex, aElements, aCount);
+
+  // need to addref all these
+  for (uint32_t i = 0; i < aCount; ++i) {
+    NS_IF_ADDREF(aElements[i]);
+  }
+}
+
+void nsCOMArray_base::ReplaceObjectAt(nsISupports* aObject, int32_t aIndex) {
+  mArray.EnsureLengthAtLeast(aIndex + 1);
+  nsISupports* oldObject = mArray[aIndex];
+  // Make sure to addref first, in case aObject == oldObject
+  NS_IF_ADDREF(mArray[aIndex] = aObject);
+  NS_IF_RELEASE(oldObject);
+}
+
+bool nsCOMArray_base::RemoveObject(nsISupports* aObject) {
+  bool result = mArray.RemoveElement(aObject);
+  if (result) {
+    NS_IF_RELEASE(aObject);
+  }
+  return result;
+}
+
+bool nsCOMArray_base::RemoveObjectAt(int32_t aIndex) {
+  if (uint32_t(aIndex) < mArray.Length()) {
+    nsISupports* element = mArray[aIndex];
+
+    mArray.RemoveElementAt(aIndex);
+    NS_IF_RELEASE(element);
+    return true;
+  }
+
+  return false;
+}
+
+void nsCOMArray_base::RemoveElementAt(uint32_t aIndex) {
+  nsISupports* element = mArray[aIndex];
+  mArray.RemoveElementAt(aIndex);
+  NS_IF_RELEASE(element);
+}
+
+bool nsCOMArray_base::RemoveObjectsAt(int32_t aIndex, int32_t aCount) {
+  if (uint32_t(aIndex) + uint32_t(aCount) <= mArray.Length()) {
+    nsTArray<nsISupports*> elementsToDestroy(aCount);
+    elementsToDestroy.AppendElements(mArray.Elements() + aIndex, aCount);
+    mArray.RemoveElementsAt(aIndex, aCount);
+    ReleaseObjects(elementsToDestroy);
+    return true;
+  }
+
+  return false;
+}
+
+void nsCOMArray_base::RemoveElementsAt(uint32_t aIndex, uint32_t aCount) {
+  nsTArray<nsISupports*> elementsToDestroy(aCount);
+  elementsToDestroy.AppendElements(mArray.Elements() + aIndex, aCount);
+  mArray.RemoveElementsAt(aIndex, aCount);
+  ReleaseObjects(elementsToDestroy);
+}
+
+// useful for destructors
+void ReleaseObjects(nsTArray<nsISupports*>& aArray) {
+  for (uint32_t i = 0; i < aArray.Length(); ++i) {
+    NS_IF_RELEASE(aArray[i]);
+  }
+}
+
+void nsCOMArray_base::Clear() {
+  nsTArray<nsISupports*> objects = std::move(mArray);
+  ReleaseObjects(objects);
+}
+
+bool nsCOMArray_base::SetCount(int32_t aNewCount) {
+  NS_ASSERTION(aNewCount >= 0, "SetCount(negative index)");
+  if (aNewCount < 0) {
+    return false;
+  }
+
+  int32_t count = mArray.Length();
+  if (count > aNewCount) {
+    RemoveObjectsAt(aNewCount, mArray.Length() - aNewCount);
+  }
+  mArray.SetLength(aNewCount);
+  return true;
+}
diff --git a/xpcom/ds/nsCOMArray.h b/xpcom/ds/nsCOMArray.h
new file mode 100644
index 0000000000..0ea1437f25
--- /dev/null
+++ b/xpcom/ds/nsCOMArray.h
@@ -0,0 +1,398 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsCOMArray_h__
+#define nsCOMArray_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/ArrayIterator.h"
+#include "mozilla/MemoryReporting.h"
+
+#include "nsCycleCollectionNoteChild.h"
+#include "nsTArray.h"
+#include "nsISupports.h"
+
+#include <iterator>
+
+// See below for the definition of nsCOMArray<T>
+
+// a class that's nsISupports-specific, so that we can contain the
+// work of this class in the XPCOM dll
+class nsCOMArray_base {
+  friend class nsArrayBase;
+
+ protected:
+  nsCOMArray_base() = default;
+  explicit nsCOMArray_base(int32_t aCount) : mArray(aCount) {}
+  nsCOMArray_base(const nsCOMArray_base& aOther);
+  nsCOMArray_base(nsCOMArray_base&& aOther) = default;
+  nsCOMArray_base& operator=(nsCOMArray_base&& aOther) = default;
+  ~nsCOMArray_base();
+
+  int32_t IndexOf(nsISupports* aObject, uint32_t aStartIndex = 0) const;
+  bool Contains(nsISupports* aObject) const { return IndexOf(aObject) != -1; }
+
+  int32_t IndexOfObject(nsISupports* aObject) const;
+  bool ContainsObject(nsISupports* aObject) const {
+    return IndexOfObject(aObject) != -1;
+  }
+
+  typedef bool (*nsBaseArrayEnumFunc)(void* aElement, void* aData);
+
+  // enumerate through the array with a callback.
+  bool EnumerateForwards(nsBaseArrayEnumFunc aFunc, void* aData) const;
+
+  bool EnumerateBackwards(nsBaseArrayEnumFunc aFunc, void* aData) const;
+
+  typedef int (*nsISupportsComparatorFunc)(nsISupports* aElement1,
+                                           nsISupports* aElement2, void* aData);
+
+  struct nsISupportsComparatorContext {
+    nsISupportsComparatorFunc mComparatorFunc;
+    void* mData;
+  };
+
+  static int VoidStarComparator(const void* aElement1, const void* aElement2,
+                                void* aData);
+  void Sort(nsISupportsComparatorFunc aFunc, void* aData);
+
+  bool InsertObjectAt(nsISupports* aObject, int32_t aIndex);
+  void InsertElementAt(uint32_t aIndex, nsISupports* aElement);
+  void InsertElementAt(uint32_t aIndex, already_AddRefed<nsISupports> aElement);
+  bool InsertObjectsAt(const nsCOMArray_base& aObjects, int32_t aIndex);
+  void InsertElementsAt(uint32_t aIndex, const nsCOMArray_base& aElements);
+  void InsertElementsAt(uint32_t aIndex, nsISupports* const* aElements,
+                        uint32_t aCount);
+  void ReplaceObjectAt(nsISupports* aObject, int32_t aIndex);
+  void ReplaceElementAt(uint32_t aIndex, nsISupports* aElement) {
+    nsISupports* oldElement = mArray[aIndex];
+    NS_IF_ADDREF(mArray[aIndex] = aElement);
+    NS_IF_RELEASE(oldElement);
+  }
+  bool AppendObject(nsISupports* aObject) {
+    return InsertObjectAt(aObject, Count());
+  }
+  void AppendElement(nsISupports* aElement) {
+    InsertElementAt(Length(), aElement);
+  }
+  void AppendElement(already_AddRefed<nsISupports> aElement) {
+    InsertElementAt(Length(), std::move(aElement));
+  }
+
+  bool AppendObjects(const nsCOMArray_base& aObjects) {
+    return InsertObjectsAt(aObjects, Count());
+  }
+  void AppendElements(const nsCOMArray_base& aElements) {
+    return InsertElementsAt(Length(), aElements);
+  }
+  void AppendElements(nsISupports* const* aElements, uint32_t aCount) {
+    return InsertElementsAt(Length(), aElements, aCount);
+  }
+  bool RemoveObject(nsISupports* aObject);
+  nsISupports** Elements() { return mArray.Elements(); }
+  void SwapElements(nsCOMArray_base& aOther) {
+    mArray.SwapElements(aOther.mArray);
+  }
+
+ public:
+  // elements in the array (including null elements!)
+  int32_t Count() const { return mArray.Length(); }
+  // nsTArray-compatible version
+  uint32_t Length() const { return mArray.Length(); }
+  bool IsEmpty() const { return mArray.IsEmpty(); }
+
+  // If the array grows, the newly created entries will all be null;
+  // if the array shrinks, the excess entries will all be released.
+  bool SetCount(int32_t aNewCount);
+  // nsTArray-compatible version
+  void TruncateLength(uint32_t aNewLength) {
+    if (mArray.Length() > aNewLength) {
+      RemoveElementsAt(aNewLength, mArray.Length() - aNewLength);
+    }
+  }
+
+  // remove all elements in the array, and call NS_RELEASE on each one
+  void Clear();
+
+  nsISupports* ObjectAt(int32_t aIndex) const { return mArray[aIndex]; }
+  // nsTArray-compatible version
+  nsISupports* ElementAt(uint32_t aIndex) const { return mArray[aIndex]; }
+
+  nsISupports* SafeObjectAt(int32_t aIndex) const {
+    return mArray.SafeElementAt(aIndex, nullptr);
+  }
+  // nsTArray-compatible version
+  nsISupports* SafeElementAt(uint32_t aIndex) const {
+    return mArray.SafeElementAt(aIndex, nullptr);
+  }
+
+  nsISupports* operator[](int32_t aIndex) const { return mArray[aIndex]; }
+
+  // remove an element at a specific position, shrinking the array
+  // as necessary
+  bool RemoveObjectAt(int32_t aIndex);
+  // nsTArray-compatible version
+  void RemoveElementAt(uint32_t aIndex);
+
+  // remove a range of elements at a specific position, shrinking the array
+  // as necessary
+  bool RemoveObjectsAt(int32_t aIndex, int32_t aCount);
+  // nsTArray-compatible version
+  void RemoveElementsAt(uint32_t aIndex, uint32_t aCount);
+
+  void SwapElementsAt(uint32_t aIndex1, uint32_t aIndex2) {
+    nsISupports* tmp = mArray[aIndex1];
+    mArray[aIndex1] = mArray[aIndex2];
+    mArray[aIndex2] = tmp;
+  }
+
+  // Ensures there is enough space to store a total of aCapacity objects.
+  // This method never deletes any objects.
+  void SetCapacity(uint32_t aCapacity) { mArray.SetCapacity(aCapacity); }
+  uint32_t Capacity() { return mArray.Capacity(); }
+
+  // Measures the size of the array's element storage. If you want to measure
+  // anything hanging off the array, you must iterate over the elements and
+  // measure them individually; hence the "Shallow" prefix. Note that because
+  // each element in an nsCOMArray<T> is actually a T* any such iteration
+  // should use a SizeOfIncludingThis() function on each element rather than a
+  // SizeOfExcludingThis() function, so that the memory taken by the T itself
+  // is included as well as anything it points to.
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return mArray.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+ private:
+  // the actual storage
+  nsTArray<nsISupports*> mArray;
+
+  // don't implement these, defaults will muck with refcounts!
+  nsCOMArray_base& operator=(const nsCOMArray_base& aOther) = delete;
+};
+
+inline void ImplCycleCollectionUnlink(nsCOMArray_base& aField) {
+  aField.Clear();
+}
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, nsCOMArray_base& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+  int32_t length = aField.Count();
+  for (int32_t i = 0; i < length; ++i) {
+    CycleCollectionNoteChild(aCallback, aField[i], aName, aFlags);
+  }
+}
+
+// a non-XPCOM, refcounting array of XPCOM objects
+// used as a member variable or stack variable - this object is NOT
+// refcounted, but the objects that it holds are
+//
+// most of the read-only accessors like ObjectAt()/etc do NOT refcount
+// on the way out. This means that you can do one of two things:
+//
+// * does an addref, but holds onto a reference
+// nsCOMPtr<T> foo = array[i];
+//
+// * avoids the refcount, but foo might go stale if array[i] is ever
+// * modified/removed. Be careful not to NS_RELEASE(foo)!
+// T* foo = array[i];
+//
+// This array will accept null as an argument for any object, and will store
+// null in the array. But that also means that methods like ObjectAt() may
+// return null when referring to an existing, but null entry in the array.
+template <class T>
+class nsCOMArray : public nsCOMArray_base {
+ public:
+  typedef int32_t index_type;
+  typedef mozilla::ArrayIterator<T*, nsCOMArray> iterator;
+  typedef mozilla::ArrayIterator<const T*, nsCOMArray> const_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  nsCOMArray() = default;
+  explicit nsCOMArray(int32_t aCount) : nsCOMArray_base(aCount) {}
+  explicit nsCOMArray(const nsCOMArray<T>& aOther) : nsCOMArray_base(aOther) {}
+  nsCOMArray(nsCOMArray<T>&& aOther) = default;
+  ~nsCOMArray() = default;
+
+  // We have a move assignment operator, but no copy assignment operator.
+  nsCOMArray<T>& operator=(nsCOMArray<T>&& aOther) = default;
+
+  // these do NOT refcount on the way out, for speed
+  T* ObjectAt(int32_t aIndex) const {
+    return static_cast<T*>(nsCOMArray_base::ObjectAt(aIndex));
+  }
+  // nsTArray-compatible version
+  T* ElementAt(uint32_t aIndex) const {
+    return static_cast<T*>(nsCOMArray_base::ElementAt(aIndex));
+  }
+
+  // these do NOT refcount on the way out, for speed
+  T* SafeObjectAt(int32_t aIndex) const {
+    return static_cast<T*>(nsCOMArray_base::SafeObjectAt(aIndex));
+  }
+  // nsTArray-compatible version
+  T* SafeElementAt(uint32_t aIndex) const {
+    return static_cast<T*>(nsCOMArray_base::SafeElementAt(aIndex));
+  }
+
+  // indexing operator for syntactic sugar
+  T* operator[](int32_t aIndex) const { return ObjectAt(aIndex); }
+
+  // index of the element in question.. does NOT refcount
+  // note: this does not check COM object identity. Use
+  // IndexOfObject() for that purpose
+  int32_t IndexOf(T* aObject, uint32_t aStartIndex = 0) const {
+    return nsCOMArray_base::IndexOf(aObject, aStartIndex);
+  }
+  bool Contains(T* aObject) const { return nsCOMArray_base::Contains(aObject); }
+
+  // index of the element in question.. be careful!
+  // this is much slower than IndexOf() because it uses
+  // QueryInterface to determine actual COM identity of the object
+  // if you need to do this frequently then consider enforcing
+  // COM object identity before adding/comparing elements
+  int32_t IndexOfObject(T* aObject) const {
+    return nsCOMArray_base::IndexOfObject(aObject);
+  }
+  bool ContainsObject(nsISupports* aObject) const {
+    return nsCOMArray_base::ContainsObject(aObject);
+  }
+
+  // inserts aObject at aIndex, shifting the objects at aIndex and
+  // later to make space
+  bool InsertObjectAt(T* aObject, int32_t aIndex) {
+    return nsCOMArray_base::InsertObjectAt(aObject, aIndex);
+  }
+  // nsTArray-compatible version
+  void InsertElementAt(uint32_t aIndex, T* aElement) {
+    nsCOMArray_base::InsertElementAt(aIndex, aElement);
+  }
+
+  // inserts the objects from aObject at aIndex, shifting the
+  // objects at aIndex and later to make space
+  bool InsertObjectsAt(const nsCOMArray<T>& aObjects, int32_t aIndex) {
+    return nsCOMArray_base::InsertObjectsAt(aObjects, aIndex);
+  }
+  // nsTArray-compatible version
+  void InsertElementsAt(uint32_t aIndex, const nsCOMArray<T>& aElements) {
+    nsCOMArray_base::InsertElementsAt(aIndex, aElements);
+  }
+  void InsertElementsAt(uint32_t aIndex, T* const* aElements, uint32_t aCount) {
+    nsCOMArray_base::InsertElementsAt(
+        aIndex, reinterpret_cast<nsISupports* const*>(aElements), aCount);
+  }
+
+  // replaces an existing element. Warning: if the array grows,
+  // the newly created entries will all be null
+  void ReplaceObjectAt(T* aObject, int32_t aIndex) {
+    nsCOMArray_base::ReplaceObjectAt(aObject, aIndex);
+  }
+  // nsTArray-compatible version
+  void ReplaceElementAt(uint32_t aIndex, T* aElement) {
+    nsCOMArray_base::ReplaceElementAt(aIndex, aElement);
+  }
+
+  typedef int (*TComparatorFunc)(T* aElement1, T* aElement2, void* aData);
+
+  struct TComparatorContext {
+    TComparatorFunc mComparatorFunc;
+    void* mData;
+  };
+
+  static int nsISupportsComparator(nsISupports* aElement1,
+                                   nsISupports* aElement2, void* aData) {
+    auto ctx = static_cast<TComparatorContext*>(aData);
+    return (*ctx->mComparatorFunc)(static_cast<T*>(aElement1),
+                                   static_cast<T*>(aElement2), ctx->mData);
+  }
+
+  void Sort(TComparatorFunc aFunc, void* aData) {
+    TComparatorContext ctx = {aFunc, aData};
+    nsCOMArray_base::Sort(nsISupportsComparator, &ctx);
+  }
+
+  // append an object, growing the array as necessary
+  bool AppendObject(T* aObject) {
+    return nsCOMArray_base::AppendObject(aObject);
+  }
+  // nsTArray-compatible version
+  void AppendElement(T* aElement) { nsCOMArray_base::AppendElement(aElement); }
+  void AppendElement(already_AddRefed<T> aElement) {
+    nsCOMArray_base::AppendElement(std::move(aElement));
+  }
+
+  // append objects, growing the array as necessary
+  bool AppendObjects(const nsCOMArray<T>& aObjects) {
+    return nsCOMArray_base::AppendObjects(aObjects);
+  }
+  // nsTArray-compatible version
+  void AppendElements(const nsCOMArray<T>& aElements) {
+    return nsCOMArray_base::AppendElements(aElements);
+  }
+  void AppendElements(T* const* aElements, uint32_t aCount) {
+    InsertElementsAt(Length(), aElements, aCount);
+  }
+
+  // remove the first instance of the given object and shrink the
+  // array as necessary
+  // Warning: if you pass null here, it will remove the first null element
+  bool RemoveObject(T* aObject) {
+    return nsCOMArray_base::RemoveObject(aObject);
+  }
+  // nsTArray-compatible version
+  bool RemoveElement(T* aElement) {
+    return nsCOMArray_base::RemoveObject(aElement);
+  }
+
+  T** Elements() { return reinterpret_cast<T**>(nsCOMArray_base::Elements()); }
+  void SwapElements(nsCOMArray<T>& aOther) {
+    nsCOMArray_base::SwapElements(aOther);
+  }
+
+  // Methods for range-based for loops.
+  iterator begin() { return iterator(*this, 0); }
+  const_iterator begin() const { return const_iterator(*this, 0); }
+  const_iterator cbegin() const { return begin(); }
+  iterator end() { return iterator(*this, Length()); }
+  const_iterator end() const { return const_iterator(*this, Length()); }
+  const_iterator cend() const { return end(); }
+
+  // Methods for reverse iterating.
+  reverse_iterator rbegin() { return reverse_iterator(end()); }
+  const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+  const_reverse_iterator crbegin() const { return rbegin(); }
+  reverse_iterator rend() { return reverse_iterator(begin()); }
+  const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+  const_reverse_iterator crend() const { return rend(); }
+
+ private:
+  // don't implement these!
+  nsCOMArray<T>& operator=(const nsCOMArray<T>& aOther) = delete;
+};
+
+template <typename T>
+inline void ImplCycleCollectionUnlink(nsCOMArray<T>& aField) {
+  aField.Clear();
+}
+
+template <typename E>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, nsCOMArray<E>& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+  int32_t length = aField.Count();
+  for (int32_t i = 0; i < length; ++i) {
+    CycleCollectionNoteChild(aCallback, aField[i], aName, aFlags);
+  }
+}
+
+#endif
diff --git a/xpcom/ds/nsCRT.cpp b/xpcom/ds/nsCRT.cpp
new file mode 100644
index 0000000000..9d2fcb7120
--- /dev/null
+++ b/xpcom/ds/nsCRT.cpp
@@ -0,0 +1,123 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * MODULE NOTES:
+ * @update  gess7/30/98
+ *
+ * Much as I hate to do it, we were using string compares wrong.
+ * Often, programmers call functions like strcmp(s1,s2), and pass
+ * one or more null strings. Rather than blow up on these, I've
+ * added quick checks to ensure that cases like this don't cause
+ * us to fail.
+ *
+ * In general, if you pass a null into any of these string compare
+ * routines, we simply return 0.
+ */
+
+#include "nsCRT.h"
+#include "nsDebug.h"
+
+//----------------------------------------------------------------------
+
+////////////////////////////////////////////////////////////////////////////////
+// My lovely strtok routine
+
+#define IS_DELIM(m, c) ((m)[(c) >> 3] & (1 << ((c)&7)))
+#define SET_DELIM(m, c) ((m)[(c) >> 3] |= (1 << ((c)&7)))
+#define DELIM_TABLE_SIZE 32
+
+char* nsCRT::strtok(char* aString, const char* aDelims, char** aNewStr) {
+  NS_ASSERTION(aString,
+               "Unlike regular strtok, the first argument cannot be null.");
+
+  char delimTable[DELIM_TABLE_SIZE];
+  uint32_t i;
+  char* result;
+  char* str = aString;
+
+  for (i = 0; i < DELIM_TABLE_SIZE; ++i) {
+    delimTable[i] = '\0';
+  }
+
+  for (i = 0; aDelims[i]; i++) {
+    SET_DELIM(delimTable, static_cast<uint8_t>(aDelims[i]));
+  }
+  NS_ASSERTION(aDelims[i] == '\0', "too many delimiters");
+
+  // skip to beginning
+  while (*str && IS_DELIM(delimTable, static_cast<uint8_t>(*str))) {
+    str++;
+  }
+  result = str;
+
+  // fix up the end of the token
+  while (*str) {
+    if (IS_DELIM(delimTable, static_cast<uint8_t>(*str))) {
+      *str++ = '\0';
+      break;
+    }
+    str++;
+  }
+  *aNewStr = str;
+
+  return str == result ? nullptr : result;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+/**
+ * Compare unichar string ptrs, stopping at the 1st null
+ * NOTE: If both are null, we return 0.
+ * NOTE: We terminate the search upon encountering a nullptr
+ *
+ * @update  gess 11/10/99
+ * @param   s1 and s2 both point to unichar strings
+ * @return  0 if they match, -1 if s1<s2; 1 if s1>s2
+ */
+int32_t nsCRT::strcmp(const char16_t* aStr1, const char16_t* aStr2) {
+  if (aStr1 && aStr2) {
+    for (;;) {
+      char16_t c1 = *aStr1++;
+      char16_t c2 = *aStr2++;
+      if (c1 != c2) {
+        if (c1 < c2) {
+          return -1;
+        }
+        return 1;
+      }
+      if (c1 == 0 || c2 == 0) {
+        break;
+      }
+    }
+  } else {
+    if (aStr1) {  // aStr2 must have been null
+      return -1;
+    }
+    if (aStr2) {  // aStr1 must have been null
+      return 1;
+    }
+  }
+  return 0;
+}
+
+// This should use NSPR but NSPR isn't exporting its PR_strtoll function
+// Until then...
+int64_t nsCRT::atoll(const char* aStr) {
+  if (!aStr) {
+    return 0;
+  }
+
+  int64_t ll = 0;
+
+  while (*aStr && *aStr >= '0' && *aStr <= '9') {
+    ll *= 10;
+    ll += *aStr - '0';
+    aStr++;
+  }
+
+  return ll;
+}
diff --git a/xpcom/ds/nsCRT.h b/xpcom/ds/nsCRT.h
new file mode 100644
index 0000000000..14b46c5059
--- /dev/null
+++ b/xpcom/ds/nsCRT.h
@@ -0,0 +1,119 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef nsCRT_h___
+#define nsCRT_h___
+
+#include <stdlib.h>
+#include <ctype.h>
+#include "plstr.h"
+#include "nscore.h"
+#include "nsCRTGlue.h"
+
+#if defined(XP_WIN)
+#  define NS_LINEBREAK "\015\012"
+#  define NS_ULINEBREAK u"\015\012"
+#  define NS_LINEBREAK_LEN 2
+#else
+#  ifdef XP_UNIX
+#    define NS_LINEBREAK "\012"
+#    define NS_ULINEBREAK u"\012"
+#    define NS_LINEBREAK_LEN 1
+#  endif /* XP_UNIX */
+#endif   /* XP_WIN */
+
+/// This is a wrapper class around all the C runtime functions.
+
+class nsCRT {
+ public:
+  enum {
+    LF = '\n' /* Line Feed */,
+    VTAB = '\v' /* Vertical Tab */,
+    CR = '\r' /* Carriage Return */
+  };
+
+  /// String comparison.
+  static int32_t strcmp(const char* aStr1, const char* aStr2) {
+    return int32_t(PL_strcmp(aStr1, aStr2));
+  }
+
+  /// Case-insensitive string comparison.
+  static int32_t strcasecmp(const char* aStr1, const char* aStr2) {
+    /* Some functions like `PL_strcasecmp` are reimplementations
+     * of the their native POSIX counterparts, which breaks libFuzzer.
+     * For this purpose, we use the natives instead when fuzzing.
+     */
+#if defined(LIBFUZZER) && defined(LINUX)
+    return int32_t(::strcasecmp(aStr1, aStr2));
+#else
+    return int32_t(PL_strcasecmp(aStr1, aStr2));
+#endif
+  }
+
+  /// Case-insensitive string comparison with length
+  static int32_t strncasecmp(const char* aStr1, const char* aStr2,
+                             uint32_t aMaxLen) {
+#if defined(LIBFUZZER) && defined(LINUX)
+    int32_t result = int32_t(::strncasecmp(aStr1, aStr2, aMaxLen));
+#else
+    int32_t result = int32_t(PL_strncasecmp(aStr1, aStr2, aMaxLen));
+#endif
+    // Egads. PL_strncasecmp is returning *very* negative numbers.
+    // Some folks expect -1,0,1, so let's temper its enthusiasm.
+    if (result < 0) {
+      result = -1;
+    }
+    return result;
+  }
+
+  /// Case-insensitive substring search.
+  static char* strcasestr(const char* aStr1, const char* aStr2) {
+#if defined(LIBFUZZER) && defined(LINUX)
+    return const_cast<char*>(::strcasestr(aStr1, aStr2));
+#else
+    return PL_strcasestr(aStr1, aStr2);
+#endif
+  }
+
+  /**
+
+    How to use this fancy (thread-safe) version of strtok:
+
+    void main(void) {
+      printf("%s\n\nTokens:\n", string);
+      // Establish string and get the first token:
+      char* newStr;
+      token = nsCRT::strtok(string, seps, &newStr);
+      while (token != nullptr) {
+        // While there are tokens in "string"
+        printf(" %s\n", token);
+        // Get next token:
+        token = nsCRT::strtok(newStr, seps, &newStr);
+      }
+    }
+    * WARNING - STRTOK WHACKS str THE FIRST TIME IT IS CALLED *
+    * MAKE A COPY OF str IF YOU NEED TO USE IT AFTER strtok() *
+  */
+  static char* strtok(char* aStr, const char* aDelims, char** aNewStr);
+
+  /// Like strcmp except for ucs2 strings
+  static int32_t strcmp(const char16_t* aStr1, const char16_t* aStr2);
+
+  // String to longlong
+  static int64_t atoll(const char* aStr);
+
+  static char ToUpper(char aChar) { return NS_ToUpper(aChar); }
+  static char ToLower(char aChar) { return NS_ToLower(aChar); }
+
+  static bool IsAsciiSpace(char16_t aChar) {
+    return NS_IsAsciiWhitespace(aChar);
+  }
+};
+
+inline bool NS_IS_SPACE(char16_t aChar) {
+  return ((int(aChar) & 0x7f) == int(aChar)) && isspace(int(aChar));
+}
+
+#endif /* nsCRT_h___ */
diff --git a/xpcom/ds/nsCharSeparatedTokenizer.cpp b/xpcom/ds/nsCharSeparatedTokenizer.cpp
new file mode 100644
index 0000000000..7288ed070f
--- /dev/null
+++ b/xpcom/ds/nsCharSeparatedTokenizer.cpp
@@ -0,0 +1,10 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsCharSeparatedTokenizer.h"
+
+template class nsTSubstringSplitter<char>;
+template class nsTSubstringSplitter<char16_t>;
diff --git a/xpcom/ds/nsCharSeparatedTokenizer.h b/xpcom/ds/nsCharSeparatedTokenizer.h
new file mode 100644
index 0000000000..5cf6992e3e
--- /dev/null
+++ b/xpcom/ds/nsCharSeparatedTokenizer.h
@@ -0,0 +1,274 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __nsCharSeparatedTokenizer_h
+#define __nsCharSeparatedTokenizer_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/RangedPtr.h"
+#include "mozilla/TypedEnumBits.h"
+
+#include "nsCRTGlue.h"
+#include "nsTDependentSubstring.h"
+
+// Flags -- only one for now. If we need more, they should be defined to
+// be 1 << 1, 1 << 2, etc. (They're masks, and aFlags is a bitfield.)
+enum class nsTokenizerFlags {
+  Default = 0,
+  SeparatorOptional = 1 << 0,
+  IncludeEmptyTokenAtEnd = 1 << 1
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(nsTokenizerFlags)
+
+/**
+ * This parses a SeparatorChar-separated string into tokens.
+ * Whitespace surrounding tokens is not treated as part of tokens, however
+ * whitespace inside a token is. If the final token is the empty string, it is
+ * not returned by default.
+ *
+ * Some examples, with SeparatorChar = ',':
+ *
+ * "foo, bar, baz" ->      "foo" "bar" "baz"
+ * "foo,bar,baz" ->        "foo" "bar" "baz"
+ * "foo , bar hi , baz" -> "foo" "bar hi" "baz"
+ * "foo, ,bar,baz" ->      "foo" "" "bar" "baz"
+ * "foo,,bar,baz" ->       "foo" "" "bar" "baz"
+ * "foo,bar,baz," ->       "foo" "bar" "baz"
+ *
+ * The function used for whitespace detection is a template argument.
+ * By default, it is NS_IsAsciiWhitespace.
+ */
+template <typename TDependentSubstringType, bool IsWhitespace(char16_t),
+          nsTokenizerFlags Flags = nsTokenizerFlags::Default>
+class nsTCharSeparatedTokenizer {
+  using CharType = typename TDependentSubstringType::char_type;
+  using SubstringType = typename TDependentSubstringType::substring_type;
+
+ public:
+  using DependentSubstringType = TDependentSubstringType;
+
+  nsTCharSeparatedTokenizer(const SubstringType& aSource,
+                            CharType aSeparatorChar)
+      : mIter(aSource.Data(), aSource.Length()),
+        mEnd(aSource.Data() + aSource.Length(), aSource.Data(),
+             aSource.Length()),
+        mSeparatorChar(aSeparatorChar),
+        mWhitespaceBeforeFirstToken(false),
+        mWhitespaceAfterCurrentToken(false),
+        mSeparatorAfterCurrentToken(false) {
+    // Skip initial whitespace
+    while (mIter < mEnd && IsWhitespace(*mIter)) {
+      mWhitespaceBeforeFirstToken = true;
+      ++mIter;
+    }
+  }
+
+  /**
+   * Checks if any more tokens are available.
+   */
+  bool hasMoreTokens() const {
+    MOZ_ASSERT(mIter == mEnd || !IsWhitespace(*mIter),
+               "Should be at beginning of token if there is one");
+
+    if constexpr (Flags & nsTokenizerFlags::IncludeEmptyTokenAtEnd) {
+      return mIter < mEnd || (mIter == mEnd && mSeparatorAfterCurrentToken);
+    } else {
+      return mIter < mEnd;
+    }
+  }
+
+  /*
+   * Returns true if there is whitespace prior to the first token.
+   */
+  bool whitespaceBeforeFirstToken() const {
+    return mWhitespaceBeforeFirstToken;
+  }
+
+  /*
+   * Returns true if there is a separator after the current token.
+   * Useful if you want to check whether the last token has a separator
+   * after it which may not be valid.
+   */
+  bool separatorAfterCurrentToken() const {
+    return mSeparatorAfterCurrentToken;
+  }
+
+  /*
+   * Returns true if there is any whitespace after the current token.
+   */
+  bool whitespaceAfterCurrentToken() const {
+    return mWhitespaceAfterCurrentToken;
+  }
+
+  /**
+   * Returns the next token.
+   */
+  const DependentSubstringType nextToken() {
+    mozilla::RangedPtr<const CharType> tokenStart = mIter;
+    mozilla::RangedPtr<const CharType> tokenEnd = mIter;
+
+    MOZ_ASSERT(mIter == mEnd || !IsWhitespace(*mIter),
+               "Should be at beginning of token if there is one");
+
+    // Search until we hit separator or end (or whitespace, if a separator
+    // isn't required -- see clause with 'break' below).
+    while (mIter < mEnd && *mIter != mSeparatorChar) {
+      // Skip to end of the current word.
+      while (mIter < mEnd && !IsWhitespace(*mIter) &&
+             *mIter != mSeparatorChar) {
+        ++mIter;
+      }
+      tokenEnd = mIter;
+
+      // Skip whitespace after the current word.
+      mWhitespaceAfterCurrentToken = false;
+      while (mIter < mEnd && IsWhitespace(*mIter)) {
+        mWhitespaceAfterCurrentToken = true;
+        ++mIter;
+      }
+      if constexpr (Flags & nsTokenizerFlags::SeparatorOptional) {
+        // We've hit (and skipped) whitespace, and that's sufficient to end
+        // our token, regardless of whether we've reached a SeparatorChar.
+        break;
+      }  // (else, we'll keep looping until we hit mEnd or SeparatorChar)
+    }
+
+    mSeparatorAfterCurrentToken = (mIter != mEnd && *mIter == mSeparatorChar);
+    MOZ_ASSERT((Flags & nsTokenizerFlags::SeparatorOptional) ||
+                   (mSeparatorAfterCurrentToken == (mIter < mEnd)),
+               "If we require a separator and haven't hit the end of "
+               "our string, then we shouldn't have left the loop "
+               "unless we hit a separator");
+
+    // Skip separator (and any whitespace after it), if we're at one.
+    if (mSeparatorAfterCurrentToken) {
+      ++mIter;
+
+      while (mIter < mEnd && IsWhitespace(*mIter)) {
+        mWhitespaceAfterCurrentToken = true;
+        ++mIter;
+      }
+    }
+
+    return Substring(tokenStart.get(), tokenEnd.get());
+  }
+
+  auto ToRange() const;
+
+ private:
+  mozilla::RangedPtr<const CharType> mIter;
+  const mozilla::RangedPtr<const CharType> mEnd;
+  const CharType mSeparatorChar;
+  bool mWhitespaceBeforeFirstToken;
+  bool mWhitespaceAfterCurrentToken;
+  bool mSeparatorAfterCurrentToken;
+};
+
+constexpr bool NS_TokenizerIgnoreNothing(char16_t) { return false; }
+
+template <bool IsWhitespace(char16_t), typename CharType,
+          nsTokenizerFlags Flags = nsTokenizerFlags::Default>
+using nsTCharSeparatedTokenizerTemplate =
+    nsTCharSeparatedTokenizer<nsTDependentSubstring<CharType>, IsWhitespace,
+                              Flags>;
+
+template <bool IsWhitespace(char16_t),
+          nsTokenizerFlags Flags = nsTokenizerFlags::Default>
+using nsCharSeparatedTokenizerTemplate =
+    nsTCharSeparatedTokenizerTemplate<IsWhitespace, char16_t, Flags>;
+
+using nsCharSeparatedTokenizer =
+    nsCharSeparatedTokenizerTemplate<NS_IsAsciiWhitespace>;
+
+template <bool IsWhitespace(char16_t),
+          nsTokenizerFlags Flags = nsTokenizerFlags::Default>
+using nsCCharSeparatedTokenizerTemplate =
+    nsTCharSeparatedTokenizerTemplate<IsWhitespace, char, Flags>;
+
+using nsCCharSeparatedTokenizer =
+    nsCCharSeparatedTokenizerTemplate<NS_IsAsciiWhitespace>;
+
+/**
+ * Adapts a char separated tokenizer for use in a range-based for loop.
+ *
+ * Use this typically only indirectly, e.g. like
+ *
+ * for (const auto& token : nsCharSeparatedTokenizer(aText, ' ').ToRange()) {
+ *    // ...
+ * }
+ */
+template <typename Tokenizer>
+class nsTokenizedRange {
+ public:
+  using DependentSubstringType = typename Tokenizer::DependentSubstringType;
+
+  explicit nsTokenizedRange(Tokenizer&& aTokenizer)
+      : mTokenizer(std::move(aTokenizer)) {}
+
+  struct EndSentinel {};
+  struct Iterator {
+    explicit Iterator(const Tokenizer& aTokenizer) : mTokenizer(aTokenizer) {
+      Next();
+    }
+
+    const DependentSubstringType& operator*() const { return *mCurrentToken; }
+
+    Iterator& operator++() {
+      Next();
+      return *this;
+    }
+
+    bool operator==(const EndSentinel&) const {
+      return mCurrentToken.isNothing();
+    }
+
+    bool operator!=(const EndSentinel&) const { return mCurrentToken.isSome(); }
+
+   private:
+    void Next() {
+      mCurrentToken.reset();
+
+      if (mTokenizer.hasMoreTokens()) {
+        mCurrentToken.emplace(mTokenizer.nextToken());
+      }
+    }
+
+    Tokenizer mTokenizer;
+    mozilla::Maybe<DependentSubstringType> mCurrentToken;
+  };
+
+  auto begin() const { return Iterator{mTokenizer}; }
+  auto end() const { return EndSentinel{}; }
+
+ private:
+  const Tokenizer mTokenizer;
+};
+
+template <typename TDependentSubstringType, bool IsWhitespace(char16_t),
+          nsTokenizerFlags Flags>
+auto nsTCharSeparatedTokenizer<TDependentSubstringType, IsWhitespace,
+                               Flags>::ToRange() const {
+  return nsTokenizedRange{nsTCharSeparatedTokenizer{*this}};
+}
+
+// You should not need to instantiate this class directly.
+// Use nsTSubstring::Split instead.
+template <typename T>
+class nsTSubstringSplitter
+    : public nsTokenizedRange<nsTCharSeparatedTokenizerTemplate<
+          NS_TokenizerIgnoreNothing, T,
+          nsTokenizerFlags::IncludeEmptyTokenAtEnd>> {
+ public:
+  using nsTokenizedRange<nsTCharSeparatedTokenizerTemplate<
+      NS_TokenizerIgnoreNothing, T,
+      nsTokenizerFlags::IncludeEmptyTokenAtEnd>>::nsTokenizedRange;
+};
+
+extern template class nsTSubstringSplitter<char>;
+extern template class nsTSubstringSplitter<char16_t>;
+
+#endif /* __nsCharSeparatedTokenizer_h */
diff --git a/xpcom/ds/nsCheapSets.h b/xpcom/ds/nsCheapSets.h
new file mode 100644
index 0000000000..3d09ccfdb7
--- /dev/null
+++ b/xpcom/ds/nsCheapSets.h
@@ -0,0 +1,155 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __nsCheapSets_h__
+#define __nsCheapSets_h__
+
+#include "nsTHashtable.h"
+#include <stdint.h>
+
+enum nsCheapSetOperator {
+  OpNext = 0,    // enumerator says continue
+  OpRemove = 1,  // enumerator says remove and continue
+};
+
+/**
+ * A set that takes up minimal size when there are 0 or 1 entries in the set.
+ * Use for cases where sizes of 0 and 1 are even slightly common.
+ */
+template <typename EntryType>
+class nsCheapSet {
+ public:
+  typedef typename EntryType::KeyType KeyType;
+  typedef nsCheapSetOperator (*Enumerator)(EntryType* aEntry, void* userArg);
+
+  nsCheapSet() : mState(ZERO) { mUnion.table = nullptr; }
+  ~nsCheapSet() { Clear(); }
+
+  /**
+   * Remove all entries.
+   */
+  void Clear() {
+    switch (mState) {
+      case ZERO:
+        break;
+      case ONE:
+        GetSingleEntry()->~EntryType();
+        break;
+      case MANY:
+        delete mUnion.table;
+        break;
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        break;
+    }
+    mState = ZERO;
+  }
+
+  void Put(const KeyType aVal);
+
+  void Remove(const KeyType aVal);
+
+  bool Contains(const KeyType aVal) {
+    switch (mState) {
+      case ZERO:
+        return false;
+      case ONE:
+        return GetSingleEntry()->KeyEquals(EntryType::KeyToPointer(aVal));
+      case MANY:
+        return !!mUnion.table->GetEntry(aVal);
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        return false;
+    }
+  }
+
+  uint32_t EnumerateEntries(Enumerator aEnumFunc, void* aUserArg) {
+    switch (mState) {
+      case ZERO:
+        return 0;
+      case ONE:
+        if (aEnumFunc(GetSingleEntry(), aUserArg) == OpRemove) {
+          GetSingleEntry()->~EntryType();
+          mState = ZERO;
+        }
+        return 1;
+      case MANY: {
+        uint32_t n = mUnion.table->Count();
+        for (auto iter = mUnion.table->Iter(); !iter.Done(); iter.Next()) {
+          auto entry = static_cast<EntryType*>(iter.Get());
+          if (aEnumFunc(entry, aUserArg) == OpRemove) {
+            iter.Remove();
+          }
+        }
+        return n;
+      }
+      default:
+        MOZ_ASSERT_UNREACHABLE("bogus state");
+        return 0;
+    }
+  }
+
+ private:
+  EntryType* GetSingleEntry() {
+    return reinterpret_cast<EntryType*>(&mUnion.singleEntry[0]);
+  }
+
+  enum SetState { ZERO, ONE, MANY };
+
+  union {
+    nsTHashtable<EntryType>* table;
+    char singleEntry[sizeof(EntryType)];
+  } mUnion;
+  enum SetState mState;
+};
+
+template <typename EntryType>
+void nsCheapSet<EntryType>::Put(const KeyType aVal) {
+  switch (mState) {
+    case ZERO:
+      new (GetSingleEntry()) EntryType(EntryType::KeyToPointer(aVal));
+      mState = ONE;
+      return;
+    case ONE: {
+      nsTHashtable<EntryType>* table = new nsTHashtable<EntryType>();
+      EntryType* entry = GetSingleEntry();
+      table->PutEntry(entry->GetKey());
+      entry->~EntryType();
+      mUnion.table = table;
+      mState = MANY;
+    }
+      [[fallthrough]];
+
+    case MANY:
+      mUnion.table->PutEntry(aVal);
+      return;
+    default:
+      MOZ_ASSERT_UNREACHABLE("bogus state");
+      return;
+  }
+}
+
+template <typename EntryType>
+void nsCheapSet<EntryType>::Remove(const KeyType aVal) {
+  switch (mState) {
+    case ZERO:
+      break;
+    case ONE:
+      if (Contains(aVal)) {
+        GetSingleEntry()->~EntryType();
+        mState = ZERO;
+      }
+      break;
+    case MANY:
+      mUnion.table->RemoveEntry(aVal);
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("bogus state");
+      break;
+  }
+}
+
+#endif
diff --git a/xpcom/ds/nsClassHashtable.h b/xpcom/ds/nsClassHashtable.h
new file mode 100644
index 0000000000..a1a2384f7e
--- /dev/null
+++ b/xpcom/ds/nsClassHashtable.h
@@ -0,0 +1,115 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsClassHashtable_h__
+#define nsClassHashtable_h__
+
+#include <utility>
+
+#include "mozilla/UniquePtr.h"
+#include "nsBaseHashtable.h"
+#include "nsHashKeys.h"
+
+/**
+ * Helper class that provides methods to wrap and unwrap the UserDataType.
+ */
+template <class T>
+class nsUniquePtrConverter {
+ public:
+  using UserDataType = T*;
+  using DataType = mozilla::UniquePtr<T>;
+
+  static UserDataType Unwrap(DataType& src) { return src.get(); }
+  static DataType Wrap(UserDataType&& src) { return DataType(std::move(src)); }
+  static DataType Wrap(const UserDataType& src) { return DataType(src); }
+};
+
+/**
+ * templated hashtable class maps keys to C++ object pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param Class the class-type being wrapped
+ * @see nsInterfaceHashtable, nsClassHashtable
+ */
+template <class KeyClass, class T>
+class nsClassHashtable : public nsBaseHashtable<KeyClass, mozilla::UniquePtr<T>,
+                                                T*, nsUniquePtrConverter<T>> {
+ public:
+  typedef typename KeyClass::KeyType KeyType;
+  typedef T* UserDataType;
+  typedef nsBaseHashtable<KeyClass, mozilla::UniquePtr<T>, T*,
+                          nsUniquePtrConverter<T>>
+      base_type;
+
+  using base_type::IsEmpty;
+  using base_type::Remove;
+
+  nsClassHashtable() = default;
+  explicit nsClassHashtable(uint32_t aInitLength) : base_type(aInitLength) {}
+
+  /**
+   * @copydoc nsBaseHashtable::Get
+   * @param aData if the key doesn't exist, pData will be set to nullptr.
+   */
+  bool Get(KeyType aKey, UserDataType* aData) const;
+
+  /**
+   * @copydoc nsBaseHashtable::Get
+   * @returns nullptr if the key is not present.
+   */
+  [[nodiscard]] UserDataType Get(KeyType aKey) const;
+};
+
+template <typename K, typename T>
+inline void ImplCycleCollectionUnlink(nsClassHashtable<K, T>& aField) {
+  aField.Clear();
+}
+
+template <typename K, typename T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    const nsClassHashtable<K, T>& aField, const char* aName,
+    uint32_t aFlags = 0) {
+  for (auto iter = aField.ConstIter(); !iter.Done(); iter.Next()) {
+    ImplCycleCollectionTraverse(aCallback, *iter.UserData(), aName, aFlags);
+  }
+}
+
+//
+// nsClassHashtable definitions
+//
+
+template <class KeyClass, class T>
+bool nsClassHashtable<KeyClass, T>::Get(KeyType aKey, T** aRetVal) const {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+
+  if (ent) {
+    if (aRetVal) {
+      *aRetVal = ent->GetData().get();
+    }
+
+    return true;
+  }
+
+  if (aRetVal) {
+    *aRetVal = nullptr;
+  }
+
+  return false;
+}
+
+template <class KeyClass, class T>
+T* nsClassHashtable<KeyClass, T>::Get(KeyType aKey) const {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+  if (!ent) {
+    return nullptr;
+  }
+
+  return ent->GetData().get();
+}
+
+#endif  // nsClassHashtable_h__
diff --git a/xpcom/ds/nsDeque.cpp b/xpcom/ds/nsDeque.cpp
new file mode 100644
index 0000000000..07f61fa471
--- /dev/null
+++ b/xpcom/ds/nsDeque.cpp
@@ -0,0 +1,265 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsDeque.h"
+#include "nsISupportsImpl.h"
+#include <string.h>
+
+#include "mozilla/CheckedInt.h"
+
+#define modulus(x, y) ((x) % (y))
+
+namespace mozilla {
+namespace detail {
+
+/**
+ * Standard constructor
+ * @param deallocator, called by Erase and ~nsDequeBase
+ */
+nsDequeBase::nsDequeBase() {
+  MOZ_COUNT_CTOR(nsDequeBase);
+  mOrigin = mSize = 0;
+  mData = mBuffer;  // don't allocate space until you must
+  mCapacity = sizeof(mBuffer) / sizeof(mBuffer[0]);
+  memset(mData, 0, mCapacity * sizeof(mBuffer[0]));
+}
+
+/**
+ * Destructor
+ */
+nsDequeBase::~nsDequeBase() {
+  MOZ_COUNT_DTOR(nsDequeBase);
+
+  if (mData && mData != mBuffer) {
+    free(mData);
+  }
+  mData = nullptr;
+}
+
+size_t nsDequeBase::SizeOfExcludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) const {
+  size_t size = 0;
+  if (mData != mBuffer) {
+    size += aMallocSizeOf(mData);
+  }
+
+  return size;
+}
+
+/**
+ * Remove all items from container without destroying them.
+ */
+void nsDequeBase::Empty() {
+  if (mSize && mData) {
+    memset(mData, 0, mCapacity * sizeof(*mData));
+  }
+  mSize = 0;
+  mOrigin = 0;
+}
+
+/**
+ * This method quadruples the size of the deque
+ * Elements in the deque are resequenced so that elements
+ * in the deque are stored sequentially
+ *
+ * @return  whether growing succeeded
+ */
+bool nsDequeBase::GrowCapacity() {
+  mozilla::CheckedInt<size_t> newCapacity = mCapacity;
+  newCapacity *= 4;
+
+  NS_ASSERTION(newCapacity.isValid(), "Overflow");
+  if (!newCapacity.isValid()) {
+    return false;
+  }
+
+  // Sanity check the new byte size.
+  mozilla::CheckedInt<size_t> newByteSize = newCapacity;
+  newByteSize *= sizeof(void*);
+
+  NS_ASSERTION(newByteSize.isValid(), "Overflow");
+  if (!newByteSize.isValid()) {
+    return false;
+  }
+
+  void** temp = (void**)malloc(newByteSize.value());
+  if (!temp) {
+    return false;
+  }
+
+  // Here's the interesting part: You can't just move the elements
+  // directly (in situ) from the old buffer to the new one.
+  // Since capacity has changed, the old origin doesn't make
+  // sense anymore. It's better to resequence the elements now.
+
+  memcpy(temp, mData + mOrigin, sizeof(void*) * (mCapacity - mOrigin));
+  memcpy(temp + (mCapacity - mOrigin), mData, sizeof(void*) * mOrigin);
+
+  if (mData != mBuffer) {
+    free(mData);
+  }
+
+  mCapacity = newCapacity.value();
+  mOrigin = 0;  // now realign the origin...
+  mData = temp;
+
+  return true;
+}
+
+/**
+ * This method adds an item to the end of the deque.
+ * This operation has the potential to cause the
+ * underlying buffer to resize.
+ *
+ * @param   aItem: new item to be added to deque
+ */
+bool nsDequeBase::Push(void* aItem, const fallible_t&) {
+  if (mSize == mCapacity && !GrowCapacity()) {
+    return false;
+  }
+  mData[modulus(mOrigin + mSize, mCapacity)] = aItem;
+  mSize++;
+  return true;
+}
+
+/**
+ * This method adds an item to the front of the deque.
+ * This operation has the potential to cause the
+ * underlying buffer to resize.
+ *
+ * --Commments for GrowCapacity() case
+ * We've grown and shifted which means that the old
+ * final element in the deque is now the first element
+ * in the deque.  This is temporary.
+ * We haven't inserted the new element at the front.
+ *
+ * To continue with the idea of having the front at zero
+ * after a grow, we move the old final item (which through
+ * the voodoo of mOrigin-- is now the first) to its final
+ * position which is conveniently the old length.
+ *
+ * Note that this case only happens when the deque is full.
+ * [And that pieces of this magic only work if the deque is full.]
+ * picture:
+ *   [ABCDEFGH] @[mOrigin:3]:D.
+ * Task: PushFront("Z")
+ * shift mOrigin so, @[mOrigin:2]:C
+ * stretch and rearrange: (mOrigin:0)
+ *   [CDEFGHAB ________ ________ ________]
+ * copy: (The second C is currently out of bounds)
+ *   [CDEFGHAB C_______ ________ ________]
+ * later we will insert Z:
+ *   [ZDEFGHAB C_______ ________ ________]
+ * and increment size: 9. (C is no longer out of bounds)
+ * --
+ * @param   aItem: new item to be added to deque
+ */
+bool nsDequeBase::PushFront(void* aItem, const fallible_t&) {
+  if (mOrigin == 0) {
+    mOrigin = mCapacity - 1;
+  } else {
+    mOrigin--;
+  }
+
+  if (mSize == mCapacity) {
+    if (!GrowCapacity()) {
+      return false;
+    }
+    /* Comments explaining this are above*/
+    mData[mSize] = mData[mOrigin];
+  }
+  mData[mOrigin] = aItem;
+  mSize++;
+  return true;
+}
+
+/**
+ * Remove and return the last item in the container.
+ *
+ * @return  ptr to last item in container
+ */
+void* nsDequeBase::Pop() {
+  void* result = nullptr;
+  if (mSize > 0) {
+    --mSize;
+    size_t offset = modulus(mSize + mOrigin, mCapacity);
+    result = mData[offset];
+    mData[offset] = nullptr;
+    if (!mSize) {
+      mOrigin = 0;
+    }
+  }
+  return result;
+}
+
+/**
+ * This method gets called you want to remove and return
+ * the first member in the container.
+ *
+ * @return  last item in container
+ */
+void* nsDequeBase::PopFront() {
+  void* result = nullptr;
+  if (mSize > 0) {
+    NS_ASSERTION(mOrigin < mCapacity, "Error: Bad origin");
+    result = mData[mOrigin];
+    mData[mOrigin++] = nullptr;  // zero it out for debugging purposes.
+    mSize--;
+    // Cycle around if we pop off the end
+    // and reset origin if when we pop the last element
+    if (mCapacity == mOrigin || !mSize) {
+      mOrigin = 0;
+    }
+  }
+  return result;
+}
+
+/**
+ * This method gets called you want to peek at the bottom
+ * member without removing it.
+ *
+ * @return  last item in container
+ */
+void* nsDequeBase::Peek() const {
+  void* result = nullptr;
+  if (mSize > 0) {
+    result = mData[modulus(mSize - 1 + mOrigin, mCapacity)];
+  }
+  return result;
+}
+
+/**
+ * This method gets called you want to peek at the topmost
+ * member without removing it.
+ *
+ * @return  last item in container
+ */
+void* nsDequeBase::PeekFront() const {
+  void* result = nullptr;
+  if (mSize > 0) {
+    result = mData[mOrigin];
+  }
+  return result;
+}
+
+/**
+ * Call this to retrieve the ith element from this container.
+ * Keep in mind that accessing the underlying elements is
+ * done in a relative fashion. Object 0 is not necessarily
+ * the first element (the first element is at mOrigin).
+ *
+ * @param   aIndex : 0 relative offset of item you want
+ * @return  void* or null
+ */
+void* nsDequeBase::ObjectAt(size_t aIndex) const {
+  void* result = nullptr;
+  if (aIndex < mSize) {
+    result = mData[modulus(mOrigin + aIndex, mCapacity)];
+  }
+  return result;
+}
+}  // namespace detail
+}  // namespace mozilla
diff --git a/xpcom/ds/nsDeque.h b/xpcom/ds/nsDeque.h
new file mode 100644
index 0000000000..4f0bf27037
--- /dev/null
+++ b/xpcom/ds/nsDeque.h
@@ -0,0 +1,538 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * MODULE NOTES:
+ *
+ * The Deque is a very small, very efficient container object
+ * than can hold items of type T*, offering the following features:
+ * - Its interface supports pushing, popping, and peeking of items at the back
+ *   or front, and retrieval from any position.
+ * - It can iterate over items via a ForEach method, range-for, or an iterator
+ *   class.
+ * - When full, it can efficiently resize dynamically.
+ *
+ * NOTE: The only bit of trickery here is that this deque is
+ * built upon a ring-buffer. Like all ring buffers, the first
+ * item may not be at index[0]. The mOrigin member determines
+ * where the first child is. This point is quietly hidden from
+ * customers of this class.
+ */
+
+#ifndef _NSDEQUE
+#define _NSDEQUE
+#include <cstddef>
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/fallible.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+#include "nsCOMPtr.h"
+#include "nsDebug.h"
+#include "nsISupports.h"
+
+namespace mozilla {
+
+namespace detail {
+/**
+ * The deque (double-ended queue) class is a common container type,
+ * whose behavior mimics a line in your favorite checkout stand.
+ * Classic CS describes the common behavior of a queue as FIFO.
+ * A deque allows insertion and removal at both ends of
+ * the container.
+ *
+ * nsDequeBase implements the untyped deque data structure while
+ * nsDeque provides the typed implementation and iterators.
+ */
+class nsDequeBase {
+ public:
+  /**
+   * Returns the number of items currently stored in
+   * this deque.
+   *
+   * @return  number of items currently in the deque
+   */
+  inline size_t GetSize() const { return mSize; }
+
+ protected:
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  /**
+   * Constructs an empty deque.
+   *
+   * @param   aDeallocator Optional deallocator functor that will be called from
+   *                       Erase() and the destructor on any remaining item.
+   *                       The deallocator is owned by the deque and will be
+   *                       deleted at destruction time.
+   */
+  explicit nsDequeBase();
+
+  /**
+   * Deque destructor. Erases all items, deletes the deallocator.
+   */
+  ~nsDequeBase();
+
+  /**
+   * Appends new member at the end of the deque.
+   *
+   * @param   aItem item to store in deque
+   * @return  true if succeeded, false if failed to resize deque as needed
+   */
+  [[nodiscard]] bool Push(void* aItem, const fallible_t&);
+
+  /**
+   * Inserts new member at the front of the deque.
+   *
+   * @param   aItem item to store in deque
+   * @return  true if succeeded, false if failed to resize deque as needed
+   */
+  [[nodiscard]] bool PushFront(void* aItem, const fallible_t&);
+
+  /**
+   * Remove and return the last item in the container.
+   *
+   * @return  the item that was the last item in container
+   */
+  void* Pop();
+
+  /**
+   * Remove and return the first item in the container.
+   *
+   * @return  the item that was first item in container
+   */
+  void* PopFront();
+
+  /**
+   * Retrieve the last item without removing it.
+   *
+   * @return  the last item in container
+   */
+  void* Peek() const;
+
+  /**
+   * Retrieve the first item without removing it.
+   *
+   * @return  the first item in container
+   */
+  void* PeekFront() const;
+
+  /**
+   * Retrieve a member from the deque without removing it.
+   *
+   * @param   index of desired item
+   * @return  item in list, or nullptr if index is outside the deque
+   */
+  void* ObjectAt(size_t aIndex) const;
+
+  bool GrowCapacity();
+
+  /**
+   * Remove all items from container without destroying them.
+   */
+  void Empty();
+
+  size_t mSize;
+  size_t mCapacity;
+  size_t mOrigin;
+  void* mBuffer[8];
+  void** mData;
+
+  nsDequeBase& operator=(const nsDequeBase& aOther) = delete;
+  nsDequeBase(const nsDequeBase& aOther) = delete;
+};
+
+// This iterator assumes that the deque itself is const, i.e., it cannot be
+// modified while the iterator is used.
+// Also it is a 'const' iterator in that it provides copies of the deque's
+// elements, and therefore it is not possible to modify the deque's contents
+// by assigning to a dereference of this iterator.
+template <typename Deque>
+class ConstDequeIterator {
+ public:
+  ConstDequeIterator(const Deque& aDeque, size_t aIndex)
+      : mDeque(aDeque), mIndex(aIndex) {}
+  ConstDequeIterator& operator++() {
+    ++mIndex;
+    return *this;
+  }
+  bool operator==(const ConstDequeIterator& aOther) const {
+    return mIndex == aOther.mIndex;
+  }
+  bool operator!=(const ConstDequeIterator& aOther) const {
+    return mIndex != aOther.mIndex;
+  }
+  typename Deque::PointerType operator*() const {
+    // Don't allow out-of-deque dereferences.
+    MOZ_RELEASE_ASSERT(mIndex < mDeque.GetSize());
+    return mDeque.ObjectAt(mIndex);
+  }
+
+ private:
+  const Deque& mDeque;
+  size_t mIndex;
+};
+
+// It is a 'const' iterator in that it provides copies of the deque's
+// elements, and therefore it is not possible to modify the deque's contents
+// by assigning to a dereference of this iterator.
+// If the deque is modified in other ways, this iterator will stay at the same
+// index, and will handle past-the-end comparisons, but not dereferencing.
+template <typename Deque>
+class ConstIterator {
+ public:
+  // Special index for the end iterator, to track the possibly-shifting
+  // deque size.
+  static const size_t EndIteratorIndex = size_t(-1);
+
+  ConstIterator(const Deque& aDeque, size_t aIndex)
+      : mDeque(aDeque), mIndex(aIndex) {}
+  ConstIterator& operator++() {
+    // End-iterator shouldn't be modified.
+    MOZ_ASSERT(mIndex != EndIteratorIndex);
+    ++mIndex;
+    return *this;
+  }
+  bool operator==(const ConstIterator& aOther) const {
+    return EffectiveIndex() == aOther.EffectiveIndex();
+  }
+  bool operator!=(const ConstIterator& aOther) const {
+    return EffectiveIndex() != aOther.EffectiveIndex();
+  }
+  typename Deque::PointerType operator*() const {
+    // Don't allow out-of-deque dereferences.
+    MOZ_RELEASE_ASSERT(mIndex < mDeque.GetSize());
+    return mDeque.ObjectAt(mIndex);
+  }
+
+ private:
+  // 0 <= index < deque.GetSize() inside the deque, deque.GetSize() otherwise.
+  // Only used when comparing indices, not to actually access items.
+  size_t EffectiveIndex() const {
+    return (mIndex < mDeque.GetSize()) ? mIndex : mDeque.GetSize();
+  }
+
+  const Deque& mDeque;
+  size_t mIndex;  // May point outside the deque!
+};
+
+}  // namespace detail
+}  // namespace mozilla
+
+/**
+ * The nsDequeFunctor class is used when you want to create
+ * callbacks between the deque and your generic code.
+ * Use these objects in a call to ForEach(), and as custom deallocators.
+ */
+template <typename T>
+class nsDequeFunctor {
+ public:
+  virtual void operator()(T* aObject) = 0;
+  virtual ~nsDequeFunctor() = default;
+};
+
+/******************************************************
+ * Here comes the nsDeque class itself...
+ ******************************************************/
+
+/**
+ * The deque (double-ended queue) class is a common container type,
+ * whose behavior mimics a line in your favorite checkout stand.
+ * Classic CS describes the common behavior of a queue as FIFO.
+ * A deque allows insertion and removal at both ends of
+ * the container.
+ *
+ * The deque stores pointers to items.
+ */
+template <typename T>
+class nsDeque : public mozilla::detail::nsDequeBase {
+  typedef mozilla::fallible_t fallible_t;
+
+ public:
+  using PointerType = T*;
+  using ConstDequeIterator = mozilla::detail::ConstDequeIterator<nsDeque<T>>;
+  using ConstIterator = mozilla::detail::ConstIterator<nsDeque<T>>;
+
+  /**
+   * Constructs an empty deque.
+   *
+   * @param   aDeallocator Optional deallocator functor that will be called from
+   *                       Erase() and the destructor on any remaining item.
+   *                       The deallocator is owned by the deque and will be
+   *                       deleted at destruction time.
+   */
+  explicit nsDeque(nsDequeFunctor<T>* aDeallocator = nullptr) {
+    MOZ_COUNT_CTOR(nsDeque);
+    mDeallocator = aDeallocator;
+  }
+
+  /**
+   * Deque destructor. Erases all items, deletes the deallocator.
+   */
+  ~nsDeque() {
+    MOZ_COUNT_DTOR(nsDeque);
+
+    Erase();
+    SetDeallocator(nullptr);
+  }
+
+  /**
+   * Appends new member at the end of the deque.
+   *
+   * @param   aItem item to store in deque
+   */
+  inline void Push(T* aItem) {
+    if (!nsDequeBase::Push(aItem, mozilla::fallible)) {
+      NS_ABORT_OOM(mSize * sizeof(T*));
+    }
+  }
+
+  /**
+   * Appends new member at the end of the deque.
+   *
+   * @param   aItem item to store in deque
+   * @return  true if succeeded, false if failed to resize deque as needed
+   */
+  [[nodiscard]] inline bool Push(T* aItem, const fallible_t& aFaillible) {
+    return nsDequeBase::Push(aItem, aFaillible);
+  }
+
+  /**
+   * Inserts new member at the front of the deque.
+   *
+   * @param   aItem item to store in deque
+   */
+  inline void PushFront(T* aItem) {
+    if (!nsDequeBase::PushFront(aItem, mozilla::fallible)) {
+      NS_ABORT_OOM(mSize * sizeof(T*));
+    }
+  }
+
+  /**
+   * Inserts new member at the front of the deque.
+   *
+   * @param   aItem item to store in deque
+   * @return  true if succeeded, false if failed to resize deque as needed
+   */
+  [[nodiscard]] bool PushFront(T* aItem, const fallible_t& aFallible) {
+    return nsDequeBase::PushFront(aItem, aFallible);
+  }
+
+  /**
+   * Remove and return the last item in the container.
+   *
+   * @return  the item that was the last item in container
+   */
+  inline T* Pop() { return static_cast<T*>(nsDequeBase::Pop()); }
+
+  /**
+   * Remove and return the first item in the container.
+   *
+   * @return  the item that was first item in container
+   */
+  inline T* PopFront() { return static_cast<T*>(nsDequeBase::PopFront()); }
+
+  /**
+   * Retrieve the last item without removing it.
+   *
+   * @return  the last item in container
+   */
+  inline T* Peek() const { return static_cast<T*>(nsDequeBase::Peek()); }
+
+  /**
+   * Retrieve the first item without removing it.
+   *
+   * @return  the first item in container
+   */
+  inline T* PeekFront() const {
+    return static_cast<T*>(nsDequeBase::PeekFront());
+  }
+
+  /**
+   * Retrieve a member from the deque without removing it.
+   *
+   * @param   index of desired item
+   * @return  item in list, or nullptr if index is outside the deque
+   */
+  inline T* ObjectAt(size_t aIndex) const {
+    if (NS_WARN_IF(aIndex >= GetSize())) {
+      return nullptr;
+    }
+    return static_cast<T*>(nsDequeBase::ObjectAt(aIndex));
+  }
+
+  /**
+   * Remove and delete all items from container.
+   * Deletes are handled by the deallocator nsDequeFunctor
+   * which is specified at deque construction.
+   */
+  void Erase() {
+    if (mDeallocator && mSize) {
+      ForEach(*mDeallocator);
+    }
+    Empty();
+  }
+
+  /**
+   * Call this method when you want to iterate through all
+   * items in the container, passing a functor along
+   * to call your code.
+   * If the deque is modified during ForEach, iteration will continue based on
+   * item indices; meaning that front operations may effectively skip over
+   * items or visit some items multiple times.
+   *
+   * @param   aFunctor object to call for each member
+   */
+  void ForEach(nsDequeFunctor<T>& aFunctor) const {
+    for (size_t i = 0; i < mSize; ++i) {
+      aFunctor(ObjectAt(i));
+    }
+  }
+
+  // If this deque is const, we can provide ConstDequeIterator's.
+  ConstDequeIterator begin() const { return ConstDequeIterator(*this, 0); }
+  ConstDequeIterator end() const { return ConstDequeIterator(*this, mSize); }
+
+  // If this deque is *not* const, we provide ConstIterator's that can handle
+  // deque size changes.
+  ConstIterator begin() { return ConstIterator(*this, 0); }
+  ConstIterator end() {
+    return ConstIterator(*this, ConstIterator::EndIteratorIndex);
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    size_t size = nsDequeBase::SizeOfExcludingThis(aMallocSizeOf);
+    if (mDeallocator) {
+      size += aMallocSizeOf(mDeallocator);
+    }
+    return size;
+  }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+ protected:
+  nsDequeFunctor<T>* mDeallocator;
+
+ private:
+  /**
+   * Copy constructor (deleted)
+   *
+   * @param aOther another deque
+   */
+  nsDeque(const nsDeque& aOther) = delete;
+
+  /**
+   * Deque assignment operator (deleted)
+   *
+   * @param aOther another deque
+   * @return *this
+   */
+  nsDeque& operator=(const nsDeque& aOther) = delete;
+
+  void SetDeallocator(nsDequeFunctor<T>* aDeallocator) {
+    delete mDeallocator;
+    mDeallocator = aDeallocator;
+  }
+};
+
+/**
+ * Specialization of nsDeque for reference counted pointers.
+ *
+ * Provides builtin reference handling as part of Push/Peek/Pop
+ */
+template <typename T>
+class nsRefPtrDeque : private nsDeque<T> {
+  typedef mozilla::fallible_t fallible_t;
+
+  class RefPtrDeallocator : public nsDequeFunctor<T> {
+   public:
+    virtual void operator()(T* aObject) override {
+      RefPtr<T> releaseMe = dont_AddRef(aObject);
+    }
+  };
+
+ public:
+  using PointerType = RefPtr<T>;
+  using ConstDequeIterator =
+      mozilla::detail::ConstDequeIterator<nsRefPtrDeque<T>>;
+  using ConstIterator = mozilla::detail::ConstIterator<nsRefPtrDeque<T>>;
+
+  explicit nsRefPtrDeque() : nsDeque<T>(new RefPtrDeallocator()) {}
+
+  inline void PushFront(already_AddRefed<T> aItem) {
+    T* item = aItem.take();
+    nsDeque<T>::PushFront(item);
+  }
+
+  inline void PushFront(T* aItem) { PushFront(do_AddRef(aItem)); }
+
+  inline void Push(T* aItem) { Push(do_AddRef(aItem)); }
+
+  inline void Push(already_AddRefed<T> aItem) {
+    T* item = aItem.take();
+    nsDeque<T>::Push(item);
+  }
+
+  inline already_AddRefed<T> PopFront() {
+    return dont_AddRef(nsDeque<T>::PopFront());
+  }
+
+  inline already_AddRefed<T> Pop() { return dont_AddRef(nsDeque<T>::Pop()); }
+
+  inline T* PeekFront() const { return nsDeque<T>::PeekFront(); }
+
+  inline T* Peek() const { return nsDeque<T>::Peek(); }
+
+  inline T* ObjectAt(size_t aIndex) const {
+    return nsDeque<T>::ObjectAt(aIndex);
+  }
+
+  inline void Erase() { nsDeque<T>::Erase(); }
+
+  // If this deque is const, we can provide ConstDequeIterator's.
+  ConstDequeIterator begin() const { return ConstDequeIterator(*this, 0); }
+  ConstDequeIterator end() const {
+    return ConstDequeIterator(*this, GetSize());
+  }
+
+  // If this deque is *not* const, we provide ConstIterator's that can handle
+  // deque size changes.
+  ConstIterator begin() { return ConstIterator(*this, 0); }
+  ConstIterator end() {
+    return ConstIterator(*this, ConstIterator::EndIteratorIndex);
+  }
+
+  inline size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return nsDeque<T>::SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  inline size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return nsDeque<T>::SizeOfIncludingThis(aMallocSizeOf);
+  }
+
+  inline size_t GetSize() const { return nsDeque<T>::GetSize(); }
+
+  /**
+   * Call this method when you want to iterate through all
+   * items in the container, passing a functor along
+   * to call your code.
+   * If the deque is modified during ForEach, iteration will continue based on
+   * item indices; meaning that front operations may effectively skip over
+   * items or visit some items multiple times.
+   *
+   * @param   aFunctor object to call for each member
+   */
+  void ForEach(nsDequeFunctor<T>& aFunctor) const {
+    size_t size = GetSize();
+    for (size_t i = 0; i < size; ++i) {
+      aFunctor(ObjectAt(i));
+    }
+  }
+};
+
+#endif
diff --git a/xpcom/ds/nsEnumeratorUtils.cpp b/xpcom/ds/nsEnumeratorUtils.cpp
new file mode 100644
index 0000000000..69631818a3
--- /dev/null
+++ b/xpcom/ds/nsEnumeratorUtils.cpp
@@ -0,0 +1,248 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Attributes.h"
+
+#include "nsEnumeratorUtils.h"
+
+#include "nsIStringEnumerator.h"
+#include "nsSimpleEnumerator.h"
+
+#include "nsCOMPtr.h"
+#include "mozilla/RefPtr.h"
+
+class EmptyEnumeratorImpl : public nsSimpleEnumerator,
+                            public nsIUTF8StringEnumerator,
+                            public nsIStringEnumerator {
+ public:
+  EmptyEnumeratorImpl() = default;
+
+  // nsISupports interface. Not really inherited, but no mRefCnt.
+  NS_DECL_ISUPPORTS_INHERITED
+
+  // nsISimpleEnumerator
+  NS_DECL_NSISIMPLEENUMERATOR
+  NS_DECL_NSIUTF8STRINGENUMERATOR
+  NS_DECL_NSISTRINGENUMERATORBASE
+  // can't use NS_DECL_NSISTRINGENUMERATOR because they share the
+  // HasMore() signature
+
+  NS_IMETHOD GetNext(nsAString& aResult) override;
+
+  static EmptyEnumeratorImpl* GetInstance() {
+    static const EmptyEnumeratorImpl kInstance;
+    return const_cast<EmptyEnumeratorImpl*>(&kInstance);
+  }
+};
+
+// nsISupports interface
+NS_IMETHODIMP_(MozExternalRefCountType)
+EmptyEnumeratorImpl::AddRef(void) { return 2; }
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+EmptyEnumeratorImpl::Release(void) { return 1; }
+
+NS_IMPL_QUERY_INTERFACE_INHERITED(EmptyEnumeratorImpl, nsSimpleEnumerator,
+                                  nsIUTF8StringEnumerator, nsIStringEnumerator)
+
+// nsISimpleEnumerator interface
+NS_IMETHODIMP
+EmptyEnumeratorImpl::HasMoreElements(bool* aResult) {
+  *aResult = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+EmptyEnumeratorImpl::HasMore(bool* aResult) {
+  *aResult = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+EmptyEnumeratorImpl::GetNext(nsISupports** aResult) { return NS_ERROR_FAILURE; }
+
+NS_IMETHODIMP
+EmptyEnumeratorImpl::GetNext(nsACString& aResult) {
+  return NS_ERROR_UNEXPECTED;
+}
+
+NS_IMETHODIMP
+EmptyEnumeratorImpl::GetNext(nsAString& aResult) { return NS_ERROR_UNEXPECTED; }
+
+NS_IMETHODIMP
+EmptyEnumeratorImpl::StringIterator(nsIJSEnumerator** aRetVal) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+nsresult NS_NewEmptyEnumerator(nsISimpleEnumerator** aResult) {
+  *aResult = EmptyEnumeratorImpl::GetInstance();
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+class nsSingletonEnumerator final : public nsSimpleEnumerator {
+ public:
+  // nsISimpleEnumerator methods
+  NS_IMETHOD HasMoreElements(bool* aResult) override;
+  NS_IMETHOD GetNext(nsISupports** aResult) override;
+
+  explicit nsSingletonEnumerator(nsISupports* aValue);
+
+ private:
+  ~nsSingletonEnumerator() override;
+
+ protected:
+  nsCOMPtr<nsISupports> mValue;
+  bool mConsumed;
+};
+
+nsSingletonEnumerator::nsSingletonEnumerator(nsISupports* aValue)
+    : mValue(aValue) {
+  mConsumed = (mValue ? false : true);
+}
+
+nsSingletonEnumerator::~nsSingletonEnumerator() = default;
+
+NS_IMETHODIMP
+nsSingletonEnumerator::HasMoreElements(bool* aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  *aResult = !mConsumed;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSingletonEnumerator::GetNext(nsISupports** aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  if (mConsumed) {
+    return NS_ERROR_FAILURE;
+  }
+
+  mConsumed = true;
+
+  *aResult = mValue;
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+nsresult NS_NewSingletonEnumerator(nsISimpleEnumerator** aResult,
+                                   nsISupports* aSingleton) {
+  RefPtr<nsSingletonEnumerator> enumer = new nsSingletonEnumerator(aSingleton);
+  enumer.forget(aResult);
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
+class nsUnionEnumerator final : public nsSimpleEnumerator {
+ public:
+  // nsISimpleEnumerator methods
+  NS_IMETHOD HasMoreElements(bool* aResult) override;
+  NS_IMETHOD GetNext(nsISupports** aResult) override;
+
+  nsUnionEnumerator(nsISimpleEnumerator* aFirstEnumerator,
+                    nsISimpleEnumerator* aSecondEnumerator);
+
+ private:
+  ~nsUnionEnumerator() override;
+
+ protected:
+  nsCOMPtr<nsISimpleEnumerator> mFirstEnumerator, mSecondEnumerator;
+  bool mConsumed;
+  bool mAtSecond;
+};
+
+nsUnionEnumerator::nsUnionEnumerator(nsISimpleEnumerator* aFirstEnumerator,
+                                     nsISimpleEnumerator* aSecondEnumerator)
+    : mFirstEnumerator(aFirstEnumerator),
+      mSecondEnumerator(aSecondEnumerator),
+      mConsumed(false),
+      mAtSecond(false) {}
+
+nsUnionEnumerator::~nsUnionEnumerator() = default;
+
+NS_IMETHODIMP
+nsUnionEnumerator::HasMoreElements(bool* aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  nsresult rv;
+
+  if (mConsumed) {
+    *aResult = false;
+    return NS_OK;
+  }
+
+  if (!mAtSecond) {
+    rv = mFirstEnumerator->HasMoreElements(aResult);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+
+    if (*aResult) {
+      return NS_OK;
+    }
+
+    mAtSecond = true;
+  }
+
+  rv = mSecondEnumerator->HasMoreElements(aResult);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  if (*aResult) {
+    return NS_OK;
+  }
+
+  *aResult = false;
+  mConsumed = true;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsUnionEnumerator::GetNext(nsISupports** aResult) {
+  MOZ_ASSERT(aResult != 0, "null ptr");
+  if (!aResult) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  if (mConsumed) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (!mAtSecond) {
+    return mFirstEnumerator->GetNext(aResult);
+  }
+
+  return mSecondEnumerator->GetNext(aResult);
+}
+
+nsresult NS_NewUnionEnumerator(nsISimpleEnumerator** aResult,
+                               nsISimpleEnumerator* aFirstEnumerator,
+                               nsISimpleEnumerator* aSecondEnumerator) {
+  *aResult = nullptr;
+  if (!aFirstEnumerator) {
+    *aResult = aSecondEnumerator;
+  } else if (!aSecondEnumerator) {
+    *aResult = aFirstEnumerator;
+  } else {
+    auto* enumer = new nsUnionEnumerator(aFirstEnumerator, aSecondEnumerator);
+    *aResult = enumer;
+  }
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
diff --git a/xpcom/ds/nsEnumeratorUtils.h b/xpcom/ds/nsEnumeratorUtils.h
new file mode 100644
index 0000000000..f7a0db099e
--- /dev/null
+++ b/xpcom/ds/nsEnumeratorUtils.h
@@ -0,0 +1,24 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsEnumeratorUtils_h__
+#define nsEnumeratorUtils_h__
+
+#include "nscore.h"
+
+class nsISupports;
+class nsISimpleEnumerator;
+
+nsresult NS_NewEmptyEnumerator(nsISimpleEnumerator** aResult);
+
+nsresult NS_NewSingletonEnumerator(nsISimpleEnumerator** aResult,
+                                   nsISupports* aSingleton);
+
+nsresult NS_NewUnionEnumerator(nsISimpleEnumerator** aResult,
+                               nsISimpleEnumerator* aFirstEnumerator,
+                               nsISimpleEnumerator* aSecondEnumerator);
+
+#endif /* nsEnumeratorUtils_h__ */
diff --git a/xpcom/ds/nsExpirationTracker.h b/xpcom/ds/nsExpirationTracker.h
new file mode 100644
index 0000000000..ec2a25f67d
--- /dev/null
+++ b/xpcom/ds/nsExpirationTracker.h
@@ -0,0 +1,618 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef NSEXPIRATIONTRACKER_H_
+#define NSEXPIRATIONTRACKER_H_
+
+#include <cstring>
+#include "MainThreadUtils.h"
+#include "nsAlgorithm.h"
+#include "nsDebug.h"
+#include "nsTArray.h"
+#include "nsITimer.h"
+#include "nsCOMPtr.h"
+#include "nsIEventTarget.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsISupports.h"
+#include "nsIThread.h"
+#include "nsThreadUtils.h"
+#include "nscore.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Likely.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefCountType.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Services.h"
+
+/**
+ * Data used to track the expiration state of an object. We promise that this
+ * is 32 bits so that objects that includes this as a field can pad and align
+ * efficiently.
+ */
+struct nsExpirationState {
+  enum {
+    NOT_TRACKED = (1U << 4) - 1,
+    MAX_INDEX_IN_GENERATION = (1U << 28) - 1
+  };
+
+  nsExpirationState() : mGeneration(NOT_TRACKED), mIndexInGeneration(0) {}
+  bool IsTracked() { return mGeneration != NOT_TRACKED; }
+
+  /**
+   * The generation that this object belongs to, or NOT_TRACKED.
+   */
+  uint32_t mGeneration : 4;
+  uint32_t mIndexInGeneration : 28;
+};
+
+/**
+ * ExpirationTracker classes:
+ * - ExpirationTrackerImpl (Thread-safe class)
+ * - nsExpirationTracker (Main-thread only class)
+ *
+ * These classes can track the lifetimes and usage of a large number of
+ * objects, and send a notification some window of time after a live object was
+ * last used. This is very useful when you manage a large number of objects
+ * and want to flush some after they haven't been used for a while.
+ * nsExpirationTracker is designed to be very space and time efficient.
+ *
+ * The type parameter T is the object type that we will track pointers to. T
+ * must include an accessible method GetExpirationState() that returns a
+ * pointer to an nsExpirationState associated with the object (preferably,
+ * stored in a field of the object).
+ *
+ * The parameter K is the number of generations that will be used. Increasing
+ * the number of generations narrows the window within which we promise
+ * to fire notifications, at a slight increase in space cost for the tracker.
+ * We require 2 <= K <= nsExpirationState::NOT_TRACKED (currently 15).
+ *
+ * To use this class, you need to inherit from it and override the
+ * NotifyExpired() method.
+ *
+ * The approach is to track objects in K generations. When an object is accessed
+ * it moves from its current generation to the newest generation. Generations
+ * are stored in a cyclic array; when a timer interrupt fires, we advance
+ * the current generation pointer to effectively age all objects very
+ * efficiently. By storing information in each object about its generation and
+ * index within its generation array, we make removal of objects from a
+ * generation very cheap.
+ *
+ * Future work:
+ * -- Add a method to change the timer period?
+ */
+
+/**
+ * Base class for ExiprationTracker implementations.
+ *
+ * nsExpirationTracker class below is a specialized class to be inherited by the
+ * instances to be accessed only on main-thread.
+ *
+ * For creating a thread-safe tracker, you can define a subclass inheriting this
+ * base class and specialize the Mutex and AutoLock to be used.
+ *
+ * For an example of using ExpirationTrackerImpl with a DataMutex
+ * @see mozilla::gfx::GradientCache.
+ *
+ */
+template <typename T, uint32_t K, typename Mutex, typename AutoLock>
+class ExpirationTrackerImpl {
+ public:
+  /**
+   * Initialize the tracker.
+   * @param aTimerPeriod the timer period in milliseconds. The guarantees
+   * provided by the tracker are defined in terms of this period. If the
+   * period is zero, then we don't use a timer and rely on someone calling
+   * AgeOneGenerationLocked explicitly.
+   * @param aName the name of the subclass for telemetry.
+   * @param aEventTarget the optional event target on main thread to label the
+   * runnable of the asynchronous invocation to NotifyExpired().
+
+   */
+  ExpirationTrackerImpl(uint32_t aTimerPeriod, const char* aName,
+                        nsIEventTarget* aEventTarget = nullptr)
+      : mTimerPeriod(aTimerPeriod),
+        mNewestGeneration(0),
+        mInAgeOneGeneration(false),
+        mName(aName),
+        mEventTarget(aEventTarget) {
+    static_assert(K >= 2 && K <= nsExpirationState::NOT_TRACKED,
+                  "Unsupported number of generations (must be 2 <= K <= 15)");
+    MOZ_ASSERT(NS_IsMainThread());
+    if (mEventTarget) {
+      bool current = false;
+      // NOTE: The following check+crash could be condensed into a
+      // MOZ_RELEASE_ASSERT, but that triggers a segfault during compilation in
+      // clang 3.8. Once we don't have to care about clang 3.8 anymore, though,
+      // we can convert to MOZ_RELEASE_ASSERT here.
+      if (MOZ_UNLIKELY(NS_FAILED(mEventTarget->IsOnCurrentThread(&current)) ||
+                       !current)) {
+        MOZ_CRASH("Provided event target must be on the main thread");
+      }
+    }
+    mObserver = new ExpirationTrackerObserver();
+    mObserver->Init(this);
+  }
+
+  virtual ~ExpirationTrackerImpl() {
+    MOZ_ASSERT(NS_IsMainThread());
+    if (mTimer) {
+      mTimer->Cancel();
+    }
+    mObserver->Destroy();
+  }
+
+  /**
+   * Add an object to be tracked. It must not already be tracked. It will
+   * be added to the newest generation, i.e., as if it was just used.
+   * @return an error on out-of-memory
+   */
+  nsresult AddObjectLocked(T* aObj, const AutoLock& aAutoLock) {
+    if (NS_WARN_IF(!aObj)) {
+      MOZ_DIAGNOSTIC_ASSERT(false, "Invalid object to add");
+      return NS_ERROR_UNEXPECTED;
+    }
+    nsExpirationState* state = aObj->GetExpirationState();
+    if (NS_WARN_IF(state->IsTracked())) {
+      MOZ_DIAGNOSTIC_ASSERT(false,
+                            "Tried to add an object that's already tracked");
+      return NS_ERROR_UNEXPECTED;
+    }
+    nsTArray<T*>& generation = mGenerations[mNewestGeneration];
+    uint32_t index = generation.Length();
+    if (index > nsExpirationState::MAX_INDEX_IN_GENERATION) {
+      NS_WARNING("More than 256M elements tracked, this is probably a problem");
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+    if (index == 0) {
+      // We might need to start the timer
+      nsresult rv = CheckStartTimerLocked(aAutoLock);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+    }
+    // XXX(Bug 1631371) Check if this should use a fallible operation as it
+    // pretended earlier.
+    generation.AppendElement(aObj);
+    state->mGeneration = mNewestGeneration;
+    state->mIndexInGeneration = index;
+    return NS_OK;
+  }
+
+  /**
+   * Remove an object from the tracker. It must currently be tracked.
+   */
+  void RemoveObjectLocked(T* aObj, const AutoLock& aAutoLock) {
+    if (NS_WARN_IF(!aObj)) {
+      MOZ_DIAGNOSTIC_ASSERT(false, "Invalid object to remove");
+      return;
+    }
+    nsExpirationState* state = aObj->GetExpirationState();
+    if (NS_WARN_IF(!state->IsTracked())) {
+      MOZ_DIAGNOSTIC_ASSERT(false,
+                            "Tried to remove an object that's not tracked");
+      return;
+    }
+    nsTArray<T*>& generation = mGenerations[state->mGeneration];
+    uint32_t index = state->mIndexInGeneration;
+    MOZ_ASSERT(generation.Length() > index && generation[index] == aObj,
+               "Object is lying about its index");
+    // Move the last object to fill the hole created by removing aObj
+    T* lastObj = generation.PopLastElement();
+    // XXX It looks weird that index might point to the element that was just
+    // removed. Is that really correct?
+    if (index < generation.Length()) {
+      generation[index] = lastObj;
+    }
+    lastObj->GetExpirationState()->mIndexInGeneration = index;
+    state->mGeneration = nsExpirationState::NOT_TRACKED;
+    // We do not check whether we need to stop the timer here. The timer
+    // will check that itself next time it fires. Checking here would not
+    // be efficient since we'd need to track all generations. Also we could
+    // thrash by incessantly creating and destroying timers if someone
+    // kept adding and removing an object from the tracker.
+  }
+
+  /**
+   * Notify that an object has been used.
+   * @return an error if we lost the object from the tracker...
+   */
+  nsresult MarkUsedLocked(T* aObj, const AutoLock& aAutoLock) {
+    nsExpirationState* state = aObj->GetExpirationState();
+    if (mNewestGeneration == state->mGeneration) {
+      return NS_OK;
+    }
+    RemoveObjectLocked(aObj, aAutoLock);
+    return AddObjectLocked(aObj, aAutoLock);
+  }
+
+  /**
+   * The timer calls this, but it can also be manually called if you want
+   * to age objects "artifically". This can result in calls to
+   * NotifyExpiredLocked.
+   */
+  void AgeOneGenerationLocked(const AutoLock& aAutoLock) {
+    if (mInAgeOneGeneration) {
+      NS_WARNING("Can't reenter AgeOneGeneration from NotifyExpired");
+      return;
+    }
+
+    mInAgeOneGeneration = true;
+    uint32_t reapGeneration =
+        mNewestGeneration > 0 ? mNewestGeneration - 1 : K - 1;
+    nsTArray<T*>& generation = mGenerations[reapGeneration];
+    // The following is rather tricky. We have to cope with objects being
+    // removed from this generation either because of a call to RemoveObject
+    // (or indirectly via MarkUsedLocked) inside NotifyExpiredLocked.
+    // Fortunately no objects can be added to this generation because it's not
+    // the newest generation. We depend on the fact that RemoveObject can only
+    // cause the indexes of objects in this generation to *decrease*, not
+    // increase. So if we start from the end and work our way backwards we are
+    // guaranteed to see each object at least once.
+    size_t index = generation.Length();
+    for (;;) {
+      // Objects could have been removed so index could be outside
+      // the array
+      index = XPCOM_MIN(index, generation.Length());
+      if (index == 0) {
+        break;
+      }
+      --index;
+      NotifyExpiredLocked(generation[index], aAutoLock);
+    }
+    // Any leftover objects from reapGeneration just end up in the new
+    // newest-generation. This is bad form, though, so warn if there are any.
+    if (!generation.IsEmpty()) {
+      NS_WARNING("Expired objects were not removed or marked used");
+    }
+    // Free excess memory used by the generation array, since we probably
+    // just removed most or all of its elements.
+    generation.Compact();
+    mNewestGeneration = reapGeneration;
+    mInAgeOneGeneration = false;
+  }
+
+  /**
+   * This just calls AgeOneGenerationLocked K times. Under normal circumstances
+   * this will result in all objects getting NotifyExpiredLocked called on them,
+   * but if NotifyExpiredLocked itself marks some objects as used, then those
+   * objects might not expire. This would be a good thing to call if we get into
+   * a critically-low memory situation.
+   */
+  void AgeAllGenerationsLocked(const AutoLock& aAutoLock) {
+    uint32_t i;
+    for (i = 0; i < K; ++i) {
+      AgeOneGenerationLocked(aAutoLock);
+    }
+  }
+
+  class Iterator {
+   private:
+    ExpirationTrackerImpl<T, K, Mutex, AutoLock>* mTracker;
+    uint32_t mGeneration;
+    uint32_t mIndex;
+
+   public:
+    Iterator(ExpirationTrackerImpl<T, K, Mutex, AutoLock>* aTracker,
+             AutoLock& aAutoLock)
+        : mTracker(aTracker), mGeneration(0), mIndex(0) {}
+
+    T* Next() {
+      while (mGeneration < K) {
+        nsTArray<T*>* generation = &mTracker->mGenerations[mGeneration];
+        if (mIndex < generation->Length()) {
+          ++mIndex;
+          return (*generation)[mIndex - 1];
+        }
+        ++mGeneration;
+        mIndex = 0;
+      }
+      return nullptr;
+    }
+  };
+
+  friend class Iterator;
+
+  bool IsEmptyLocked(const AutoLock& aAutoLock) const {
+    for (uint32_t i = 0; i < K; ++i) {
+      if (!mGenerations[i].IsEmpty()) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  size_t Length(const AutoLock& aAutoLock) const {
+    size_t len = 0;
+    for (uint32_t i = 0; i < K; ++i) {
+      len += mGenerations[i].Length();
+    }
+    return len;
+  }
+
+  // @return The amount of memory used by this ExpirationTrackerImpl, excluding
+  // sizeof(*this). If you want to measure anything hanging off the mGenerations
+  // array, you must iterate over the elements and measure them individually;
+  // hence the "Shallow" prefix.
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    size_t bytes = 0;
+    for (uint32_t i = 0; i < K; ++i) {
+      bytes += mGenerations[i].ShallowSizeOfExcludingThis(aMallocSizeOf);
+    }
+    return bytes;
+  }
+
+ protected:
+  /**
+   * This must be overridden to catch notifications. It is called whenever
+   * we detect that an object has not been used for at least (K-1)*mTimerPeriod
+   * milliseconds. If timer events are not delayed, it will be called within
+   * roughly K*mTimerPeriod milliseconds after the last use.
+   * (Unless AgeOneGenerationLocked or AgeAllGenerationsLocked have been called
+   * to accelerate the aging process.)
+   *
+   * NOTE: These bounds ignore delays in timer firings due to actual work being
+   * performed by the browser. We use a slack timer so there is always at least
+   * mTimerPeriod milliseconds between firings, which gives us
+   * (K-1)*mTimerPeriod as a pretty solid lower bound. The upper bound is rather
+   * loose, however. If the maximum amount by which any given timer firing is
+   * delayed is D, then the upper bound before NotifyExpiredLocked is called is
+   * K*(mTimerPeriod + D).
+   *
+   * The NotifyExpiredLocked call is expected to remove the object from the
+   * tracker, but it need not. The object (or other objects) could be
+   * "resurrected" by calling MarkUsedLocked() on them, or they might just not
+   * be removed. Any objects left over that have not been resurrected or removed
+   * are placed in the new newest-generation, but this is considered "bad form"
+   * and should be avoided (we'll issue a warning). (This recycling counts
+   * as "a use" for the purposes of the expiry guarantee above...)
+   *
+   * For robustness and simplicity, we allow objects to be notified more than
+   * once here in the same timer tick.
+   */
+  virtual void NotifyExpiredLocked(T*, const AutoLock&) = 0;
+
+  /**
+   * This may be overridden to perform any post-aging work that needs to be
+   * done while still holding the lock. It will be called once after each timer
+   * event, and each low memory event has been handled.
+   */
+  virtual void NotifyHandlerEndLocked(const AutoLock&){};
+
+  /**
+   * This may be overridden to perform any post-aging work that needs to be
+   * done outside the lock. It will be called once after each
+   * NotifyEndTransactionLocked call.
+   */
+  virtual void NotifyHandlerEnd(){};
+
+  virtual Mutex& GetMutex() = 0;
+
+ private:
+  class ExpirationTrackerObserver;
+  RefPtr<ExpirationTrackerObserver> mObserver;
+  nsTArray<T*> mGenerations[K];
+  nsCOMPtr<nsITimer> mTimer;
+  uint32_t mTimerPeriod;
+  uint32_t mNewestGeneration;
+  bool mInAgeOneGeneration;
+  const char* const mName;  // Used for timer firing profiling.
+  const nsCOMPtr<nsIEventTarget> mEventTarget;
+
+  /**
+   * Whenever "memory-pressure" is observed, it calls AgeAllGenerationsLocked()
+   * to minimize memory usage.
+   */
+  class ExpirationTrackerObserver final : public nsIObserver {
+   public:
+    void Init(ExpirationTrackerImpl<T, K, Mutex, AutoLock>* aObj) {
+      mOwner = aObj;
+      nsCOMPtr<nsIObserverService> obs =
+          mozilla::services::GetObserverService();
+      if (obs) {
+        obs->AddObserver(this, "memory-pressure", false);
+      }
+    }
+    void Destroy() {
+      mOwner = nullptr;
+      nsCOMPtr<nsIObserverService> obs =
+          mozilla::services::GetObserverService();
+      if (obs) {
+        obs->RemoveObserver(this, "memory-pressure");
+      }
+    }
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSIOBSERVER
+   private:
+    ExpirationTrackerImpl<T, K, Mutex, AutoLock>* mOwner;
+  };
+
+  void HandleLowMemory() {
+    {
+      AutoLock lock(GetMutex());
+      AgeAllGenerationsLocked(lock);
+      NotifyHandlerEndLocked(lock);
+    }
+    NotifyHandlerEnd();
+  }
+
+  void HandleTimeout() {
+    {
+      AutoLock lock(GetMutex());
+      AgeOneGenerationLocked(lock);
+      // Cancel the timer if we have no objects to track
+      if (IsEmptyLocked(lock)) {
+        mTimer->Cancel();
+        mTimer = nullptr;
+      }
+      NotifyHandlerEndLocked(lock);
+    }
+    NotifyHandlerEnd();
+  }
+
+  static void TimerCallback(nsITimer* aTimer, void* aThis) {
+    ExpirationTrackerImpl* tracker = static_cast<ExpirationTrackerImpl*>(aThis);
+    tracker->HandleTimeout();
+  }
+
+  nsresult CheckStartTimerLocked(const AutoLock& aAutoLock) {
+    if (mTimer || !mTimerPeriod) {
+      return NS_OK;
+    }
+    nsCOMPtr<nsIEventTarget> target = mEventTarget;
+    if (!target && !NS_IsMainThread()) {
+      // TimerCallback should always be run on the main thread to prevent races
+      // to the destruction of the tracker.
+      target = do_GetMainThread();
+      NS_ENSURE_STATE(target);
+    }
+
+    return NS_NewTimerWithFuncCallback(
+        getter_AddRefs(mTimer), TimerCallback, this, mTimerPeriod,
+        nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY, mName, target);
+  }
+};
+
+namespace detail {
+
+class PlaceholderLock {
+ public:
+  void Lock() {}
+  void Unlock() {}
+};
+
+class PlaceholderAutoLock {
+ public:
+  explicit PlaceholderAutoLock(PlaceholderLock&) {}
+  ~PlaceholderAutoLock() = default;
+};
+
+template <typename T, uint32_t K>
+using SingleThreadedExpirationTracker =
+    ExpirationTrackerImpl<T, K, PlaceholderLock, PlaceholderAutoLock>;
+
+}  // namespace detail
+
+template <typename T, uint32_t K>
+class nsExpirationTracker
+    : protected ::detail::SingleThreadedExpirationTracker<T, K> {
+  typedef ::detail::PlaceholderLock Lock;
+  typedef ::detail::PlaceholderAutoLock AutoLock;
+
+  Lock mLock;
+
+  AutoLock FakeLock() {
+    MOZ_DIAGNOSTIC_ASSERT(NS_IsMainThread());
+    return AutoLock(mLock);
+  }
+
+  Lock& GetMutex() override {
+    MOZ_DIAGNOSTIC_ASSERT(NS_IsMainThread());
+    return mLock;
+  }
+
+  void NotifyExpiredLocked(T* aObject, const AutoLock&) override {
+    NotifyExpired(aObject);
+  }
+
+  /**
+   * Since there are no users of these callbacks in the single threaded case,
+   * we mark them as final with the hope that the compiler can optimize the
+   * method calls out entirely.
+   */
+  void NotifyHandlerEndLocked(const AutoLock&) final {}
+  void NotifyHandlerEnd() final {}
+
+ protected:
+  virtual void NotifyExpired(T* aObj) = 0;
+
+ public:
+  nsExpirationTracker(uint32_t aTimerPeriod, const char* aName,
+                      nsIEventTarget* aEventTarget = nullptr)
+      : ::detail::SingleThreadedExpirationTracker<T, K>(aTimerPeriod, aName,
+                                                        aEventTarget) {}
+
+  virtual ~nsExpirationTracker() = default;
+
+  nsresult AddObject(T* aObj) {
+    return this->AddObjectLocked(aObj, FakeLock());
+  }
+
+  void RemoveObject(T* aObj) { this->RemoveObjectLocked(aObj, FakeLock()); }
+
+  nsresult MarkUsed(T* aObj) { return this->MarkUsedLocked(aObj, FakeLock()); }
+
+  void AgeOneGeneration() { this->AgeOneGenerationLocked(FakeLock()); }
+
+  void AgeAllGenerations() { this->AgeAllGenerationsLocked(FakeLock()); }
+
+  class Iterator {
+   private:
+    AutoLock mAutoLock;
+    typename ExpirationTrackerImpl<T, K, Lock, AutoLock>::Iterator mIterator;
+
+   public:
+    explicit Iterator(nsExpirationTracker<T, K>* aTracker)
+        : mAutoLock(aTracker->GetMutex()), mIterator(aTracker, mAutoLock) {}
+
+    T* Next() { return mIterator.Next(); }
+  };
+
+  friend class Iterator;
+
+  bool IsEmpty() { return this->IsEmptyLocked(FakeLock()); }
+};
+
+template <typename T, uint32_t K, typename Mutex, typename AutoLock>
+NS_IMETHODIMP ExpirationTrackerImpl<T, K, Mutex, AutoLock>::
+    ExpirationTrackerObserver::Observe(nsISupports* aSubject,
+                                       const char* aTopic,
+                                       const char16_t* aData) {
+  if (!strcmp(aTopic, "memory-pressure") && mOwner) {
+    mOwner->HandleLowMemory();
+  }
+  return NS_OK;
+}
+
+template <class T, uint32_t K, typename Mutex, typename AutoLock>
+NS_IMETHODIMP_(MozExternalRefCountType)
+ExpirationTrackerImpl<T, K, Mutex, AutoLock>::ExpirationTrackerObserver::AddRef(
+    void) {
+  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+  NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+  ++mRefCnt;
+  NS_LOG_ADDREF(this, mRefCnt, "ExpirationTrackerObserver", sizeof(*this));
+  return mRefCnt;
+}
+
+template <class T, uint32_t K, typename Mutex, typename AutoLock>
+NS_IMETHODIMP_(MozExternalRefCountType)
+ExpirationTrackerImpl<T, K, Mutex,
+                      AutoLock>::ExpirationTrackerObserver::Release(void) {
+  MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
+  NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+  --mRefCnt;
+  NS_LOG_RELEASE(this, mRefCnt, "ExpirationTrackerObserver");
+  if (mRefCnt == 0) {
+    NS_ASSERT_OWNINGTHREAD(ExpirationTrackerObserver);
+    mRefCnt = 1; /* stabilize */
+    delete (this);
+    return 0;
+  }
+  return mRefCnt;
+}
+
+template <class T, uint32_t K, typename Mutex, typename AutoLock>
+NS_IMETHODIMP ExpirationTrackerImpl<T, K, Mutex, AutoLock>::
+    ExpirationTrackerObserver::QueryInterface(REFNSIID aIID,
+                                              void** aInstancePtr) {
+  NS_ASSERTION(aInstancePtr, "QueryInterface requires a non-NULL destination!");
+  nsresult rv = NS_ERROR_FAILURE;
+  NS_INTERFACE_TABLE(ExpirationTrackerObserver, nsIObserver)
+  return rv;
+}
+
+#endif /*NSEXPIRATIONTRACKER_H_*/
diff --git a/xpcom/ds/nsGkAtoms.cpp b/xpcom/ds/nsGkAtoms.cpp
new file mode 100644
index 0000000000..402e0eb779
--- /dev/null
+++ b/xpcom/ds/nsGkAtoms.cpp
@@ -0,0 +1,70 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsGkAtoms.h"
+
+namespace mozilla {
+namespace detail {
+
+// Because this is `constexpr` it ends up in read-only memory where it can be
+// shared between processes.
+extern constexpr GkAtoms gGkAtoms = {
+// The initialization of each atom's string.
+//
+// Expansion of the example GK_ATOM entries in nsGkAtoms.h:
+//
+//   u"a",
+//   u"bb",
+//   u"Ccc",
+//
+#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) \
+  u"" value_,
+#include "nsGkAtomList.h"
+#undef GK_ATOM
+    {
+// The initialization of the atoms themselves.
+//
+// Note that |value_| is an 8-bit string, and so |sizeof(value_)| is equal
+// to the number of chars (including the terminating '\0'). The |u""| prefix
+// converts |value_| to a 16-bit string.
+//
+// Expansion of the example GK_ATOM entries in nsGkAtoms.h:
+//
+//   nsStaticAtom(
+//     1,
+//     0x01234567,
+//     offsetof(GkAtoms, mAtoms[static_cast<size_t>(GkAtoms::Atoms::a)]) -
+//       offsetof(GkAtoms, a_string),
+//     true),
+//
+//   nsStaticAtom(
+//     2,
+//     0x12345678,
+//     offsetof(GkAtoms, mAtoms[static_cast<size_t>(GkAtoms::Atoms::bb)]) -
+//       offsetof(GkAtoms, bb_string),
+//     false),
+//
+//   nsStaticAtom(
+//     3,
+//     0x23456789,
+//     offsetof(GkAtoms, mAtoms[static_cast<size_t>(GkAtoms::Atoms::Ccc)]) -
+//       offsetof(GkAtoms, Ccc_string),
+//     false),
+//
+#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_)     \
+  nsStaticAtom(                                                               \
+      sizeof(value_) - 1, hash_,                                              \
+      offsetof(GkAtoms, mAtoms[static_cast<size_t>(GkAtoms::Atoms::name_)]) - \
+          offsetof(GkAtoms, name_##_string),                                  \
+      is_ascii_lower_),
+#include "nsGkAtomList.h"
+#undef GK_ATOM
+    }};
+
+}  // namespace detail
+}  // namespace mozilla
+
+const nsStaticAtom* const nsGkAtoms::sAtoms = mozilla::detail::gGkAtoms.mAtoms;
diff --git a/xpcom/ds/nsGkAtoms.h b/xpcom/ds/nsGkAtoms.h
new file mode 100644
index 0000000000..eba33bf95c
--- /dev/null
+++ b/xpcom/ds/nsGkAtoms.h
@@ -0,0 +1,192 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsGkAtoms_h___
+#define nsGkAtoms_h___
+
+#include "nsAtom.h"
+
+// Static atoms are structured carefully to satisfy a lot of constraints.
+//
+// - We have ~2300 static atoms.
+//
+// - We want them to be constexpr so they end up in .rodata, and thus shared
+//   between processes, minimizing memory usage.
+//
+// - We need them to be in an array, so we can iterate over them (for
+//   registration and lookups).
+//
+// - Each static atom has a string literal associated with it. We can't use a
+//   pointer to the string literal because then the atoms won't end up in
+//   .rodata. Therefore the string literals and the atoms must be arranged in a
+//   way such that a numeric index can be used instead. This numeric index
+//   (nsStaticAtom::mStringOffset) must be computable at compile-time to keep
+//   the static atom constexpr. It should also not be too large (a uint32_t is
+//   reasonable).
+//
+// - Each static atom stores the hash value of its associated string literal;
+//   it's used in various ways. The hash value must be specified at
+//   compile-time, to keep the static atom constexpr.
+//
+// - As well as accessing each static atom via array indexing, we need an
+//   individual pointer, e.g. nsGkAtoms::foo. We want this to be constexpr so
+//   it doesn't take up any space in memory.
+//
+// - The array of static atoms can't be in a .h file, because it's a huge
+//   constexpr expression, which would blow out compile times. But the
+//   individual pointers for the static atoms must be in a .h file so they are
+//   public.
+//
+// nsGkAtoms below defines static atoms in a way that satisfies these
+// constraints. It uses nsGkAtomList.h, which defines the names and values of
+// the atoms. nsGkAtomList.h is generated by StaticAtoms.py and has entries
+// that look like this:
+//
+//   GK_ATOM(a, "a", 0x01234567, nsStaticAtom, Atom)
+//   GK_ATOM(bb, "bb", 0x12345678, nsCSSPseudoElementStaticAtom,
+//           PseudoElementAtom)
+//   GK_ATOM(Ccc, "Ccc", 0x23456789, nsCSSAnonBoxPseudoStaticAtom,
+//           InheritingAnonBoxAtom)
+//
+// Comments throughout this file and nsGkAtoms.cpp show how these entries get
+// expanded by macros.
+
+// Trivial subclasses of nsStaticAtom so that function signatures can require
+// an atom from a specific atom list.
+#define DEFINE_STATIC_ATOM_SUBCLASS(name_)                              \
+  class name_ : public nsStaticAtom {                                   \
+   public:                                                              \
+    constexpr name_(uint32_t aLength, uint32_t aHash, uint32_t aOffset, \
+                    bool aIsAsciiLowercase)                             \
+        : nsStaticAtom(aLength, aHash, aOffset, aIsAsciiLowercase) {}   \
+  };
+
+DEFINE_STATIC_ATOM_SUBCLASS(nsCSSAnonBoxPseudoStaticAtom)
+DEFINE_STATIC_ATOM_SUBCLASS(nsCSSPseudoElementStaticAtom)
+
+#undef DEFINE_STATIC_ATOM_SUBCLASS
+
+namespace mozilla {
+namespace detail {
+
+// This `detail` class contains the atom strings and the atom objects. Because
+// they are together in a class, the `mStringOffset` field of the atoms will be
+// small and can be initialized at compile time.
+//
+// A `detail` namespace is used because the things within it aren't directly
+// referenced by external users of these static atoms.
+struct GkAtoms {
+// The declaration of each atom's string.
+//
+// Expansion of the example GK_ATOM entries from above:
+//
+//   const char16_t a_string[sizeof("a")];
+//   const char16_t bb_string[sizeof("bb")];
+//   const char16_t Ccc_string[sizeof("Ccc")];
+//
+#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) \
+  const char16_t name_##_string[sizeof(value_)];
+#include "nsGkAtomList.h"
+#undef GK_ATOM
+
+  // The enum value for each atom.
+  enum class Atoms {
+// Expansion of the example GK_ATOM entries above:
+//
+//   a,
+//   bb,
+//   Ccc,
+//
+#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) name_,
+#include "nsGkAtomList.h"
+#undef GK_ATOM
+    AtomsCount
+  };
+
+  const nsStaticAtom mAtoms[static_cast<size_t>(Atoms::AtomsCount)];
+};
+
+// The offset from the start of the GkAtoms object to the start of the
+// nsStaticAtom array inside it.  This is used in Rust to avoid problems
+// with lld-link.exe on Windows when rust-bindgen generates a non-opaque
+// version of GkAtoms.
+//
+// https://bugzilla.mozilla.org/show_bug.cgi?id=1517685
+const ptrdiff_t kGkAtomsArrayOffset = offsetof(GkAtoms, mAtoms);
+
+// The GkAtoms instance is `extern const` so it can be defined in a .cpp file.
+//
+// XXX: The NS_EXTERNAL_VIS is necessary to work around an apparent GCC bug:
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87494
+#if defined(__GNUC__) && !defined(__clang__)
+extern NS_EXTERNAL_VIS const GkAtoms gGkAtoms;
+#else
+extern const GkAtoms gGkAtoms;
+#endif
+
+}  // namespace detail
+}  // namespace mozilla
+
+// This class holds the pointers to the individual atoms.
+class nsGkAtoms {
+ private:
+  friend void NS_InitAtomTable();
+
+  // This is a useful handle to the array of atoms, used below and also
+  // possibly by Rust code.
+  static const nsStaticAtom* const sAtoms;
+
+  // The number of atoms, used below.
+  static constexpr size_t sAtomsLen =
+      static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::AtomsCount);
+
+ public:
+  static nsStaticAtom* GetAtomByIndex(size_t aIndex) {
+    MOZ_ASSERT(aIndex < sAtomsLen);
+    return const_cast<nsStaticAtom*>(&sAtoms[aIndex]);
+  }
+
+  static size_t IndexOf(const nsStaticAtom* atom) {
+    nsStaticAtom* firstAtom = GetAtomByIndex(0);
+    size_t ret = atom - firstAtom;
+    MOZ_ASSERT(ret < sAtomsLen);
+    return ret;
+  }
+
+// The definition of the pointer to each static atom.
+//
+// These types are not `static constexpr <type>* const` -- even though these
+// atoms are immutable -- because they are often passed to functions with
+// `nsAtom*` parameters that can be passed both dynamic and static atoms.
+//
+// Expansion of the example GK_ATOM entries above:
+//
+//   static constexpr nsStaticAtom* a =
+//     const_cast<nsStaticAtom*>(
+//       &mozilla::detail::gGkAtoms.mAtoms[
+//         static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::a)]);
+//
+//   static constexpr nsStaticAtom* bb =
+//     const_cast<nsStaticAtom*>(
+//       &mozilla::detail::gGkAtoms.mAtoms[
+//         static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::bb)]);
+//
+//   static constexpr nsStaticAtom* Ccc =
+//     const_cast<nsStaticAtom*>(
+//       &mozilla::detail::gGkAtoms.mAtoms[
+//         static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::Ccc)]);
+//
+#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) \
+  static constexpr nsStaticAtom* name_ = const_cast<nsStaticAtom*>(       \
+      &mozilla::detail::gGkAtoms.mAtoms[static_cast<size_t>(              \
+          mozilla::detail::GkAtoms::Atoms::name_)]);
+#include "nsGkAtomList.h"
+#undef GK_ATOM
+};
+
+inline bool nsAtom::IsEmpty() const { return this == nsGkAtoms::_empty; }
+
+#endif /* nsGkAtoms_h___ */
diff --git a/xpcom/ds/nsHashKeys.h b/xpcom/ds/nsHashKeys.h
new file mode 100644
index 0000000000..fb0a8244c4
--- /dev/null
+++ b/xpcom/ds/nsHashKeys.h
@@ -0,0 +1,636 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTHashKeys_h__
+#define nsTHashKeys_h__
+
+#include "nsID.h"
+#include "nsISupports.h"
+#include "nsCOMPtr.h"
+#include "PLDHashTable.h"
+#include <new>
+
+#include "nsString.h"
+#include "nsCRTGlue.h"
+#include "nsUnicharUtils.h"
+#include "nsPointerHashKeys.h"
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include <type_traits>
+#include <utility>
+
+#include "mozilla/HashFunctions.h"
+
+namespace mozilla {
+
+// These are defined analogously to the HashString overloads in mfbt.
+
+inline uint32_t HashString(const nsAString& aStr) {
+  return HashString(aStr.BeginReading(), aStr.Length());
+}
+
+inline uint32_t HashString(const nsACString& aStr) {
+  return HashString(aStr.BeginReading(), aStr.Length());
+}
+
+}  // namespace mozilla
+
+/** @file nsHashKeys.h
+ * standard HashKey classes for nsBaseHashtable and relatives. Each of these
+ * classes follows the nsTHashtable::EntryType specification
+ *
+ * Lightweight keytypes provided here:
+ * nsStringHashKey
+ * nsCStringHashKey
+ * nsUint32HashKey
+ * nsUint64HashKey
+ * nsFloatHashKey
+ * IntPtrHashKey
+ * nsPtrHashKey
+ * nsClearingPtrHashKey
+ * nsVoidPtrHashKey
+ * nsClearingVoidPtrHashKey
+ * nsISupportsHashKey
+ * nsIDHashKey
+ * nsDepCharHashKey
+ * nsCharPtrHashKey
+ * nsUnicharPtrHashKey
+ * nsGenericHashKey
+ */
+
+/**
+ * hashkey wrapper using nsAString KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsStringHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const nsAString& KeyType;
+  typedef const nsAString* KeyTypePointer;
+
+  explicit nsStringHashKey(KeyTypePointer aStr) : mStr(*aStr) {}
+  nsStringHashKey(const nsStringHashKey&) = delete;
+  nsStringHashKey(nsStringHashKey&& aToMove)
+      : PLDHashEntryHdr(std::move(aToMove)), mStr(std::move(aToMove.mStr)) {}
+  ~nsStringHashKey() = default;
+
+  KeyType GetKey() const { return mStr; }
+  bool KeyEquals(const KeyTypePointer aKey) const { return mStr.Equals(*aKey); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(const KeyTypePointer aKey) {
+    return mozilla::HashString(*aKey);
+  }
+
+#ifdef MOZILLA_INTERNAL_API
+  // To avoid double-counting, only measure the string if it is unshared.
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return GetKey().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+  }
+#endif
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  nsString mStr;
+};
+
+#ifdef MOZILLA_INTERNAL_API
+
+namespace mozilla::detail {
+
+template <class CharT, bool Unicode = true>
+struct comparatorTraits {};
+
+template <>
+struct comparatorTraits<char, false> {
+  static int caseInsensitiveCompare(const char* aLhs, const char* aRhs,
+                                    size_t aLhsLength, size_t aRhsLength) {
+    return nsCaseInsensitiveCStringComparator(aLhs, aRhs, aLhsLength,
+                                              aRhsLength);
+  };
+};
+
+template <>
+struct comparatorTraits<char, true> {
+  static int caseInsensitiveCompare(const char* aLhs, const char* aRhs,
+                                    size_t aLhsLength, size_t aRhsLength) {
+    return nsCaseInsensitiveUTF8StringComparator(aLhs, aRhs, aLhsLength,
+                                                 aRhsLength);
+  };
+};
+
+template <>
+struct comparatorTraits<char16_t, true> {
+  static int caseInsensitiveCompare(const char16_t* aLhs, const char16_t* aRhs,
+                                    size_t aLhsLength, size_t aRhsLength) {
+    return nsCaseInsensitiveStringComparator(aLhs, aRhs, aLhsLength,
+                                             aRhsLength);
+  };
+};
+
+}  // namespace mozilla::detail
+
+/**
+ * This is internal-API only because nsCaseInsensitive{C}StringComparator is
+ * internal-only.
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+
+template <typename T, bool Unicode>
+class nsTStringCaseInsensitiveHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const nsTSubstring<T>& KeyType;
+  typedef const nsTSubstring<T>* KeyTypePointer;
+
+  explicit nsTStringCaseInsensitiveHashKey(KeyTypePointer aStr) : mStr(*aStr) {
+    // take it easy just deal HashKey
+  }
+
+  nsTStringCaseInsensitiveHashKey(const nsTStringCaseInsensitiveHashKey&) =
+      delete;
+  nsTStringCaseInsensitiveHashKey(nsTStringCaseInsensitiveHashKey&& aToMove)
+      : PLDHashEntryHdr(std::move(aToMove)), mStr(std::move(aToMove.mStr)) {}
+  ~nsTStringCaseInsensitiveHashKey() = default;
+
+  KeyType GetKey() const { return mStr; }
+  bool KeyEquals(const KeyTypePointer aKey) const {
+    using comparator = typename mozilla::detail::comparatorTraits<T, Unicode>;
+    return mStr.Equals(*aKey, comparator::caseInsensitiveCompare);
+  }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(const KeyTypePointer aKey) {
+    nsTAutoString<T> tmKey(*aKey);
+    ToLowerCase(tmKey);
+    return mozilla::HashString(tmKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+  // To avoid double-counting, only measure the string if it is unshared.
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return GetKey().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+  }
+
+ private:
+  const nsTString<T> mStr;
+};
+
+using nsStringCaseInsensitiveHashKey =
+    nsTStringCaseInsensitiveHashKey<char16_t, true>;
+using nsCStringASCIICaseInsensitiveHashKey =
+    nsTStringCaseInsensitiveHashKey<char, false>;
+using nsCStringUTF8CaseInsensitiveHashKey =
+    nsTStringCaseInsensitiveHashKey<char, true>;
+
+#endif  // MOZILLA_INTERNAL_API
+
+/**
+ * hashkey wrapper using nsACString KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsCStringHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const nsACString& KeyType;
+  typedef const nsACString* KeyTypePointer;
+
+  explicit nsCStringHashKey(const nsACString* aStr) : mStr(*aStr) {}
+  nsCStringHashKey(nsCStringHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mStr(std::move(aOther.mStr)) {}
+  ~nsCStringHashKey() = default;
+
+  KeyType GetKey() const { return mStr; }
+  bool KeyEquals(KeyTypePointer aKey) const { return mStr.Equals(*aKey); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return mozilla::HashString(*aKey);
+  }
+
+#ifdef MOZILLA_INTERNAL_API
+  // To avoid double-counting, only measure the string if it is unshared.
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return GetKey().SizeOfExcludingThisIfUnshared(aMallocSizeOf);
+  }
+#endif
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const nsCString mStr;
+};
+
+/**
+ * hashkey wrapper using integral or enum KeyTypes
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+template <typename T,
+          std::enable_if_t<std::is_integral_v<T> || std::is_enum_v<T>, int> = 0>
+class nsIntegralHashKey : public PLDHashEntryHdr {
+ public:
+  using KeyType = const T&;
+  using KeyTypePointer = const T*;
+
+  explicit nsIntegralHashKey(KeyTypePointer aKey) : mValue(*aKey) {}
+  nsIntegralHashKey(nsIntegralHashKey&& aOther) noexcept
+      : PLDHashEntryHdr(std::move(aOther)), mValue(aOther.mValue) {}
+  ~nsIntegralHashKey() = default;
+
+  KeyType GetKey() const { return mValue; }
+  bool KeyEquals(KeyTypePointer aKey) const { return *aKey == mValue; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return mozilla::HashGeneric(*aKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const T mValue;
+};
+
+/**
+ * hashkey wrapper using uint32_t KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+using nsUint32HashKey = nsIntegralHashKey<uint32_t>;
+
+/**
+ * hashkey wrapper using uint64_t KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+using nsUint64HashKey = nsIntegralHashKey<uint64_t>;
+
+/**
+ * hashkey wrapper using float KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsFloatHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const float& KeyType;
+  typedef const float* KeyTypePointer;
+
+  explicit nsFloatHashKey(KeyTypePointer aKey) : mValue(*aKey) {}
+  nsFloatHashKey(nsFloatHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mValue(std::move(aOther.mValue)) {}
+  ~nsFloatHashKey() = default;
+
+  KeyType GetKey() const { return mValue; }
+  bool KeyEquals(KeyTypePointer aKey) const { return *aKey == mValue; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return *reinterpret_cast<const uint32_t*>(aKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const float mValue;
+};
+
+/**
+ * hashkey wrapper using intptr_t KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+using IntPtrHashKey = nsIntegralHashKey<intptr_t>;
+
+/**
+ * hashkey wrapper using nsISupports* KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsISupportsHashKey : public PLDHashEntryHdr {
+ public:
+  typedef nsISupports* KeyType;
+  typedef const nsISupports* KeyTypePointer;
+
+  explicit nsISupportsHashKey(const nsISupports* aKey)
+      : mSupports(const_cast<nsISupports*>(aKey)) {}
+  nsISupportsHashKey(nsISupportsHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)),
+        mSupports(std::move(aOther.mSupports)) {}
+  ~nsISupportsHashKey() = default;
+
+  KeyType GetKey() const { return mSupports; }
+  bool KeyEquals(KeyTypePointer aKey) const { return aKey == mSupports; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return NS_PTR_TO_UINT32(aKey) >> 2;
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  nsCOMPtr<nsISupports> mSupports;
+};
+
+/**
+ * hashkey wrapper using refcounted * KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+template <class T>
+class nsRefPtrHashKey : public PLDHashEntryHdr {
+ public:
+  typedef T* KeyType;
+  typedef const T* KeyTypePointer;
+
+  explicit nsRefPtrHashKey(const T* aKey) : mKey(const_cast<T*>(aKey)) {}
+  nsRefPtrHashKey(nsRefPtrHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mKey(std::move(aOther.mKey)) {}
+  ~nsRefPtrHashKey() = default;
+
+  KeyType GetKey() const { return mKey; }
+  bool KeyEquals(KeyTypePointer aKey) const { return aKey == mKey; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return NS_PTR_TO_UINT32(aKey) >> 2;
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  RefPtr<T> mKey;
+};
+
+template <class T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, nsRefPtrHashKey<T>& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  CycleCollectionNoteChild(aCallback, aField.GetKey(), aName, aFlags);
+}
+
+/**
+ * hashkey wrapper using T* KeyType that sets key to nullptr upon
+ * destruction. Relevant only in cases where a memory pointer-scanner
+ * like valgrind might get confused about stale references.
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+
+template <class T>
+class nsClearingPtrHashKey : public nsPtrHashKey<T> {
+ public:
+  explicit nsClearingPtrHashKey(const T* aKey) : nsPtrHashKey<T>(aKey) {}
+  nsClearingPtrHashKey(nsClearingPtrHashKey&& aToMove)
+      : nsPtrHashKey<T>(std::move(aToMove)) {}
+  ~nsClearingPtrHashKey() { nsPtrHashKey<T>::mKey = nullptr; }
+};
+
+typedef nsClearingPtrHashKey<const void> nsClearingVoidPtrHashKey;
+
+/**
+ * hashkey wrapper using a function pointer KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+template <class T>
+class nsFuncPtrHashKey : public PLDHashEntryHdr {
+ public:
+  typedef T& KeyType;
+  typedef const T* KeyTypePointer;
+
+  explicit nsFuncPtrHashKey(const T* aKey) : mKey(*const_cast<T*>(aKey)) {}
+  nsFuncPtrHashKey(const nsFuncPtrHashKey<T>& aToCopy) : mKey(aToCopy.mKey) {}
+  ~nsFuncPtrHashKey() = default;
+
+  KeyType GetKey() const { return const_cast<T&>(mKey); }
+  bool KeyEquals(KeyTypePointer aKey) const { return *aKey == mKey; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return NS_PTR_TO_UINT32(*aKey) >> 2;
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ protected:
+  T mKey;
+};
+
+/**
+ * hashkey wrapper using nsID KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsIDHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const nsID& KeyType;
+  typedef const nsID* KeyTypePointer;
+
+  explicit nsIDHashKey(const nsID* aInID) : mID(*aInID) {}
+  nsIDHashKey(nsIDHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mID(std::move(aOther.mID)) {}
+  ~nsIDHashKey() = default;
+
+  KeyType GetKey() const { return mID; }
+  bool KeyEquals(KeyTypePointer aKey) const { return aKey->Equals(mID); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    // Hash the nsID object's raw bytes.
+    return mozilla::HashBytes(aKey, sizeof(KeyType));
+  }
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  nsID mID;
+};
+
+/**
+ * hashkey wrapper using nsID* KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsIDPointerHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const nsID* KeyType;
+  typedef const nsID* KeyTypePointer;
+
+  explicit nsIDPointerHashKey(const nsID* aInID) : mID(aInID) {}
+  nsIDPointerHashKey(nsIDPointerHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mID(aOther.mID) {}
+  ~nsIDPointerHashKey() = default;
+
+  KeyType GetKey() const { return mID; }
+  bool KeyEquals(KeyTypePointer aKey) const { return aKey->Equals(*mID); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    // Hash the nsID object's raw bytes.
+    return mozilla::HashBytes(aKey, sizeof(*aKey));
+  }
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const nsID* mID;
+};
+
+/**
+ * hashkey wrapper for "dependent" const char*; this class does not "own"
+ * its string pointer.
+ *
+ * This class must only be used if the strings have a lifetime longer than
+ * the hashtable they occupy. This normally occurs only for static
+ * strings or strings that have been arena-allocated.
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+class nsDepCharHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const char* KeyType;
+  typedef const char* KeyTypePointer;
+
+  explicit nsDepCharHashKey(const char* aKey) : mKey(aKey) {}
+  nsDepCharHashKey(nsDepCharHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mKey(std::move(aOther.mKey)) {}
+  ~nsDepCharHashKey() = default;
+
+  const char* GetKey() const { return mKey; }
+  bool KeyEquals(const char* aKey) const { return !strcmp(mKey, aKey); }
+
+  static const char* KeyToPointer(const char* aKey) { return aKey; }
+  static PLDHashNumber HashKey(const char* aKey) {
+    return mozilla::HashString(aKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const char* mKey;
+};
+
+/**
+ * hashkey wrapper for const char*; at construction, this class duplicates
+ * a string pointed to by the pointer so that it doesn't matter whether or not
+ * the string lives longer than the hash table.
+ */
+class nsCharPtrHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const char* KeyType;
+  typedef const char* KeyTypePointer;
+
+  explicit nsCharPtrHashKey(const char* aKey) : mKey(strdup(aKey)) {}
+
+  nsCharPtrHashKey(const nsCharPtrHashKey&) = delete;
+  nsCharPtrHashKey(nsCharPtrHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mKey(aOther.mKey) {
+    aOther.mKey = nullptr;
+  }
+
+  ~nsCharPtrHashKey() {
+    if (mKey) {
+      free(const_cast<char*>(mKey));
+    }
+  }
+
+  const char* GetKey() const { return mKey; }
+  bool KeyEquals(KeyTypePointer aKey) const { return !strcmp(mKey, aKey); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return mozilla::HashString(aKey);
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(mKey);
+  }
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const char* mKey;
+};
+
+/**
+ * hashkey wrapper for const char16_t*; at construction, this class duplicates
+ * a string pointed to by the pointer so that it doesn't matter whether or not
+ * the string lives longer than the hash table.
+ */
+class nsUnicharPtrHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const char16_t* KeyType;
+  typedef const char16_t* KeyTypePointer;
+
+  explicit nsUnicharPtrHashKey(const char16_t* aKey) : mKey(NS_xstrdup(aKey)) {}
+  nsUnicharPtrHashKey(const nsUnicharPtrHashKey& aToCopy) = delete;
+  nsUnicharPtrHashKey(nsUnicharPtrHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mKey(aOther.mKey) {
+    aOther.mKey = nullptr;
+  }
+
+  ~nsUnicharPtrHashKey() {
+    if (mKey) {
+      free(const_cast<char16_t*>(mKey));
+    }
+  }
+
+  const char16_t* GetKey() const { return mKey; }
+  bool KeyEquals(KeyTypePointer aKey) const { return !NS_strcmp(mKey, aKey); }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return mozilla::HashString(aKey);
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(mKey);
+  }
+
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  const char16_t* mKey;
+};
+
+namespace mozilla {
+
+template <typename T>
+PLDHashNumber Hash(const T& aValue) {
+  return aValue.Hash();
+}
+
+}  // namespace mozilla
+
+/**
+ * Hashtable key class to use with objects for which Hash() and operator==()
+ * are defined.
+ */
+template <typename T>
+class nsGenericHashKey : public PLDHashEntryHdr {
+ public:
+  typedef const T& KeyType;
+  typedef const T* KeyTypePointer;
+
+  explicit nsGenericHashKey(KeyTypePointer aKey) : mKey(*aKey) {}
+  nsGenericHashKey(const nsGenericHashKey&) = delete;
+  nsGenericHashKey(nsGenericHashKey&& aOther)
+      : PLDHashEntryHdr(std::move(aOther)), mKey(std::move(aOther.mKey)) {}
+
+  KeyType GetKey() const { return mKey; }
+  bool KeyEquals(KeyTypePointer aKey) const { return *aKey == mKey; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return &aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return ::mozilla::Hash(*aKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ private:
+  T mKey;
+};
+
+#endif  // nsTHashKeys_h__
diff --git a/xpcom/ds/nsHashPropertyBag.cpp b/xpcom/ds/nsHashPropertyBag.cpp
new file mode 100644
index 0000000000..68dd612c57
--- /dev/null
+++ b/xpcom/ds/nsHashPropertyBag.cpp
@@ -0,0 +1,366 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsHashPropertyBag.h"
+
+#include <utility>
+
+#include "mozilla/Attributes.h"
+#include "mozilla/SimpleEnumerator.h"
+#include "nsArray.h"
+#include "nsArrayEnumerator.h"
+#include "nsIProperty.h"
+#include "nsIVariant.h"
+#include "nsThreadUtils.h"
+#include "nsVariant.h"
+
+using mozilla::MakeRefPtr;
+using mozilla::SimpleEnumerator;
+using mozilla::Unused;
+
+extern "C" {
+
+// This function uses C linkage because it's exposed to Rust to support the
+// `HashPropertyBag` wrapper in the `storage_variant` crate.
+void NS_NewHashPropertyBag(nsIWritablePropertyBag** aBag) {
+  MakeRefPtr<nsHashPropertyBag>().forget(aBag);
+}
+
+}  // extern "C"
+
+/*
+ * nsHashPropertyBagBase implementation.
+ */
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::HasKey(const nsAString& aName, bool* aResult) {
+  *aResult = mPropertyHash.Get(aName, nullptr);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::Get(const nsAString& aName, nsIVariant** aResult) {
+  if (!mPropertyHash.Get(aName, aResult)) {
+    *aResult = nullptr;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetProperty(const nsAString& aName,
+                                   nsIVariant** aResult) {
+  bool isFound = mPropertyHash.Get(aName, aResult);
+  if (!isFound) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::SetProperty(const nsAString& aName, nsIVariant* aValue) {
+  if (NS_WARN_IF(!aValue)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  mPropertyHash.InsertOrUpdate(aName, aValue);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::DeleteProperty(const nsAString& aName) {
+  return mPropertyHash.Remove(aName) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+//
+// nsSimpleProperty class and impl; used for GetEnumerator
+//
+
+class nsSimpleProperty final : public nsIProperty {
+  ~nsSimpleProperty() = default;
+
+ public:
+  nsSimpleProperty(const nsAString& aName, nsIVariant* aValue)
+      : mName(aName), mValue(aValue) {}
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIPROPERTY
+ protected:
+  nsString mName;
+  nsCOMPtr<nsIVariant> mValue;
+};
+
+NS_IMPL_ISUPPORTS(nsSimpleProperty, nsIProperty)
+
+NS_IMETHODIMP
+nsSimpleProperty::GetName(nsAString& aName) {
+  aName.Assign(mName);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSimpleProperty::GetValue(nsIVariant** aValue) {
+  NS_IF_ADDREF(*aValue = mValue);
+  return NS_OK;
+}
+
+// end nsSimpleProperty
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetEnumerator(nsISimpleEnumerator** aResult) {
+  nsCOMPtr<nsIMutableArray> propertyArray = nsArray::Create();
+  if (!propertyArray) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  for (auto iter = mPropertyHash.Iter(); !iter.Done(); iter.Next()) {
+    const nsAString& key = iter.Key();
+    nsIVariant* data = iter.UserData();
+    nsSimpleProperty* sprop = new nsSimpleProperty(key, data);
+    propertyArray->AppendElement(sprop);
+  }
+
+  return NS_NewArrayEnumerator(aResult, propertyArray, NS_GET_IID(nsIProperty));
+}
+
+#define IMPL_GETSETPROPERTY_AS(Name, Type)                          \
+  NS_IMETHODIMP                                                     \
+  nsHashPropertyBagBase::GetPropertyAs##Name(const nsAString& prop, \
+                                             Type* _retval) {       \
+    nsIVariant* v = mPropertyHash.GetWeak(prop);                    \
+    if (!v) return NS_ERROR_NOT_AVAILABLE;                          \
+    return v->GetAs##Name(_retval);                                 \
+  }                                                                 \
+                                                                    \
+  NS_IMETHODIMP                                                     \
+  nsHashPropertyBagBase::SetPropertyAs##Name(const nsAString& prop, \
+                                             Type value) {          \
+    nsCOMPtr<nsIWritableVariant> var = new nsVariant();             \
+    var->SetAs##Name(value);                                        \
+    return SetProperty(prop, var);                                  \
+  }
+
+IMPL_GETSETPROPERTY_AS(Int32, int32_t)
+IMPL_GETSETPROPERTY_AS(Uint32, uint32_t)
+IMPL_GETSETPROPERTY_AS(Int64, int64_t)
+IMPL_GETSETPROPERTY_AS(Uint64, uint64_t)
+IMPL_GETSETPROPERTY_AS(Double, double)
+IMPL_GETSETPROPERTY_AS(Bool, bool)
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetPropertyAsAString(const nsAString& aProp,
+                                            nsAString& aResult) {
+  nsIVariant* v = mPropertyHash.GetWeak(aProp);
+  if (!v) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  return v->GetAsAString(aResult);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetPropertyAsACString(const nsAString& aProp,
+                                             nsACString& aResult) {
+  nsIVariant* v = mPropertyHash.GetWeak(aProp);
+  if (!v) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  return v->GetAsACString(aResult);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetPropertyAsAUTF8String(const nsAString& aProp,
+                                                nsACString& aResult) {
+  nsIVariant* v = mPropertyHash.GetWeak(aProp);
+  if (!v) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  return v->GetAsAUTF8String(aResult);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::GetPropertyAsInterface(const nsAString& aProp,
+                                              const nsIID& aIID,
+                                              void** aResult) {
+  nsIVariant* v = mPropertyHash.GetWeak(aProp);
+  if (!v) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  nsCOMPtr<nsISupports> val;
+  nsresult rv = v->GetAsISupports(getter_AddRefs(val));
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  if (!val) {
+    // We have a value, but it's null
+    *aResult = nullptr;
+    return NS_OK;
+  }
+  return val->QueryInterface(aIID, aResult);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::SetPropertyAsAString(const nsAString& aProp,
+                                            const nsAString& aValue) {
+  nsCOMPtr<nsIWritableVariant> var = new nsVariant();
+  var->SetAsAString(aValue);
+  return SetProperty(aProp, var);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::SetPropertyAsACString(const nsAString& aProp,
+                                             const nsACString& aValue) {
+  nsCOMPtr<nsIWritableVariant> var = new nsVariant();
+  var->SetAsACString(aValue);
+  return SetProperty(aProp, var);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::SetPropertyAsAUTF8String(const nsAString& aProp,
+                                                const nsACString& aValue) {
+  nsCOMPtr<nsIWritableVariant> var = new nsVariant();
+  var->SetAsAUTF8String(aValue);
+  return SetProperty(aProp, var);
+}
+
+NS_IMETHODIMP
+nsHashPropertyBagBase::SetPropertyAsInterface(const nsAString& aProp,
+                                              nsISupports* aValue) {
+  nsCOMPtr<nsIWritableVariant> var = new nsVariant();
+  var->SetAsISupports(aValue);
+  return SetProperty(aProp, var);
+}
+
+void nsHashPropertyBagBase::CopyFrom(const nsHashPropertyBagBase* aOther) {
+  for (const auto& entry : aOther->mPropertyHash) {
+    SetProperty(entry.GetKey(), entry.GetWeak());
+  }
+}
+
+void nsHashPropertyBagBase::CopyFrom(nsIPropertyBag* aOther) {
+  CopyFrom(this, aOther);
+}
+
+/* static */ void nsHashPropertyBagBase::CopyFrom(nsIWritablePropertyBag* aTo,
+                                                  nsIPropertyBag* aFrom) {
+  if (aTo && aFrom) {
+    nsCOMPtr<nsISimpleEnumerator> enumerator;
+    if (NS_SUCCEEDED(aFrom->GetEnumerator(getter_AddRefs(enumerator)))) {
+      for (auto& property : SimpleEnumerator<nsIProperty>(enumerator)) {
+        nsString name;
+        nsCOMPtr<nsIVariant> value;
+        Unused << NS_WARN_IF(NS_FAILED(property->GetName(name)));
+        Unused << NS_WARN_IF(
+            NS_FAILED(property->GetValue(getter_AddRefs(value))));
+        Unused << NS_WARN_IF(
+            NS_FAILED(aTo->SetProperty(std::move(name), value)));
+      }
+    } else {
+      NS_WARNING("Unable to copy nsIPropertyBag");
+    }
+  }
+}
+
+nsresult nsGetProperty::operator()(const nsIID& aIID,
+                                   void** aInstancePtr) const {
+  nsresult rv;
+
+  if (mPropBag) {
+    rv = mPropBag->GetPropertyAsInterface(mPropName, aIID, aInstancePtr);
+  } else {
+    rv = NS_ERROR_NULL_POINTER;
+    *aInstancePtr = 0;
+  }
+
+  if (mErrorPtr) {
+    *mErrorPtr = rv;
+  }
+  return rv;
+}
+
+/*
+ * nsHashPropertyBag implementation.
+ */
+
+NS_IMPL_ADDREF(nsHashPropertyBag)
+NS_IMPL_RELEASE(nsHashPropertyBag)
+
+NS_INTERFACE_MAP_BEGIN(nsHashPropertyBag)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsIPropertyBag, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY(nsIPropertyBag2)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag2)
+NS_INTERFACE_MAP_END
+
+/*
+ * We need to ensure that the hashtable is destroyed on the main thread, as
+ * the nsIVariant values are main-thread only objects.
+ */
+class ProxyHashtableDestructor final : public mozilla::Runnable {
+ public:
+  using HashtableType = nsInterfaceHashtable<nsStringHashKey, nsIVariant>;
+  explicit ProxyHashtableDestructor(HashtableType&& aTable)
+      : mozilla::Runnable("ProxyHashtableDestructor"),
+        mPropertyHash(std::move(aTable)) {}
+
+  NS_IMETHODIMP
+  Run() override {
+    MOZ_ASSERT(NS_IsMainThread());
+    HashtableType table(std::move(mPropertyHash));
+    return NS_OK;
+  }
+
+ private:
+  HashtableType mPropertyHash;
+};
+
+nsHashPropertyBag::~nsHashPropertyBag() {
+  if (!NS_IsMainThread()) {
+    RefPtr<ProxyHashtableDestructor> runnable =
+        new ProxyHashtableDestructor(std::move(mPropertyHash));
+    MOZ_ALWAYS_SUCCEEDS(NS_DispatchToMainThread(runnable));
+  }
+}
+
+/*
+ * nsHashPropertyBagOMT implementation
+ */
+NS_IMPL_ADDREF(nsHashPropertyBagOMT)
+NS_IMPL_RELEASE(nsHashPropertyBagOMT)
+
+NS_INTERFACE_MAP_BEGIN(nsHashPropertyBagOMT)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsIPropertyBag, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY(nsIPropertyBag2)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag2)
+NS_INTERFACE_MAP_END
+
+nsHashPropertyBagOMT::nsHashPropertyBagOMT() {
+  // nsHashPropertyBagOMT is supposed to be used off-main thread. If you need a
+  // single threaded property bag on the main thread, you should consider using
+  // nsHashPropertyBagCC instead, to prevent leaks.
+  MOZ_ASSERT(!NS_IsMainThread());
+}
+
+/*
+ * nsHashPropertyBagCC implementation.
+ */
+
+NS_IMPL_CYCLE_COLLECTION(nsHashPropertyBagCC, mPropertyHash)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsHashPropertyBagCC)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsHashPropertyBagCC)
+
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsHashPropertyBagCC)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsIPropertyBag, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIWritablePropertyBag)
+  NS_INTERFACE_MAP_ENTRY(nsIPropertyBag2)
+  NS_INTERFACE_MAP_ENTRY(nsIWritablePropertyBag2)
+NS_INTERFACE_MAP_END
diff --git a/xpcom/ds/nsHashPropertyBag.h b/xpcom/ds/nsHashPropertyBag.h
new file mode 100644
index 0000000000..7a2e6b0ac8
--- /dev/null
+++ b/xpcom/ds/nsHashPropertyBag.h
@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsHashPropertyBag_h___
+#define nsHashPropertyBag_h___
+
+#include "nsIVariant.h"
+#include "nsIWritablePropertyBag.h"
+#include "nsIWritablePropertyBag2.h"
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsInterfaceHashtable.h"
+
+class nsHashPropertyBagBase : public nsIWritablePropertyBag,
+                              public nsIWritablePropertyBag2 {
+ public:
+  nsHashPropertyBagBase() = default;
+
+  void CopyFrom(const nsHashPropertyBagBase* aOther);
+  void CopyFrom(nsIPropertyBag* aOther);
+  static void CopyFrom(nsIWritablePropertyBag* aTo, nsIPropertyBag* aFrom);
+
+  NS_DECL_NSIPROPERTYBAG
+  NS_DECL_NSIPROPERTYBAG2
+
+  NS_DECL_NSIWRITABLEPROPERTYBAG
+  NS_DECL_NSIWRITABLEPROPERTYBAG2
+
+ protected:
+  // a hash table of string -> nsIVariant
+  nsInterfaceHashtable<nsStringHashKey, nsIVariant> mPropertyHash;
+};
+
+class nsHashPropertyBag : public nsHashPropertyBagBase {
+ public:
+  nsHashPropertyBag() = default;
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+ protected:
+  virtual ~nsHashPropertyBag();
+};
+
+/*
+ * Off Main Thread variant of nsHashPropertyBag. Instances of this class
+ * should not be created on a main thread, nor should it contain main thread
+ * only objects, such as XPCVariants. The purpose of this class is to provide a
+ * way to use the property bag off main thread.
+ * Note: this class needs to be created and destroyed on the same thread and
+ * should be used single threaded.
+ */
+class nsHashPropertyBagOMT final : public nsHashPropertyBagBase {
+ public:
+  nsHashPropertyBagOMT();
+  NS_DECL_ISUPPORTS
+
+ protected:
+  // Doesn't need to dispatch to main thread because it cannot contain
+  // XPCVariants
+  virtual ~nsHashPropertyBagOMT() = default;
+};
+
+/* A cycle collected nsHashPropertyBag for main-thread-only use. */
+class nsHashPropertyBagCC final : public nsHashPropertyBagBase {
+ public:
+  nsHashPropertyBagCC() = default;
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS_AMBIGUOUS(nsHashPropertyBagCC,
+                                           nsIWritablePropertyBag)
+ protected:
+  virtual ~nsHashPropertyBagCC() = default;
+};
+
+inline nsISupports* ToSupports(nsHashPropertyBagBase* aPropertyBag) {
+  return static_cast<nsIWritablePropertyBag*>(aPropertyBag);
+}
+
+#endif /* nsHashPropertyBag_h___ */
diff --git a/xpcom/ds/nsHashtablesFwd.h b/xpcom/ds/nsHashtablesFwd.h
new file mode 100644
index 0000000000..080f79887d
--- /dev/null
+++ b/xpcom/ds/nsHashtablesFwd.h
@@ -0,0 +1,94 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef XPCOM_DS_NSHASHTABLESFWD_H_
+#define XPCOM_DS_NSHASHTABLESFWD_H_
+
+#include "mozilla/Attributes.h"
+
+struct PLDHashEntryHdr;
+
+template <class T>
+class MOZ_IS_REFPTR nsCOMPtr;
+
+template <class T>
+class MOZ_IS_REFPTR RefPtr;
+
+template <class EntryType>
+class MOZ_NEEDS_NO_VTABLE_TYPE nsTHashtable;
+
+template <class DataType, class UserDataType>
+class nsDefaultConverter;
+
+template <class KeyClass, class DataType, class UserDataType,
+          class Converter = nsDefaultConverter<DataType, UserDataType>>
+class nsBaseHashtable;
+
+template <class KeyClass, class T>
+class nsClassHashtable;
+
+template <class KeyClass, class PtrType>
+class nsRefCountedHashtable;
+
+/**
+ * templated hashtable class maps keys to interface pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @deprecated This is going to be removed. Use nsTHashMap instead.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param Interface the interface-type being wrapped
+ * @see nsClassHashtable, nsTHashMap
+ */
+template <class KeyClass, class Interface>
+using nsInterfaceHashtable =
+    nsRefCountedHashtable<KeyClass, nsCOMPtr<Interface>>;
+
+/**
+ * templated hashtable class maps keys to reference pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @deprecated This is going to be removed. Use nsTHashMap instead.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param PtrType the reference-type being wrapped
+ * @see nsClassHashtable, nsTHashMap
+ */
+template <class KeyClass, class ClassType>
+using nsRefPtrHashtable = nsRefCountedHashtable<KeyClass, RefPtr<ClassType>>;
+
+namespace mozilla::detail {
+template <class KeyType, class = void>
+struct nsKeyClass;
+}  // namespace mozilla::detail
+
+/**
+ * A universal hash map that maps some KeyType to some DataType. It can be used
+ * for any DataType, including RefPtr<T>, nsCOMPtr<T> and UniquePtr<T>.
+ *
+ * For the default hash keys types, the appropriate hash key class is determined
+ * automatically, so you can just specify `nsTHashMap<uint32_t,
+ * RefPtr<Foo>>`, for example.
+ *
+ * If you require custom hash behaviour (e.g. case insensitive string handling),
+ * you can still specify a hash key class derived from PLDHashEntryHdr
+ * explicitly.
+ *
+ * If you need to use a custom UserDataType, use nsBaseHashtable (or
+ * nsTHashtable) directly. However, you should double-check if that's really
+ * necessary.
+ */
+template <class KeyType, class DataType>
+using nsTHashMap =
+    nsBaseHashtable<typename mozilla::detail::nsKeyClass<KeyType>::type,
+                    DataType, DataType>;
+
+template <class KeyClass>
+class nsTBaseHashSet;
+
+template <class KeyType>
+using nsTHashSet =
+    nsTBaseHashSet<typename mozilla::detail::nsKeyClass<KeyType>::type>;
+
+#endif  // XPCOM_DS_NSHASHTABLESFWD_H_
diff --git a/xpcom/ds/nsIArray.idl b/xpcom/ds/nsIArray.idl
new file mode 100644
index 0000000000..7e514649cb
--- /dev/null
+++ b/xpcom/ds/nsIArray.idl
@@ -0,0 +1,103 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsISimpleEnumerator;
+
+/**
+ * nsIArray
+ *
+ * An indexed collection of elements. Provides basic functionality for
+ * retrieving elements at a specific position, searching for
+ * elements. Indexes are zero-based, such that the last element in the
+ * array is stored at the index length-1.
+ *
+ * For an array which can be modified, see nsIMutableArray below.
+ *
+ * Neither interface makes any attempt to protect the individual
+ * elements from modification. The convention is that the elements of
+ * the array should not be modified. Documentation within a specific
+ * interface should describe variations from this convention.
+ *
+ * It is also convention that if an interface provides access to an
+ * nsIArray, that the array should not be QueryInterfaced to an
+ * nsIMutableArray for modification. If the interface in question had
+ * intended the array to be modified, it would have returned an
+ * nsIMutableArray!
+ *
+ * null is a valid entry in the array, and as such any nsISupports
+ * parameters may be null, except where noted.
+ */
+[scriptable, builtinclass, uuid(114744d9-c369-456e-b55a-52fe52880d2d)]
+interface nsIArray : nsISupports
+{
+    /**
+     * length
+     *
+     * number of elements in the array.
+     */
+    readonly attribute unsigned long length;
+
+    /**
+     * queryElementAt()
+     *
+     * Retrieve a specific element of the array, and QueryInterface it
+     * to the specified interface. null is a valid result for
+     * this method, but exceptions are thrown in other circumstances
+     *
+     * @param index position of element
+     * @param uuid the IID of the requested interface
+     * @param result the object, QI'd to the requested interface
+     *
+     * @throws NS_ERROR_NO_INTERFACE when an entry exists at the
+     *         specified index, but the requested interface is not
+     *         available.
+     * @throws NS_ERROR_ILLEGAL_VALUE when index > length-1
+     *
+     */
+    void queryElementAt(in unsigned long index,
+                        in nsIIDRef uuid,
+                        [iid_is(uuid), retval] out nsQIResult result);
+
+    /**
+     * indexOf()
+     *
+     * Get the position of a specific element. Note that since null is
+     * a valid input, exceptions are used to indicate that an element
+     * is not found.
+     *
+     * @param startIndex The initial element to search in the array
+     *                   To start at the beginning, use 0 as the
+     *                   startIndex
+     * @param element    The element you are looking for
+     * @returns a number >= startIndex which is the position of the
+     *          element in the array.
+     * @throws NS_ERROR_FAILURE if the element was not in the array.
+     */
+    unsigned long indexOf(in unsigned long startIndex,
+                          in nsISupports element);
+
+    /**
+     * enumerate the array
+     *
+     * @returns a new enumerator positioned at the start of the array
+     * @throws NS_ERROR_FAILURE if the array is empty (to make it easy
+     *         to detect errors), or NS_ERROR_OUT_OF_MEMORY if out of memory.
+     */
+    [binaryname(ScriptedEnumerate), optional_argc]
+    nsISimpleEnumerator enumerate([optional] in nsIIDRef aElemIID);
+
+    [noscript]
+    nsISimpleEnumerator enumerateImpl(in nsIDRef aElemIID);
+
+    %{C++
+    nsresult
+    Enumerate(nsISimpleEnumerator** aRetVal, const nsID& aElemIID = NS_GET_IID(nsISupports))
+    {
+      return EnumerateImpl(aElemIID, aRetVal);
+    }
+    %}
+};
diff --git a/xpcom/ds/nsIArrayExtensions.idl b/xpcom/ds/nsIArrayExtensions.idl
new file mode 100644
index 0000000000..872a5c018d
--- /dev/null
+++ b/xpcom/ds/nsIArrayExtensions.idl
@@ -0,0 +1,51 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIArray.idl"
+
+/**
+ * Helper interface for allowing scripts to treat nsIArray instances as if
+ * they were nsISupportsArray instances while iterating.
+ *
+ * nsISupportsArray is convenient to iterate over in JavaScript:
+ *
+ * for (let i = 0; i < array.Count(); ++i) {
+ *   let elem = array.GetElementAt(i);
+ *   ...
+ * }
+ *
+ * but doing the same with nsIArray is somewhat less convenient, since
+ * queryElementAt is not nearly so nice to use from JavaScript.  So we provide
+ * this extension interface so interfaces that currently return
+ * nsISupportsArray can start returning nsIArrayExtensions and all JavaScript
+ * should Just Work. Eventually we'll roll this interface into nsIArray
+ * itself, possibly getting rid of the Count() method, as it duplicates
+ * nsIArray functionality.
+ */
+[scriptable, builtinclass, uuid(261d442e-050c-453d-8aaa-b3f23bcc528b)]
+interface nsIArrayExtensions : nsIArray
+{
+  /**
+   * Count()
+   *
+   * Retrieves the length of the array. This is an alias for the
+   * |nsIArray.length| attribute.
+   */
+  uint32_t Count();
+
+  /**
+   * GetElementAt()
+   *
+   * Retrieve a specific element of the array. null is a valid result for
+   * this method.
+   *
+   * Note: If the index is out of bounds null will be returned.
+   *       This differs from the behavior of nsIArray.queryElementAt() which
+   *       will throw if an invalid index is specified.
+   *
+   * @param index position of element
+   */
+  nsISupports GetElementAt(in uint32_t index);
+};
diff --git a/xpcom/ds/nsIINIParser.idl b/xpcom/ds/nsIINIParser.idl
new file mode 100644
index 0000000000..434490bf42
--- /dev/null
+++ b/xpcom/ds/nsIINIParser.idl
@@ -0,0 +1,61 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIUTF8StringEnumerator;
+interface nsIFile;
+
+[scriptable, uuid(7eb955f6-3e78-4d39-b72f-c1bf12a94bce)]
+interface nsIINIParser : nsISupports
+{
+  /**
+   * Initializes an INI file from string data
+   */
+  void initFromString(in AUTF8String aData);
+
+  /**
+   * Enumerates the [section]s available in the INI file.
+   */
+  nsIUTF8StringEnumerator getSections();
+
+  /**
+   * Enumerates the keys available within a section.
+   */
+  nsIUTF8StringEnumerator getKeys(in AUTF8String aSection);
+
+  /**
+   * Get the value of a string for a particular section and key.
+   */
+  AUTF8String getString(in AUTF8String aSection, in AUTF8String aKey);
+};
+
+[scriptable, uuid(b67bb24b-31a3-4a6a-a5d9-0485c9af5a04)]
+interface nsIINIParserWriter : nsISupports
+{
+
+  /**
+   * Set the value of a string for a particular section and key.
+   */
+  void setString(in AUTF8String aSection, in AUTF8String aKey, in AUTF8String aValue);
+
+  /**
+   * Write to the INI file.
+   */
+  void writeFile(in nsIFile aINIFile);
+
+  /**
+   * Return the formatted INI file contents
+   */
+  AUTF8String writeToString();
+};
+
+[scriptable, uuid(ccae7ea5-1218-4b51-aecb-c2d8ecd46af9)]
+interface nsIINIParserFactory : nsISupports
+{
+  /**
+   * Create an iniparser instance from a local file.
+   */
+  nsIINIParser createINIParser([optional] in nsIFile aINIFile);
+};
diff --git a/xpcom/ds/nsIMutableArray.idl b/xpcom/ds/nsIMutableArray.idl
new file mode 100644
index 0000000000..3f06ecbeb1
--- /dev/null
+++ b/xpcom/ds/nsIMutableArray.idl
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIArrayExtensions.idl"
+
+/**
+ * nsIMutableArray
+ * A separate set of methods that will act on the array. Consumers of
+ * nsIArray should not QueryInterface to nsIMutableArray unless they
+ * own the array.
+ *
+ * As above, it is legal to add null elements to the array. Note also
+ * that null elements can be created as a side effect of
+ * insertElementAt(). Conversely, if insertElementAt() is never used,
+ * and null elements are never explicitly added to the array, then it
+ * is guaranteed that queryElementAt() will never return a null value.
+ *
+ * Any of these methods may throw NS_ERROR_OUT_OF_MEMORY when the
+ * array must grow to complete the call, but the allocation fails.
+ */
+[scriptable, builtinclass, uuid(af059da0-c85b-40ec-af07-ae4bfdc192cc)]
+interface nsIMutableArray : nsIArrayExtensions
+{
+    /**
+     * appendElement()
+     *
+     * Append an element at the end of the array.
+     *
+     * @param element The element to append.
+     */
+    void appendElement(in nsISupports element);
+
+    /**
+     * removeElementAt()
+     *
+     * Remove an element at a specific position, moving all elements
+     * stored at a higher position down one.
+     * To remove a specific element, use indexOf() to find the index
+     * first, then call removeElementAt().
+     *
+     * @param index the position of the item
+     *
+     */
+    void removeElementAt(in unsigned long index);
+
+    /**
+     * insertElementAt()
+     *
+     * Insert an element at the given position, moving the element
+     * currently located in that position, and all elements in higher
+     * position, up by one.
+     *
+     * @param element The element to insert
+     * @param index   The position in the array:
+     *                If the position is lower than the current length
+     *                of the array, the elements at that position and
+     *                onwards are bumped one position up.
+     *                If the position is equal to the current length
+     *                of the array, the new element is appended.
+     *                An index lower than 0 or higher than the current
+     *                length of the array is invalid and will be ignored.
+     */
+    void insertElementAt(in nsISupports element, in unsigned long index);
+
+    /**
+     * replaceElementAt()
+     *
+     * Replace the element at the given position.
+     *
+     * @param element The new element to insert
+     * @param index   The position in the array
+     *                If the position is lower than the current length
+     *                of the array, an existing element will be replaced.
+     *                If the position is equal to the current length
+     *                of the array, the new element is appended.
+     *                If the position is higher than the current length
+     *                of the array, empty elements are appended followed
+     *                by the new element at the specified position.
+     *                An index lower than 0 is invalid and will be ignored.
+     */
+    void replaceElementAt(in nsISupports element, in unsigned long index);
+
+
+    /**
+     * clear()
+     *
+     * clear the entire array, releasing all stored objects
+     */
+    void clear();
+};
diff --git a/xpcom/ds/nsINIParserImpl.cpp b/xpcom/ds/nsINIParserImpl.cpp
new file mode 100644
index 0000000000..6ed52947d8
--- /dev/null
+++ b/xpcom/ds/nsINIParserImpl.cpp
@@ -0,0 +1,143 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsINIParserImpl.h"
+
+#include "nsINIParser.h"
+#include "nsStringEnumerator.h"
+#include "nsTArray.h"
+#include "mozilla/Attributes.h"
+
+class nsINIParserImpl final : public nsIINIParser, public nsIINIParserWriter {
+  ~nsINIParserImpl() = default;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIINIPARSER
+  NS_DECL_NSIINIPARSERWRITER
+
+  nsresult Init(nsIFile* aINIFile) { return mParser.Init(aINIFile); }
+
+ private:
+  nsINIParser mParser;
+  bool ContainsNull(const nsACString& aStr);
+};
+
+NS_IMPL_ISUPPORTS(nsINIParserFactory, nsIINIParserFactory)
+
+NS_IMETHODIMP
+nsINIParserFactory::CreateINIParser(nsIFile* aINIFile, nsIINIParser** aResult) {
+  *aResult = nullptr;
+
+  RefPtr<nsINIParserImpl> p(new nsINIParserImpl());
+
+  if (aINIFile) {
+    nsresult rv = p->Init(aINIFile);
+    if (NS_FAILED(rv)) {
+      return rv;
+    }
+  }
+
+  p.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(nsINIParserImpl, nsIINIParser, nsIINIParserWriter)
+
+bool nsINIParserImpl::ContainsNull(const nsACString& aStr) {
+  return aStr.CountChar('\0') > 0;
+}
+
+static bool SectionCB(const char* aSection, void* aClosure) {
+  nsTArray<nsCString>* strings = static_cast<nsTArray<nsCString>*>(aClosure);
+  strings->AppendElement()->Assign(aSection);
+  return true;
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::GetSections(nsIUTF8StringEnumerator** aResult) {
+  nsTArray<nsCString>* strings = new nsTArray<nsCString>;
+
+  nsresult rv = mParser.GetSections(SectionCB, strings);
+  if (NS_SUCCEEDED(rv)) {
+    rv = NS_NewAdoptingUTF8StringEnumerator(aResult, strings);
+  }
+
+  if (NS_FAILED(rv)) {
+    delete strings;
+  }
+
+  return rv;
+}
+
+static bool KeyCB(const char* aKey, const char* aValue, void* aClosure) {
+  nsTArray<nsCString>* strings = static_cast<nsTArray<nsCString>*>(aClosure);
+  strings->AppendElement()->Assign(aKey);
+  return true;
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::GetKeys(const nsACString& aSection,
+                         nsIUTF8StringEnumerator** aResult) {
+  if (ContainsNull(aSection)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsTArray<nsCString>* strings = new nsTArray<nsCString>;
+
+  nsresult rv =
+      mParser.GetStrings(PromiseFlatCString(aSection).get(), KeyCB, strings);
+  if (NS_SUCCEEDED(rv)) {
+    rv = NS_NewAdoptingUTF8StringEnumerator(aResult, strings);
+  }
+
+  if (NS_FAILED(rv)) {
+    delete strings;
+  }
+
+  return rv;
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::GetString(const nsACString& aSection, const nsACString& aKey,
+                           nsACString& aResult) {
+  if (ContainsNull(aSection) || ContainsNull(aKey)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  return mParser.GetString(PromiseFlatCString(aSection).get(),
+                           PromiseFlatCString(aKey).get(), aResult);
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::InitFromString(const nsACString& aData) {
+  return mParser.InitFromString(nsCString(aData));
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::SetString(const nsACString& aSection, const nsACString& aKey,
+                           const nsACString& aValue) {
+  if (ContainsNull(aSection) || ContainsNull(aKey) || ContainsNull(aValue)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  return mParser.SetString(PromiseFlatCString(aSection).get(),
+                           PromiseFlatCString(aKey).get(),
+                           PromiseFlatCString(aValue).get());
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::WriteFile(nsIFile* aINIFile) {
+  return mParser.WriteToFile(aINIFile);
+}
+
+NS_IMETHODIMP
+nsINIParserImpl::WriteToString(nsACString& aOutput) {
+  aOutput.Truncate();
+  mParser.WriteToString(aOutput);
+
+  return NS_OK;
+}
diff --git a/xpcom/ds/nsINIParserImpl.h b/xpcom/ds/nsINIParserImpl.h
new file mode 100644
index 0000000000..1a60fd1071
--- /dev/null
+++ b/xpcom/ds/nsINIParserImpl.h
@@ -0,0 +1,23 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsINIParserImpl_h__
+#define nsINIParserImpl_h__
+
+#include "nsIINIParser.h"
+#include "mozilla/Attributes.h"
+
+#define NS_INIPARSERFACTORY_CONTRACTID "@mozilla.org/xpcom/ini-parser-factory;1"
+
+class nsINIParserFactory final : public nsIINIParserFactory {
+  ~nsINIParserFactory() = default;
+
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIINIPARSERFACTORY
+};
+
+#endif  // nsINIParserImpl_h__
diff --git a/xpcom/ds/nsIObserver.idl b/xpcom/ds/nsIObserver.idl
new file mode 100644
index 0000000000..773424d0a7
--- /dev/null
+++ b/xpcom/ds/nsIObserver.idl
@@ -0,0 +1,37 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * This interface is implemented by an object that wants
+ * to observe an event corresponding to a topic.
+ */
+
+[scriptable, function, uuid(DB242E01-E4D9-11d2-9DDE-000064657374)]
+interface nsIObserver : nsISupports {
+
+   /**
+    * Observe will be called when there is a notification for the
+    * topic |aTopic|.  This assumes that the object implementing
+    * this interface has been registered with an observer service
+    * such as the nsIObserverService.
+    *
+    * If you expect multiple topics/subjects, the impl is
+    * responsible for filtering.
+    *
+    * You should not modify, add, remove, or enumerate
+    * notifications in the implemention of observe.
+    *
+    * @param aSubject : Notification specific interface pointer.
+    * @param aTopic   : The notification topic or subject.
+    * @param aData    : Notification specific wide string.
+    *                    subject event.
+    */
+    void observe( in nsISupports aSubject,
+                  in string      aTopic,
+                  in wstring     aData );
+
+};
diff --git a/xpcom/ds/nsIObserverService.idl b/xpcom/ds/nsIObserverService.idl
new file mode 100644
index 0000000000..5ca51d5d44
--- /dev/null
+++ b/xpcom/ds/nsIObserverService.idl
@@ -0,0 +1,109 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIObserver;
+interface nsISimpleEnumerator;
+
+/**
+ * nsIObserverService
+ *
+ * Service allows a client listener (nsIObserver) to register and unregister for
+ * notifications of specific string referenced topic. Service also provides a
+ * way to notify registered listeners and a way to enumerate registered client
+ * listeners.
+ */
+
+[scriptable, builtinclass, uuid(D07F5192-E3D1-11d2-8ACD-00105A1B8860)]
+interface nsIObserverService : nsISupports
+{
+
+    /**
+     * AddObserver
+     *
+     * Registers a given listener for a notifications regarding the specified
+     * topic.
+     *
+     * @param anObserve : The interface pointer which will receive notifications.
+     * @param aTopic    : The notification topic or subject.
+     * @param ownsWeak  : If set to false, the nsIObserverService will hold a
+     *                    strong reference to |anObserver|.  If set to true and
+     *                    |anObserver| supports the nsIWeakReference interface,
+     *                    a weak reference will be held.  Otherwise an error will be
+     *                    returned.
+     */
+    void addObserver( in nsIObserver anObserver, in string aTopic,
+                      [optional] in boolean ownsWeak);
+
+    /**
+     * removeObserver
+     *
+     * Unregisters a given listener from notifications regarding the specified
+     * topic.
+     *
+     * @param anObserver : The interface pointer which will stop recieving
+     *                     notifications.
+     * @param aTopic     : The notification topic or subject.
+     */
+    void removeObserver( in nsIObserver anObserver, in string aTopic );
+
+    /**
+     * notifyObservers
+     *
+     * Notifies all registered listeners of the given topic.
+     * Must not be used with shutdown topics (will assert
+     * on the parent process).
+     *
+     * @param aSubject : Notification specific interface pointer.
+     * @param aTopic   : The notification topic or subject.
+     * @param someData : Notification specific wide string.
+     */
+    void notifyObservers( in nsISupports aSubject,
+                          in string aTopic,
+                          [optional] in wstring someData );
+
+    /**
+     * hasObservers
+     *
+     * Checks to see if there are registered listeners for the given topic.
+     *
+     * Implemented in "nsObserverService.cpp".
+     *
+     * @param aTopic   : The notification topic or subject.
+     * @param aFound : An out parameter; True if there are registered observers,
+     * False otherwise.
+     */
+    [noscript, notxpcom, nostdcall] boolean hasObservers(in string aTopic);
+
+    %{C++
+    /**
+     * notifyWhenScriptSafe
+     *
+     * Notifies all registered listeners of the given topic once it is safe to
+     * run script.
+     *
+     * Implemented in "nsObserverService.cpp".
+     *
+     * @param aSubject : Notification specific interface pointer.
+     * @param aTopic   : The notification topic or subject.
+     * @param someData : Notification specific wide string.
+     */
+    nsresult NotifyWhenScriptSafe(nsISupports* aSubject,
+                                  const char* aTopic,
+                                  const char16_t* aData = nullptr);
+    %}
+
+    /**
+     * enumerateObservers
+     *
+     * Returns an enumeration of all registered listeners.
+     *
+     * @param aTopic   : The notification topic or subject.
+     */
+    nsISimpleEnumerator enumerateObservers( in string aTopic );
+
+
+};
diff --git a/xpcom/ds/nsIPersistentProperties.h b/xpcom/ds/nsIPersistentProperties.h
new file mode 100644
index 0000000000..eef30c2ff2
--- /dev/null
+++ b/xpcom/ds/nsIPersistentProperties.h
@@ -0,0 +1,13 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __gen_nsIPersistentProperties_h__
+#define __gen_nsIPersistentProperties_h__
+
+// "soft" switch over to an IDL generated header file
+#include "nsIPersistentProperties2.h"
+
+#endif /* __gen_nsIPersistentProperties_h__ */
diff --git a/xpcom/ds/nsIPersistentProperties2.idl b/xpcom/ds/nsIPersistentProperties2.idl
new file mode 100644
index 0000000000..7b07ba33ab
--- /dev/null
+++ b/xpcom/ds/nsIPersistentProperties2.idl
@@ -0,0 +1,59 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsIProperties.idl"
+
+interface nsIInputStream;
+interface nsIOutputStream;
+interface nsISimpleEnumerator;
+
+%{C++
+#include "mozilla/MemoryReporting.h"
+%}
+
+native MallocSizeOf(mozilla::MallocSizeOf);
+
+[scriptable, uuid(283EE646-1AEF-11D4-98B3-00C04fA0CE9A)]
+interface nsIPropertyElement : nsISupports {
+  attribute AUTF8String key;
+  attribute AString value;
+};
+
+[scriptable, builtinclass, uuid(706867af-0400-4faa-beb1-0dae87308784)]
+interface nsIPersistentProperties : nsIProperties
+{
+  /**
+   * load a set of name/value pairs from the input stream
+   * names and values should be in UTF8
+   */
+  void load(in nsIInputStream input);
+
+  /**
+   * output the values to the stream - results will be in UTF8
+   */
+  void save(in nsIOutputStream output, in AUTF8String header);
+
+  /**
+   * get an enumeration of nsIPropertyElement objects,
+   * which are read-only (i.e. setting properties on the element will
+   * not make changes back into the source nsIPersistentProperties
+   */
+  nsISimpleEnumerator enumerate();
+
+  /**
+   * shortcut to nsIProperty's get() which retrieves a string value
+   * directly (and thus faster)
+   */
+  AString getStringProperty(in AUTF8String key);
+
+  /**
+   * shortcut to nsIProperty's set() which sets a string value
+   * directly (and thus faster). If the given property already exists,
+   * then the old value will be returned
+   */
+  AString setStringProperty(in AUTF8String key, in AString value);
+
+  [notxpcom, nostdcall] size_t sizeOfIncludingThis(in MallocSizeOf aMallocSizeOf);
+};
diff --git a/xpcom/ds/nsIProperties.idl b/xpcom/ds/nsIProperties.idl
new file mode 100644
index 0000000000..50d833e231
--- /dev/null
+++ b/xpcom/ds/nsIProperties.idl
@@ -0,0 +1,46 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/*
+ * Simple mapping service interface.
+ */
+
+[scriptable, uuid(78650582-4e93-4b60-8e85-26ebd3eb14ca)]
+interface nsIProperties : nsISupports
+{
+    /**
+     * Gets a property with a given name.
+     *
+     * @throws NS_ERROR_FAILURE if a property with that name doesn't exist.
+     * @throws NS_ERROR_NO_INTERFACE if the found property fails to QI to the
+     * given iid.
+     */
+    void get(in string prop, in nsIIDRef iid,
+             [iid_is(iid),retval] out nsQIResult result);
+
+    /**
+     * Sets a property with a given name to a given value.
+     */
+    void set(in string prop, in nsISupports value);
+
+    /**
+     * Returns true if the property with the given name exists.
+     */
+    boolean has(in string prop);
+
+    /**
+     * Undefines a property.
+     * @throws NS_ERROR_FAILURE if a property with that name doesn't
+     * already exist.
+     */
+    void undefine(in string prop);
+
+    /**
+     *  Returns an array of the keys.
+     */
+    Array<ACString> getKeys();
+};
diff --git a/xpcom/ds/nsIProperty.idl b/xpcom/ds/nsIProperty.idl
new file mode 100644
index 0000000000..9e6e1e487e
--- /dev/null
+++ b/xpcom/ds/nsIProperty.idl
@@ -0,0 +1,25 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsIVariant based Property support. */
+
+#include "nsISupports.idl"
+
+interface nsIVariant;
+
+[scriptable, uuid(6dcf9030-a49f-11d5-910d-0010a4e73d9a)]
+interface nsIProperty : nsISupports
+{
+    /**
+     * Get the name of the property.
+     */
+    readonly attribute AString    name;
+
+    /**
+     * Get the value of the property.
+     */
+    readonly attribute nsIVariant value;
+};
diff --git a/xpcom/ds/nsIPropertyBag.idl b/xpcom/ds/nsIPropertyBag.idl
new file mode 100644
index 0000000000..57bd263451
--- /dev/null
+++ b/xpcom/ds/nsIPropertyBag.idl
@@ -0,0 +1,28 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsIVariant based Property Bag support. */
+
+#include "nsISupports.idl"
+
+interface nsIVariant;
+interface nsISimpleEnumerator;
+
+[scriptable, uuid(bfcd37b0-a49f-11d5-910d-0010a4e73d9a)]
+interface nsIPropertyBag : nsISupports
+{
+    /**
+     * Get a nsISimpleEnumerator whose elements are nsIProperty objects.
+     */
+    readonly attribute nsISimpleEnumerator enumerator;
+
+    /**
+     * Get a property value for the given name.
+     * @throws NS_ERROR_FAILURE if a property with that name doesn't
+     * exist.
+     */
+    nsIVariant getProperty(in AString name);
+};
diff --git a/xpcom/ds/nsIPropertyBag2.idl b/xpcom/ds/nsIPropertyBag2.idl
new file mode 100644
index 0000000000..3232dce0ce
--- /dev/null
+++ b/xpcom/ds/nsIPropertyBag2.idl
@@ -0,0 +1,80 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsIVariant based Property Bag support. */
+
+#include "nsIPropertyBag.idl"
+
+[scriptable, uuid(625cfd1e-da1e-4417-9ee9-dbc8e0b3fd79)]
+interface nsIPropertyBag2 : nsIPropertyBag
+{
+  // Accessing a property as a different type may attempt conversion to the
+  // requested value
+
+  int32_t     getPropertyAsInt32       (in AString prop);
+  uint32_t    getPropertyAsUint32      (in AString prop);
+  int64_t     getPropertyAsInt64       (in AString prop);
+  uint64_t    getPropertyAsUint64      (in AString prop);
+  double      getPropertyAsDouble      (in AString prop);
+  AString     getPropertyAsAString     (in AString prop);
+  ACString    getPropertyAsACString    (in AString prop);
+  AUTF8String getPropertyAsAUTF8String (in AString prop);
+  boolean     getPropertyAsBool        (in AString prop);
+
+  /**
+   * This method returns null if the value exists, but is null.
+   *
+   * Note: C++ callers should not use this method. They should use the
+   * typesafe `do_GetProperty` wrapper instead.
+   */
+  void        getPropertyAsInterface   (in AString prop,
+                                        in nsIIDRef iid,
+                                        [iid_is(iid), retval] out nsQIResult result);
+
+  /**
+   * This method returns null if the value does not exist,
+   * or exists but is null.
+   */
+  nsIVariant  get                      (in AString prop);
+
+  /**
+   * Check for the existence of a key.
+   */
+  boolean     hasKey                   (in AString prop);
+};
+
+
+%{C++
+#include "nsCOMPtr.h"
+#include "nsAString.h"
+
+class MOZ_STACK_CLASS nsGetProperty final : public nsCOMPtr_helper {
+ public:
+  nsGetProperty(nsIPropertyBag2* aPropBag, const nsAString& aPropName, nsresult* aError)
+      : mPropBag(aPropBag), mPropName(aPropName), mErrorPtr(aError) {}
+
+  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const override;
+
+ private:
+  nsIPropertyBag2* MOZ_NON_OWNING_REF mPropBag;
+  const nsAString& mPropName;
+  nsresult* mErrorPtr;
+};
+
+/**
+ * A typesafe wrapper around nsIPropertyBag2::GetPropertyAsInterface. Similar
+ * to the `do_QueryInterface` family of functions, when assigned to a
+ * `nsCOMPtr` of a given type, attempts to query the given property to that
+ * type.
+ *
+ * If `aError` is passed, the return value of `GetPropertyAsInterface` is
+ * stored in it.
+ */
+inline const nsGetProperty do_GetProperty(nsIPropertyBag2* aPropBag,
+                                          const nsAString& aPropName,
+                                          nsresult* aError = 0) {
+  return nsGetProperty(aPropBag, aPropName, aError);
+}
+
+%}
diff --git a/xpcom/ds/nsISerializable.idl b/xpcom/ds/nsISerializable.idl
new file mode 100644
index 0000000000..1920fa1b16
--- /dev/null
+++ b/xpcom/ds/nsISerializable.idl
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIObjectInputStream;
+interface nsIObjectOutputStream;
+
+[scriptable, uuid(91cca981-c26d-44a8-bebe-d9ed4891503a)]
+interface nsISerializable : nsISupports
+{
+    /**
+     * Initialize the object implementing nsISerializable, which must have
+     * been freshly constructed via CreateInstance.  All data members that
+     * can't be set to default values must have been serialized by write,
+     * and should be read from aInputStream in the same order by this method.
+     */
+    [must_use] void read(in nsIObjectInputStream aInputStream);
+
+    /**
+     * Serialize the object implementing nsISerializable to aOutputStream, by
+     * writing each data member that must be recovered later to reconstitute
+     * a working replica of this object, in a canonical member and byte order,
+     * to aOutputStream.
+     *
+     * NB: a class that implements nsISerializable *must* also implement
+     * nsIClassInfo, in particular nsIClassInfo::GetClassID.
+     */
+    void write(in nsIObjectOutputStream aOutputStream);
+};
diff --git a/xpcom/ds/nsISimpleEnumerator.idl b/xpcom/ds/nsISimpleEnumerator.idl
new file mode 100644
index 0000000000..1a2b47705d
--- /dev/null
+++ b/xpcom/ds/nsISimpleEnumerator.idl
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+/**
+ * Used to enumerate over elements defined by its implementor.
+ * Although hasMoreElements() can be called independently of getNext(),
+ * getNext() must be pre-ceeded by a call to hasMoreElements(). There is
+ * no way to "reset" an enumerator, once you obtain one.
+ *
+ * @version 1.0
+ */
+
+/**
+ * A wrapper for an nsISimpleEnumerator instance which implements the
+ * JavaScript iteration protocol.
+ */
+[scriptable, uuid(4432e8ae-d4d3-42a6-a4d1-829f1c29512b)]
+interface nsIJSEnumerator : nsISupports {
+  [symbol]
+  nsIJSEnumerator iterator();
+
+  [implicit_jscontext]
+  jsval next();
+};
+
+[scriptable, uuid(796f340d-0a2a-490b-9c60-640765e99782)]
+interface nsISimpleEnumeratorBase : nsISupports {
+  /**
+   * Returns a JavaScript iterator for all remaining entries in the enumerator.
+   * Each entry is typically queried to the appropriate interface for the
+   * enumerator.
+   */
+  [symbol]
+  nsIJSEnumerator iterator();
+
+  /**
+   * Returns JavaScript iterator for all remaining entries in the enumerator.
+   * Each entry is queried only to the supplied interface. If any element
+   * fails to query to that interface, the error is propagated to the caller.
+   */
+  nsIJSEnumerator entries(in nsIIDRef aIface);
+};
+
+[scriptable, uuid(D1899240-F9D2-11D2-BDD6-000064657374)]
+interface nsISimpleEnumerator : nsISimpleEnumeratorBase {
+  /**
+   * Called to determine whether or not the enumerator has
+   * any elements that can be returned via getNext(). This method
+   * is generally used to determine whether or not to initiate or
+   * continue iteration over the enumerator, though it can be
+   * called without subsequent getNext() calls. Does not affect
+   * internal state of enumerator.
+   *
+   * @see getNext()
+   * @return true if there are remaining elements in the enumerator.
+   *         false if there are no more elements in the enumerator.
+   */
+  boolean hasMoreElements();
+
+  /**
+   * Called to retrieve the next element in the enumerator. The "next"
+   * element is the first element upon the first call. Must be
+   * pre-ceeded by a call to hasMoreElements() which returns PR_TRUE.
+   * This method is generally called within a loop to iterate over
+   * the elements in the enumerator.
+   *
+   * @see hasMoreElements()
+   * @throws NS_ERROR_FAILURE if there are no more elements
+   *                          to enumerate.
+   * @return the next element in the enumeration.
+   */
+  nsISupports getNext();
+};
diff --git a/xpcom/ds/nsIStringEnumerator.idl b/xpcom/ds/nsIStringEnumerator.idl
new file mode 100644
index 0000000000..96ab966bff
--- /dev/null
+++ b/xpcom/ds/nsIStringEnumerator.idl
@@ -0,0 +1,37 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+interface nsIJSEnumerator;
+
+/**
+ * Used to enumerate over an ordered list of strings.
+ */
+
+/**
+ * Base class for C++-implemented string iterators. JS implementors need not
+ * be queryable to it.
+ */
+[scriptable, uuid(f5213d15-a4d1-4fb7-8a48-d69ccb7fb0eb)]
+interface nsIStringEnumeratorBase : nsISupports
+{
+    [symbol, binaryname(StringIterator)]
+    nsIJSEnumerator iterator();
+};
+
+[scriptable, uuid(50d3ef6c-9380-4f06-9fb2-95488f7d141c)]
+interface nsIStringEnumerator : nsIStringEnumeratorBase
+{
+    boolean hasMore();
+    AString getNext();
+};
+
+[scriptable, uuid(9bdf1010-3695-4907-95ed-83d0410ec307)]
+interface nsIUTF8StringEnumerator : nsIStringEnumeratorBase
+{
+    boolean hasMore();
+    AUTF8String getNext();
+};
diff --git a/xpcom/ds/nsISupportsIterators.idl b/xpcom/ds/nsISupportsIterators.idl
new file mode 100644
index 0000000000..8d47375d57
--- /dev/null
+++ b/xpcom/ds/nsISupportsIterators.idl
@@ -0,0 +1,292 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsISupportsIterators.idl --- IDL defining general purpose iterators */
+
+
+#include "nsISupports.idl"
+
+
+  /*
+    ...
+  */
+
+
+  /**
+   * ...
+   */
+[scriptable, uuid(7330650e-1dd2-11b2-a0c2-9ff86ee97bed)]
+interface nsIOutputIterator : nsISupports
+  {
+      /**
+       * Put |anElementToPut| into the underlying container or sequence at the position currently pointed to by this iterator.
+       * The iterator and the underlying container or sequence cooperate to |Release()|
+       * the replaced element, if any and if necessary, and to |AddRef()| the new element.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anElementToPut the element to place into the underlying container or sequence
+       */
+    void putElement( in nsISupports anElementToPut );
+
+      /**
+       * Advance this iterator to the next position in the underlying container or sequence.
+       */
+    void stepForward();
+  };
+
+  /**
+   * ...
+   */
+[scriptable, uuid(85585e12-1dd2-11b2-a930-f6929058269a)]
+interface nsIInputIterator : nsISupports
+  {
+      /**
+       * Retrieve (and |AddRef()|) the element this iterator currently points to.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @result a new reference to the element this iterator currently points to (if any)
+       */
+    nsISupports getElement();
+
+      /**
+       * Advance this iterator to the next position in the underlying container or sequence.
+       */
+    void stepForward();
+
+      /**
+       * Test if |anotherIterator| points to the same position in the underlying container or sequence.
+       *
+       * The result is undefined if |anotherIterator| was not created by or for the same underlying container or sequence.
+       *
+       * @param anotherIterator another iterator to compare against, created by or for the same underlying container or sequence
+       * @result true if |anotherIterator| points to the same position in the underlying container or sequence
+       */
+    boolean isEqualTo( in nsISupports anotherIterator );
+
+      /**
+       * Create a new iterator pointing to the same position in the underlying container or sequence to which this iterator currently points.
+       * The returned iterator is suitable for use in a subsequent call to |isEqualTo()| against this iterator.
+       *
+       * @result a new iterator pointing at the same position in the same underlying container or sequence as this iterator
+       */
+    nsISupports clone();
+  };
+
+  /**
+   * ...
+   */
+[scriptable, uuid(8da01646-1dd2-11b2-98a7-c7009045be7e)]
+interface nsIForwardIterator : nsISupports
+  {
+      /**
+       * Retrieve (and |AddRef()|) the element this iterator currently points to.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @result a new reference to the element this iterator currently points to (if any)
+       */
+    nsISupports getElement();
+
+      /**
+       * Put |anElementToPut| into the underlying container or sequence at the position currently pointed to by this iterator.
+       * The iterator and the underlying container or sequence cooperate to |Release()|
+       * the replaced element, if any and if necessary, and to |AddRef()| the new element.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anElementToPut the element to place into the underlying container or sequence
+       */
+    void putElement( in nsISupports anElementToPut );
+
+      /**
+       * Advance this iterator to the next position in the underlying container or sequence.
+       */
+    void stepForward();
+
+      /**
+       * Test if |anotherIterator| points to the same position in the underlying container or sequence.
+       *
+       * The result is undefined if |anotherIterator| was not created by or for the same underlying container or sequence.
+       *
+       * @param anotherIterator another iterator to compare against, created by or for the same underlying container or sequence
+       * @result true if |anotherIterator| points to the same position in the underlying container or sequence
+       */
+    boolean isEqualTo( in nsISupports anotherIterator );
+
+      /**
+       * Create a new iterator pointing to the same position in the underlying container or sequence to which this iterator currently points.
+       * The returned iterator is suitable for use in a subsequent call to |isEqualTo()| against this iterator.
+       *
+       * @result a new iterator pointing at the same position in the same underlying container or sequence as this iterator
+       */
+    nsISupports clone();
+  };
+
+  /**
+   * ...
+   */
+[scriptable, uuid(948defaa-1dd1-11b2-89f6-8ce81f5ebda9)]
+interface nsIBidirectionalIterator : nsISupports
+  {
+      /**
+       * Retrieve (and |AddRef()|) the element this iterator currently points to.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @result a new reference to the element this iterator currently points to (if any)
+       */
+    nsISupports getElement();
+
+      /**
+       * Put |anElementToPut| into the underlying container or sequence at the position currently pointed to by this iterator.
+       * The iterator and the underlying container or sequence cooperate to |Release()|
+       * the replaced element, if any and if necessary, and to |AddRef()| the new element.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anElementToPut the element to place into the underlying container or sequence
+       */
+    void putElement( in nsISupports anElementToPut );
+
+      /**
+       * Advance this iterator to the next position in the underlying container or sequence.
+       */
+    void stepForward();
+
+      /**
+       * Move this iterator to the previous position in the underlying container or sequence.
+       */
+    void stepBackward();
+
+      /**
+       * Test if |anotherIterator| points to the same position in the underlying container or sequence.
+       *
+       * The result is undefined if |anotherIterator| was not created by or for the same underlying container or sequence.
+       *
+       * @param anotherIterator another iterator to compare against, created by or for the same underlying container or sequence
+       * @result true if |anotherIterator| points to the same position in the underlying container or sequence
+       */
+    boolean isEqualTo( in nsISupports anotherIterator );
+
+      /**
+       * Create a new iterator pointing to the same position in the underlying container or sequence to which this iterator currently points.
+       * The returned iterator is suitable for use in a subsequent call to |isEqualTo()| against this iterator.
+       *
+       * @result a new iterator pointing at the same position in the same underlying container or sequence as this iterator
+       */
+    nsISupports clone();
+  };
+
+  /**
+   * ...
+   */
+[scriptable, uuid(9bd6fdb0-1dd1-11b2-9101-d15375968230)]
+interface nsIRandomAccessIterator : nsISupports
+  {
+      /**
+       * Retrieve (and |AddRef()|) the element this iterator currently points to.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @result a new reference to the element this iterator currently points to (if any)
+       */
+    nsISupports getElement();
+
+      /**
+       * Retrieve (and |AddRef()|) an element at some offset from where this iterator currently points.
+       * The offset may be negative.  |getElementAt(0)| is equivalent to |getElement()|.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anOffset a |0|-based offset from the position to which this iterator currently points
+       * @result a new reference to the indicated element (if any)
+       */
+    nsISupports getElementAt( in int32_t anOffset );
+
+      /**
+       * Put |anElementToPut| into the underlying container or sequence at the position currently pointed to by this iterator.
+       * The iterator and the underlying container or sequence cooperate to |Release()|
+       * the replaced element, if any and if necessary, and to |AddRef()| the new element.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anElementToPut the element to place into the underlying container or sequence
+       */
+    void putElement( in nsISupports anElementToPut );
+
+      /**
+       * Put |anElementToPut| into the underlying container or sequence at the position |anOffset| away from that currently pointed to by this iterator.
+       * The iterator and the underlying container or sequence cooperate to |Release()|
+       * the replaced element, if any and if necessary, and to |AddRef()| the new element.
+       * |putElementAt(0, obj)| is equivalent to |putElement(obj)|.
+       *
+       * The result is undefined if this iterator currently points outside the
+       * useful range of the underlying container or sequence.
+       *
+       * @param anOffset a |0|-based offset from the position to which this iterator currently points
+       * @param anElementToPut the element to place into the underlying container or sequence
+       */
+    void putElementAt( in int32_t anOffset, in nsISupports anElementToPut );
+
+      /**
+       * Advance this iterator to the next position in the underlying container or sequence.
+       */
+    void stepForward();
+
+      /**
+       * Move this iterator by |anOffset| positions in the underlying container or sequence.
+       * |anOffset| may be negative.  |stepForwardBy(1)| is equivalent to |stepForward()|.
+       * |stepForwardBy(0)| is a no-op.
+       *
+       * @param anOffset a |0|-based offset from the position to which this iterator currently points
+       */
+    void stepForwardBy( in int32_t anOffset );
+
+      /**
+       * Move this iterator to the previous position in the underlying container or sequence.
+       */
+    void stepBackward();
+
+      /**
+       * Move this iterator backwards by |anOffset| positions in the underlying container or sequence.
+       * |anOffset| may be negative.  |stepBackwardBy(1)| is equivalent to |stepBackward()|.
+       * |stepBackwardBy(n)| is equivalent to |stepForwardBy(-n)|.  |stepBackwardBy(0)| is a no-op.
+       *
+       * @param anOffset a |0|-based offset from the position to which this iterator currently points
+       */
+    void stepBackwardBy( in int32_t anOffset );
+
+      /**
+       * Test if |anotherIterator| points to the same position in the underlying container or sequence.
+       *
+       * The result is undefined if |anotherIterator| was not created by or for the same underlying container or sequence.
+       *
+       * @param anotherIterator another iterator to compare against, created by or for the same underlying container or sequence
+       * @result true if |anotherIterator| points to the same position in the underlying container or sequence
+       */
+    boolean isEqualTo( in nsISupports anotherIterator );
+
+      /**
+       * Create a new iterator pointing to the same position in the underlying container or sequence to which this iterator currently points.
+       * The returned iterator is suitable for use in a subsequent call to |isEqualTo()| against this iterator.
+       *
+       * @result a new iterator pointing at the same position in the same underlying container or sequence as this iterator
+       */
+    nsISupports clone();
+  };
+
+%{C++
+%}
diff --git a/xpcom/ds/nsISupportsPrimitives.idl b/xpcom/ds/nsISupportsPrimitives.idl
new file mode 100644
index 0000000000..d661ce3b21
--- /dev/null
+++ b/xpcom/ds/nsISupportsPrimitives.idl
@@ -0,0 +1,222 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsISupports wrappers for single primitive pieces of data. */
+
+#include "nsISupports.idl"
+
+/**
+ * Primitive base interface.
+ *
+ * These first three are pointer types and do data copying
+ * using the nsIMemory. Be careful!
+ */
+
+[scriptable, builtinclass, uuid(d0d4b136-1dd1-11b2-9371-f0727ef827c0)]
+interface nsISupportsPrimitive : nsISupports
+{
+    const unsigned short TYPE_ID                = 1;
+    const unsigned short TYPE_CSTRING           = 2;
+    const unsigned short TYPE_STRING            = 3;
+    const unsigned short TYPE_PRBOOL            = 4;
+    const unsigned short TYPE_PRUINT8           = 5;
+    const unsigned short TYPE_PRUINT16          = 6;
+    const unsigned short TYPE_PRUINT32          = 7;
+    const unsigned short TYPE_PRUINT64          = 8;
+    const unsigned short TYPE_PRTIME            = 9;
+    const unsigned short TYPE_CHAR              = 10;
+    const unsigned short TYPE_PRINT16           = 11;
+    const unsigned short TYPE_PRINT32           = 12;
+    const unsigned short TYPE_PRINT64           = 13;
+    const unsigned short TYPE_FLOAT             = 14;
+    const unsigned short TYPE_DOUBLE            = 15;
+    // 16 was for TYPE_VOID
+    const unsigned short TYPE_INTERFACE_POINTER = 17;
+
+    readonly attribute unsigned short type;
+};
+
+/**
+ * Scriptable storage for nsID structures
+ */
+
+[scriptable, builtinclass, uuid(d18290a0-4a1c-11d3-9890-006008962422)]
+interface nsISupportsID : nsISupportsPrimitive
+{
+    attribute nsIDPtr data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for ASCII strings
+ */
+
+[scriptable, builtinclass, uuid(d65ff270-4a1c-11d3-9890-006008962422)]
+interface nsISupportsCString : nsISupportsPrimitive
+{
+    attribute ACString data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for Unicode strings
+ */
+
+[scriptable, builtinclass, uuid(d79dc970-4a1c-11d3-9890-006008962422)]
+interface nsISupportsString : nsISupportsPrimitive
+{
+    attribute AString data;
+    wstring toString();
+};
+
+/**
+ * The rest are truly primitive and are passed by value
+ */
+
+/**
+ * Scriptable storage for booleans
+ */
+
+[scriptable, builtinclass, uuid(ddc3b490-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRBool : nsISupportsPrimitive
+{
+    attribute boolean data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for 8-bit integers
+ */
+
+[scriptable, builtinclass, uuid(dec2e4e0-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRUint8 : nsISupportsPrimitive
+{
+    attribute uint8_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for unsigned 16-bit integers
+ */
+
+[scriptable, builtinclass, uuid(dfacb090-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRUint16 : nsISupportsPrimitive
+{
+    attribute uint16_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for unsigned 32-bit integers
+ */
+
+[scriptable, builtinclass, uuid(e01dc470-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRUint32 : nsISupportsPrimitive
+{
+    attribute uint32_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for 64-bit integers
+ */
+
+[scriptable, builtinclass, uuid(e13567c0-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRUint64 : nsISupportsPrimitive
+{
+    attribute uint64_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for NSPR date/time values
+ */
+
+[scriptable, builtinclass, uuid(e2563630-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRTime : nsISupportsPrimitive
+{
+    attribute PRTime data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for single character values
+ * (often used to store an ASCII character)
+ */
+
+[scriptable, builtinclass, uuid(e2b05e40-4a1c-11d3-9890-006008962422)]
+interface nsISupportsChar : nsISupportsPrimitive
+{
+    attribute char data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for 16-bit integers
+ */
+
+[scriptable, builtinclass, uuid(e30d94b0-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRInt16 : nsISupportsPrimitive
+{
+    attribute int16_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for 32-bit integers
+ */
+
+[scriptable, builtinclass, uuid(e36c5250-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRInt32 : nsISupportsPrimitive
+{
+    attribute int32_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for 64-bit integers
+ */
+
+[scriptable, builtinclass, uuid(e3cb0ff0-4a1c-11d3-9890-006008962422)]
+interface nsISupportsPRInt64 : nsISupportsPrimitive
+{
+    attribute int64_t data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for floating point numbers
+ */
+
+[scriptable, builtinclass, uuid(abeaa390-4ac0-11d3-baea-00805f8a5dd7)]
+interface nsISupportsFloat : nsISupportsPrimitive
+{
+    attribute float data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for doubles
+ */
+
+[scriptable, builtinclass, uuid(b32523a0-4ac0-11d3-baea-00805f8a5dd7)]
+interface nsISupportsDouble : nsISupportsPrimitive
+{
+    attribute double data;
+    string toString();
+};
+
+/**
+ * Scriptable storage for other XPCOM objects
+ */
+
+[scriptable, builtinclass, uuid(995ea724-1dd1-11b2-9211-c21bdd3e7ed0)]
+interface nsISupportsInterfacePointer : nsISupportsPrimitive
+{
+    attribute nsISupports data;
+    attribute nsIDPtr dataIID;
+
+    string toString();
+};
diff --git a/xpcom/ds/nsIVariant.idl b/xpcom/ds/nsIVariant.idl
new file mode 100644
index 0000000000..0d7f7e70c3
--- /dev/null
+++ b/xpcom/ds/nsIVariant.idl
@@ -0,0 +1,162 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* The long avoided variant support for xpcom. */
+
+#include "nsISupports.idl"
+
+%{C++
+#include "xptinfo.h"
+
+// This enum class used to be a const-only XPIDL interface, containing literal
+// integer descriptions of the different fields. Instead, it now directly
+// references the nsXPTTypeTag variants VTYPE_ are intended to match.
+struct nsIDataType
+{
+    enum {
+        // These MUST match the declarations in xptinfo.h.
+        // Otherwise the world is likely to explode.
+        VTYPE_INT8              = TD_INT8             ,
+        VTYPE_INT16             = TD_INT16            ,
+        VTYPE_INT32             = TD_INT32            ,
+        VTYPE_INT64             = TD_INT64            ,
+        VTYPE_UINT8             = TD_UINT8            ,
+        VTYPE_UINT16            = TD_UINT16           ,
+        VTYPE_UINT32            = TD_UINT32           ,
+        VTYPE_UINT64            = TD_UINT64           ,
+        VTYPE_FLOAT             = TD_FLOAT            ,
+        VTYPE_DOUBLE            = TD_DOUBLE           ,
+        VTYPE_BOOL              = TD_BOOL             ,
+        VTYPE_CHAR              = TD_CHAR             ,
+        VTYPE_WCHAR             = TD_WCHAR            ,
+        VTYPE_VOID              = TD_VOID             ,
+        VTYPE_ID                = TD_NSIDPTR          ,
+        VTYPE_CHAR_STR          = TD_PSTRING          ,
+        VTYPE_WCHAR_STR         = TD_PWSTRING         ,
+        VTYPE_INTERFACE         = TD_INTERFACE_TYPE   ,
+        VTYPE_INTERFACE_IS      = TD_INTERFACE_IS_TYPE,
+        VTYPE_ARRAY             = TD_LEGACY_ARRAY     ,
+        VTYPE_STRING_SIZE_IS    = TD_PSTRING_SIZE_IS  ,
+        VTYPE_WSTRING_SIZE_IS   = TD_PWSTRING_SIZE_IS ,
+        VTYPE_UTF8STRING        = TD_UTF8STRING       ,
+        VTYPE_CSTRING           = TD_CSTRING          ,
+        VTYPE_ASTRING           = TD_ASTRING          ,
+
+        // Non-xpt variant types
+        VTYPE_EMPTY_ARRAY       = 254                 ,
+        VTYPE_EMPTY             = 255
+    };
+};
+%}
+
+
+/**
+ * XPConnect has magic to transparently convert between nsIVariant and JS types.
+ * We mark the interface [scriptable] so that JS can use methods
+ * that refer to this interface. But we mark all the methods and attributes
+ * [noscript] since any nsIVariant object will be automatically converted to a
+ * JS type anyway.
+ */
+
+[scriptable, builtinclass, uuid(81e4c2de-acac-4ad6-901a-b5fb1b851a0d)]
+interface nsIVariant : nsISupports
+{
+    [notxpcom,nostdcall] readonly attribute uint16_t     dataType;
+
+    [noscript] uint8_t      getAsInt8();
+    [noscript] int16_t      getAsInt16();
+    [noscript] int32_t      getAsInt32();
+    [noscript] int64_t      getAsInt64();
+    [noscript] uint8_t      getAsUint8();
+    [noscript] uint16_t     getAsUint16();
+    [noscript] uint32_t     getAsUint32();
+    [noscript] uint64_t     getAsUint64();
+    [noscript] float        getAsFloat();
+    [noscript] double       getAsDouble();
+    [noscript] boolean      getAsBool();
+    [noscript] char         getAsChar();
+    [noscript] wchar        getAsWChar();
+    [notxpcom] nsresult     getAsID(out nsID retval);
+    [noscript] AString      getAsAString();
+    [noscript] ACString     getAsACString();
+    [noscript] AUTF8String  getAsAUTF8String();
+    [noscript] string       getAsString();
+    [noscript] wstring      getAsWString();
+    [noscript] nsISupports  getAsISupports();
+    [noscript] jsval        getAsJSVal();
+
+    [noscript] void getAsInterface(out nsIIDPtr iid,
+                                   [iid_is(iid), retval] out nsQIResult iface);
+
+    [notxpcom] nsresult getAsArray(out uint16_t type, out nsIID iid,
+                                   out uint32_t count, out voidPtr ptr);
+
+    [noscript] void getAsStringWithSize(out uint32_t size,
+                                        [size_is(size), retval] out string str);
+
+    [noscript] void getAsWStringWithSize(out uint32_t size,
+                                         [size_is(size), retval] out wstring str);
+};
+
+/**
+ * An object that implements nsIVariant may or may NOT also implement this
+ * nsIWritableVariant.
+ *
+ * If the 'writable' attribute is false then attempts to call any of the 'set'
+ * methods can be expected to fail. Setting the 'writable' attribute may or
+ * may not succeed.
+ *
+ */
+
+[scriptable, builtinclass, uuid(5586a590-8c82-11d5-90f3-0010a4e73d9a)]
+interface nsIWritableVariant : nsIVariant
+{
+    attribute boolean writable;
+
+    void setAsInt8(in uint8_t aValue);
+    void setAsInt16(in int16_t aValue);
+    void setAsInt32(in int32_t aValue);
+    void setAsInt64(in int64_t aValue);
+    void setAsUint8(in uint8_t aValue);
+    void setAsUint16(in uint16_t aValue);
+    void setAsUint32(in uint32_t aValue);
+    void setAsUint64(in uint64_t aValue);
+    void setAsFloat(in float aValue);
+    void setAsDouble(in double aValue);
+    void setAsBool(in boolean aValue);
+    void setAsChar(in char aValue);
+    void setAsWChar(in wchar aValue);
+    void setAsID(in nsIDRef aValue);
+    void setAsAString(in AString aValue);
+    void setAsACString(in ACString aValue);
+    void setAsAUTF8String(in AUTF8String aValue);
+    void setAsString(in string aValue);
+    void setAsWString(in wstring aValue);
+    void setAsISupports(in nsISupports aValue);
+
+    void setAsInterface(in nsIIDRef iid,
+                        [iid_is(iid)] in nsQIResult iface);
+
+    [noscript] void setAsArray(in uint16_t type, in nsIIDPtr iid,
+                               in uint32_t count, in voidPtr ptr);
+
+    void setAsStringWithSize(in uint32_t size,
+                             [size_is(size)] in string str);
+
+    void setAsWStringWithSize(in uint32_t size,
+                              [size_is(size)] in wstring str);
+
+    void setAsVoid();
+    void setAsEmpty();
+    void setAsEmptyArray();
+
+    void setFromVariant(in nsIVariant aValue);
+};
+
+%{C++
+// The contractID for the generic implementation built in to xpcom.
+#define NS_VARIANT_CONTRACTID "@mozilla.org/variant;1"
+%}
diff --git a/xpcom/ds/nsIWindowsRegKey.idl b/xpcom/ds/nsIWindowsRegKey.idl
new file mode 100644
index 0000000000..197ba69c6d
--- /dev/null
+++ b/xpcom/ds/nsIWindowsRegKey.idl
@@ -0,0 +1,336 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsISupports.idl"
+
+%{C++
+#include <windows.h>
+%}
+
+native HKEY(HKEY);
+
+/**
+ * This interface is designed to provide scriptable access to the Windows
+ * registry system ("With Great Power Comes Great Responsibility").  The
+ * interface represents a single key in the registry.
+ *
+ * This interface is highly Win32 specific.
+ */
+[scriptable, uuid(2555b930-d64f-437e-9be7-0a2cb252c1f4)]
+interface nsIWindowsRegKey : nsISupports
+{
+  /**
+   * Root keys.  The values for these keys correspond to the values from
+   * WinReg.h in the MS Platform SDK.  The ROOT_KEY_ prefix corresponds to the
+   * HKEY_ prefix in the MS Platform SDK.
+   *
+   * This interface is not restricted to using only these root keys.
+   */
+  const unsigned long ROOT_KEY_CLASSES_ROOT     = 0x80000000;
+  const unsigned long ROOT_KEY_CURRENT_USER     = 0x80000001;
+  const unsigned long ROOT_KEY_LOCAL_MACHINE    = 0x80000002;
+
+  /**
+   * Values for the mode parameter passed to the open and create methods.
+   * The values defined here correspond to the REGSAM values defined in
+   * WinNT.h in the MS Platform SDK, where ACCESS_ is replaced with KEY_.
+   *
+   * This interface is not restricted to using only these access types.
+   */
+  const unsigned long ACCESS_BASIC              = 0x00020000;
+  const unsigned long ACCESS_QUERY_VALUE        = 0x00000001;
+  const unsigned long ACCESS_SET_VALUE          = 0x00000002;
+  const unsigned long ACCESS_CREATE_SUB_KEY     = 0x00000004;
+  const unsigned long ACCESS_ENUMERATE_SUB_KEYS = 0x00000008;
+  const unsigned long ACCESS_NOTIFY             = 0x00000010;
+  const unsigned long ACCESS_READ               = ACCESS_BASIC |
+                                                  ACCESS_QUERY_VALUE |
+                                                  ACCESS_ENUMERATE_SUB_KEYS |
+                                                  ACCESS_NOTIFY;
+  const unsigned long ACCESS_WRITE              = ACCESS_BASIC |
+                                                  ACCESS_SET_VALUE |
+                                                  ACCESS_CREATE_SUB_KEY;
+  const unsigned long ACCESS_ALL                = ACCESS_READ |
+                                                  ACCESS_WRITE;
+  const unsigned long WOW64_32                  = 0x00000200;
+  const unsigned long WOW64_64                  = 0x00000100;
+
+
+  /**
+   * Values for the type of a registry value.  The numeric values of these
+   * constants are taken directly from WinNT.h in the MS Platform SDK.
+   * The Microsoft documentation should be consulted for the exact meaning of
+   * these value types.
+   *
+   * This interface is somewhat restricted to using only these value types.
+   * There is no method that is directly equivalent to RegQueryValueEx or
+   * RegSetValueEx.  In particular, this interface does not support the
+   * REG_MULTI_SZ and REG_EXPAND_SZ value types.  It is still possible to
+   * enumerate values that have other types (i.e., getValueType may return a
+   * type not defined below).
+   */
+  const unsigned long TYPE_NONE   = 0;  // REG_NONE
+  const unsigned long TYPE_STRING = 1;  // REG_SZ
+  const unsigned long TYPE_BINARY = 3;  // REG_BINARY
+  const unsigned long TYPE_INT    = 4;  // REG_DWORD
+  const unsigned long TYPE_INT64  = 11; // REG_QWORD
+
+  /**
+   * This attribute exposes the native HKEY and is available to provide C++
+   * consumers with the flexibility of making other Windows registry API calls
+   * that are not exposed via this interface.
+   *
+   * It is possible to initialize this object by setting an HKEY on it.  In
+   * that case, it is the responsibility of the consumer setting the HKEY to
+   * ensure that it is a valid HKEY.
+   *
+   * WARNING: Setting the key does not close the old key.
+   */
+  [noscript] attribute HKEY key;
+
+  /**
+   * This method closes the key.  If the key is already closed, then this
+   * method does nothing.
+   */
+  void close();
+
+  /**
+   * This method opens an existing key.  This method fails if the key
+   * does not exist.
+   *
+   * NOTE: On 32-bit Windows, it is valid to pass any HKEY as the rootKey
+   * parameter of this function.  However, for compatibility with 64-bit
+   * Windows, that usage should probably be avoided in favor of openChild.
+   *
+   * @param rootKey
+   *        A root key defined above or any valid HKEY on 32-bit Windows.
+   * @param relPath
+   *        A relative path from the given root key.
+   * @param mode
+   *        Access mode, which is a bit-wise OR of the ACCESS_ values defined
+   *        above.
+   */
+  void open(in unsigned long rootKey, in AString relPath, in unsigned long mode);
+
+  /**
+   * This method opens an existing key or creates a new key.
+   *
+   * NOTE: On 32-bit Windows, it is valid to pass any HKEY as the rootKey
+   * parameter of this function.  However, for compatibility with 64-bit
+   * Windows, that usage should probably be avoided in favor of createChild.
+   *
+   * @param rootKey
+   *        A root key defined above or any valid HKEY on 32-bit Windows.
+   * @param relPath
+   *        A relative path from the given root key.
+   * @param mode
+   *        Access mode, which is a bit-wise OR of the ACCESS_ values defined
+   *        above.
+   */
+  void create(in unsigned long rootKey, in AString relPath, in unsigned long mode);
+
+  /**
+   * This method opens a subkey relative to this key.  This method fails if the
+   * key does not exist.
+   *
+   * @return nsIWindowsRegKey for the newly opened subkey.
+   */
+  nsIWindowsRegKey openChild(in AString relPath, in unsigned long mode);
+
+  /**
+   * This method opens or creates a subkey relative to this key.
+   *
+   * @return nsIWindowsRegKey for the newly opened or created subkey.
+   */
+  nsIWindowsRegKey createChild(in AString relPath, in unsigned long mode);
+
+  /**
+   * This attribute returns the number of child keys.
+   */
+  readonly attribute unsigned long childCount;
+
+  /**
+   * This method returns the name of the n'th child key.
+   *
+   * @param index
+   *        The index of the requested child key.
+   */
+  AString getChildName(in unsigned long index);
+
+  /**
+   * This method checks to see if the key has a child by the given name.
+   *
+   * @param name
+   *        The name of the requested child key.
+   */
+  boolean hasChild(in AString name);
+
+  /**
+   * This attribute returns the number of values under this key.
+   */
+  readonly attribute unsigned long valueCount;
+
+  /**
+   * This method returns the name of the n'th value under this key.
+   *
+   * @param index
+   *        The index of the requested value.
+   */
+  AString getValueName(in unsigned long index);
+
+  /**
+   * This method checks to see if the key has a value by the given name.
+   *
+   * @param name
+   *        The name of the requested value.
+   */
+  boolean hasValue(in AString name);
+
+  /**
+   * This method removes a child key and all of its values.  This method will
+   * fail if the key has any children of its own.
+   *
+   * @param relPath
+   *        The relative path from this key to the key to be removed.
+   */
+  void removeChild(in AString relPath);
+
+  /**
+   * This method removes the value with the given name.
+   *
+   * @param name
+   *        The name of the value to be removed.
+   */
+  void removeValue(in AString name);
+
+  /**
+   * This method returns the type of the value with the given name.  The return
+   * value is one of the "TYPE_" constants defined above.
+   *
+   * @param name
+   *        The name of the value to query.
+   */
+  unsigned long getValueType(in AString name);
+
+  /**
+   * This method reads the string contents of the named value as a Unicode
+   * string.
+   *
+   * @param name
+   *        The name of the value to query.  This parameter can be the empty
+   *        string to request the key's default value.
+   */
+  AString readStringValue(in AString name);
+
+  /**
+   * This method reads the integer contents of the named value.
+   *
+   * @param name
+   *        The name of the value to query.
+   */
+  unsigned long readIntValue(in AString name);
+
+  /**
+   * This method reads the 64-bit integer contents of the named value.
+   *
+   * @param name
+   *        The name of the value to query.
+   */
+  unsigned long long readInt64Value(in AString name);
+
+  /**
+   * This method reads the binary contents of the named value under this key.
+   *
+   * JavaScript callers should take care with the result of this method since
+   * it will be byte-expanded to form a JS string.  (The binary data will be
+   * treated as an ISO-Latin-1 character string, which it is not).
+   *
+   * @param name
+   *        The name of the value to query.
+   */
+  ACString readBinaryValue(in AString name);
+
+  /**
+   * This method writes the unicode string contents of the named value.  The
+   * value will be created if it does not already exist.
+   *
+   * @param name
+   *        The name of the value to modify.  This parameter can be the empty
+   *        string to modify the key's default value.
+   * @param data
+   *        The data for the value to modify.
+   */
+  void writeStringValue(in AString name, in AString data);
+
+  /**
+   * This method writes the integer contents of the named value.  The value
+   * will be created if it does not already exist.
+   *
+   * @param name
+   *        The name of the value to modify.
+   * @param data
+   *        The data for the value to modify.
+   */
+  void writeIntValue(in AString name, in unsigned long data);
+
+  /**
+   * This method writes the 64-bit integer contents of the named value.  The
+   * value will be created if it does not already exist.
+   *
+   * @param name
+   *        The name of the value to modify.
+   * @param data
+   *        The data for the value to modify.
+   */
+  void writeInt64Value(in AString name, in unsigned long long data);
+
+  /**
+   * This method writes the binary contents of the named value.  The value will
+   * be created if it does not already exist.
+   *
+   * JavaScript callers should take care with the value passed to this method
+   * since it will be truncated from a JS string (unicode) to a ISO-Latin-1
+   * string.  (The binary data will be treated as an ISO-Latin-1 character
+   * string, which it is not).  So, JavaScript callers should only pass
+   * character values in the range \u0000 to \u00FF, or else data loss will
+   * occur.
+   *
+   * @param name
+   *        The name of the value to modify.
+   * @param data
+   *        The data for the value to modify.
+   */
+  void writeBinaryValue(in AString name, in ACString data);
+
+  /**
+   * This method starts watching the key to see if any of its values have
+   * changed.  The key must have been opened with mode including ACCESS_NOTIFY.
+   * If recurse is true, then this key and any of its descendant keys are
+   * watched.  Otherwise, only this key is watched.
+   *
+   * @param recurse
+   *        Indicates whether or not to also watch child keys.
+   */
+  void startWatching(in boolean recurse);
+
+  /**
+   * This method stops any watching of the key initiated by a call to
+   * startWatching.  This method does nothing if the key is not being watched.
+   */
+  void stopWatching();
+
+  /**
+   * This method returns true if the key is being watched for changes (i.e.,
+   * if startWatching() was called).
+   */
+  boolean isWatching();
+
+  /**
+   * This method returns true if the key has changed and false otherwise.
+   * This method will always return false if startWatching was not called.
+   */
+  boolean hasChanged();
+};
diff --git a/xpcom/ds/nsIWritablePropertyBag.idl b/xpcom/ds/nsIWritablePropertyBag.idl
new file mode 100644
index 0000000000..e916b7ccd6
--- /dev/null
+++ b/xpcom/ds/nsIWritablePropertyBag.idl
@@ -0,0 +1,27 @@
+/* -*- Mode: IDL; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsIVariant based writable Property Bag support. */
+
+#include "nsIPropertyBag.idl"
+
+[scriptable, uuid(96fc4671-eeb4-4823-9421-e50fb70ad353)]
+interface nsIWritablePropertyBag : nsIPropertyBag
+{
+    /**
+     * Set a property with the given name to the given value.  If
+     * a property already exists with the given name, it is
+     * overwritten.
+     */
+    void setProperty(in AString name, in nsIVariant value);
+
+    /**
+     * Delete a property with the given name.
+     * @throws NS_ERROR_FAILURE if a property with that name doesn't
+     * exist.
+     */
+    void deleteProperty(in AString name);
+};
diff --git a/xpcom/ds/nsIWritablePropertyBag2.idl b/xpcom/ds/nsIWritablePropertyBag2.idl
new file mode 100644
index 0000000000..50f093905d
--- /dev/null
+++ b/xpcom/ds/nsIWritablePropertyBag2.idl
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* nsIVariant based Property Bag support. */
+
+#include "nsIPropertyBag2.idl"
+
+[scriptable, uuid(9cfd1587-360e-4957-a58f-4c2b1c5e7ed9)]
+interface nsIWritablePropertyBag2 : nsIPropertyBag2
+{
+  void        setPropertyAsInt32       (in AString prop, in int32_t value);
+  void        setPropertyAsUint32      (in AString prop, in uint32_t value);
+  void        setPropertyAsInt64       (in AString prop, in int64_t value);
+  void        setPropertyAsUint64      (in AString prop, in uint64_t value);
+  void        setPropertyAsDouble      (in AString prop, in double value);
+  void        setPropertyAsAString     (in AString prop, in AString value);
+  void        setPropertyAsACString    (in AString prop, in ACString value);
+  void        setPropertyAsAUTF8String (in AString prop, in AUTF8String value);
+  void        setPropertyAsBool        (in AString prop, in boolean value);
+  void        setPropertyAsInterface   (in AString prop, in nsISupports value);
+};
diff --git a/xpcom/ds/nsInterfaceHashtable.h b/xpcom/ds/nsInterfaceHashtable.h
new file mode 100644
index 0000000000..644864cf8f
--- /dev/null
+++ b/xpcom/ds/nsInterfaceHashtable.h
@@ -0,0 +1,14 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsInterfaceHashtable_h__
+#define nsInterfaceHashtable_h__
+
+#include "nsRefCountedHashtable.h"
+#include "nsHashKeys.h"
+#include "nsCOMPtr.h"
+
+#endif  // nsInterfaceHashtable_h__
diff --git a/xpcom/ds/nsMathUtils.h b/xpcom/ds/nsMathUtils.h
new file mode 100644
index 0000000000..527e0c3eb2
--- /dev/null
+++ b/xpcom/ds/nsMathUtils.h
@@ -0,0 +1,109 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsMathUtils_h__
+#define nsMathUtils_h__
+
+#include "nscore.h"
+#include <cmath>
+#include <float.h>
+
+#if defined(XP_SOLARIS)
+#  include <ieeefp.h>
+#endif
+
+/*
+ * round
+ */
+inline double NS_round(double aNum) {
+  return aNum >= 0.0 ? floor(aNum + 0.5) : ceil(aNum - 0.5);
+}
+inline float NS_roundf(float aNum) {
+  return aNum >= 0.0f ? floorf(aNum + 0.5f) : ceilf(aNum - 0.5f);
+}
+inline int32_t NS_lround(double aNum) {
+  return aNum >= 0.0 ? int32_t(aNum + 0.5) : int32_t(aNum - 0.5);
+}
+
+/* NS_roundup30 rounds towards infinity for positive and       */
+/* negative numbers.                                           */
+
+#if defined(XP_WIN) && defined(_M_IX86) && !defined(__GNUC__) && \
+    !defined(__clang__)
+inline int32_t NS_lroundup30(float x) {
+  /* Code derived from Laurent de Soras' paper at             */
+  /* http://ldesoras.free.fr/doc/articles/rounding_en.pdf     */
+
+  /* Rounding up on Windows is expensive using the float to   */
+  /* int conversion and the floor function. A faster          */
+  /* approach is to use f87 rounding while assuming the       */
+  /* default rounding mode of rounding to the nearest         */
+  /* integer. This rounding mode, however, actually rounds    */
+  /* to the nearest integer so we add the floating point      */
+  /* number to itself and add our rounding factor before      */
+  /* doing the conversion to an integer. We then do a right   */
+  /* shift of one bit on the integer to divide by two.        */
+
+  /* This routine doesn't handle numbers larger in magnitude  */
+  /* than 2^30 but this is fine for NSToCoordRound because    */
+  /* Coords are limited to 2^30 in magnitude.                 */
+
+  static const double round_to_nearest = 0.5f;
+  int i;
+
+  __asm {
+    fld     x                   ; load fp argument
+    fadd    st, st(0)           ; double it
+    fadd    round_to_nearest    ; add the rounding factor
+    fistp   dword ptr i         ; convert the result to int
+  }
+  return i >> 1; /* divide by 2 */
+}
+#endif /* XP_WIN && _M_IX86 && !__GNUC__ */
+
+inline int32_t NS_lroundf(float aNum) {
+  return aNum >= 0.0f ? int32_t(aNum + 0.5f) : int32_t(aNum - 0.5f);
+}
+
+/*
+ * hypot.  We don't need a super accurate version of this, if a platform
+ * turns up with none of the possibilities below it would be okay to fall
+ * back to sqrt(x*x + y*y).
+ */
+inline double NS_hypot(double aNum1, double aNum2) {
+#ifdef __GNUC__
+  return __builtin_hypot(aNum1, aNum2);
+#elif defined _WIN32
+  return _hypot(aNum1, aNum2);
+#else
+  return hypot(aNum1, aNum2);
+#endif
+}
+
+/**
+ * Check whether a floating point number is finite (not +/-infinity and not a
+ * NaN value).
+ */
+inline bool NS_finite(double aNum) {
+#ifdef WIN32
+  // NOTE: '!!' casts an int to bool without spamming MSVC warning C4800.
+  return !!_finite(aNum);
+#else
+  return std::isfinite(aNum);
+#endif
+}
+
+/**
+ * Returns the result of the modulo of x by y using a floored division.
+ * fmod(x, y) is using a truncated division.
+ * The main difference is that the result of this method will have the sign of
+ * y while the result of fmod(x, y) will have the sign of x.
+ */
+inline double NS_floorModulo(double aNum1, double aNum2) {
+  return (aNum1 - aNum2 * floor(aNum1 / aNum2));
+}
+
+#endif
diff --git a/xpcom/ds/nsObserverList.cpp b/xpcom/ds/nsObserverList.cpp
new file mode 100644
index 0000000000..ba3eaaa2aa
--- /dev/null
+++ b/xpcom/ds/nsObserverList.cpp
@@ -0,0 +1,92 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsObserverList.h"
+
+#include "mozilla/ResultExtensions.h"
+#include "nsCOMArray.h"
+#include "xpcpublic.h"
+
+nsresult nsObserverList::AddObserver(nsIObserver* anObserver, bool ownsWeak) {
+  NS_ASSERTION(anObserver, "Null input");
+
+  MOZ_TRY(mObservers.AppendWeakElement(anObserver, ownsWeak));
+  return NS_OK;
+}
+
+nsresult nsObserverList::RemoveObserver(nsIObserver* anObserver) {
+  NS_ASSERTION(anObserver, "Null input");
+
+  MOZ_TRY(mObservers.RemoveWeakElement(anObserver));
+  return NS_OK;
+}
+
+void nsObserverList::GetObserverList(nsISimpleEnumerator** anEnumerator) {
+  RefPtr<nsObserverEnumerator> e(new nsObserverEnumerator(this));
+  e.forget(anEnumerator);
+}
+
+nsCOMArray<nsIObserver> nsObserverList::ReverseCloneObserverArray() {
+  nsCOMArray<nsIObserver> array;
+  array.SetCapacity(mObservers.Length());
+
+  // XXX This could also use RemoveElementsBy if we lifted the promise to fill
+  // aArray in reverse order. Although there shouldn't be anyone explicitly
+  // relying on the order, changing this might cause subtle problems, so we
+  // better leave it as it is.
+
+  for (int32_t i = mObservers.Length() - 1; i >= 0; --i) {
+    nsCOMPtr<nsIObserver> observer = mObservers[i].GetValue();
+    if (observer) {
+      array.AppendElement(observer.forget());
+    } else {
+      // the object has gone away, remove the weakref
+      mObservers.RemoveElementAt(i);
+    }
+  }
+
+  return array;
+}
+
+void nsObserverList::AppendStrongObservers(nsCOMArray<nsIObserver>& aArray) {
+  aArray.SetCapacity(aArray.Length() + mObservers.Length());
+
+  for (int32_t i = mObservers.Length() - 1; i >= 0; --i) {
+    if (!mObservers[i].IsWeak()) {
+      nsCOMPtr<nsIObserver> observer = mObservers[i].GetValue();
+      aArray.AppendObject(observer);
+    }
+  }
+}
+
+void nsObserverList::NotifyObservers(nsISupports* aSubject, const char* aTopic,
+                                     const char16_t* someData) {
+  const nsCOMArray<nsIObserver> observers = ReverseCloneObserverArray();
+
+  for (int32_t i = 0; i < observers.Count(); ++i) {
+    observers[i]->Observe(aSubject, aTopic, someData);
+  }
+}
+
+nsObserverEnumerator::nsObserverEnumerator(nsObserverList* aObserverList)
+    : mIndex(0), mObservers(aObserverList->ReverseCloneObserverArray()) {}
+
+NS_IMETHODIMP
+nsObserverEnumerator::HasMoreElements(bool* aResult) {
+  *aResult = (mIndex < mObservers.Count());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsObserverEnumerator::GetNext(nsISupports** aResult) {
+  if (mIndex == mObservers.Count()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  NS_ADDREF(*aResult = mObservers[mIndex]);
+  ++mIndex;
+  return NS_OK;
+}
diff --git a/xpcom/ds/nsObserverList.h b/xpcom/ds/nsObserverList.h
new file mode 100644
index 0000000000..9f209e40d6
--- /dev/null
+++ b/xpcom/ds/nsObserverList.h
@@ -0,0 +1,67 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsObserverList_h___
+#define nsObserverList_h___
+
+#include "nsISupports.h"
+#include "nsCOMArray.h"
+#include "nsIObserver.h"
+#include "nsHashKeys.h"
+#include "nsMaybeWeakPtr.h"
+#include "nsSimpleEnumerator.h"
+#include "mozilla/Attributes.h"
+
+class nsObserverList : public nsCharPtrHashKey {
+  friend class nsObserverService;
+
+ public:
+  explicit nsObserverList(const char* aKey) : nsCharPtrHashKey(aKey) {
+    MOZ_COUNT_CTOR(nsObserverList);
+  }
+
+  nsObserverList(nsObserverList&& aOther)
+      : nsCharPtrHashKey(std::move(aOther)),
+        mObservers(std::move(aOther.mObservers)) {
+    MOZ_COUNT_CTOR(nsObserverList);
+  }
+
+  MOZ_COUNTED_DTOR(nsObserverList)
+
+  [[nodiscard]] nsresult AddObserver(nsIObserver* aObserver, bool aOwnsWeak);
+  [[nodiscard]] nsresult RemoveObserver(nsIObserver* aObserver);
+
+  void NotifyObservers(nsISupports* aSubject, const char* aTopic,
+                       const char16_t* aSomeData);
+  void GetObserverList(nsISimpleEnumerator** aEnumerator);
+
+  // Clone an array with the observers of this category.
+  // The array is filled in last-added-first order.
+  nsCOMArray<nsIObserver> ReverseCloneObserverArray();
+
+  // Like FillObserverArray(), but only for strongly held observers.
+  void AppendStrongObservers(nsCOMArray<nsIObserver>& aArray);
+
+ private:
+  nsMaybeWeakPtrArray<nsIObserver> mObservers;
+};
+
+class nsObserverEnumerator final : public nsSimpleEnumerator {
+ public:
+  NS_DECL_NSISIMPLEENUMERATOR
+
+  explicit nsObserverEnumerator(nsObserverList* aObserverList);
+
+  const nsID& DefaultInterface() override { return NS_GET_IID(nsIObserver); }
+
+ private:
+  ~nsObserverEnumerator() override = default;
+
+  int32_t mIndex;  // Counts up from 0
+  nsCOMArray<nsIObserver> mObservers;
+};
+
+#endif /* nsObserverList_h___ */
diff --git a/xpcom/ds/nsObserverService.cpp b/xpcom/ds/nsObserverService.cpp
new file mode 100644
index 0000000000..8f7a09d1aa
--- /dev/null
+++ b/xpcom/ds/nsObserverService.cpp
@@ -0,0 +1,367 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Logging.h"
+#include "nsComponentManagerUtils.h"
+#include "nsContentUtils.h"
+#include "nsIConsoleService.h"
+#include "nsIObserverService.h"
+#include "nsIObserver.h"
+#include "nsIScriptError.h"
+#include "nsObserverService.h"
+#include "nsObserverList.h"
+#include "nsServiceManagerUtils.h"
+#include "nsThreadUtils.h"
+#include "nsEnumeratorUtils.h"
+#include "xpcpublic.h"
+#include "mozilla/AppShutdown.h"
+#include "mozilla/net/NeckoCommon.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/TimeStamp.h"
+#include "nsString.h"
+
+static const uint32_t kMinTelemetryNotifyObserversLatencyMs = 1;
+
+// Log module for nsObserverService logging...
+//
+// To enable logging (see prlog.h for full details):
+//
+//    set MOZ_LOG=ObserverService:5
+//    set MOZ_LOG_FILE=service.log
+//
+// This enables LogLevel::Debug level information and places all output in
+// the file service.log.
+static mozilla::LazyLogModule sObserverServiceLog("ObserverService");
+#define LOG(x) MOZ_LOG(sObserverServiceLog, mozilla::LogLevel::Debug, x)
+
+using namespace mozilla;
+
+NS_IMETHODIMP
+nsObserverService::CollectReports(nsIHandleReportCallback* aHandleReport,
+                                  nsISupports* aData, bool aAnonymize) {
+  struct SuspectObserver {
+    SuspectObserver(const char* aTopic, size_t aReferentCount)
+        : mTopic(aTopic), mReferentCount(aReferentCount) {}
+    const char* mTopic;
+    size_t mReferentCount;
+  };
+
+  size_t totalNumStrong = 0;
+  size_t totalNumWeakAlive = 0;
+  size_t totalNumWeakDead = 0;
+  nsTArray<SuspectObserver> suspectObservers;
+
+  for (auto iter = mObserverTopicTable.Iter(); !iter.Done(); iter.Next()) {
+    nsObserverList* observerList = iter.Get();
+    if (!observerList) {
+      continue;
+    }
+
+    size_t topicNumStrong = 0;
+    size_t topicNumWeakAlive = 0;
+    size_t topicNumWeakDead = 0;
+
+    nsMaybeWeakPtrArray<nsIObserver>& observers = observerList->mObservers;
+    for (uint32_t i = 0; i < observers.Length(); i++) {
+      if (observers[i].IsWeak()) {
+        nsCOMPtr<nsIObserver> ref = observers[i].GetValue();
+        if (ref) {
+          topicNumWeakAlive++;
+        } else {
+          topicNumWeakDead++;
+        }
+      } else {
+        topicNumStrong++;
+      }
+    }
+
+    totalNumStrong += topicNumStrong;
+    totalNumWeakAlive += topicNumWeakAlive;
+    totalNumWeakDead += topicNumWeakDead;
+
+    // Keep track of topics that have a suspiciously large number
+    // of referents (symptom of leaks).
+    size_t topicTotal = topicNumStrong + topicNumWeakAlive + topicNumWeakDead;
+    if (topicTotal > kSuspectReferentCount) {
+      SuspectObserver suspect(observerList->GetKey(), topicTotal);
+      suspectObservers.AppendElement(suspect);
+    }
+  }
+
+  // These aren't privacy-sensitive and so don't need anonymizing.
+  for (uint32_t i = 0; i < suspectObservers.Length(); i++) {
+    SuspectObserver& suspect = suspectObservers[i];
+    nsPrintfCString suspectPath("observer-service-suspect/referent(topic=%s)",
+                                suspect.mTopic);
+    aHandleReport->Callback(
+        /* process */ ""_ns, suspectPath, KIND_OTHER, UNITS_COUNT,
+        suspect.mReferentCount,
+        nsLiteralCString("A topic with a suspiciously large number of "
+                         "referents.  This may be symptomatic of a leak "
+                         "if the number of referents is high with "
+                         "respect to the number of windows."),
+        aData);
+  }
+
+  MOZ_COLLECT_REPORT(
+      "observer-service/referent/strong", KIND_OTHER, UNITS_COUNT,
+      totalNumStrong,
+      "The number of strong references held by the observer service.");
+
+  MOZ_COLLECT_REPORT(
+      "observer-service/referent/weak/alive", KIND_OTHER, UNITS_COUNT,
+      totalNumWeakAlive,
+      "The number of weak references held by the observer service that are "
+      "still alive.");
+
+  MOZ_COLLECT_REPORT(
+      "observer-service/referent/weak/dead", KIND_OTHER, UNITS_COUNT,
+      totalNumWeakDead,
+      "The number of weak references held by the observer service that are "
+      "dead.");
+
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// nsObserverService Implementation
+
+NS_IMPL_ISUPPORTS(nsObserverService, nsIObserverService, nsObserverService,
+                  nsIMemoryReporter)
+
+nsObserverService::nsObserverService() : mShuttingDown(false) {}
+
+nsObserverService::~nsObserverService(void) { Shutdown(); }
+
+void nsObserverService::RegisterReporter() { RegisterWeakMemoryReporter(this); }
+
+void nsObserverService::Shutdown() {
+  if (mShuttingDown) {
+    return;
+  }
+
+  mShuttingDown = true;
+  UnregisterWeakMemoryReporter(this);
+  mObserverTopicTable.Clear();
+}
+
+nsresult nsObserverService::Create(const nsIID& aIID, void** aInstancePtr) {
+  LOG(("nsObserverService::Create()"));
+
+  RefPtr<nsObserverService> os = new nsObserverService();
+
+  // The memory reporter can not be immediately registered here because
+  // the nsMemoryReporterManager may attempt to get the nsObserverService
+  // during initialization, causing a recursive GetService.
+  NS_DispatchToCurrentThread(
+      NewRunnableMethod("nsObserverService::RegisterReporter", os,
+                        &nsObserverService::RegisterReporter));
+
+  return os->QueryInterface(aIID, aInstancePtr);
+}
+
+nsresult nsObserverService::EnsureValidCall() const {
+  if (!NS_IsMainThread()) {
+    MOZ_CRASH("Using observer service off the main thread!");
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mShuttingDown) {
+    NS_ERROR("Using observer service after XPCOM shutdown!");
+    return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+  }
+
+  return NS_OK;
+}
+
+nsresult nsObserverService::FilterHttpOnTopics(const char* aTopic) {
+  // Specifically allow http-on-opening-request and http-on-stop-request in the
+  // child process; see bug 1269765.
+  if (mozilla::net::IsNeckoChild() && !strncmp(aTopic, "http-on-", 8) &&
+      strcmp(aTopic, "http-on-failed-opening-request") &&
+      strcmp(aTopic, "http-on-opening-request") &&
+      strcmp(aTopic, "http-on-stop-request") &&
+      strcmp(aTopic, "http-on-image-cache-response")) {
+    nsCOMPtr<nsIConsoleService> console(
+        do_GetService(NS_CONSOLESERVICE_CONTRACTID));
+    nsCOMPtr<nsIScriptError> error(
+        do_CreateInstance(NS_SCRIPTERROR_CONTRACTID));
+    error->Init(u"http-on-* observers only work in the parent process"_ns,
+                u""_ns, u""_ns, 0, 0, nsIScriptError::warningFlag,
+                "chrome javascript"_ns, false /* from private window */,
+                true /* from chrome context */);
+    console->LogMessage(error);
+
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsObserverService::AddObserver(nsIObserver* aObserver, const char* aTopic,
+                               bool aOwnsWeak) {
+  LOG(("nsObserverService::AddObserver(%p: %s, %s)", (void*)aObserver, aTopic,
+       aOwnsWeak ? "weak" : "strong"));
+
+  MOZ_TRY(EnsureValidCall());
+  if (NS_WARN_IF(!aObserver) || NS_WARN_IF(!aTopic)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  MOZ_TRY(FilterHttpOnTopics(aTopic));
+
+  nsObserverList* observerList = mObserverTopicTable.PutEntry(aTopic);
+  if (!observerList) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return observerList->AddObserver(aObserver, aOwnsWeak);
+}
+
+NS_IMETHODIMP
+nsObserverService::RemoveObserver(nsIObserver* aObserver, const char* aTopic) {
+  LOG(("nsObserverService::RemoveObserver(%p: %s)", (void*)aObserver, aTopic));
+
+  if (mShuttingDown) {
+    // The service is shutting down. Let's ignore this call.
+    return NS_OK;
+  }
+
+  MOZ_TRY(EnsureValidCall());
+  if (NS_WARN_IF(!aObserver) || NS_WARN_IF(!aTopic)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsObserverList* observerList = mObserverTopicTable.GetEntry(aTopic);
+  if (!observerList) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return observerList->RemoveObserver(aObserver);
+}
+
+NS_IMETHODIMP
+nsObserverService::EnumerateObservers(const char* aTopic,
+                                      nsISimpleEnumerator** anEnumerator) {
+  LOG(("nsObserverService::EnumerateObservers(%s)", aTopic));
+
+  MOZ_TRY(EnsureValidCall());
+  if (NS_WARN_IF(!anEnumerator) || NS_WARN_IF(!aTopic)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsObserverList* observerList = mObserverTopicTable.GetEntry(aTopic);
+  if (!observerList) {
+    return NS_NewEmptyEnumerator(anEnumerator);
+  }
+
+  observerList->GetObserverList(anEnumerator);
+  return NS_OK;
+}
+
+// Enumerate observers of aTopic and call Observe on each.
+NS_IMETHODIMP nsObserverService::NotifyObservers(nsISupports* aSubject,
+                                                 const char* aTopic,
+                                                 const char16_t* aSomeData) {
+  LOG(("nsObserverService::NotifyObservers(%s)", aTopic));
+
+  MOZ_TRY(EnsureValidCall());
+  if (NS_WARN_IF(!aTopic)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  MOZ_ASSERT(AppShutdown::IsNoOrLegalShutdownTopic(aTopic));
+
+  mozilla::TimeStamp start = TimeStamp::Now();
+
+  AUTO_PROFILER_MARKER_TEXT("NotifyObservers", OTHER, MarkerStack::Capture(),
+                            nsDependentCString(aTopic));
+  AUTO_PROFILER_LABEL_DYNAMIC_CSTR_NONSENSITIVE(
+      "nsObserverService::NotifyObservers", OTHER, aTopic);
+
+  nsObserverList* observerList = mObserverTopicTable.GetEntry(aTopic);
+  if (observerList) {
+    observerList->NotifyObservers(aSubject, aTopic, aSomeData);
+  }
+
+  uint32_t latencyMs = round((TimeStamp::Now() - start).ToMilliseconds());
+  if (latencyMs >= kMinTelemetryNotifyObserversLatencyMs) {
+    Telemetry::Accumulate(Telemetry::NOTIFY_OBSERVERS_LATENCY_MS,
+                          nsDependentCString(aTopic), latencyMs);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsObserverService::UnmarkGrayStrongObservers() {
+  MOZ_TRY(EnsureValidCall());
+
+  nsCOMArray<nsIObserver> strongObservers;
+  for (auto iter = mObserverTopicTable.Iter(); !iter.Done(); iter.Next()) {
+    nsObserverList* aObserverList = iter.Get();
+    if (aObserverList) {
+      aObserverList->AppendStrongObservers(strongObservers);
+    }
+  }
+
+  for (uint32_t i = 0; i < strongObservers.Length(); ++i) {
+    xpc_TryUnmarkWrappedGrayObject(strongObservers[i]);
+  }
+
+  return NS_OK;
+}
+
+bool nsObserverService::HasObservers(const char* aTopic) {
+  return mObserverTopicTable.Contains(aTopic);
+}
+
+namespace {
+
+class NotifyWhenScriptSafeRunnable : public mozilla::Runnable {
+ public:
+  NotifyWhenScriptSafeRunnable(nsIObserverService* aObs, nsISupports* aSubject,
+                               const char* aTopic, const char16_t* aData)
+      : mozilla::Runnable("NotifyWhenScriptSafeRunnable"),
+        mObs(aObs),
+        mSubject(aSubject),
+        mTopic(aTopic) {
+    if (aData) {
+      mData.Assign(aData);
+    } else {
+      mData.SetIsVoid(true);
+    }
+  }
+
+  NS_IMETHOD Run() override {
+    const char16_t* data = mData.IsVoid() ? nullptr : mData.get();
+    return mObs->NotifyObservers(mSubject, mTopic.get(), data);
+  }
+
+ private:
+  nsCOMPtr<nsIObserverService> mObs;
+  nsCOMPtr<nsISupports> mSubject;
+  nsCString mTopic;
+  nsString mData;
+};
+
+}  // namespace
+
+nsresult nsIObserverService::NotifyWhenScriptSafe(nsISupports* aSubject,
+                                                  const char* aTopic,
+                                                  const char16_t* aData) {
+  if (nsContentUtils::IsSafeToRunScript()) {
+    return NotifyObservers(aSubject, aTopic, aData);
+  }
+
+  nsContentUtils::AddScriptRunner(MakeAndAddRef<NotifyWhenScriptSafeRunnable>(
+      this, aSubject, aTopic, aData));
+  return NS_OK;
+}
diff --git a/xpcom/ds/nsObserverService.h b/xpcom/ds/nsObserverService.h
new file mode 100644
index 0000000000..f4e445995b
--- /dev/null
+++ b/xpcom/ds/nsObserverService.h
@@ -0,0 +1,56 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsObserverService_h___
+#define nsObserverService_h___
+
+#include "nsIObserverService.h"
+#include "nsObserverList.h"
+#include "nsIMemoryReporter.h"
+#include "nsTHashtable.h"
+#include "mozilla/Attributes.h"
+
+// {D07F5195-E3D1-11d2-8ACD-00105A1B8860}
+#define NS_OBSERVERSERVICE_CID                      \
+  {                                                 \
+    0xd07f5195, 0xe3d1, 0x11d2, {                   \
+      0x8a, 0xcd, 0x0, 0x10, 0x5a, 0x1b, 0x88, 0x60 \
+    }                                               \
+  }
+
+class nsObserverService final : public nsIObserverService,
+                                public nsIMemoryReporter {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_OBSERVERSERVICE_CID)
+
+  nsObserverService();
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVERSERVICE
+  NS_DECL_NSIMEMORYREPORTER
+
+  void Shutdown();
+
+  [[nodiscard]] static nsresult Create(const nsIID& aIID, void** aInstancePtr);
+
+  // Unmark any strongly held observers implemented in JS so the cycle
+  // collector will not traverse them.
+  NS_IMETHOD UnmarkGrayStrongObservers();
+
+ private:
+  ~nsObserverService(void);
+  void RegisterReporter();
+  nsresult EnsureValidCall() const;
+  nsresult FilterHttpOnTopics(const char* aTopic);
+
+  static const size_t kSuspectReferentCount = 100;
+  bool mShuttingDown;
+  nsTHashtable<nsObserverList> mObserverTopicTable;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsObserverService, NS_OBSERVERSERVICE_CID)
+
+#endif /* nsObserverService_h___ */
diff --git a/xpcom/ds/nsPersistentProperties.cpp b/xpcom/ds/nsPersistentProperties.cpp
new file mode 100644
index 0000000000..4e83870742
--- /dev/null
+++ b/xpcom/ds/nsPersistentProperties.cpp
@@ -0,0 +1,584 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsArrayEnumerator.h"
+#include "nsID.h"
+#include "nsCOMArray.h"
+#include "nsUnicharInputStream.h"
+#include "nsPrintfCString.h"
+
+#include "nsPersistentProperties.h"
+#include "nsIProperties.h"
+
+#include "mozilla/ArenaAllocatorExtensions.h"
+
+using mozilla::ArenaStrdup;
+
+struct PropertyTableEntry : public PLDHashEntryHdr {
+  // both of these are arena-allocated
+  const char* mKey;
+  const char16_t* mValue;
+};
+
+static const struct PLDHashTableOps property_HashTableOps = {
+    PLDHashTable::HashStringKey,
+    PLDHashTable::MatchStringKey,
+    PLDHashTable::MoveEntryStub,
+    PLDHashTable::ClearEntryStub,
+    nullptr,
+};
+
+//
+// parser stuff
+//
+enum EParserState {
+  eParserState_AwaitingKey,
+  eParserState_Key,
+  eParserState_AwaitingValue,
+  eParserState_Value,
+  eParserState_Comment
+};
+
+enum EParserSpecial {
+  eParserSpecial_None,     // not parsing a special character
+  eParserSpecial_Escaped,  // awaiting a special character
+  eParserSpecial_Unicode   // parsing a \Uxxx value
+};
+
+class MOZ_STACK_CLASS nsPropertiesParser {
+ public:
+  explicit nsPropertiesParser(nsIPersistentProperties* aProps)
+      : mUnicodeValuesRead(0),
+        mUnicodeValue(u'\0'),
+        mHaveMultiLine(false),
+        mMultiLineCanSkipN(false),
+        mMinLength(0),
+        mState(eParserState_AwaitingKey),
+        mSpecialState(eParserSpecial_None),
+        mProps(aProps) {}
+
+  void FinishValueState(nsAString& aOldValue) {
+    static const char trimThese[] = " \t";
+    mKey.Trim(trimThese, false, true);
+
+    // This is really ugly hack but it should be fast
+    char16_t backup_char;
+    uint32_t minLength = mMinLength;
+    if (minLength) {
+      backup_char = mValue[minLength - 1];
+      mValue.SetCharAt('x', minLength - 1);
+    }
+    mValue.Trim(trimThese, false, true);
+    if (minLength) {
+      mValue.SetCharAt(backup_char, minLength - 1);
+    }
+
+    mProps->SetStringProperty(NS_ConvertUTF16toUTF8(mKey), mValue, aOldValue);
+    mSpecialState = eParserSpecial_None;
+    WaitForKey();
+  }
+
+  EParserState GetState() { return mState; }
+
+  static nsresult SegmentWriter(nsIUnicharInputStream* aStream, void* aClosure,
+                                const char16_t* aFromSegment,
+                                uint32_t aToOffset, uint32_t aCount,
+                                uint32_t* aWriteCount);
+
+  nsresult ParseBuffer(const char16_t* aBuffer, uint32_t aBufferLength);
+
+ private:
+  bool ParseValueCharacter(
+      char16_t aChar,                // character that is just being parsed
+      const char16_t* aCur,          // pointer to character aChar in the buffer
+      const char16_t*& aTokenStart,  // string copying is done in blocks as big
+                                     // as possible, aTokenStart points to the
+                                     // beginning of this block
+      nsAString& aOldValue);  // when duplicate property is found, new value
+                              // is stored into hashtable and the old one is
+                              // placed in this variable
+
+  void WaitForKey() { mState = eParserState_AwaitingKey; }
+
+  void EnterKeyState() {
+    mKey.Truncate();
+    mState = eParserState_Key;
+  }
+
+  void WaitForValue() { mState = eParserState_AwaitingValue; }
+
+  void EnterValueState() {
+    mValue.Truncate();
+    mMinLength = 0;
+    mState = eParserState_Value;
+    mSpecialState = eParserSpecial_None;
+  }
+
+  void EnterCommentState() { mState = eParserState_Comment; }
+
+  nsAutoString mKey;
+  nsAutoString mValue;
+
+  uint32_t mUnicodeValuesRead;  // should be 4!
+  char16_t mUnicodeValue;       // currently parsed unicode value
+  bool mHaveMultiLine;          // is TRUE when last processed characters form
+                                // any of following sequences:
+                                //  - "\\\r"
+                                //  - "\\\n"
+                                //  - "\\\r\n"
+                                //  - any sequence above followed by any
+                                //    combination of ' ' and '\t'
+  bool mMultiLineCanSkipN;      // TRUE if "\\\r" was detected
+  uint32_t mMinLength;          // limit right trimming at the end to not trim
+                                // escaped whitespaces
+  EParserState mState;
+  // if we see a '\' then we enter this special state
+  EParserSpecial mSpecialState;
+  nsCOMPtr<nsIPersistentProperties> mProps;
+};
+
+inline bool IsWhiteSpace(char16_t aChar) {
+  return (aChar == ' ') || (aChar == '\t') || (aChar == '\r') ||
+         (aChar == '\n');
+}
+
+inline bool IsEOL(char16_t aChar) { return (aChar == '\r') || (aChar == '\n'); }
+
+bool nsPropertiesParser::ParseValueCharacter(char16_t aChar,
+                                             const char16_t* aCur,
+                                             const char16_t*& aTokenStart,
+                                             nsAString& aOldValue) {
+  switch (mSpecialState) {
+    // the normal state - look for special characters
+    case eParserSpecial_None:
+      switch (aChar) {
+        case '\\':
+          if (mHaveMultiLine) {
+            // there is nothing to append to mValue yet
+            mHaveMultiLine = false;
+          } else {
+            mValue += Substring(aTokenStart, aCur);
+          }
+
+          mSpecialState = eParserSpecial_Escaped;
+          break;
+
+        case '\n':
+          // if we detected multiline and got only "\\\r" ignore next "\n" if
+          // any
+          if (mHaveMultiLine && mMultiLineCanSkipN) {
+            // but don't allow another '\n' to be skipped
+            mMultiLineCanSkipN = false;
+            // Now there is nothing to append to the mValue since we are
+            // skipping whitespaces at the beginning of the new line of the
+            // multiline property. Set aTokenStart properly to ensure that
+            // nothing is appended if we find regular line-end or the end of the
+            // buffer.
+            aTokenStart = aCur + 1;
+            break;
+          }
+          [[fallthrough]];
+
+        case '\r':
+          // we're done! We have a key and value
+          mValue += Substring(aTokenStart, aCur);
+          FinishValueState(aOldValue);
+          mHaveMultiLine = false;
+          break;
+
+        default:
+          // there is nothing to do with normal characters,
+          // but handle multilines correctly
+          if (mHaveMultiLine) {
+            if (aChar == ' ' || aChar == '\t') {
+              // don't allow another '\n' to be skipped
+              mMultiLineCanSkipN = false;
+              // Now there is nothing to append to the mValue since we are
+              // skipping whitespaces at the beginning of the new line of the
+              // multiline property. Set aTokenStart properly to ensure that
+              // nothing is appended if we find regular line-end or the end of
+              // the buffer.
+              aTokenStart = aCur + 1;
+              break;
+            }
+            mHaveMultiLine = false;
+            aTokenStart = aCur;
+          }
+          break;  // from switch on (aChar)
+      }
+      break;  // from switch on (mSpecialState)
+
+    // saw a \ character, so parse the character after that
+    case eParserSpecial_Escaped:
+      // probably want to start parsing at the next token
+      // other characters, like 'u' might override this
+      aTokenStart = aCur + 1;
+      mSpecialState = eParserSpecial_None;
+
+      switch (aChar) {
+        // the easy characters - \t, \n, and so forth
+        case 't':
+          mValue += char16_t('\t');
+          mMinLength = mValue.Length();
+          break;
+        case 'n':
+          mValue += char16_t('\n');
+          mMinLength = mValue.Length();
+          break;
+        case 'r':
+          mValue += char16_t('\r');
+          mMinLength = mValue.Length();
+          break;
+        case '\\':
+          mValue += char16_t('\\');
+          break;
+
+        // switch to unicode mode!
+        case 'u':
+        case 'U':
+          mSpecialState = eParserSpecial_Unicode;
+          mUnicodeValuesRead = 0;
+          mUnicodeValue = 0;
+          break;
+
+        // a \ immediately followed by a newline means we're going multiline
+        case '\r':
+        case '\n':
+          mHaveMultiLine = true;
+          mMultiLineCanSkipN = (aChar == '\r');
+          mSpecialState = eParserSpecial_None;
+          break;
+
+        default:
+          // don't recognize the character, so just append it
+          mValue += aChar;
+          break;
+      }
+      break;
+
+    // we're in the middle of parsing a 4-character unicode value
+    // like \u5f39
+    case eParserSpecial_Unicode:
+      if ('0' <= aChar && aChar <= '9') {
+        mUnicodeValue = (mUnicodeValue << 4) | (aChar - '0');
+      } else if ('a' <= aChar && aChar <= 'f') {
+        mUnicodeValue = (mUnicodeValue << 4) | (aChar - 'a' + 0x0a);
+      } else if ('A' <= aChar && aChar <= 'F') {
+        mUnicodeValue = (mUnicodeValue << 4) | (aChar - 'A' + 0x0a);
+      } else {
+        // non-hex character. Append what we have, and move on.
+        mValue += mUnicodeValue;
+        mMinLength = mValue.Length();
+        mSpecialState = eParserSpecial_None;
+
+        // leave aTokenStart at this unknown character, so it gets appended
+        aTokenStart = aCur;
+
+        // ensure parsing this non-hex character again
+        return false;
+      }
+
+      if (++mUnicodeValuesRead >= 4) {
+        aTokenStart = aCur + 1;
+        mSpecialState = eParserSpecial_None;
+        mValue += mUnicodeValue;
+        mMinLength = mValue.Length();
+      }
+
+      break;
+  }
+
+  return true;
+}
+
+nsresult nsPropertiesParser::SegmentWriter(nsIUnicharInputStream* aStream,
+                                           void* aClosure,
+                                           const char16_t* aFromSegment,
+                                           uint32_t aToOffset, uint32_t aCount,
+                                           uint32_t* aWriteCount) {
+  nsPropertiesParser* parser = static_cast<nsPropertiesParser*>(aClosure);
+  parser->ParseBuffer(aFromSegment, aCount);
+
+  *aWriteCount = aCount;
+  return NS_OK;
+}
+
+nsresult nsPropertiesParser::ParseBuffer(const char16_t* aBuffer,
+                                         uint32_t aBufferLength) {
+  const char16_t* cur = aBuffer;
+  const char16_t* end = aBuffer + aBufferLength;
+
+  // points to the start/end of the current key or value
+  const char16_t* tokenStart = nullptr;
+
+  // if we're in the middle of parsing a key or value, make sure
+  // the current token points to the beginning of the current buffer
+  if (mState == eParserState_Key || mState == eParserState_Value) {
+    tokenStart = aBuffer;
+  }
+
+  nsAutoString oldValue;
+
+  while (cur != end) {
+    char16_t c = *cur;
+
+    switch (mState) {
+      case eParserState_AwaitingKey:
+        if (c == '#' || c == '!') {
+          EnterCommentState();
+        }
+
+        else if (!IsWhiteSpace(c)) {
+          // not a comment, not whitespace, we must have found a key!
+          EnterKeyState();
+          tokenStart = cur;
+        }
+        break;
+
+      case eParserState_Key:
+        if (c == '=' || c == ':') {
+          mKey += Substring(tokenStart, cur);
+          WaitForValue();
+        }
+        break;
+
+      case eParserState_AwaitingValue:
+        if (IsEOL(c)) {
+          // no value at all! mimic the normal value-ending
+          EnterValueState();
+          FinishValueState(oldValue);
+        }
+
+        // ignore white space leading up to the value
+        else if (!IsWhiteSpace(c)) {
+          tokenStart = cur;
+          EnterValueState();
+
+          // make sure to handle this first character
+          if (ParseValueCharacter(c, cur, tokenStart, oldValue)) {
+            cur++;
+          }
+          // If the character isn't consumed, don't do cur++ and parse
+          // the character again. This can happen f.e. for char 'X' in sequence
+          // "\u00X". This character can be control character and must be
+          // processed again.
+          continue;
+        }
+        break;
+
+      case eParserState_Value:
+        if (ParseValueCharacter(c, cur, tokenStart, oldValue)) {
+          cur++;
+        }
+        // See few lines above for reason of doing this
+        continue;
+
+      case eParserState_Comment:
+        // stay in this state till we hit EOL
+        if (c == '\r' || c == '\n') {
+          WaitForKey();
+        }
+        break;
+    }
+
+    // finally, advance to the next character
+    cur++;
+  }
+
+  // if we're still parsing the value and are in eParserSpecial_None, then
+  // append whatever we have..
+  if (mState == eParserState_Value && tokenStart &&
+      mSpecialState == eParserSpecial_None) {
+    mValue += Substring(tokenStart, cur);
+  }
+  // if we're still parsing the key, then append whatever we have..
+  else if (mState == eParserState_Key && tokenStart) {
+    mKey += Substring(tokenStart, cur);
+  }
+
+  return NS_OK;
+}
+
+nsPersistentProperties::nsPersistentProperties()
+    : mIn(nullptr),
+      mTable(&property_HashTableOps, sizeof(PropertyTableEntry), 16),
+      mArena() {}
+
+nsPersistentProperties::~nsPersistentProperties() = default;
+
+size_t nsPersistentProperties::SizeOfIncludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) {
+  // The memory used by mTable is accounted for in mArena.
+  size_t n = 0;
+  n += mArena.SizeOfExcludingThis(aMallocSizeOf);
+  n += mTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  return aMallocSizeOf(this) + n;
+}
+
+NS_IMPL_ISUPPORTS(nsPersistentProperties, nsIPersistentProperties,
+                  nsIProperties)
+
+NS_IMETHODIMP
+nsPersistentProperties::Load(nsIInputStream* aIn) {
+  nsresult rv = NS_NewUnicharInputStream(aIn, getter_AddRefs(mIn));
+
+  if (rv != NS_OK) {
+    NS_WARNING("Error creating UnicharInputStream");
+    return NS_ERROR_FAILURE;
+  }
+
+  nsPropertiesParser parser(this);
+
+  uint32_t nProcessed;
+  // If this 4096 is changed to some other value, make sure to adjust
+  // the bug121341.properties test file accordingly.
+  while (NS_SUCCEEDED(rv = mIn->ReadSegments(nsPropertiesParser::SegmentWriter,
+                                             &parser, 4096, &nProcessed)) &&
+         nProcessed != 0)
+    ;
+  mIn = nullptr;
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // We may have an unprocessed value at this point
+  // if the last line did not have a proper line ending.
+  if (parser.GetState() == eParserState_Value) {
+    nsAutoString oldValue;
+    parser.FinishValueState(oldValue);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::SetStringProperty(const nsACString& aKey,
+                                          const nsAString& aNewValue,
+                                          nsAString& aOldValue) {
+  const nsCString& flatKey = PromiseFlatCString(aKey);
+  auto entry = static_cast<PropertyTableEntry*>(mTable.Add(flatKey.get()));
+
+  if (entry->mKey) {
+    aOldValue = entry->mValue;
+    NS_WARNING(
+        nsPrintfCString("the property %s already exists", flatKey.get()).get());
+  } else {
+    aOldValue.Truncate();
+  }
+
+  entry->mKey = ArenaStrdup(flatKey, mArena);
+  entry->mValue = ArenaStrdup(aNewValue, mArena);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::Save(nsIOutputStream* aOut, const nsACString& aHeader) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::GetStringProperty(const nsACString& aKey,
+                                          nsAString& aValue) {
+  const nsCString& flatKey = PromiseFlatCString(aKey);
+
+  auto entry = static_cast<PropertyTableEntry*>(mTable.Search(flatKey.get()));
+  if (!entry) {
+    return NS_ERROR_FAILURE;
+  }
+
+  aValue = entry->mValue;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::Enumerate(nsISimpleEnumerator** aResult) {
+  nsCOMArray<nsIPropertyElement> props;
+
+  // We know the necessary size; we can avoid growing it while adding elements
+  props.SetCapacity(mTable.EntryCount());
+
+  // Step through hash entries populating a transient array
+  for (auto iter = mTable.Iter(); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<PropertyTableEntry*>(iter.Get());
+
+    RefPtr<nsPropertyElement> element = new nsPropertyElement(
+        nsDependentCString(entry->mKey), nsDependentString(entry->mValue));
+
+    if (!props.AppendObject(element)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+  }
+
+  return NS_NewArrayEnumerator(aResult, props, NS_GET_IID(nsIPropertyElement));
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// XXX Some day we'll unify the nsIPersistentProperties interface with
+// nsIProperties, but until now...
+
+NS_IMETHODIMP
+nsPersistentProperties::Get(const char* aProp, const nsIID& aUUID,
+                            void** aResult) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::Set(const char* aProp, nsISupports* value) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+NS_IMETHODIMP
+nsPersistentProperties::Undefine(const char* aProp) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::Has(const char* aProp, bool* aResult) {
+  *aResult = !!mTable.Search(aProp);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPersistentProperties::GetKeys(nsTArray<nsCString>& aKeys) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// PropertyElement
+////////////////////////////////////////////////////////////////////////////////
+
+nsresult nsPropertyElement::Create(REFNSIID aIID, void** aResult) {
+  RefPtr<nsPropertyElement> propElem = new nsPropertyElement();
+  return propElem->QueryInterface(aIID, aResult);
+}
+
+NS_IMPL_ISUPPORTS(nsPropertyElement, nsIPropertyElement)
+
+NS_IMETHODIMP
+nsPropertyElement::GetKey(nsACString& aReturnKey) {
+  aReturnKey = mKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPropertyElement::GetValue(nsAString& aReturnValue) {
+  aReturnValue = mValue;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPropertyElement::SetKey(const nsACString& aKey) {
+  mKey = aKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsPropertyElement::SetValue(const nsAString& aValue) {
+  mValue = aValue;
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
diff --git a/xpcom/ds/nsPersistentProperties.h b/xpcom/ds/nsPersistentProperties.h
new file mode 100644
index 0000000000..e35f938371
--- /dev/null
+++ b/xpcom/ds/nsPersistentProperties.h
@@ -0,0 +1,57 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsPersistentProperties_h___
+#define nsPersistentProperties_h___
+
+#include "nsIPersistentProperties2.h"
+#include "PLDHashTable.h"
+#include "nsString.h"
+#include "nsCOMPtr.h"
+#include "mozilla/ArenaAllocator.h"
+#include "mozilla/Attributes.h"
+
+class nsIUnicharInputStream;
+
+class nsPersistentProperties final : public nsIPersistentProperties {
+ public:
+  nsPersistentProperties();
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIPROPERTIES
+  NS_DECL_NSIPERSISTENTPROPERTIES
+
+ private:
+  ~nsPersistentProperties();
+
+ protected:
+  nsCOMPtr<nsIUnicharInputStream> mIn;
+
+  PLDHashTable mTable;
+  mozilla::ArenaAllocator<2048, 4> mArena;
+};
+
+class nsPropertyElement final : public nsIPropertyElement {
+ public:
+  nsPropertyElement() = default;
+
+  nsPropertyElement(const nsACString& aKey, const nsAString& aValue)
+      : mKey(aKey), mValue(aValue) {}
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIPROPERTYELEMENT
+
+  static nsresult Create(REFNSIID aIID, void** aResult);
+
+ private:
+  ~nsPropertyElement() = default;
+
+ protected:
+  nsCString mKey;
+  nsString mValue;
+};
+
+#endif /* nsPersistentProperties_h___ */
diff --git a/xpcom/ds/nsPointerHashKeys.h b/xpcom/ds/nsPointerHashKeys.h
new file mode 100644
index 0000000000..5cf2e8c4d3
--- /dev/null
+++ b/xpcom/ds/nsPointerHashKeys.h
@@ -0,0 +1,50 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Definitions for nsPtrHashKey<T> and nsVoidPtrHashKey.  */
+
+#ifndef nsPointerHashKeys_h
+#define nsPointerHashKeys_h
+
+#include "nscore.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/HashFunctions.h"
+
+#include "PLDHashTable.h"
+
+/**
+ * hashkey wrapper using T* KeyType
+ *
+ * @see nsTHashtable::EntryType for specification
+ */
+template <class T>
+class nsPtrHashKey : public PLDHashEntryHdr {
+ public:
+  typedef T* KeyType;
+  typedef const T* KeyTypePointer;
+
+  explicit nsPtrHashKey(const T* aKey) : mKey(const_cast<T*>(aKey)) {}
+  nsPtrHashKey(nsPtrHashKey<T>&& aToMove)
+      : PLDHashEntryHdr(std::move(aToMove)), mKey(std::move(aToMove.mKey)) {}
+  ~nsPtrHashKey() = default;
+
+  KeyType GetKey() const { return mKey; }
+  bool KeyEquals(KeyTypePointer aKey) const { return aKey == mKey; }
+
+  static KeyTypePointer KeyToPointer(KeyType aKey) { return aKey; }
+  static PLDHashNumber HashKey(KeyTypePointer aKey) {
+    return mozilla::HashGeneric(aKey);
+  }
+  enum { ALLOW_MEMMOVE = true };
+
+ protected:
+  T* MOZ_NON_OWNING_REF mKey;
+};
+
+typedef nsPtrHashKey<const void> nsVoidPtrHashKey;
+
+#endif  // nsPointerHashKeys_h
diff --git a/xpcom/ds/nsProperties.cpp b/xpcom/ds/nsProperties.cpp
new file mode 100644
index 0000000000..2094981634
--- /dev/null
+++ b/xpcom/ds/nsProperties.cpp
@@ -0,0 +1,61 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsProperties.h"
+
+////////////////////////////////////////////////////////////////////////////////
+
+NS_IMPL_ISUPPORTS(nsProperties, nsIProperties)
+
+NS_IMETHODIMP
+nsProperties::Get(const char* prop, const nsIID& uuid, void** result) {
+  if (NS_WARN_IF(!prop)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsCOMPtr<nsISupports> value;
+  if (!nsProperties_HashBase::Get(prop, getter_AddRefs(value))) {
+    return NS_ERROR_FAILURE;
+  }
+  return (value) ? value->QueryInterface(uuid, result) : NS_ERROR_NO_INTERFACE;
+}
+
+NS_IMETHODIMP
+nsProperties::Set(const char* prop, nsISupports* value) {
+  if (NS_WARN_IF(!prop)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  InsertOrUpdate(prop, value);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProperties::Undefine(const char* prop) {
+  if (NS_WARN_IF(!prop)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  return nsProperties_HashBase::Remove(prop) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsProperties::Has(const char* prop, bool* result) {
+  if (NS_WARN_IF(!prop)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *result = nsProperties_HashBase::Contains(prop);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProperties::GetKeys(nsTArray<nsCString>& aKeys) {
+  mozilla::AppendToArray(aKeys, this->Keys());
+
+  return NS_OK;
+}
+
+////////////////////////////////////////////////////////////////////////////////
diff --git a/xpcom/ds/nsProperties.h b/xpcom/ds/nsProperties.h
new file mode 100644
index 0000000000..c45c7361ea
--- /dev/null
+++ b/xpcom/ds/nsProperties.h
@@ -0,0 +1,29 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsProperties_h___
+#define nsProperties_h___
+
+#include "nsIProperties.h"
+#include "nsInterfaceHashtable.h"
+#include "nsHashKeys.h"
+#include "mozilla/Attributes.h"
+
+typedef nsInterfaceHashtable<nsCharPtrHashKey, nsISupports>
+    nsProperties_HashBase;
+
+class nsProperties final : public nsIProperties, public nsProperties_HashBase {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIPROPERTIES
+
+  nsProperties() = default;
+
+ private:
+  ~nsProperties() = default;
+};
+
+#endif /* nsProperties_h___ */
diff --git a/xpcom/ds/nsQuickSort.cpp b/xpcom/ds/nsQuickSort.cpp
new file mode 100644
index 0000000000..1fb3dede88
--- /dev/null
+++ b/xpcom/ds/nsQuickSort.cpp
@@ -0,0 +1,175 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+
+/*-
+ * Copyright (c) 1992, 1993
+ *	The Regents of the University of California.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/* We need this because Solaris' version of qsort is broken and
+ * causes array bounds reads.
+ */
+
+#include <stdlib.h>
+#include "nsAlgorithm.h"
+#include "nsQuickSort.h"
+
+extern "C" {
+
+#if !defined(DEBUG) && (defined(__cplusplus) || defined(__gcc))
+#  ifndef INLINE
+#    define INLINE inline
+#  endif
+#else
+#  define INLINE
+#endif
+
+typedef int cmp_t(const void*, const void*, void*);
+static INLINE char* med3(char*, char*, char*, cmp_t*, void*);
+static INLINE void swapfunc(char*, char*, int, int);
+
+/*
+ * Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
+ */
+#define swapcode(TYPE, parmi, parmj, n) \
+  {                                     \
+    long i = (n) / sizeof(TYPE);        \
+    TYPE* pi = (TYPE*)(parmi);          \
+    TYPE* pj = (TYPE*)(parmj);          \
+    do {                                \
+      TYPE t = *pi;                     \
+      *pi++ = *pj;                      \
+      *pj++ = t;                        \
+    } while (--i > 0);                  \
+  }
+
+#define SWAPINIT(a, es)                                                    \
+  swaptype = ((char*)a - (char*)0) % sizeof(long) || es % sizeof(long) ? 2 \
+             : es == sizeof(long)                                      ? 0 \
+                                                                       : 1;
+
+static INLINE void swapfunc(char* a, char* b, int n, int swaptype) {
+  if (swaptype <= 1) swapcode(long, a, b, n) else swapcode(char, a, b, n)
+}
+
+#define swap(a, b)             \
+  if (swaptype == 0) {         \
+    long t = *(long*)(a);      \
+    *(long*)(a) = *(long*)(b); \
+    *(long*)(b) = t;           \
+  } else                       \
+    swapfunc((char*)a, (char*)b, (int)es, swaptype)
+
+#define vecswap(a, b, n) \
+  if ((n) > 0) swapfunc((char*)a, (char*)b, (int)n, swaptype)
+
+static INLINE char* med3(char* a, char* b, char* c, cmp_t* cmp, void* data) {
+  return cmp(a, b, data) < 0
+             ? (cmp(b, c, data) < 0 ? b : (cmp(a, c, data) < 0 ? c : a))
+             : (cmp(b, c, data) > 0 ? b : (cmp(a, c, data) < 0 ? a : c));
+}
+
+void NS_QuickSort(void* a, unsigned int n, unsigned int es, cmp_t* cmp,
+                  void* data) {
+  char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
+  int d, r, swaptype;
+
+loop:
+  SWAPINIT(a, es);
+  /* Use insertion sort when input is small */
+  if (n < 7) {
+    for (pm = (char*)a + es; pm < (char*)a + n * es; pm += es)
+      for (pl = pm; pl > (char*)a && cmp(pl - es, pl, data) > 0; pl -= es)
+        swap(pl, pl - es);
+    return;
+  }
+  /* Choose pivot */
+  pm = (char*)a + (n / 2) * es;
+  if (n > 7) {
+    pl = (char*)a;
+    pn = (char*)a + (n - 1) * es;
+    if (n > 40) {
+      d = (n / 8) * es;
+      pl = med3(pl, pl + d, pl + 2 * d, cmp, data);
+      pm = med3(pm - d, pm, pm + d, cmp, data);
+      pn = med3(pn - 2 * d, pn - d, pn, cmp, data);
+    }
+    pm = med3(pl, pm, pn, cmp, data);
+  }
+  swap(a, pm);
+  pa = pb = (char*)a + es;
+
+  pc = pd = (char*)a + (n - 1) * es;
+  /* loop invariants:
+   * [a, pa) = pivot
+   * [pa, pb) < pivot
+   * [pb, pc + es) unprocessed
+   * [pc + es, pd + es) > pivot
+   * [pd + es, pn) = pivot
+   */
+  for (;;) {
+    while (pb <= pc && (r = cmp(pb, a, data)) <= 0) {
+      if (r == 0) {
+        swap(pa, pb);
+        pa += es;
+      }
+      pb += es;
+    }
+    while (pb <= pc && (r = cmp(pc, a, data)) >= 0) {
+      if (r == 0) {
+        swap(pc, pd);
+        pd -= es;
+      }
+      pc -= es;
+    }
+    if (pb > pc) break;
+    swap(pb, pc);
+    pb += es;
+    pc -= es;
+  }
+  /* Move pivot values */
+  pn = (char*)a + n * es;
+  r = XPCOM_MIN(pa - (char*)a, pb - pa);
+  vecswap(a, pb - r, r);
+  r = XPCOM_MIN<size_t>(pd - pc, pn - pd - es);
+  vecswap(pb, pn - r, r);
+  /* Recursively process partitioned items */
+  if ((r = pb - pa) > (int)es) NS_QuickSort(a, r / es, es, cmp, data);
+  if ((r = pd - pc) > (int)es) {
+    /* Iterate rather than recurse to save stack space */
+    a = pn - r;
+    n = r / es;
+    goto loop;
+  }
+  /*		NS_QuickSort(pn - r, r / es, es, cmp, data);*/
+}
+}
+
+#undef INLINE
+#undef swapcode
+#undef SWAPINIT
+#undef swap
+#undef vecswap
diff --git a/xpcom/ds/nsQuickSort.h b/xpcom/ds/nsQuickSort.h
new file mode 100644
index 0000000000..c2b842bea3
--- /dev/null
+++ b/xpcom/ds/nsQuickSort.h
@@ -0,0 +1,39 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* We need this because Solaris' version of qsort is broken and
+ * causes array bounds reads.
+ */
+
+#ifndef nsQuickSort_h___
+#define nsQuickSort_h___
+
+#include "nscore.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Parameters:
+ *  1. the array to sort
+ *  2. the number of elements in the array
+ *  3. the size of each array element
+ *  4. comparison function taking two elements and parameter #5 and
+ *     returning an integer:
+ *      + less than zero if the first element should be before the second
+ *      + 0 if the order of the elements does not matter
+ *      + greater than zero if the second element should be before the first
+ *  5. extra data to pass to comparison function
+ */
+void NS_QuickSort(void*, unsigned int, unsigned int,
+                  int (*)(const void*, const void*, void*), void*);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* nsQuickSort_h___ */
diff --git a/xpcom/ds/nsRefCountedHashtable.h b/xpcom/ds/nsRefCountedHashtable.h
new file mode 100644
index 0000000000..e3a0456a09
--- /dev/null
+++ b/xpcom/ds/nsRefCountedHashtable.h
@@ -0,0 +1,245 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef XPCOM_DS_NSREFCOUNTEDHASHTABLE_H_
+#define XPCOM_DS_NSREFCOUNTEDHASHTABLE_H_
+
+#include "nsBaseHashtable.h"
+#include "nsHashKeys.h"
+
+/**
+ * templated hashtable class maps keys to reference pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param PtrType the reference-type being wrapped
+ * @see nsClassHashtable, nsTHashMap
+ */
+template <class KeyClass, class PtrType>
+class nsRefCountedHashtable
+    : public nsBaseHashtable<
+          KeyClass, PtrType,
+          typename mozilla::detail::SmartPtrTraits<PtrType>::RawPointerType> {
+ public:
+  using KeyType = typename KeyClass::KeyType;
+  using SmartPtrTraits = mozilla::detail::SmartPtrTraits<PtrType>;
+  using PointeeType = typename SmartPtrTraits::PointeeType;
+  using RawPointerType = typename SmartPtrTraits::RawPointerType;
+  using base_type = nsBaseHashtable<KeyClass, PtrType, RawPointerType>;
+
+  using base_type::base_type;
+
+  static_assert(SmartPtrTraits::IsRefCounted);
+
+  /**
+   * @copydoc nsBaseHashtable::Get
+   * @param aData This is an XPCOM getter, so aData is already_addrefed.
+   *   If the key doesn't exist, *aData will be set to nullptr.
+   */
+  bool Get(KeyType aKey, RawPointerType* aData) const;
+
+  /**
+   * @copydoc nsBaseHashtable::Get
+   */
+  [[nodiscard]] already_AddRefed<PointeeType> Get(KeyType aKey) const;
+
+  /**
+   * Gets a weak reference to the hashtable entry.
+   * @param aFound If not nullptr, will be set to true if the entry is found,
+   *               to false otherwise.
+   * @return The entry, or nullptr if not found. Do not release this pointer!
+   */
+  [[nodiscard]] RawPointerType GetWeak(KeyType aKey,
+                                       bool* aFound = nullptr) const;
+
+  using base_type::InsertOrUpdate;
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_base_of_v<PointeeType, U>>>
+  void InsertOrUpdate(
+      KeyType aKey,
+      typename SmartPtrTraits::template OtherSmartPtrType<U>&& aData);
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_base_of_v<PointeeType, U>>>
+  [[nodiscard]] bool InsertOrUpdate(
+      KeyType aKey,
+      typename SmartPtrTraits::template OtherSmartPtrType<U>&& aData,
+      const mozilla::fallible_t&);
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_base_of_v<PointeeType, U>>>
+  void InsertOrUpdate(KeyType aKey, already_AddRefed<U>&& aData);
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_base_of_v<PointeeType, U>>>
+  [[nodiscard]] bool InsertOrUpdate(KeyType aKey, already_AddRefed<U>&& aData,
+                                    const mozilla::fallible_t&);
+
+  /**
+   * Remove the entry associated with aKey (if any), optionally _moving_ its
+   * current value into *aData, thereby avoiding calls to AddRef and Release.
+   * Return true if found.
+   * @param aKey the key to remove from the hashtable
+   * @param aData where to move the value (if non-null).  If an entry is not
+   *              found it will be set to nullptr.
+   * @return true if an entry for aKey was found (and removed)
+   */
+  inline bool Remove(KeyType aKey, RawPointerType* aData = nullptr);
+
+  nsRefCountedHashtable Clone() const {
+    return this->template CloneAs<nsRefCountedHashtable>();
+  }
+};
+
+template <typename K, typename T>
+inline void ImplCycleCollectionUnlink(nsRefCountedHashtable<K, T>& aField) {
+  aField.Clear();
+}
+
+template <typename K, typename T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsRefCountedHashtable<K, T>& aField, const char* aName,
+    uint32_t aFlags = 0) {
+  for (auto iter = aField.ConstIter(); !iter.Done(); iter.Next()) {
+    CycleCollectionNoteChild(aCallback, iter.UserData(), aName, aFlags);
+  }
+}
+
+//
+// nsRefCountedHashtable definitions
+//
+
+template <class KeyClass, class PtrType>
+bool nsRefCountedHashtable<KeyClass, PtrType>::Get(
+    KeyType aKey, RawPointerType* aRefPtr) const {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+
+  if (ent) {
+    if (aRefPtr) {
+      *aRefPtr = ent->GetData();
+
+      NS_IF_ADDREF(*aRefPtr);
+    }
+
+    return true;
+  }
+
+  // if the key doesn't exist, set *aRefPtr to null
+  // so that it is a valid XPCOM getter
+  if (aRefPtr) {
+    *aRefPtr = nullptr;
+  }
+
+  return false;
+}
+
+template <class KeyClass, class PtrType>
+already_AddRefed<typename nsRefCountedHashtable<KeyClass, PtrType>::PointeeType>
+nsRefCountedHashtable<KeyClass, PtrType>::Get(KeyType aKey) const {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+  if (!ent) {
+    return nullptr;
+  }
+
+  PtrType copy = ent->GetData();
+  return copy.forget();
+}
+
+template <class KeyClass, class PtrType>
+typename nsRefCountedHashtable<KeyClass, PtrType>::RawPointerType
+nsRefCountedHashtable<KeyClass, PtrType>::GetWeak(KeyType aKey,
+                                                  bool* aFound) const {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+
+  if (ent) {
+    if (aFound) {
+      *aFound = true;
+    }
+
+    return ent->GetData();
+  }
+
+  // Key does not exist, return nullptr and set aFound to false
+  if (aFound) {
+    *aFound = false;
+  }
+
+  return nullptr;
+}
+
+template <class KeyClass, class PtrType>
+template <typename U, typename>
+void nsRefCountedHashtable<KeyClass, PtrType>::InsertOrUpdate(
+    KeyType aKey,
+    typename SmartPtrTraits::template OtherSmartPtrType<U>&& aData) {
+  if (!InsertOrUpdate(aKey, std::move(aData), mozilla::fallible)) {
+    NS_ABORT_OOM(this->mTable.EntrySize() * this->mTable.EntryCount());
+  }
+}
+
+template <class KeyClass, class PtrType>
+template <typename U, typename>
+bool nsRefCountedHashtable<KeyClass, PtrType>::InsertOrUpdate(
+    KeyType aKey,
+    typename SmartPtrTraits::template OtherSmartPtrType<U>&& aData,
+    const mozilla::fallible_t&) {
+  typename base_type::EntryType* ent = this->PutEntry(aKey, mozilla::fallible);
+
+  if (!ent) {
+    return false;
+  }
+
+  ent->SetData(std::move(aData));
+
+  return true;
+}
+
+template <class KeyClass, class PtrType>
+template <typename U, typename>
+void nsRefCountedHashtable<KeyClass, PtrType>::InsertOrUpdate(
+    KeyType aKey, already_AddRefed<U>&& aData) {
+  if (!InsertOrUpdate(aKey, std::move(aData), mozilla::fallible)) {
+    NS_ABORT_OOM(this->mTable.EntrySize() * this->mTable.EntryCount());
+  }
+}
+
+template <class KeyClass, class PtrType>
+template <typename U, typename>
+bool nsRefCountedHashtable<KeyClass, PtrType>::InsertOrUpdate(
+    KeyType aKey, already_AddRefed<U>&& aData, const mozilla::fallible_t&) {
+  typename base_type::EntryType* ent = this->PutEntry(aKey, mozilla::fallible);
+
+  if (!ent) {
+    return false;
+  }
+
+  ent->SetData(std::move(aData));
+
+  return true;
+}
+
+template <class KeyClass, class PtrType>
+bool nsRefCountedHashtable<KeyClass, PtrType>::Remove(KeyType aKey,
+                                                      RawPointerType* aRefPtr) {
+  typename base_type::EntryType* ent = this->GetEntry(aKey);
+
+  if (ent) {
+    if (aRefPtr) {
+      ent->GetModifiableData()->forget(aRefPtr);
+    }
+    this->RemoveEntry(ent);
+    return true;
+  }
+
+  if (aRefPtr) {
+    *aRefPtr = nullptr;
+  }
+  return false;
+}
+
+#endif  // XPCOM_DS_NSREFCOUNTEDHASHTABLE_H_
diff --git a/xpcom/ds/nsRefPtrHashtable.h b/xpcom/ds/nsRefPtrHashtable.h
new file mode 100644
index 0000000000..55b946bf7b
--- /dev/null
+++ b/xpcom/ds/nsRefPtrHashtable.h
@@ -0,0 +1,12 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsRefPtrHashtable_h__
+#define nsRefPtrHashtable_h__
+
+#include "nsRefCountedHashtable.h"
+
+#endif  // nsRefPtrHashtable_h__
diff --git a/xpcom/ds/nsSimpleEnumerator.cpp b/xpcom/ds/nsSimpleEnumerator.cpp
new file mode 100644
index 0000000000..7f7ca9d674
--- /dev/null
+++ b/xpcom/ds/nsSimpleEnumerator.cpp
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsSimpleEnumerator.h"
+
+#include "mozilla/dom/IteratorResultBinding.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "mozilla/ResultExtensions.h"
+#include "nsContentUtils.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+namespace {
+
+class JSEnumerator final : public nsIJSEnumerator {
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIJSENUMERATOR
+
+  explicit JSEnumerator(nsISimpleEnumerator* aEnumerator, const nsID& aIID)
+      : mEnumerator(aEnumerator), mIID(aIID) {}
+
+ private:
+  ~JSEnumerator() = default;
+
+  nsCOMPtr<nsISimpleEnumerator> mEnumerator;
+  const nsID mIID;
+};
+
+}  // anonymous namespace
+
+nsresult JSEnumerator::Iterator(nsIJSEnumerator** aResult) {
+  RefPtr<JSEnumerator> result(this);
+  result.forget(aResult);
+  return NS_OK;
+}
+
+nsresult JSEnumerator::Next(JSContext* aCx, JS::MutableHandleValue aResult) {
+  RootedDictionary<IteratorResult> result(aCx);
+
+  nsCOMPtr<nsISupports> elem;
+  if (NS_FAILED(mEnumerator->GetNext(getter_AddRefs(elem)))) {
+    result.mDone = true;
+  } else {
+    result.mDone = false;
+
+    JS::RootedValue value(aCx);
+    MOZ_TRY(nsContentUtils::WrapNative(aCx, elem, &mIID, &value));
+    result.mValue = value;
+  }
+
+  if (!ToJSValue(aCx, result, aResult)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(JSEnumerator, nsIJSEnumerator)
+
+nsresult nsSimpleEnumerator::Iterator(nsIJSEnumerator** aResult) {
+  auto result = MakeRefPtr<JSEnumerator>(this, DefaultInterface());
+  result.forget(aResult);
+  return NS_OK;
+}
+
+nsresult nsSimpleEnumerator::Entries(const nsIID& aIface,
+                                     nsIJSEnumerator** aResult) {
+  auto result = MakeRefPtr<JSEnumerator>(this, aIface);
+  result.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(nsSimpleEnumerator, nsISimpleEnumerator,
+                  nsISimpleEnumeratorBase)
diff --git a/xpcom/ds/nsSimpleEnumerator.h b/xpcom/ds/nsSimpleEnumerator.h
new file mode 100644
index 0000000000..60ab6419bc
--- /dev/null
+++ b/xpcom/ds/nsSimpleEnumerator.h
@@ -0,0 +1,22 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsSimpleEnumerator_h
+#define nsSimpleEnumerator_h
+
+#include "nsISimpleEnumerator.h"
+
+class nsSimpleEnumerator : public nsISimpleEnumerator {
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISIMPLEENUMERATORBASE
+
+  virtual const nsID& DefaultInterface() { return NS_GET_IID(nsISupports); }
+
+ protected:
+  virtual ~nsSimpleEnumerator() = default;
+};
+
+#endif
diff --git a/xpcom/ds/nsStaticAtomUtils.h b/xpcom/ds/nsStaticAtomUtils.h
new file mode 100644
index 0000000000..8c766f63e1
--- /dev/null
+++ b/xpcom/ds/nsStaticAtomUtils.h
@@ -0,0 +1,36 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsStaticAtomUtils_h
+#define nsStaticAtomUtils_h
+
+#include <stdint.h>
+#include "nsAtom.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Maybe.h"
+
+// This class holds basic operations on arrays of static atoms.
+class nsStaticAtomUtils {
+ public:
+  static mozilla::Maybe<uint32_t> Lookup(nsAtom* aAtom,
+                                         const nsStaticAtom* aAtoms,
+                                         uint32_t aCount) {
+    if (aAtom->IsStatic()) {
+      ptrdiff_t index = aAtom->AsStatic() - aAtoms;
+      if (index >= 0 && index < static_cast<ptrdiff_t>(aCount)) {
+        return mozilla::Some(static_cast<uint32_t>(index));
+      }
+    }
+    return mozilla::Nothing();
+  }
+
+  static bool IsMember(nsAtom* aAtom, const nsStaticAtom* aAtoms,
+                       uint32_t aCount) {
+    return Lookup(aAtom, aAtoms, aCount).isSome();
+  }
+};
+
+#endif  // nsStaticAtomUtils_h
diff --git a/xpcom/ds/nsStaticNameTable.cpp b/xpcom/ds/nsStaticNameTable.cpp
new file mode 100644
index 0000000000..2ee731c153
--- /dev/null
+++ b/xpcom/ds/nsStaticNameTable.cpp
@@ -0,0 +1,180 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Class to manage lookup of static names in a table. */
+
+#include "mozilla/HashFunctions.h"
+#include "mozilla/TextUtils.h"
+
+#include "nsCRT.h"
+
+#include "nscore.h"
+#include "nsISupportsImpl.h"
+
+#include "nsStaticNameTable.h"
+
+using namespace mozilla;
+
+struct NameTableKey {
+  NameTableKey(const nsDependentCString aNameArray[], const nsCString* aKeyStr)
+      : mNameArray(aNameArray), mIsUnichar(false) {
+    mKeyStr.m1b = aKeyStr;
+  }
+
+  NameTableKey(const nsDependentCString aNameArray[], const nsString* aKeyStr)
+      : mNameArray(aNameArray), mIsUnichar(true) {
+    mKeyStr.m2b = aKeyStr;
+  }
+
+  const nsDependentCString* mNameArray;
+  union {
+    const nsCString* m1b;
+    const nsString* m2b;
+  } mKeyStr;
+  bool mIsUnichar;
+};
+
+struct NameTableEntry : public PLDHashEntryHdr {
+  int32_t mIndex;
+};
+
+static bool matchNameKeysCaseInsensitive(const PLDHashEntryHdr* aHdr,
+                                         const void* aVoidKey) {
+  auto entry = static_cast<const NameTableEntry*>(aHdr);
+  auto key = static_cast<const NameTableKey*>(aVoidKey);
+  const nsDependentCString* name = &key->mNameArray[entry->mIndex];
+
+  return key->mIsUnichar ? key->mKeyStr.m2b->LowerCaseEqualsASCII(
+                               name->get(), name->Length())
+                         : key->mKeyStr.m1b->LowerCaseEqualsASCII(
+                               name->get(), name->Length());
+}
+
+/*
+ * caseInsensitiveHashKey is just like PLDHashTable::HashStringKey except it
+ * uses (*s & ~0x20) instead of simply *s.  This means that "aFOO" and
+ * "afoo" and "aFoo" will all hash to the same thing.  It also means
+ * that some strings that aren't case-insensensitively equal will hash
+ * to the same value, but it's just a hash function so it doesn't
+ * matter.
+ */
+static PLDHashNumber caseInsensitiveStringHashKey(const void* aKey) {
+  PLDHashNumber h = 0;
+  const NameTableKey* tableKey = static_cast<const NameTableKey*>(aKey);
+  if (tableKey->mIsUnichar) {
+    for (const char16_t* s = tableKey->mKeyStr.m2b->get(); *s != '\0'; s++) {
+      h = AddToHash(h, *s & ~0x20);
+    }
+  } else {
+    for (const unsigned char* s = reinterpret_cast<const unsigned char*>(
+             tableKey->mKeyStr.m1b->get());
+         *s != '\0'; s++) {
+      h = AddToHash(h, *s & ~0x20);
+    }
+  }
+  return h;
+}
+
+static const struct PLDHashTableOps nametable_CaseInsensitiveHashTableOps = {
+    caseInsensitiveStringHashKey,
+    matchNameKeysCaseInsensitive,
+    PLDHashTable::MoveEntryStub,
+    PLDHashTable::ClearEntryStub,
+    nullptr,
+};
+
+nsStaticCaseInsensitiveNameTable::nsStaticCaseInsensitiveNameTable(
+    const char* const aNames[], int32_t aLength)
+    : mNameArray(nullptr),
+      mNameTable(&nametable_CaseInsensitiveHashTableOps, sizeof(NameTableEntry),
+                 aLength),
+      mNullStr("") {
+  MOZ_COUNT_CTOR(nsStaticCaseInsensitiveNameTable);
+
+  MOZ_ASSERT(aNames, "null name table");
+  MOZ_ASSERT(aLength, "0 length");
+
+  mNameArray =
+      (nsDependentCString*)moz_xmalloc(aLength * sizeof(nsDependentCString));
+
+  for (int32_t index = 0; index < aLength; ++index) {
+    const char* raw = aNames[index];
+#ifdef DEBUG
+    {
+      // verify invariants of contents
+      nsAutoCString temp1(raw);
+      nsDependentCString temp2(raw);
+      ToLowerCase(temp1);
+      MOZ_ASSERT(temp1.Equals(temp2), "upper case char in table");
+      MOZ_ASSERT(IsAsciiNullTerminated(raw),
+                 "non-ascii string in table -- "
+                 "case-insensitive matching won't work right");
+    }
+#endif
+    // use placement-new to initialize the string object
+    nsDependentCString* strPtr = &mNameArray[index];
+    new (strPtr) nsDependentCString(raw);
+
+    NameTableKey key(mNameArray, strPtr);
+
+    auto entry = static_cast<NameTableEntry*>(mNameTable.Add(&key, fallible));
+    if (!entry) {
+      continue;
+    }
+
+    // If the entry already exists it's unlikely but possible that its index is
+    // zero, in which case this assertion won't fail. But if the index is
+    // non-zero (highly likely) then it will fail. In other words, this
+    // assertion is likely but not guaranteed to detect if an entry is already
+    // used.
+    MOZ_ASSERT(entry->mIndex == 0, "Entry already exists!");
+
+    entry->mIndex = index;
+  }
+  mNameTable.MarkImmutable();
+}
+
+nsStaticCaseInsensitiveNameTable::~nsStaticCaseInsensitiveNameTable() {
+  // manually call the destructor on placement-new'ed objects
+  for (uint32_t index = 0; index < mNameTable.EntryCount(); index++) {
+    mNameArray[index].~nsDependentCString();
+  }
+  free((void*)mNameArray);
+  MOZ_COUNT_DTOR(nsStaticCaseInsensitiveNameTable);
+}
+
+int32_t nsStaticCaseInsensitiveNameTable::Lookup(
+    const nsACString& aName) const {
+  NS_ASSERTION(mNameArray, "not inited");
+
+  const nsCString& str = PromiseFlatCString(aName);
+
+  NameTableKey key(mNameArray, &str);
+  auto entry = static_cast<NameTableEntry*>(mNameTable.Search(&key));
+
+  return entry ? entry->mIndex : nsStaticCaseInsensitiveNameTable::NOT_FOUND;
+}
+
+int32_t nsStaticCaseInsensitiveNameTable::Lookup(const nsAString& aName) const {
+  NS_ASSERTION(mNameArray, "not inited");
+
+  const nsString& str = PromiseFlatString(aName);
+
+  NameTableKey key(mNameArray, &str);
+  auto entry = static_cast<NameTableEntry*>(mNameTable.Search(&key));
+
+  return entry ? entry->mIndex : nsStaticCaseInsensitiveNameTable::NOT_FOUND;
+}
+
+const nsCString& nsStaticCaseInsensitiveNameTable::GetStringValue(
+    int32_t aIndex) {
+  NS_ASSERTION(mNameArray, "not inited");
+
+  if ((NOT_FOUND < aIndex) && ((uint32_t)aIndex < mNameTable.EntryCount())) {
+    return mNameArray[aIndex];
+  }
+  return mNullStr;
+}
diff --git a/xpcom/ds/nsStaticNameTable.h b/xpcom/ds/nsStaticNameTable.h
new file mode 100644
index 0000000000..302d82c1e4
--- /dev/null
+++ b/xpcom/ds/nsStaticNameTable.h
@@ -0,0 +1,48 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Classes to manage lookup of static names in a table. */
+
+#ifndef nsStaticNameTable_h___
+#define nsStaticNameTable_h___
+
+#include "PLDHashTable.h"
+#include "nsString.h"
+
+/* This class supports case insensitive lookup.
+ *
+ * It differs from atom tables:
+ * - It supports case insensitive lookup.
+ * - It has minimal footprint by not copying the string table.
+ * - It does no locking.
+ * - It returns zero based indexes and const nsCString& as required by its
+ *   callers in the parser.
+ * - It is not an xpcom interface - meant for fast lookup in static tables.
+ *
+ * ***REQUIREMENTS***
+ * - It *requires* that all entries in the table be lowercase only.
+ * - It *requires* that the table of strings be in memory that lives at least
+ *    as long as this table object - typically a static string array.
+ */
+
+class nsStaticCaseInsensitiveNameTable {
+ public:
+  enum { NOT_FOUND = -1 };
+
+  int32_t Lookup(const nsACString& aName) const;
+  int32_t Lookup(const nsAString& aName) const;
+  const nsCString& GetStringValue(int32_t aIndex);
+
+  nsStaticCaseInsensitiveNameTable(const char* const aNames[], int32_t aLength);
+  ~nsStaticCaseInsensitiveNameTable();
+
+ private:
+  nsDependentCString* mNameArray;
+  PLDHashTable mNameTable;
+  nsDependentCString mNullStr;
+};
+
+#endif /* nsStaticNameTable_h___ */
diff --git a/xpcom/ds/nsStringEnumerator.cpp b/xpcom/ds/nsStringEnumerator.cpp
new file mode 100644
index 0000000000..7b8cf72d00
--- /dev/null
+++ b/xpcom/ds/nsStringEnumerator.cpp
@@ -0,0 +1,311 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsStringEnumerator.h"
+#include "nsSimpleEnumerator.h"
+#include "nsSupportsPrimitives.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/dom/IteratorResultBinding.h"
+#include "mozilla/dom/RootedDictionary.h"
+#include "mozilla/dom/ToJSValue.h"
+#include "nsTArray.h"
+
+using namespace mozilla;
+using namespace mozilla::dom;
+
+namespace {
+
+class JSStringEnumerator final : public nsIJSEnumerator {
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIJSENUMERATOR
+
+  explicit JSStringEnumerator(nsIStringEnumerator* aEnumerator)
+      : mEnumerator(aEnumerator) {
+    MOZ_ASSERT(mEnumerator);
+  }
+
+ private:
+  ~JSStringEnumerator() = default;
+
+  nsCOMPtr<nsIStringEnumerator> mEnumerator;
+};
+
+}  // anonymous namespace
+
+nsresult JSStringEnumerator::Iterator(nsIJSEnumerator** aResult) {
+  RefPtr<JSStringEnumerator> result(this);
+  result.forget(aResult);
+  return NS_OK;
+}
+
+nsresult JSStringEnumerator::Next(JSContext* aCx,
+                                  JS::MutableHandleValue aResult) {
+  RootedDictionary<IteratorResult> result(aCx);
+
+  nsAutoString elem;
+  if (NS_FAILED(mEnumerator->GetNext(elem))) {
+    result.mDone = true;
+  } else {
+    result.mDone = false;
+
+    if (!ToJSValue(
+            aCx, elem,
+            JS::MutableHandleValue::fromMarkedLocation(&result.mValue))) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+  }
+
+  if (!ToJSValue(aCx, result, aResult)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(JSStringEnumerator, nsIJSEnumerator)
+
+//
+// nsStringEnumeratorBase
+//
+
+nsresult nsStringEnumeratorBase::GetNext(nsAString& aResult) {
+  nsAutoCString str;
+  MOZ_TRY(GetNext(str));
+
+  CopyUTF8toUTF16(str, aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStringEnumeratorBase::StringIterator(nsIJSEnumerator** aRetVal) {
+  auto result = MakeRefPtr<JSStringEnumerator>(this);
+  result.forget(aRetVal);
+  return NS_OK;
+}
+
+//
+// nsStringEnumerator
+//
+
+class nsStringEnumerator final : public nsSimpleEnumerator,
+                                 public nsIStringEnumerator,
+                                 public nsIUTF8StringEnumerator {
+ public:
+  nsStringEnumerator(const nsTArray<nsString>* aArray, bool aOwnsArray)
+      : mArray(aArray), mIndex(0), mOwnsArray(aOwnsArray), mIsUnicode(true) {}
+
+  nsStringEnumerator(const nsTArray<nsCString>* aArray, bool aOwnsArray)
+      : mCArray(aArray), mIndex(0), mOwnsArray(aOwnsArray), mIsUnicode(false) {}
+
+  nsStringEnumerator(const nsTArray<nsString>* aArray, nsISupports* aOwner)
+      : mArray(aArray),
+        mIndex(0),
+        mOwner(aOwner),
+        mOwnsArray(false),
+        mIsUnicode(true) {}
+
+  nsStringEnumerator(const nsTArray<nsCString>* aArray, nsISupports* aOwner)
+      : mCArray(aArray),
+        mIndex(0),
+        mOwner(aOwner),
+        mOwnsArray(false),
+        mIsUnicode(false) {}
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIUTF8STRINGENUMERATOR
+  NS_DECL_NSISTRINGENUMERATORBASE
+
+  // have to declare nsIStringEnumerator manually, because of
+  // overlapping method names
+  NS_IMETHOD GetNext(nsAString& aResult) override;
+  NS_DECL_NSISIMPLEENUMERATOR
+
+  const nsID& DefaultInterface() override {
+    if (mIsUnicode) {
+      return NS_GET_IID(nsISupportsString);
+    }
+    return NS_GET_IID(nsISupportsCString);
+  }
+
+ private:
+  ~nsStringEnumerator() {
+    if (mOwnsArray) {
+      // const-casting is safe here, because the NS_New*
+      // constructors make sure mOwnsArray is consistent with
+      // the constness of the objects
+      if (mIsUnicode) {
+        delete const_cast<nsTArray<nsString>*>(mArray);
+      } else {
+        delete const_cast<nsTArray<nsCString>*>(mCArray);
+      }
+    }
+  }
+
+  union {
+    const nsTArray<nsString>* mArray;
+    const nsTArray<nsCString>* mCArray;
+  };
+
+  inline uint32_t Count() {
+    return mIsUnicode ? mArray->Length() : mCArray->Length();
+  }
+
+  uint32_t mIndex;
+
+  // the owner allows us to hold a strong reference to the object
+  // that owns the array. Having a non-null value in mOwner implies
+  // that mOwnsArray is false, because we rely on the real owner
+  // to release the array
+  nsCOMPtr<nsISupports> mOwner;
+  bool mOwnsArray;
+  bool mIsUnicode;
+};
+
+NS_IMPL_ISUPPORTS_INHERITED(nsStringEnumerator, nsSimpleEnumerator,
+                            nsIStringEnumerator, nsIUTF8StringEnumerator)
+
+NS_IMETHODIMP
+nsStringEnumerator::HasMore(bool* aResult) {
+  *aResult = mIndex < Count();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStringEnumerator::HasMoreElements(bool* aResult) { return HasMore(aResult); }
+
+NS_IMETHODIMP
+nsStringEnumerator::GetNext(nsISupports** aResult) {
+  if (mIndex >= mArray->Length()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (mIsUnicode) {
+    nsSupportsString* stringImpl = new nsSupportsString();
+
+    stringImpl->SetData(mArray->ElementAt(mIndex++));
+    *aResult = stringImpl;
+  } else {
+    nsSupportsCString* cstringImpl = new nsSupportsCString();
+
+    cstringImpl->SetData(mCArray->ElementAt(mIndex++));
+    *aResult = cstringImpl;
+  }
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStringEnumerator::GetNext(nsAString& aResult) {
+  if (NS_WARN_IF(mIndex >= Count())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mIsUnicode) {
+    aResult = mArray->ElementAt(mIndex++);
+  } else {
+    CopyUTF8toUTF16(mCArray->ElementAt(mIndex++), aResult);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStringEnumerator::GetNext(nsACString& aResult) {
+  if (NS_WARN_IF(mIndex >= Count())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (mIsUnicode) {
+    CopyUTF16toUTF8(mArray->ElementAt(mIndex++), aResult);
+  } else {
+    aResult = mCArray->ElementAt(mIndex++);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsStringEnumerator::StringIterator(nsIJSEnumerator** aRetVal) {
+  auto result = MakeRefPtr<JSStringEnumerator>(this);
+  result.forget(aRetVal);
+  return NS_OK;
+}
+
+template <class T>
+static inline nsresult StringEnumeratorTail(T** aResult) {
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+//
+// constructors
+//
+
+nsresult NS_NewStringEnumerator(nsIStringEnumerator** aResult,
+                                const nsTArray<nsString>* aArray,
+                                nsISupports* aOwner) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, aOwner);
+  return StringEnumeratorTail(aResult);
+}
+
+nsresult NS_NewUTF8StringEnumerator(nsIUTF8StringEnumerator** aResult,
+                                    const nsTArray<nsCString>* aArray,
+                                    nsISupports* aOwner) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, aOwner);
+  return StringEnumeratorTail(aResult);
+}
+
+nsresult NS_NewAdoptingStringEnumerator(nsIStringEnumerator** aResult,
+                                        nsTArray<nsString>* aArray) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, true);
+  return StringEnumeratorTail(aResult);
+}
+
+nsresult NS_NewAdoptingUTF8StringEnumerator(nsIUTF8StringEnumerator** aResult,
+                                            nsTArray<nsCString>* aArray) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, true);
+  return StringEnumeratorTail(aResult);
+}
+
+// const ones internally just forward to the non-const equivalents
+nsresult NS_NewStringEnumerator(nsIStringEnumerator** aResult,
+                                const nsTArray<nsString>* aArray) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, false);
+  return StringEnumeratorTail(aResult);
+}
+
+nsresult NS_NewUTF8StringEnumerator(nsIUTF8StringEnumerator** aResult,
+                                    const nsTArray<nsCString>* aArray) {
+  if (NS_WARN_IF(!aResult) || NS_WARN_IF(!aArray)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = new nsStringEnumerator(aArray, false);
+  return StringEnumeratorTail(aResult);
+}
diff --git a/xpcom/ds/nsStringEnumerator.h b/xpcom/ds/nsStringEnumerator.h
new file mode 100644
index 0000000000..0061227c34
--- /dev/null
+++ b/xpcom/ds/nsStringEnumerator.h
@@ -0,0 +1,102 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsStringEnumerator_h
+#define nsStringEnumerator_h
+
+#include "nsIStringEnumerator.h"
+#include "nsStringFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+class nsStringEnumeratorBase : public nsIStringEnumerator,
+                               public nsIUTF8StringEnumerator {
+ public:
+  NS_DECL_NSISTRINGENUMERATORBASE
+
+  NS_IMETHOD GetNext(nsAString&) override;
+
+  using nsIUTF8StringEnumerator::GetNext;
+
+ protected:
+  virtual ~nsStringEnumeratorBase() = default;
+};
+
+// nsIStringEnumerator/nsIUTF8StringEnumerator implementations
+//
+// Currently all implementations support both interfaces. The
+// constructors below provide the most common interface for the given
+// type (i.e. nsIStringEnumerator for char16_t* strings, and so
+// forth) but any resulting enumerators can be queried to the other
+// type. Internally, the enumerators will hold onto the type that was
+// passed in and do conversion if GetNext() for the other type of
+// string is called.
+
+// There are a few different types of enumerators:
+
+//
+// These enumerators hold a pointer to the array. Be careful
+// because modifying the array may confuse the iterator, especially if
+// you insert or remove elements in the middle of the array.
+//
+
+// The non-adopting enumerator requires that the array sticks around
+// at least as long as the enumerator does. These are for constant
+// string arrays that the enumerator does not own, this could be used
+// in VERY specialized cases such as when the provider KNOWS that the
+// string enumerator will be consumed immediately, or will at least
+// outlast the array.
+// For example:
+//
+// nsTArray<nsCString> array;
+// array.AppendCString("abc");
+// array.AppendCString("def");
+// NS_NewStringEnumerator(&enumerator, &array, true);
+//
+// // call some internal method which iterates the enumerator
+// InternalMethod(enumerator);
+// NS_RELEASE(enumerator);
+//
+[[nodiscard]] nsresult NS_NewStringEnumerator(nsIStringEnumerator** aResult,
+                                              const nsTArray<nsString>* aArray,
+                                              nsISupports* aOwner);
+[[nodiscard]] nsresult NS_NewUTF8StringEnumerator(
+    nsIUTF8StringEnumerator** aResult, const nsTArray<nsCString>* aArray);
+
+[[nodiscard]] nsresult NS_NewStringEnumerator(nsIStringEnumerator** aResult,
+                                              const nsTArray<nsString>* aArray);
+
+// Adopting string enumerators assume ownership of the array and will
+// call |operator delete| on the array when the enumerator is destroyed
+// this is useful when the provider creates an array solely for the
+// purpose of creating the enumerator.
+// For example:
+//
+// nsTArray<nsCString>* array = new nsTArray<nsCString>;
+// array->AppendString("abcd");
+// NS_NewAdoptingStringEnumerator(&result, array);
+[[nodiscard]] nsresult NS_NewAdoptingStringEnumerator(
+    nsIStringEnumerator** aResult, nsTArray<nsString>* aArray);
+
+[[nodiscard]] nsresult NS_NewAdoptingUTF8StringEnumerator(
+    nsIUTF8StringEnumerator** aResult, nsTArray<nsCString>* aArray);
+
+// these versions take a refcounted "owner" which will be addreffed
+// when the enumerator is created, and destroyed when the enumerator
+// is released. This allows providers to give non-owning pointers to
+// ns*StringArray member variables without worrying about lifetime
+// issues
+// For example:
+//
+// nsresult MyClass::Enumerate(nsIUTF8StringEnumerator** aResult) {
+//     mCategoryList->AppendString("abcd");
+//     return NS_NewStringEnumerator(aResult, mCategoryList, this);
+// }
+//
+[[nodiscard]] nsresult NS_NewUTF8StringEnumerator(
+    nsIUTF8StringEnumerator** aResult, const nsTArray<nsCString>* aArray,
+    nsISupports* aOwner);
+
+#endif  // defined nsStringEnumerator_h
diff --git a/xpcom/ds/nsSupportsPrimitives.cpp b/xpcom/ds/nsSupportsPrimitives.cpp
new file mode 100644
index 0000000000..1d0db73187
--- /dev/null
+++ b/xpcom/ds/nsSupportsPrimitives.cpp
@@ -0,0 +1,603 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsSupportsPrimitives.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/Sprintf.h"
+#include <algorithm>
+
+template <typename T>
+static char* DataToString(const char* aFormat, T aData) {
+  static const int size = 32;
+  char buf[size];
+
+  SprintfLiteral(buf, aFormat, aData);
+
+  return moz_xstrdup(buf);
+}
+
+/*****************************************************************************
+ * nsSupportsCString
+ *****************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsCString, nsISupportsCString, nsISupportsPrimitive)
+
+NS_IMETHODIMP
+nsSupportsCString::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_CSTRING;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsCString::GetData(nsACString& aData) {
+  aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsCString::ToString(char** aResult) {
+  *aResult = ToNewCString(mData, mozilla::fallible);
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsCString::SetData(const nsACString& aData) {
+  bool ok = mData.Assign(aData, mozilla::fallible);
+  if (!ok) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+/*****************************************************************************
+ * nsSupportsString
+ *****************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsString, nsISupportsString, nsISupportsPrimitive)
+
+NS_IMETHODIMP
+nsSupportsString::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_STRING;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsString::GetData(nsAString& aData) {
+  aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsString::ToString(char16_t** aResult) {
+  *aResult = ToNewUnicode(mData, mozilla::fallible);
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsString::SetData(const nsAString& aData) {
+  bool ok = mData.Assign(aData, mozilla::fallible);
+  if (!ok) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRBool, nsISupportsPRBool, nsISupportsPrimitive)
+
+nsSupportsPRBool::nsSupportsPRBool() : mData(false) {}
+
+NS_IMETHODIMP
+nsSupportsPRBool::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRBOOL;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRBool::GetData(bool* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRBool::SetData(bool aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRBool::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = moz_xstrdup(mData ? "true" : "false");
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRUint8, nsISupportsPRUint8, nsISupportsPrimitive)
+
+nsSupportsPRUint8::nsSupportsPRUint8() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRUint8::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRUINT8;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint8::GetData(uint8_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint8::SetData(uint8_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint8::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%u", static_cast<unsigned int>(mData));
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRUint16, nsISupportsPRUint16, nsISupportsPrimitive)
+
+nsSupportsPRUint16::nsSupportsPRUint16() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRUint16::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRUINT16;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint16::GetData(uint16_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint16::SetData(uint16_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint16::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%u", static_cast<unsigned int>(mData));
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRUint32, nsISupportsPRUint32, nsISupportsPrimitive)
+
+nsSupportsPRUint32::nsSupportsPRUint32() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRUint32::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRUINT32;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint32::GetData(uint32_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint32::SetData(uint32_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint32::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%u", mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRUint64, nsISupportsPRUint64, nsISupportsPrimitive)
+
+nsSupportsPRUint64::nsSupportsPRUint64() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRUint64::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRUINT64;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint64::GetData(uint64_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint64::SetData(uint64_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRUint64::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%llu", mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRTime, nsISupportsPRTime, nsISupportsPrimitive)
+
+nsSupportsPRTime::nsSupportsPRTime() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRTime::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRTIME;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRTime::GetData(PRTime* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRTime::SetData(PRTime aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRTime::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%" PRIu64, mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsChar, nsISupportsChar, nsISupportsPrimitive)
+
+nsSupportsChar::nsSupportsChar() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsChar::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_CHAR;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsChar::GetData(char* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsChar::SetData(char aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsChar::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = static_cast<char*>(moz_xmalloc(2 * sizeof(char)));
+  *aResult[0] = mData;
+  *aResult[1] = '\0';
+
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRInt16, nsISupportsPRInt16, nsISupportsPrimitive)
+
+nsSupportsPRInt16::nsSupportsPRInt16() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRInt16::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRINT16;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt16::GetData(int16_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt16::SetData(int16_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt16::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%d", static_cast<int>(mData));
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRInt32, nsISupportsPRInt32, nsISupportsPrimitive)
+
+nsSupportsPRInt32::nsSupportsPRInt32() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRInt32::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRINT32;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt32::GetData(int32_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt32::SetData(int32_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt32::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%d", mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsPRInt64, nsISupportsPRInt64, nsISupportsPrimitive)
+
+nsSupportsPRInt64::nsSupportsPRInt64() : mData(0) {}
+
+NS_IMETHODIMP
+nsSupportsPRInt64::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_PRINT64;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt64::GetData(int64_t* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt64::SetData(int64_t aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsPRInt64::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%" PRId64, mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsFloat, nsISupportsFloat, nsISupportsPrimitive)
+
+nsSupportsFloat::nsSupportsFloat() : mData(float(0.0)) {}
+
+NS_IMETHODIMP
+nsSupportsFloat::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_FLOAT;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsFloat::GetData(float* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsFloat::SetData(float aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsFloat::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%f", static_cast<double>(mData));
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsDouble, nsISupportsDouble, nsISupportsPrimitive)
+
+nsSupportsDouble::nsSupportsDouble() : mData(double(0.0)) {}
+
+NS_IMETHODIMP
+nsSupportsDouble::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_DOUBLE;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDouble::GetData(double* aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDouble::SetData(double aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDouble::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+  *aResult = DataToString("%f", mData);
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsInterfacePointer, nsISupportsInterfacePointer,
+                  nsISupportsPrimitive)
+
+nsSupportsInterfacePointer::nsSupportsInterfacePointer() : mIID(nullptr) {}
+
+nsSupportsInterfacePointer::~nsSupportsInterfacePointer() {
+  if (mIID) {
+    free(mIID);
+  }
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::GetType(uint16_t* aType) {
+  NS_ASSERTION(aType, "Bad pointer");
+  *aType = TYPE_INTERFACE_POINTER;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::GetData(nsISupports** aData) {
+  NS_ASSERTION(aData, "Bad pointer");
+  *aData = mData;
+  NS_IF_ADDREF(*aData);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::SetData(nsISupports* aData) {
+  mData = aData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::GetDataIID(nsID** aIID) {
+  NS_ASSERTION(aIID, "Bad pointer");
+
+  *aIID = mIID ? mIID->Clone() : nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::SetDataIID(const nsID* aIID) {
+  if (mIID) {
+    free(mIID);
+  }
+
+  mIID = aIID ? aIID->Clone() : nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsInterfacePointer::ToString(char** aResult) {
+  NS_ASSERTION(aResult, "Bad pointer");
+
+  // jband sez: think about asking nsIInterfaceInfoManager whether
+  // the interface has a known human-readable name
+  *aResult = moz_xstrdup("[interface pointer]");
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+NS_IMPL_ISUPPORTS(nsSupportsDependentCString, nsISupportsCString,
+                  nsISupportsPrimitive)
+
+nsSupportsDependentCString::nsSupportsDependentCString(const char* aStr)
+    : mData(aStr) {}
+
+NS_IMETHODIMP
+nsSupportsDependentCString::GetType(uint16_t* aType) {
+  if (NS_WARN_IF(!aType)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aType = TYPE_CSTRING;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDependentCString::GetData(nsACString& aData) {
+  aData = mData;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDependentCString::ToString(char** aResult) {
+  if (NS_WARN_IF(!aResult)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  *aResult = ToNewCString(mData, mozilla::fallible);
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsSupportsDependentCString::SetData(const nsACString& aData) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
diff --git a/xpcom/ds/nsSupportsPrimitives.h b/xpcom/ds/nsSupportsPrimitives.h
new file mode 100644
index 0000000000..28d768a135
--- /dev/null
+++ b/xpcom/ds/nsSupportsPrimitives.h
@@ -0,0 +1,279 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsSupportsPrimitives_h__
+#define nsSupportsPrimitives_h__
+
+#include "mozilla/Attributes.h"
+
+#include "nsISupportsPrimitives.h"
+#include "nsCOMPtr.h"
+#include "nsString.h"
+
+/***************************************************************************/
+
+class nsSupportsCString final : public nsISupportsCString {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSCSTRING
+
+  nsSupportsCString() = default;
+
+ private:
+  ~nsSupportsCString() = default;
+
+  nsCString mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsString final : public nsISupportsString {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSSTRING
+
+  nsSupportsString() = default;
+
+ private:
+  ~nsSupportsString() = default;
+
+  nsString mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRBool final : public nsISupportsPRBool {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRBOOL
+
+  nsSupportsPRBool();
+
+ private:
+  ~nsSupportsPRBool() = default;
+
+  bool mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRUint8 final : public nsISupportsPRUint8 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRUINT8
+
+  nsSupportsPRUint8();
+
+ private:
+  ~nsSupportsPRUint8() = default;
+
+  uint8_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRUint16 final : public nsISupportsPRUint16 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRUINT16
+
+  nsSupportsPRUint16();
+
+ private:
+  ~nsSupportsPRUint16() = default;
+
+  uint16_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRUint32 final : public nsISupportsPRUint32 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRUINT32
+
+  nsSupportsPRUint32();
+
+ private:
+  ~nsSupportsPRUint32() = default;
+
+  uint32_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRUint64 final : public nsISupportsPRUint64 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRUINT64
+
+  nsSupportsPRUint64();
+
+ private:
+  ~nsSupportsPRUint64() = default;
+
+  uint64_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRTime final : public nsISupportsPRTime {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRTIME
+
+  nsSupportsPRTime();
+
+ private:
+  ~nsSupportsPRTime() = default;
+
+  PRTime mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsChar final : public nsISupportsChar {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSCHAR
+
+  nsSupportsChar();
+
+ private:
+  ~nsSupportsChar() = default;
+
+  char mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRInt16 final : public nsISupportsPRInt16 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRINT16
+
+  nsSupportsPRInt16();
+
+ private:
+  ~nsSupportsPRInt16() = default;
+
+  int16_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRInt32 final : public nsISupportsPRInt32 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRINT32
+
+  nsSupportsPRInt32();
+
+ private:
+  ~nsSupportsPRInt32() = default;
+
+  int32_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsPRInt64 final : public nsISupportsPRInt64 {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSPRINT64
+
+  nsSupportsPRInt64();
+
+ private:
+  ~nsSupportsPRInt64() = default;
+
+  int64_t mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsFloat final : public nsISupportsFloat {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSFLOAT
+
+  nsSupportsFloat();
+
+ private:
+  ~nsSupportsFloat() = default;
+
+  float mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsDouble final : public nsISupportsDouble {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSDOUBLE
+
+  nsSupportsDouble();
+
+ private:
+  ~nsSupportsDouble() = default;
+
+  double mData;
+};
+
+/***************************************************************************/
+
+class nsSupportsInterfacePointer final : public nsISupportsInterfacePointer {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSINTERFACEPOINTER
+
+  nsSupportsInterfacePointer();
+
+ private:
+  ~nsSupportsInterfacePointer();
+
+  nsCOMPtr<nsISupports> mData;
+  nsID* mIID;
+};
+
+/***************************************************************************/
+
+/**
+ * Wraps a static const char* buffer for use with nsISupportsCString
+ *
+ * Only use this class with static buffers, or arena-allocated buffers of
+ * permanent lifetime!
+ */
+class nsSupportsDependentCString final : public nsISupportsCString {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSISUPPORTSPRIMITIVE
+  NS_DECL_NSISUPPORTSCSTRING
+
+  explicit nsSupportsDependentCString(const char* aStr);
+
+ private:
+  ~nsSupportsDependentCString() = default;
+
+  nsDependentCString mData;
+};
+
+#endif /* nsSupportsPrimitives_h__ */
diff --git a/xpcom/ds/nsTArray-inl.h b/xpcom/ds/nsTArray-inl.h
new file mode 100644
index 0000000000..801d3fab01
--- /dev/null
+++ b/xpcom/ds/nsTArray-inl.h
@@ -0,0 +1,691 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTArray_h__
+#  error "Don't include this file directly"
+#endif
+
+// NOTE: We don't use MOZ_COUNT_CTOR/MOZ_COUNT_DTOR to perform leak checking of
+// nsTArray_base objects intentionally for the following reasons:
+// * The leak logging isn't as useful as other types of logging, as
+//   nsTArray_base is frequently relocated without invoking a constructor, such
+//   as when stored within another nsTArray. This means that
+//   XPCOM_MEM_LOG_CLASSES cannot be used to identify specific leaks of nsTArray
+//   objects.
+// * The nsTArray type is layout compatible with the ThinVec crate with the
+//   correct flags, and ThinVec does not currently perform leak logging.
+//   This means that if a large number of arrays are transferred between Rust
+//   and C++ code using ThinVec, for example within another ThinVec, they
+//   will not be logged correctly and might appear as e.g. negative leaks.
+// * Leaks which have been found thanks to the leak logging added by this
+//   type have often not been significant, and/or have needed to be
+//   circumvented using some other mechanism. Most leaks found with this type
+//   in them also include other types which will continue to be tracked.
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc, RelocationStrategy>::nsTArray_base() : mHdr(EmptyHdr()) {}
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc, RelocationStrategy>::~nsTArray_base() {
+  if (!HasEmptyHeader() && !UsesAutoArrayBuffer()) {
+    Alloc::Free(mHdr);
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc, RelocationStrategy>::nsTArray_base(const nsTArray_base&)
+    : mHdr(EmptyHdr()) {
+  // Actual copying happens through nsTArray_CopyEnabler, we just need to do the
+  // initialization of mHdr.
+}
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc, RelocationStrategy>&
+nsTArray_base<Alloc, RelocationStrategy>::operator=(const nsTArray_base&) {
+  // Actual copying happens through nsTArray_CopyEnabler, so do nothing here (do
+  // not copy mHdr).
+  return *this;
+}
+
+template <class Alloc, class RelocationStrategy>
+const nsTArrayHeader*
+nsTArray_base<Alloc, RelocationStrategy>::GetAutoArrayBufferUnsafe(
+    size_t aElemAlign) const {
+  // Assuming |this| points to an nsAutoArray, we want to get a pointer to
+  // mAutoBuf.  So just cast |this| to nsAutoArray* and read &mAutoBuf!
+
+  const void* autoBuf =
+      &reinterpret_cast<const AutoTArray<nsTArray<uint32_t>, 1>*>(this)
+           ->mAutoBuf;
+
+  // If we're on a 32-bit system and aElemAlign is 8, we need to adjust our
+  // pointer to take into account the extra alignment in the auto array.
+
+  static_assert(
+      sizeof(void*) != 4 || (MOZ_ALIGNOF(mozilla::AlignedElem<8>) == 8 &&
+                             sizeof(AutoTArray<mozilla::AlignedElem<8>, 1>) ==
+                                 sizeof(void*) + sizeof(nsTArrayHeader) + 4 +
+                                     sizeof(mozilla::AlignedElem<8>)),
+      "auto array padding wasn't what we expected");
+
+  // We don't support alignments greater than 8 bytes.
+  MOZ_ASSERT(aElemAlign <= 4 || aElemAlign == 8, "unsupported alignment.");
+  if (sizeof(void*) == 4 && aElemAlign == 8) {
+    autoBuf = reinterpret_cast<const char*>(autoBuf) + 4;
+  }
+
+  return reinterpret_cast<const Header*>(autoBuf);
+}
+
+template <class Alloc, class RelocationStrategy>
+bool nsTArray_base<Alloc, RelocationStrategy>::UsesAutoArrayBuffer() const {
+  if (!mHdr->mIsAutoArray) {
+    return false;
+  }
+
+  // This is nuts.  If we were sane, we'd pass aElemAlign as a parameter to
+  // this function.  Unfortunately this function is called in nsTArray_base's
+  // destructor, at which point we don't know value_type's alignment.
+  //
+  // We'll fall on our face and return true when we should say false if
+  //
+  //   * we're not using our auto buffer,
+  //   * aElemAlign == 4, and
+  //   * mHdr == GetAutoArrayBuffer(8).
+  //
+  // This could happen if |*this| lives on the heap and malloc allocated our
+  // buffer on the heap adjacent to |*this|.
+  //
+  // However, we can show that this can't happen.  If |this| is an auto array
+  // (as we ensured at the beginning of the method), GetAutoArrayBuffer(8)
+  // always points to memory owned by |*this|, because (as we assert below)
+  //
+  //   * GetAutoArrayBuffer(8) is at most 4 bytes past GetAutoArrayBuffer(4),
+  //     and
+  //   * sizeof(nsTArrayHeader) > 4.
+  //
+  // Since AutoTArray always contains an nsTArrayHeader,
+  // GetAutoArrayBuffer(8) will always point inside the auto array object,
+  // even if it doesn't point at the beginning of the header.
+  //
+  // Note that this means that we can't store elements with alignment 16 in an
+  // nsTArray, because GetAutoArrayBuffer(16) could lie outside the memory
+  // owned by this AutoTArray.  We statically assert that value_type's
+  // alignment is 8 bytes or less in AutoTArray.
+
+  static_assert(sizeof(nsTArrayHeader) > 4, "see comment above");
+
+#ifdef DEBUG
+  ptrdiff_t diff = reinterpret_cast<const char*>(GetAutoArrayBuffer(8)) -
+                   reinterpret_cast<const char*>(GetAutoArrayBuffer(4));
+  MOZ_ASSERT(diff >= 0 && diff <= 4,
+             "GetAutoArrayBuffer doesn't do what we expect.");
+#endif
+
+  return mHdr == GetAutoArrayBuffer(4) || mHdr == GetAutoArrayBuffer(8);
+}
+
+// defined in nsTArray.cpp
+bool IsTwiceTheRequiredBytesRepresentableAsUint32(size_t aCapacity,
+                                                  size_t aElemSize);
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+typename ActualAlloc::ResultTypeProxy
+nsTArray_base<Alloc, RelocationStrategy>::ExtendCapacity(size_type aLength,
+                                                         size_type aCount,
+                                                         size_type aElemSize) {
+  mozilla::CheckedInt<size_type> newLength = aLength;
+  newLength += aCount;
+
+  if (!newLength.isValid()) {
+    return ActualAlloc::FailureResult();
+  }
+
+  return this->EnsureCapacity<ActualAlloc>(newLength.value(), aElemSize);
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+typename ActualAlloc::ResultTypeProxy
+nsTArray_base<Alloc, RelocationStrategy>::EnsureCapacity(size_type aCapacity,
+                                                         size_type aElemSize) {
+  // This should be the most common case so test this first
+  if (aCapacity <= mHdr->mCapacity) {
+    return ActualAlloc::SuccessResult();
+  }
+
+  // If the requested memory allocation exceeds size_type(-1)/2, then
+  // our doubling algorithm may not be able to allocate it.
+  // Additionally, if it exceeds uint32_t(-1) then we couldn't fit in the
+  // Header::mCapacity member. Just bail out in cases like that.  We don't want
+  // to be allocating 2 GB+ arrays anyway.
+  if (!IsTwiceTheRequiredBytesRepresentableAsUint32(aCapacity, aElemSize)) {
+    ActualAlloc::SizeTooBig((size_t)aCapacity * aElemSize);
+    return ActualAlloc::FailureResult();
+  }
+
+  size_t reqSize = sizeof(Header) + aCapacity * aElemSize;
+
+  if (HasEmptyHeader()) {
+    // Malloc() new data
+    Header* header = static_cast<Header*>(ActualAlloc::Malloc(reqSize));
+    if (!header) {
+      return ActualAlloc::FailureResult();
+    }
+    header->mLength = 0;
+    header->mCapacity = aCapacity;
+    header->mIsAutoArray = 0;
+    mHdr = header;
+
+    return ActualAlloc::SuccessResult();
+  }
+
+  // We increase our capacity so that the allocated buffer grows exponentially,
+  // which gives us amortized O(1) appending. Below the threshold, we use
+  // powers-of-two. Above the threshold, we grow by at least 1.125, rounding up
+  // to the nearest MiB.
+  const size_t slowGrowthThreshold = 8 * 1024 * 1024;
+
+  size_t bytesToAlloc;
+  if (reqSize >= slowGrowthThreshold) {
+    size_t currSize = sizeof(Header) + Capacity() * aElemSize;
+    size_t minNewSize = currSize + (currSize >> 3);  // multiply by 1.125
+    bytesToAlloc = reqSize > minNewSize ? reqSize : minNewSize;
+
+    // Round up to the next multiple of MiB.
+    const size_t MiB = 1 << 20;
+    bytesToAlloc = MiB * ((bytesToAlloc + MiB - 1) / MiB);
+  } else {
+    // Round up to the next power of two.
+    bytesToAlloc = mozilla::RoundUpPow2(reqSize);
+  }
+
+  Header* header;
+  if (UsesAutoArrayBuffer() || !RelocationStrategy::allowRealloc) {
+    // Malloc() and copy
+    header = static_cast<Header*>(ActualAlloc::Malloc(bytesToAlloc));
+    if (!header) {
+      return ActualAlloc::FailureResult();
+    }
+
+    RelocationStrategy::RelocateNonOverlappingRegionWithHeader(
+        header, mHdr, Length(), aElemSize);
+
+    if (!UsesAutoArrayBuffer()) {
+      ActualAlloc::Free(mHdr);
+    }
+  } else {
+    // Realloc() existing data
+    header = static_cast<Header*>(ActualAlloc::Realloc(mHdr, bytesToAlloc));
+    if (!header) {
+      return ActualAlloc::FailureResult();
+    }
+  }
+
+  // How many elements can we fit in bytesToAlloc?
+  size_t newCapacity = (bytesToAlloc - sizeof(Header)) / aElemSize;
+  MOZ_ASSERT(newCapacity >= aCapacity, "Didn't enlarge the array enough!");
+  header->mCapacity = newCapacity;
+
+  mHdr = header;
+
+  return ActualAlloc::SuccessResult();
+}
+
+// We don't need use Alloc template parameter specified here because failure to
+// shrink the capacity will leave the array unchanged.
+template <class Alloc, class RelocationStrategy>
+void nsTArray_base<Alloc, RelocationStrategy>::ShrinkCapacity(
+    size_type aElemSize, size_t aElemAlign) {
+  if (HasEmptyHeader() || UsesAutoArrayBuffer()) {
+    return;
+  }
+
+  if (mHdr->mLength >= mHdr->mCapacity) {  // should never be greater than...
+    return;
+  }
+
+  size_type length = Length();
+
+  if (IsAutoArray() && GetAutoArrayBuffer(aElemAlign)->mCapacity >= length) {
+    Header* header = GetAutoArrayBuffer(aElemAlign);
+
+    // Move the data, but don't copy the header to avoid overwriting mCapacity.
+    header->mLength = length;
+    RelocationStrategy::RelocateNonOverlappingRegion(header + 1, mHdr + 1,
+                                                     length, aElemSize);
+
+    nsTArrayFallibleAllocator::Free(mHdr);
+    mHdr = header;
+    return;
+  }
+
+  if (length == 0) {
+    MOZ_ASSERT(!IsAutoArray(), "autoarray should have fit 0 elements");
+    nsTArrayFallibleAllocator::Free(mHdr);
+    mHdr = EmptyHdr();
+    return;
+  }
+
+  size_type newSize = sizeof(Header) + length * aElemSize;
+
+  Header* newHeader;
+  if (!RelocationStrategy::allowRealloc) {
+    // Malloc() and copy.
+    newHeader =
+        static_cast<Header*>(nsTArrayFallibleAllocator::Malloc(newSize));
+    if (!newHeader) {
+      return;
+    }
+
+    RelocationStrategy::RelocateNonOverlappingRegionWithHeader(
+        newHeader, mHdr, Length(), aElemSize);
+
+    nsTArrayFallibleAllocator::Free(mHdr);
+  } else {
+    // Realloc() existing data.
+    newHeader =
+        static_cast<Header*>(nsTArrayFallibleAllocator::Realloc(mHdr, newSize));
+    if (!newHeader) {
+      return;
+    }
+  }
+
+  mHdr = newHeader;
+  mHdr->mCapacity = length;
+}
+
+template <class Alloc, class RelocationStrategy>
+void nsTArray_base<Alloc, RelocationStrategy>::ShrinkCapacityToZero(
+    size_type aElemSize, size_t aElemAlign) {
+  MOZ_ASSERT(mHdr->mLength == 0);
+
+  if (HasEmptyHeader() || UsesAutoArrayBuffer()) {
+    return;
+  }
+
+  const bool isAutoArray = IsAutoArray();
+
+  nsTArrayFallibleAllocator::Free(mHdr);
+
+  if (isAutoArray) {
+    mHdr = GetAutoArrayBufferUnsafe(aElemAlign);
+    mHdr->mLength = 0;
+  } else {
+    mHdr = EmptyHdr();
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+void nsTArray_base<Alloc, RelocationStrategy>::ShiftData(index_type aStart,
+                                                         size_type aOldLen,
+                                                         size_type aNewLen,
+                                                         size_type aElemSize,
+                                                         size_t aElemAlign) {
+  if (aOldLen == aNewLen) {
+    return;
+  }
+
+  // Determine how many elements need to be shifted
+  size_type num = mHdr->mLength - (aStart + aOldLen);
+
+  // Compute the resulting length of the array
+  mHdr->mLength += aNewLen - aOldLen;
+  if (mHdr->mLength == 0) {
+    ShrinkCapacityToZero(aElemSize, aElemAlign);
+  } else {
+    // Maybe nothing needs to be shifted
+    if (num == 0) {
+      return;
+    }
+    // Perform shift (change units to bytes first)
+    aStart *= aElemSize;
+    aNewLen *= aElemSize;
+    aOldLen *= aElemSize;
+    char* baseAddr = reinterpret_cast<char*>(mHdr + 1) + aStart;
+    RelocationStrategy::RelocateOverlappingRegion(
+        baseAddr + aNewLen, baseAddr + aOldLen, num, aElemSize);
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+void nsTArray_base<Alloc, RelocationStrategy>::SwapFromEnd(index_type aStart,
+                                                           size_type aCount,
+                                                           size_type aElemSize,
+                                                           size_t aElemAlign) {
+  // This method is part of the implementation of
+  // nsTArray::SwapRemoveElement{s,}At. For more information, read the
+  // documentation on that method.
+  if (aCount == 0) {
+    return;
+  }
+
+  // We are going to be removing aCount elements. Update our length to point to
+  // the new end of the array.
+  size_type oldLength = mHdr->mLength;
+  mHdr->mLength -= aCount;
+
+  if (mHdr->mLength == 0) {
+    // If we have no elements remaining in the array, we can free our buffer.
+    ShrinkCapacityToZero(aElemSize, aElemAlign);
+    return;
+  }
+
+  // Determine how many elements we need to move from the end of the array into
+  // the now-removed section. This will either be the number of elements which
+  // were removed (if there are more elements in the tail of the array), or the
+  // entire tail of the array, whichever is smaller.
+  size_type relocCount = std::min(aCount, mHdr->mLength - aStart);
+  if (relocCount == 0) {
+    return;
+  }
+
+  // Move the elements which are now stranded after the end of the array back
+  // into the now-vacated memory.
+  index_type sourceBytes = (oldLength - relocCount) * aElemSize;
+  index_type destBytes = aStart * aElemSize;
+
+  // Perform the final copy. This is guaranteed to be a non-overlapping copy
+  // as our source contains only still-valid entries, and the destination
+  // contains only invalid entries which need to be overwritten.
+  MOZ_ASSERT(sourceBytes >= destBytes,
+             "The source should be after the destination.");
+  MOZ_ASSERT(sourceBytes - destBytes >= relocCount * aElemSize,
+             "The range should be nonoverlapping");
+
+  char* baseAddr = reinterpret_cast<char*>(mHdr + 1);
+  RelocationStrategy::RelocateNonOverlappingRegion(
+      baseAddr + destBytes, baseAddr + sourceBytes, relocCount, aElemSize);
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+typename ActualAlloc::ResultTypeProxy
+nsTArray_base<Alloc, RelocationStrategy>::InsertSlotsAt(index_type aIndex,
+                                                        size_type aCount,
+                                                        size_type aElemSize,
+                                                        size_t aElemAlign) {
+  if (MOZ_UNLIKELY(aIndex > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+  }
+
+  if (!ActualAlloc::Successful(
+          this->ExtendCapacity<ActualAlloc>(Length(), aCount, aElemSize))) {
+    return ActualAlloc::FailureResult();
+  }
+
+  // Move the existing elements as needed.  Note that this will
+  // change our mLength, so no need to call IncrementLength.
+  ShiftData<ActualAlloc>(aIndex, 0, aCount, aElemSize, aElemAlign);
+
+  return ActualAlloc::SuccessResult();
+}
+
+// nsTArray_base::IsAutoArrayRestorer is an RAII class which takes
+// |nsTArray_base &array| in its constructor.  When it's destructed, it ensures
+// that
+//
+//   * array.mIsAutoArray has the same value as it did when we started, and
+//   * if array has an auto buffer and mHdr would otherwise point to
+//     sEmptyTArrayHeader, array.mHdr points to array's auto buffer.
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc, RelocationStrategy>::IsAutoArrayRestorer::
+    IsAutoArrayRestorer(nsTArray_base<Alloc, RelocationStrategy>& aArray,
+                        size_t aElemAlign)
+    : mArray(aArray), mElemAlign(aElemAlign), mIsAuto(aArray.IsAutoArray()) {}
+
+template <class Alloc, class RelocationStrategy>
+nsTArray_base<Alloc,
+              RelocationStrategy>::IsAutoArrayRestorer::~IsAutoArrayRestorer() {
+  // Careful: We don't want to set mIsAutoArray = 1 on sEmptyTArrayHeader.
+  if (mIsAuto && mArray.HasEmptyHeader()) {
+    // Call GetAutoArrayBufferUnsafe() because GetAutoArrayBuffer() asserts
+    // that mHdr->mIsAutoArray is true, which surely isn't the case here.
+    mArray.mHdr = mArray.GetAutoArrayBufferUnsafe(mElemAlign);
+    mArray.mHdr->mLength = 0;
+  } else if (!mArray.HasEmptyHeader()) {
+    mArray.mHdr->mIsAutoArray = mIsAuto;
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc, class Allocator>
+typename ActualAlloc::ResultTypeProxy
+nsTArray_base<Alloc, RelocationStrategy>::SwapArrayElements(
+    nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+    size_t aElemAlign) {
+  // EnsureNotUsingAutoArrayBuffer will set mHdr = sEmptyTArrayHeader even if we
+  // have an auto buffer.  We need to point mHdr back to our auto buffer before
+  // we return, otherwise we'll forget that we have an auto buffer at all!
+  // IsAutoArrayRestorer takes care of this for us.
+
+  IsAutoArrayRestorer ourAutoRestorer(*this, aElemAlign);
+  typename nsTArray_base<Allocator, RelocationStrategy>::IsAutoArrayRestorer
+      otherAutoRestorer(aOther, aElemAlign);
+
+  // If neither array uses an auto buffer which is big enough to store the
+  // other array's elements, then ensure that both arrays use malloc'ed storage
+  // and swap their mHdr pointers.
+  if ((!UsesAutoArrayBuffer() || Capacity() < aOther.Length()) &&
+      (!aOther.UsesAutoArrayBuffer() || aOther.Capacity() < Length())) {
+    if (!EnsureNotUsingAutoArrayBuffer<ActualAlloc>(aElemSize) ||
+        !aOther.template EnsureNotUsingAutoArrayBuffer<ActualAlloc>(
+            aElemSize)) {
+      return ActualAlloc::FailureResult();
+    }
+
+    Header* temp = mHdr;
+    mHdr = aOther.mHdr;
+    aOther.mHdr = temp;
+
+    return ActualAlloc::SuccessResult();
+  }
+
+  // Swap the two arrays by copying, since at least one is using an auto
+  // buffer which is large enough to hold all of the aOther's elements.  We'll
+  // copy the shorter array into temporary storage.
+  //
+  // (We could do better than this in some circumstances.  Suppose we're
+  // swapping arrays X and Y.  X has space for 2 elements in its auto buffer,
+  // but currently has length 4, so it's using malloc'ed storage.  Y has length
+  // 2.  When we swap X and Y, we don't need to use a temporary buffer; we can
+  // write Y straight into X's auto buffer, write X's malloc'ed buffer on top
+  // of Y, and then switch X to using its auto buffer.)
+
+  if (!ActualAlloc::Successful(
+          EnsureCapacity<ActualAlloc>(aOther.Length(), aElemSize)) ||
+      !Allocator::Successful(
+          aOther.template EnsureCapacity<Allocator>(Length(), aElemSize))) {
+    return ActualAlloc::FailureResult();
+  }
+
+  // The EnsureCapacity calls above shouldn't have caused *both* arrays to
+  // switch from their auto buffers to malloc'ed space.
+  MOZ_ASSERT(UsesAutoArrayBuffer() || aOther.UsesAutoArrayBuffer(),
+             "One of the arrays should be using its auto buffer.");
+
+  size_type smallerLength = XPCOM_MIN(Length(), aOther.Length());
+  size_type largerLength = XPCOM_MAX(Length(), aOther.Length());
+  void* smallerElements;
+  void* largerElements;
+  if (Length() <= aOther.Length()) {
+    smallerElements = Hdr() + 1;
+    largerElements = aOther.Hdr() + 1;
+  } else {
+    smallerElements = aOther.Hdr() + 1;
+    largerElements = Hdr() + 1;
+  }
+
+  // Allocate temporary storage for the smaller of the two arrays.  We want to
+  // allocate this space on the stack, if it's not too large.  Sounds like a
+  // job for AutoTArray!  (One of the two arrays we're swapping is using an
+  // auto buffer, so we're likely not allocating a lot of space here.  But one
+  // could, in theory, allocate a huge AutoTArray on the heap.)
+  AutoTArray<uint8_t, 64 * sizeof(void*)> temp;
+  if (!ActualAlloc::Successful(temp.template EnsureCapacity<ActualAlloc>(
+          smallerLength * aElemSize, sizeof(uint8_t)))) {
+    return ActualAlloc::FailureResult();
+  }
+
+  RelocationStrategy::RelocateNonOverlappingRegion(
+      temp.Elements(), smallerElements, smallerLength, aElemSize);
+  RelocationStrategy::RelocateNonOverlappingRegion(
+      smallerElements, largerElements, largerLength, aElemSize);
+  RelocationStrategy::RelocateNonOverlappingRegion(
+      largerElements, temp.Elements(), smallerLength, aElemSize);
+
+  // Swap the arrays' lengths.
+  MOZ_ASSERT((aOther.Length() == 0 || !HasEmptyHeader()) &&
+                 (Length() == 0 || !aOther.HasEmptyHeader()),
+             "Don't set sEmptyTArrayHeader's length.");
+  size_type tempLength = Length();
+
+  // Avoid writing to EmptyHdr, since it can trigger false
+  // positives with TSan.
+  if (!HasEmptyHeader()) {
+    mHdr->mLength = aOther.Length();
+  }
+  if (!aOther.HasEmptyHeader()) {
+    aOther.mHdr->mLength = tempLength;
+  }
+
+  return ActualAlloc::SuccessResult();
+}
+
+template <class Alloc, class RelocationStrategy>
+template <class Allocator>
+void nsTArray_base<Alloc, RelocationStrategy>::MoveInit(
+    nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+    size_t aElemAlign) {
+  // This method is similar to SwapArrayElements, but specialized for the case
+  // where the target array is empty with no allocated heap storage. It is
+  // provided and used to simplify template instantiation and enable better code
+  // generation.
+
+  MOZ_ASSERT(Length() == 0);
+  MOZ_ASSERT(Capacity() == 0 || (IsAutoArray() && UsesAutoArrayBuffer()));
+
+  // EnsureNotUsingAutoArrayBuffer will set mHdr = sEmptyTArrayHeader even if we
+  // have an auto buffer.  We need to point mHdr back to our auto buffer before
+  // we return, otherwise we'll forget that we have an auto buffer at all!
+  // IsAutoArrayRestorer takes care of this for us.
+
+  IsAutoArrayRestorer ourAutoRestorer(*this, aElemAlign);
+  typename nsTArray_base<Allocator, RelocationStrategy>::IsAutoArrayRestorer
+      otherAutoRestorer(aOther, aElemAlign);
+
+  // If neither array uses an auto buffer which is big enough to store the
+  // other array's elements, then ensure that both arrays use malloc'ed storage
+  // and swap their mHdr pointers.
+  if ((!IsAutoArray() || Capacity() < aOther.Length()) &&
+      !aOther.UsesAutoArrayBuffer()) {
+    mHdr = aOther.mHdr;
+
+    aOther.mHdr = EmptyHdr();
+
+    return;
+  }
+
+  // Move the data by copying, since at least one has an auto
+  // buffer which is large enough to hold all of the aOther's elements.
+
+  EnsureCapacity<nsTArrayInfallibleAllocator>(aOther.Length(), aElemSize);
+
+  // The EnsureCapacity calls above shouldn't have caused *both* arrays to
+  // switch from their auto buffers to malloc'ed space.
+  MOZ_ASSERT(UsesAutoArrayBuffer() || aOther.UsesAutoArrayBuffer(),
+             "One of the arrays should be using its auto buffer.");
+
+  RelocationStrategy::RelocateNonOverlappingRegion(Hdr() + 1, aOther.Hdr() + 1,
+                                                   aOther.Length(), aElemSize);
+
+  // Swap the arrays' lengths.
+  MOZ_ASSERT((aOther.Length() == 0 || !HasEmptyHeader()) &&
+                 (Length() == 0 || !aOther.HasEmptyHeader()),
+             "Don't set sEmptyTArrayHeader's length.");
+
+  // Avoid writing to EmptyHdr, since it can trigger false
+  // positives with TSan.
+  if (!HasEmptyHeader()) {
+    mHdr->mLength = aOther.Length();
+  }
+  if (!aOther.HasEmptyHeader()) {
+    aOther.mHdr->mLength = 0;
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+template <class Allocator>
+void nsTArray_base<Alloc, RelocationStrategy>::MoveConstructNonAutoArray(
+    nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+    size_t aElemAlign) {
+  // We know that we are not an (Copyable)AutoTArray and we know that we are
+  // empty, so don't use SwapArrayElements which doesn't know either of these
+  // facts and is very complex.
+
+  if (aOther.IsEmpty()) {
+    return;
+  }
+
+  // aOther might be an (Copyable)AutoTArray though, and it might use its inline
+  // buffer.
+  const bool otherUsesAutoArrayBuffer = aOther.UsesAutoArrayBuffer();
+  if (otherUsesAutoArrayBuffer) {
+    // Use nsTArrayInfallibleAllocator regardless of Alloc because this is
+    // called from a move constructor, which cannot report an error to the
+    // caller.
+    aOther.template EnsureNotUsingAutoArrayBuffer<nsTArrayInfallibleAllocator>(
+        aElemSize);
+  }
+
+  const bool otherIsAuto = otherUsesAutoArrayBuffer || aOther.IsAutoArray();
+  mHdr = aOther.mHdr;
+  // We might write to mHdr, so ensure it's not the static empty header. aOther
+  // shouldn't have been empty if we get here anyway.
+  MOZ_ASSERT(!HasEmptyHeader());
+
+  if (otherIsAuto) {
+    mHdr->mIsAutoArray = false;
+    aOther.mHdr = aOther.GetAutoArrayBufferUnsafe(aElemAlign);
+    aOther.mHdr->mLength = 0;
+  } else {
+    aOther.mHdr = aOther.EmptyHdr();
+  }
+}
+
+template <class Alloc, class RelocationStrategy>
+template <typename ActualAlloc>
+bool nsTArray_base<Alloc, RelocationStrategy>::EnsureNotUsingAutoArrayBuffer(
+    size_type aElemSize) {
+  if (UsesAutoArrayBuffer()) {
+    // If you call this on a 0-length array, we'll set that array's mHdr to
+    // sEmptyTArrayHeader, in flagrant violation of the AutoTArray invariants.
+    // It's up to you to set it back!  (If you don't, the AutoTArray will
+    // forget that it has an auto buffer.)
+    if (Length() == 0) {
+      mHdr = EmptyHdr();
+      return true;
+    }
+
+    size_type size = sizeof(Header) + Length() * aElemSize;
+
+    Header* header = static_cast<Header*>(ActualAlloc::Malloc(size));
+    if (!header) {
+      return false;
+    }
+
+    RelocationStrategy::RelocateNonOverlappingRegionWithHeader(
+        header, mHdr, Length(), aElemSize);
+    header->mCapacity = Length();
+    mHdr = header;
+  }
+
+  return true;
+}
diff --git a/xpcom/ds/nsTArray.cpp b/xpcom/ds/nsTArray.cpp
new file mode 100644
index 0000000000..50326fb331
--- /dev/null
+++ b/xpcom/ds/nsTArray.cpp
@@ -0,0 +1,28 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <string.h>
+#include "nsTArray.h"
+#include "nsXPCOM.h"
+#include "nsCycleCollectionNoteChild.h"
+#include "nsDebug.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/IntegerPrintfMacros.h"
+
+// Ensure this is sufficiently aligned so that Elements() and co don't create
+// unaligned pointers, or slices with unaligned pointers for empty arrays, see
+// https://github.com/servo/servo/issues/22613.
+alignas(8) const nsTArrayHeader sEmptyTArrayHeader = {0, 0, 0};
+
+bool IsTwiceTheRequiredBytesRepresentableAsUint32(size_t aCapacity,
+                                                  size_t aElemSize) {
+  using mozilla::CheckedUint32;
+  return ((CheckedUint32(aCapacity) * aElemSize) * 2).isValid();
+}
+
+void ::detail::SetCycleCollectionArrayFlag(uint32_t& aFlags) {
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+}
diff --git a/xpcom/ds/nsTArray.h b/xpcom/ds/nsTArray.h
new file mode 100644
index 0000000000..ed8c05943c
--- /dev/null
+++ b/xpcom/ds/nsTArray.h
@@ -0,0 +1,3345 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTArray_h__
+#define nsTArray_h__
+
+#include <string.h>
+
+#include <algorithm>
+#include <functional>
+#include <initializer_list>
+#include <iterator>
+#include <new>
+#include <ostream>
+#include <type_traits>
+#include <utility>
+
+#include "mozilla/Alignment.h"
+#include "mozilla/ArrayIterator.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/BinarySearch.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/FunctionTypeTraits.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/Span.h"
+#include "mozilla/fallible.h"
+#include "mozilla/mozalloc.h"
+#include "nsAlgorithm.h"
+#include "nsDebug.h"
+#include "nsISupports.h"
+#include "nsQuickSort.h"
+#include "nsRegionFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+namespace JS {
+template <class T>
+class Heap;
+} /* namespace JS */
+
+class nsCycleCollectionTraversalCallback;
+class nsRegion;
+
+namespace mozilla::a11y {
+class BatchData;
+}
+
+namespace mozilla {
+namespace layers {
+class Animation;
+class FrameStats;
+struct PropertyAnimationGroup;
+struct TileClient;
+}  // namespace layers
+}  // namespace mozilla
+
+namespace mozilla {
+struct SerializedStructuredCloneBuffer;
+class SourceBufferTask;
+}  // namespace mozilla
+
+namespace mozilla::dom::binding_detail {
+template <typename, typename>
+class RecordEntry;
+}
+
+namespace mozilla::dom::ipc {
+class StructuredCloneData;
+}  // namespace mozilla::dom::ipc
+
+namespace mozilla::dom {
+class ClonedMessageData;
+class MessageData;
+class MessagePortIdentifier;
+struct MozPluginParameter;
+template <typename T>
+struct Nullable;
+class OwningFileOrDirectory;
+class OwningStringOrBooleanOrObject;
+class OwningUTF8StringOrDouble;
+class Pref;
+class RefMessageData;
+class ResponsiveImageCandidate;
+class ServiceWorkerRegistrationData;
+namespace indexedDB {
+class SerializedStructuredCloneReadInfo;
+class ObjectStoreCursorResponse;
+class IndexCursorResponse;
+}  // namespace indexedDB
+}  // namespace mozilla::dom
+
+namespace mozilla::ipc {
+class ContentSecurityPolicy;
+template <class T>
+class Endpoint;
+}  // namespace mozilla::ipc
+
+class JSStructuredCloneData;
+
+template <class T>
+class RefPtr;
+
+//
+// nsTArray<E> is a resizable array class, like std::vector.
+//
+// Unlike std::vector, which follows C++'s construction/destruction rules,
+// By default, nsTArray assumes that instances of E can be relocated safely
+// using memory utils (memcpy/memmove).
+//
+// The public classes defined in this header are
+//
+//   nsTArray<E>,
+//   CopyableTArray<E>,
+//   FallibleTArray<E>,
+//   AutoTArray<E, N>,
+//   CopyableAutoTArray<E, N>
+//
+// nsTArray, CopyableTArray, AutoTArray and CopyableAutoTArray are infallible by
+// default. To opt-in to fallible behaviour, use the `mozilla::fallible`
+// parameter and check the return value.
+//
+// CopyableTArray and CopyableAutoTArray< are copy-constructible and
+// copy-assignable. Use these only when syntactically necessary to avoid implcit
+// unintentional copies. nsTArray/AutoTArray can be conveniently copied using
+// the Clone() member function. Consider using std::move where possible.
+//
+// If you just want to declare the nsTArray types (e.g., if you're in a header
+// file and don't need the full nsTArray definitions) consider including
+// nsTArrayForwardDeclare.h instead of nsTArray.h.
+//
+// The template parameter E specifies the type of the elements and has the
+// following requirements:
+//
+//   E MUST be safely memmove()'able.
+//   E MUST define a copy-constructor.
+//   E MAY define operator< for sorting.
+//   E MAY define operator== for searching.
+//
+// (Note that the memmove requirement may be relaxed for certain types - see
+// nsTArray_RelocationStrategy below.)
+//
+// There is a public type value_type defined as E within each array class, and
+// we reference the type under this name below.
+//
+// For member functions taking a Comparator instance, Comparator must be either
+// a functor with a tri-state comparison function with a signature compatible to
+//
+//   /** @return negative iff a < b, 0 iff a == b, positive iff a > b */
+//   int (const value_type& a, const value_type& b);
+//
+// or a class defining member functions with signatures compatible to:
+//
+//   class Comparator {
+//     public:
+//       /** @return True if the elements are equals; false otherwise. */
+//       bool Equals(const value_type& a, const value_type& b) const;
+//
+//       /** @return True if (a < b); false otherwise. */
+//       bool LessThan(const value_type& a, const value_type& b) const;
+//   };
+//
+// The Equals member function is used for searching, and the LessThan member
+// function is used for searching and sorting.  Note that some member functions,
+// e.g. Compare, are templates where a different type Item can be used for the
+// element to compare to. In that case, the signatures must be compatible to
+// allow those comparisons, but the details are not documented here.
+//
+
+//
+// nsTArrayFallibleResult and nsTArrayInfallibleResult types are proxy types
+// which are used because you cannot use a templated type which is bound to
+// void as an argument to a void function.  In order to work around that, we
+// encode either a void or a boolean inside these proxy objects, and pass them
+// to the aforementioned function instead, and then use the type information to
+// decide what to do in the function.
+//
+// Note that public nsTArray methods should never return a proxy type.  Such
+// types are only meant to be used in the internal nsTArray helper methods.
+// Public methods returning non-proxy types cannot be called from other
+// nsTArray members.
+//
+struct nsTArrayFallibleResult {
+  // Note: allows implicit conversions from and to bool
+  MOZ_IMPLICIT constexpr nsTArrayFallibleResult(bool aResult)
+      : mResult(aResult) {}
+
+  MOZ_IMPLICIT constexpr operator bool() { return mResult; }
+
+ private:
+  bool mResult;
+};
+
+struct nsTArrayInfallibleResult {};
+
+//
+// nsTArray*Allocators must all use the same |free()|, to allow swap()'ing
+// between fallible and infallible variants.
+//
+
+struct nsTArrayFallibleAllocatorBase {
+  typedef bool ResultType;
+  typedef nsTArrayFallibleResult ResultTypeProxy;
+
+  static constexpr ResultType Result(ResultTypeProxy aResult) {
+    return aResult;
+  }
+  static constexpr bool Successful(ResultTypeProxy aResult) { return aResult; }
+  static constexpr ResultTypeProxy SuccessResult() { return true; }
+  static constexpr ResultTypeProxy FailureResult() { return false; }
+  static constexpr ResultType ConvertBoolToResultType(bool aValue) {
+    return aValue;
+  }
+};
+
+struct nsTArrayInfallibleAllocatorBase {
+  typedef void ResultType;
+  typedef nsTArrayInfallibleResult ResultTypeProxy;
+
+  static constexpr ResultType Result(ResultTypeProxy aResult) {}
+  static constexpr bool Successful(ResultTypeProxy) { return true; }
+  static constexpr ResultTypeProxy SuccessResult() { return ResultTypeProxy(); }
+
+  [[noreturn]] static ResultTypeProxy FailureResult() {
+    MOZ_CRASH("Infallible nsTArray should never fail");
+  }
+
+  template <typename T>
+  static constexpr ResultType ConvertBoolToResultType(T aValue) {
+    if (!aValue) {
+      MOZ_CRASH("infallible nsTArray should never convert false to ResultType");
+    }
+  }
+
+  template <typename T>
+  static constexpr ResultType ConvertBoolToResultType(
+      const mozilla::NotNull<T>& aValue) {}
+};
+
+struct nsTArrayFallibleAllocator : nsTArrayFallibleAllocatorBase {
+  static void* Malloc(size_t aSize) { return malloc(aSize); }
+  static void* Realloc(void* aPtr, size_t aSize) {
+    return realloc(aPtr, aSize);
+  }
+
+  static void Free(void* aPtr) { free(aPtr); }
+  static void SizeTooBig(size_t) {}
+};
+
+struct nsTArrayInfallibleAllocator : nsTArrayInfallibleAllocatorBase {
+  static void* Malloc(size_t aSize) MOZ_NONNULL_RETURN {
+    return moz_xmalloc(aSize);
+  }
+  static void* Realloc(void* aPtr, size_t aSize) MOZ_NONNULL_RETURN {
+    return moz_xrealloc(aPtr, aSize);
+  }
+
+  static void Free(void* aPtr) { free(aPtr); }
+  static void SizeTooBig(size_t aSize) { NS_ABORT_OOM(aSize); }
+};
+
+// nsTArray_base stores elements into the space allocated beyond
+// sizeof(*this).  This is done to minimize the size of the nsTArray
+// object when it is empty.
+struct nsTArrayHeader {
+  uint32_t mLength;
+  uint32_t mCapacity : 31;
+  uint32_t mIsAutoArray : 1;
+};
+
+extern "C" {
+extern const nsTArrayHeader sEmptyTArrayHeader;
+}
+
+namespace detail {
+// nsTArray_CopyDisabler disables copy operations.
+class nsTArray_CopyDisabler {
+ public:
+  nsTArray_CopyDisabler() = default;
+
+  nsTArray_CopyDisabler(const nsTArray_CopyDisabler&) = delete;
+  nsTArray_CopyDisabler& operator=(const nsTArray_CopyDisabler&) = delete;
+};
+
+}  // namespace detail
+
+// This class provides a SafeElementAt method to nsTArray<E*> which does
+// not take a second default value parameter.
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper : public ::detail::nsTArray_CopyDisabler {
+  typedef E* elem_type;
+  typedef size_t index_type;
+
+  // No implementation is provided for these two methods, and that is on
+  // purpose, since we don't support these functions on non-pointer type
+  // instantiations.
+  elem_type& SafeElementAt(index_type aIndex);
+  const elem_type& SafeElementAt(index_type aIndex) const;
+};
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<E*, Derived>
+    : public ::detail::nsTArray_CopyDisabler {
+  typedef E* elem_type;
+  // typedef const E* const_elem_type;   XXX: see below
+  typedef size_t index_type;
+
+  elem_type SafeElementAt(index_type aIndex) {
+    return static_cast<Derived*>(this)->SafeElementAt(aIndex, nullptr);
+  }
+
+  // XXX: Probably should return const_elem_type, but callsites must be fixed.
+  // Also, the use of const_elem_type for nsTArray<xpcGCCallback> in
+  // xpcprivate.h causes build failures on Windows because xpcGCCallback is a
+  // function pointer and MSVC doesn't like qualifying it with |const|.
+  elem_type SafeElementAt(index_type aIndex) const {
+    return static_cast<const Derived*>(this)->SafeElementAt(aIndex, nullptr);
+  }
+};
+
+// E is a smart pointer type; the
+// smart pointer can act as its element_type*.
+template <class E, class Derived>
+struct nsTArray_SafeElementAtSmartPtrHelper
+    : public ::detail::nsTArray_CopyDisabler {
+  typedef typename E::element_type* elem_type;
+  typedef const typename E::element_type* const_elem_type;
+  typedef size_t index_type;
+
+  elem_type SafeElementAt(index_type aIndex) {
+    auto* derived = static_cast<Derived*>(this);
+    if (aIndex < derived->Length()) {
+      return derived->Elements()[aIndex];
+    }
+    return nullptr;
+  }
+
+  // XXX: Probably should return const_elem_type, but callsites must be fixed.
+  elem_type SafeElementAt(index_type aIndex) const {
+    auto* derived = static_cast<const Derived*>(this);
+    if (aIndex < derived->Length()) {
+      return derived->Elements()[aIndex];
+    }
+    return nullptr;
+  }
+};
+
+template <class T>
+class nsCOMPtr;
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<nsCOMPtr<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<nsCOMPtr<E>, Derived> {};
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<RefPtr<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<RefPtr<E>, Derived> {};
+
+namespace mozilla {
+template <class T>
+class OwningNonNull;
+}  // namespace mozilla
+
+template <class E, class Derived>
+struct nsTArray_SafeElementAtHelper<mozilla::OwningNonNull<E>, Derived>
+    : public nsTArray_SafeElementAtSmartPtrHelper<mozilla::OwningNonNull<E>,
+                                                  Derived> {};
+
+// Servo bindings.
+extern "C" void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity,
+                                           size_t aElementSize);
+extern "C" void Gecko_ClearPODTArray(void* aArray, size_t aElementSize,
+                                     size_t aElementAlign);
+
+//
+// This class serves as a base class for nsTArray.  It shouldn't be used
+// directly.  It holds common implementation code that does not depend on the
+// element type of the nsTArray.
+//
+template <class Alloc, class RelocationStrategy>
+class nsTArray_base {
+  // Allow swapping elements with |nsTArray_base|s created using a
+  // different allocator.  This is kosher because all allocators use
+  // the same free().
+  template <class XAlloc, class XRelocationStrategy>
+  friend class nsTArray_base;
+
+  // Needed for AppendElements from an array with a different allocator, which
+  // calls ShiftData.
+  template <class E, class XAlloc>
+  friend class nsTArray_Impl;
+
+  friend void Gecko_EnsureTArrayCapacity(void* aArray, size_t aCapacity,
+                                         size_t aElemSize);
+  friend void Gecko_ClearPODTArray(void* aTArray, size_t aElementSize,
+                                   size_t aElementAlign);
+
+ protected:
+  typedef nsTArrayHeader Header;
+
+ public:
+  typedef size_t size_type;
+  typedef size_t index_type;
+
+  // @return The number of elements in the array.
+  size_type Length() const { return mHdr->mLength; }
+
+  // @return True if the array is empty or false otherwise.
+  bool IsEmpty() const { return Length() == 0; }
+
+  // @return The number of elements that can fit in the array without forcing
+  // the array to be re-allocated.  The length of an array is always less
+  // than or equal to its capacity.
+  size_type Capacity() const { return mHdr->mCapacity; }
+
+#ifdef DEBUG
+  void* DebugGetHeader() const { return mHdr; }
+#endif
+
+ protected:
+  nsTArray_base();
+
+  ~nsTArray_base();
+
+  nsTArray_base(const nsTArray_base&);
+  nsTArray_base& operator=(const nsTArray_base&);
+
+  // Resize the storage if necessary to achieve the requested capacity.
+  // @param aCapacity The requested number of array elements.
+  // @param aElemSize The size of an array element.
+  // @return False if insufficient memory is available; true otherwise.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy EnsureCapacity(size_type aCapacity,
+                                                       size_type aElemSize);
+
+  // Extend the storage to accommodate aCount extra elements.
+  // @param aLength The current size of the array.
+  // @param aCount The number of elements to add.
+  // @param aElemSize The size of an array element.
+  // @return False if insufficient memory is available or the new length
+  //   would overflow; true otherwise.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy ExtendCapacity(size_type aLength,
+                                                       size_type aCount,
+                                                       size_type aElemSize);
+
+  // Tries to resize the storage to the minimum required amount. If this fails,
+  // the array is left as-is.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  void ShrinkCapacity(size_type aElemSize, size_t aElemAlign);
+
+  // Resizes the storage to 0. This may only be called when Length() is already
+  // 0.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  void ShrinkCapacityToZero(size_type aElemSize, size_t aElemAlign);
+
+  // This method may be called to resize a "gap" in the array by shifting
+  // elements around.  It updates mLength appropriately.  If the resulting
+  // array has zero elements, then the array's memory is free'd.
+  // @param aStart     The starting index of the gap.
+  // @param aOldLen    The current length of the gap.
+  // @param aNewLen    The desired length of the gap.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  void ShiftData(index_type aStart, size_type aOldLen, size_type aNewLen,
+                 size_type aElemSize, size_t aElemAlign);
+
+  // This method may be called to swap elements from the end of the array to
+  // fill a "gap" in the array. If the resulting array has zero elements, then
+  // the array's memory is free'd.
+  // @param aStart     The starting index of the gap.
+  // @param aCount     The length of the gap.
+  // @param aElemSize  The size of an array element.
+  // @param aElemAlign The alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  void SwapFromEnd(index_type aStart, size_type aCount, size_type aElemSize,
+                   size_t aElemAlign);
+
+  // This method increments the length member of the array's header.
+  // Note that mHdr may actually be sEmptyTArrayHeader in the case where a
+  // zero-length array is inserted into our array. But then aNum should
+  // always be 0.
+  void IncrementLength(size_t aNum) {
+    if (HasEmptyHeader()) {
+      if (MOZ_UNLIKELY(aNum != 0)) {
+        // Writing a non-zero length to the empty header would be extremely bad.
+        MOZ_CRASH();
+      }
+    } else {
+      mHdr->mLength += aNum;
+    }
+  }
+
+  // This method inserts blank slots into the array.
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of slots to insert
+  // @param aElementSize the size of an array element.
+  // @param aElemAlign the alignment in bytes of an array element.
+  template <typename ActualAlloc>
+  typename ActualAlloc::ResultTypeProxy InsertSlotsAt(index_type aIndex,
+                                                      size_type aCount,
+                                                      size_type aElementSize,
+                                                      size_t aElemAlign);
+
+  template <typename ActualAlloc, class Allocator>
+  typename ActualAlloc::ResultTypeProxy SwapArrayElements(
+      nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+      size_t aElemAlign);
+
+  template <class Allocator>
+  void MoveConstructNonAutoArray(
+      nsTArray_base<Allocator, RelocationStrategy>& aOther, size_type aElemSize,
+      size_t aElemAlign);
+
+  template <class Allocator>
+  void MoveInit(nsTArray_base<Allocator, RelocationStrategy>& aOther,
+                size_type aElemSize, size_t aElemAlign);
+
+  // This is an RAII class used in SwapArrayElements.
+  class IsAutoArrayRestorer {
+   public:
+    IsAutoArrayRestorer(nsTArray_base<Alloc, RelocationStrategy>& aArray,
+                        size_t aElemAlign);
+    ~IsAutoArrayRestorer();
+
+   private:
+    nsTArray_base<Alloc, RelocationStrategy>& mArray;
+    size_t mElemAlign;
+    bool mIsAuto;
+  };
+
+  // Helper function for SwapArrayElements. Ensures that if the array
+  // is an AutoTArray that it doesn't use the built-in buffer.
+  template <typename ActualAlloc>
+  bool EnsureNotUsingAutoArrayBuffer(size_type aElemSize);
+
+  // Returns true if this nsTArray is an AutoTArray with a built-in buffer.
+  bool IsAutoArray() const { return mHdr->mIsAutoArray; }
+
+  // Returns a Header for the built-in buffer of this AutoTArray.
+  Header* GetAutoArrayBuffer(size_t aElemAlign) {
+    MOZ_ASSERT(IsAutoArray(), "Should be an auto array to call this");
+    return GetAutoArrayBufferUnsafe(aElemAlign);
+  }
+  const Header* GetAutoArrayBuffer(size_t aElemAlign) const {
+    MOZ_ASSERT(IsAutoArray(), "Should be an auto array to call this");
+    return GetAutoArrayBufferUnsafe(aElemAlign);
+  }
+
+  // Returns a Header for the built-in buffer of this AutoTArray, but doesn't
+  // assert that we are an AutoTArray.
+  Header* GetAutoArrayBufferUnsafe(size_t aElemAlign) {
+    return const_cast<Header*>(
+        static_cast<const nsTArray_base<Alloc, RelocationStrategy>*>(this)
+            ->GetAutoArrayBufferUnsafe(aElemAlign));
+  }
+  const Header* GetAutoArrayBufferUnsafe(size_t aElemAlign) const;
+
+  // Returns true if this is an AutoTArray and it currently uses the
+  // built-in buffer to store its elements.
+  bool UsesAutoArrayBuffer() const;
+
+  // The array's elements (prefixed with a Header).  This pointer is never
+  // null.  If the array is empty, then this will point to sEmptyTArrayHeader.
+  Header* mHdr;
+
+  Header* Hdr() const MOZ_NONNULL_RETURN { return mHdr; }
+  Header** PtrToHdr() MOZ_NONNULL_RETURN { return &mHdr; }
+  static Header* EmptyHdr() MOZ_NONNULL_RETURN {
+    return const_cast<Header*>(&sEmptyTArrayHeader);
+  }
+
+  [[nodiscard]] bool HasEmptyHeader() const { return mHdr == EmptyHdr(); }
+};
+
+namespace detail {
+
+// Used for argument checking in nsTArrayElementTraits::Emplace.
+template <typename... T>
+struct ChooseFirst;
+
+template <>
+struct ChooseFirst<> {
+  // Choose a default type that is guaranteed to not match E* for any
+  // nsTArray<E>.
+  typedef void Type;
+};
+
+template <typename A, typename... Args>
+struct ChooseFirst<A, Args...> {
+  typedef A Type;
+};
+
+}  // namespace detail
+
+//
+// This class defines convenience functions for element specific operations.
+// Specialize this template if necessary.
+//
+template <class E>
+class nsTArrayElementTraits {
+ public:
+  // Invoke the default constructor in place.
+  static inline void Construct(E* aE) {
+    // Do NOT call "E()"! That triggers C++ "default initialization"
+    // which zeroes out POD ("plain old data") types such as regular
+    // ints.  We don't want that because it can be a performance issue
+    // and people don't expect it; nsTArray should work like a regular
+    // C/C++ array in this respect.
+    new (static_cast<void*>(aE)) E;
+  }
+  // Invoke the copy-constructor in place.
+  template <class A>
+  static inline void Construct(E* aE, A&& aArg) {
+    using E_NoCV = std::remove_cv_t<E>;
+    using A_NoCV = std::remove_cv_t<A>;
+    static_assert(!std::is_same_v<E_NoCV*, A_NoCV>,
+                  "For safety, we disallow constructing nsTArray<E> elements "
+                  "from E* pointers. See bug 960591.");
+    new (static_cast<void*>(aE)) E(std::forward<A>(aArg));
+  }
+  // Construct in place.
+  template <class... Args>
+  static inline void Emplace(E* aE, Args&&... aArgs) {
+    using E_NoCV = std::remove_cv_t<E>;
+    using A_NoCV =
+        std::remove_cv_t<typename ::detail::ChooseFirst<Args...>::Type>;
+    static_assert(!std::is_same_v<E_NoCV*, A_NoCV>,
+                  "For safety, we disallow constructing nsTArray<E> elements "
+                  "from E* pointers. See bug 960591.");
+    new (static_cast<void*>(aE)) E(std::forward<Args>(aArgs)...);
+  }
+  // Invoke the destructor in place.
+  static inline void Destruct(E* aE) { aE->~E(); }
+};
+
+// The default comparator used by nsTArray
+template <class A, class B>
+class nsDefaultComparator {
+ public:
+  bool Equals(const A& aA, const B& aB) const { return aA == aB; }
+  bool LessThan(const A& aA, const B& aB) const { return aA < aB; }
+};
+
+template <bool IsTriviallyCopyConstructible, bool IsSameType>
+struct AssignRangeAlgorithm {
+  template <class Item, class ElemType, class IndexType, class SizeType>
+  static void implementation(ElemType* aElements, IndexType aStart,
+                             SizeType aCount, const Item* aValues) {
+    ElemType* iter = aElements + aStart;
+    ElemType* end = iter + aCount;
+    for (; iter != end; ++iter, ++aValues) {
+      nsTArrayElementTraits<ElemType>::Construct(iter, *aValues);
+    }
+  }
+};
+
+template <>
+struct AssignRangeAlgorithm<true, true> {
+  template <class Item, class ElemType, class IndexType, class SizeType>
+  static void implementation(ElemType* aElements, IndexType aStart,
+                             SizeType aCount, const Item* aValues) {
+    if (aValues) {
+      memcpy(aElements + aStart, aValues, aCount * sizeof(ElemType));
+    }
+  }
+};
+
+//
+// Normally elements are copied with memcpy and memmove, but for some element
+// types that is problematic.  The nsTArray_RelocationStrategy template class
+// can be specialized to ensure that copying calls constructors and destructors
+// instead, as is done below for JS::Heap<E> elements.
+//
+
+//
+// A class that defines how to copy elements using memcpy/memmove.
+//
+struct nsTArray_RelocateUsingMemutils {
+  const static bool allowRealloc = true;
+
+  static void RelocateNonOverlappingRegionWithHeader(void* aDest,
+                                                     const void* aSrc,
+                                                     size_t aCount,
+                                                     size_t aElemSize) {
+    memcpy(aDest, aSrc, sizeof(nsTArrayHeader) + aCount * aElemSize);
+  }
+
+  static void RelocateOverlappingRegion(void* aDest, void* aSrc, size_t aCount,
+                                        size_t aElemSize) {
+    memmove(aDest, aSrc, aCount * aElemSize);
+  }
+
+  static void RelocateNonOverlappingRegion(void* aDest, void* aSrc,
+                                           size_t aCount, size_t aElemSize) {
+    memcpy(aDest, aSrc, aCount * aElemSize);
+  }
+};
+
+//
+// A template class that defines how to relocate elements using the type's move
+// constructor and destructor appropriately.
+//
+template <class ElemType>
+struct nsTArray_RelocateUsingMoveConstructor {
+  typedef nsTArrayElementTraits<ElemType> traits;
+
+  const static bool allowRealloc = false;
+
+  static void RelocateNonOverlappingRegionWithHeader(void* aDest, void* aSrc,
+                                                     size_t aCount,
+                                                     size_t aElemSize) {
+    nsTArrayHeader* destHeader = static_cast<nsTArrayHeader*>(aDest);
+    nsTArrayHeader* srcHeader = static_cast<nsTArrayHeader*>(aSrc);
+    *destHeader = *srcHeader;
+    RelocateNonOverlappingRegion(
+        static_cast<uint8_t*>(aDest) + sizeof(nsTArrayHeader),
+        static_cast<uint8_t*>(aSrc) + sizeof(nsTArrayHeader), aCount,
+        aElemSize);
+  }
+
+  // RelocateNonOverlappingRegion and RelocateOverlappingRegion are defined by
+  // analogy with memmove and memcpy that are used for relocation of
+  // trivially-relocatable types through nsTArray_RelocateUsingMemutils. What
+  // they actually do is slightly different: RelocateOverlappingRegion checks to
+  // see which direction the movement needs to take place, whether from
+  // back-to-front of the range to be moved or from front-to-back.
+  // RelocateNonOverlappingRegion assumes that relocating front-to-back is
+  // always valid.  They use RelocateRegionForward and RelocateRegionBackward,
+  // which are analogous to std::move and std::move_backward respectively,
+  // except they don't move-assign the destination from the source but
+  // move-construct the destination from the source and destroy the source.
+  static void RelocateOverlappingRegion(void* aDest, void* aSrc, size_t aCount,
+                                        size_t aElemSize) {
+    ElemType* destBegin = static_cast<ElemType*>(aDest);
+    ElemType* srcBegin = static_cast<ElemType*>(aSrc);
+
+    // If destination and source are the same, this is a no-op.
+    // In practice, we don't do this.
+    if (destBegin == srcBegin) {
+      return;
+    }
+
+    ElemType* srcEnd = srcBegin + aCount;
+    ElemType* destEnd = destBegin + aCount;
+
+    // Figure out whether to relocate back-to-front or front-to-back.
+    if (srcEnd > destBegin && srcEnd < destEnd) {
+      RelocateRegionBackward(srcBegin, srcEnd, destEnd);
+    } else {
+      RelocateRegionForward(srcBegin, srcEnd, destBegin);
+    }
+  }
+
+  static void RelocateNonOverlappingRegion(void* aDest, void* aSrc,
+                                           size_t aCount, size_t aElemSize) {
+    ElemType* destBegin = static_cast<ElemType*>(aDest);
+    ElemType* srcBegin = static_cast<ElemType*>(aSrc);
+    ElemType* srcEnd = srcBegin + aCount;
+#ifdef DEBUG
+    ElemType* destEnd = destBegin + aCount;
+    MOZ_ASSERT(srcEnd <= destBegin || srcBegin >= destEnd);
+#endif
+    RelocateRegionForward(srcBegin, srcEnd, destBegin);
+  }
+
+ private:
+  static void RelocateRegionForward(ElemType* srcBegin, ElemType* srcEnd,
+                                    ElemType* destBegin) {
+    ElemType* srcElem = srcBegin;
+    ElemType* destElem = destBegin;
+
+    while (srcElem != srcEnd) {
+      RelocateElement(srcElem, destElem);
+      ++destElem;
+      ++srcElem;
+    }
+  }
+
+  static void RelocateRegionBackward(ElemType* srcBegin, ElemType* srcEnd,
+                                     ElemType* destEnd) {
+    ElemType* srcElem = srcEnd;
+    ElemType* destElem = destEnd;
+    while (srcElem != srcBegin) {
+      --destElem;
+      --srcElem;
+      RelocateElement(srcElem, destElem);
+    }
+  }
+
+  static void RelocateElement(ElemType* srcElem, ElemType* destElem) {
+    traits::Construct(destElem, std::move(*srcElem));
+    traits::Destruct(srcElem);
+  }
+};
+
+//
+// The default behaviour is to use memcpy/memmove for everything.
+//
+template <class E>
+struct MOZ_NEEDS_MEMMOVABLE_TYPE nsTArray_RelocationStrategy {
+  using Type = nsTArray_RelocateUsingMemutils;
+};
+
+//
+// Some classes require constructors/destructors to be called, so they are
+// specialized here.
+//
+#define MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(E)     \
+  template <>                                              \
+  struct nsTArray_RelocationStrategy<E> {                  \
+    using Type = nsTArray_RelocateUsingMoveConstructor<E>; \
+  };
+
+#define MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(T) \
+  template <typename S>                                             \
+  struct nsTArray_RelocationStrategy<T<S>> {                        \
+    using Type = nsTArray_RelocateUsingMoveConstructor<T<S>>;       \
+  };
+
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(JS::Heap)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(std::function)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR_FOR_TEMPLATE(mozilla::ipc::Endpoint)
+
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(nsRegion)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(nsIntRegion)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::layers::TileClient)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::SerializedStructuredCloneBuffer)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::ipc::StructuredCloneData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::ClonedMessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::ObjectStoreCursorResponse)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::IndexCursorResponse)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(
+    mozilla::dom::indexedDB::SerializedStructuredCloneReadInfo);
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(JSStructuredCloneData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::MessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::dom::RefMessageData)
+MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR(mozilla::SourceBufferTask)
+
+//
+// Base class for nsTArray_Impl that is templated on element type and derived
+// nsTArray_Impl class, to allow extra conversions to be added for specific
+// types.
+//
+template <class E, class Derived>
+struct nsTArray_TypedBase : public nsTArray_SafeElementAtHelper<E, Derived> {};
+
+//
+// Specialization of nsTArray_TypedBase for arrays containing JS::Heap<E>
+// elements.
+//
+// These conversions are safe because JS::Heap<E> and E share the same
+// representation, and since the result of the conversions are const references
+// we won't miss any barriers.
+//
+// The static_cast is necessary to obtain the correct address for the derived
+// class since we are a base class used in multiple inheritance.
+//
+template <class E, class Derived>
+struct nsTArray_TypedBase<JS::Heap<E>, Derived>
+    : public nsTArray_SafeElementAtHelper<JS::Heap<E>, Derived> {
+  operator const nsTArray<E>&() {
+    static_assert(sizeof(E) == sizeof(JS::Heap<E>),
+                  "JS::Heap<E> must be binary compatible with E.");
+    Derived* self = static_cast<Derived*>(this);
+    return *reinterpret_cast<nsTArray<E>*>(self);
+  }
+
+  operator const FallibleTArray<E>&() {
+    Derived* self = static_cast<Derived*>(this);
+    return *reinterpret_cast<FallibleTArray<E>*>(self);
+  }
+};
+
+namespace detail {
+
+// These helpers allow us to differentiate between tri-state comparator
+// functions and classes with LessThan() and Equal() methods. If an object, when
+// called as a function with two instances of our element type, returns an int,
+// we treat it as a tri-state comparator.
+//
+// T is the type of the comparator object we want to check. U is the array
+// element type that we'll be comparing.
+//
+// V is never passed, and is only used to allow us to specialize on the return
+// value of the comparator function.
+template <typename T, typename U, typename V = int>
+struct IsCompareMethod : std::false_type {};
+
+template <typename T, typename U>
+struct IsCompareMethod<
+    T, U, decltype(std::declval<T>()(std::declval<U>(), std::declval<U>()))>
+    : std::true_type {};
+
+// These two wrappers allow us to use either a tri-state comparator, or an
+// object with Equals() and LessThan() methods interchangeably. They provide a
+// tri-state Compare() method, and Equals() method, and a LessThan() method.
+//
+// Depending on the type of the underlying comparator, they either pass these
+// through directly, or synthesize them from the methods available on the
+// comparator.
+//
+// Callers should always use the most-specific of these methods that match their
+// purpose.
+
+// Comparator wrapper for a tri-state comparator function
+template <typename T, typename U, bool IsCompare = IsCompareMethod<T, U>::value>
+struct CompareWrapper {
+#ifdef _MSC_VER
+#  pragma warning(push)
+#  pragma warning(disable : 4180) /* Silence "qualifier applied to function \
+                                     type has no meaning" warning */
+#endif
+  MOZ_IMPLICIT CompareWrapper(const T& aComparator)
+      : mComparator(aComparator) {}
+
+  template <typename A, typename B>
+  int Compare(A& aLeft, B& aRight) const {
+    return mComparator(aLeft, aRight);
+  }
+
+  template <typename A, typename B>
+  bool Equals(A& aLeft, B& aRight) const {
+    return Compare(aLeft, aRight) == 0;
+  }
+
+  template <typename A, typename B>
+  bool LessThan(A& aLeft, B& aRight) const {
+    return Compare(aLeft, aRight) < 0;
+  }
+
+  const T& mComparator;
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
+};
+
+// Comparator wrapper for a class with Equals() and LessThan() methods.
+template <typename T, typename U>
+struct CompareWrapper<T, U, false> {
+  MOZ_IMPLICIT CompareWrapper(const T& aComparator)
+      : mComparator(aComparator) {}
+
+  template <typename A, typename B>
+  int Compare(A& aLeft, B& aRight) const {
+    if (Equals(aLeft, aRight)) {
+      return 0;
+    }
+    return LessThan(aLeft, aRight) ? -1 : 1;
+  }
+
+  template <typename A, typename B>
+  bool Equals(A& aLeft, B& aRight) const {
+    return mComparator.Equals(aLeft, aRight);
+  }
+
+  template <typename A, typename B>
+  bool LessThan(A& aLeft, B& aRight) const {
+    return mComparator.LessThan(aLeft, aRight);
+  }
+
+  const T& mComparator;
+};
+
+}  // namespace detail
+
+//
+// nsTArray_Impl contains most of the guts supporting nsTArray, FallibleTArray,
+// AutoTArray.
+//
+// The only situation in which you might need to use nsTArray_Impl in your code
+// is if you're writing code which mutates a TArray which may or may not be
+// infallible.
+//
+// Code which merely reads from a TArray which may or may not be infallible can
+// simply cast the TArray to |const nsTArray&|; both fallible and infallible
+// TArrays can be cast to |const nsTArray&|.
+//
+template <class E, class Alloc>
+class nsTArray_Impl
+    : public nsTArray_base<Alloc,
+                           typename nsTArray_RelocationStrategy<E>::Type>,
+      public nsTArray_TypedBase<E, nsTArray_Impl<E, Alloc>> {
+ private:
+  friend class nsTArray<E>;
+
+  typedef nsTArrayFallibleAllocator FallibleAlloc;
+  typedef nsTArrayInfallibleAllocator InfallibleAlloc;
+
+ public:
+  typedef typename nsTArray_RelocationStrategy<E>::Type relocation_type;
+  typedef nsTArray_base<Alloc, relocation_type> base_type;
+  typedef typename base_type::size_type size_type;
+  typedef typename base_type::index_type index_type;
+  typedef E value_type;
+  typedef nsTArray_Impl<E, Alloc> self_type;
+  typedef nsTArrayElementTraits<E> elem_traits;
+  typedef nsTArray_SafeElementAtHelper<E, self_type> safeelementat_helper_type;
+  typedef mozilla::ArrayIterator<value_type&, self_type> iterator;
+  typedef mozilla::ArrayIterator<const value_type&, self_type> const_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  using base_type::EmptyHdr;
+  using safeelementat_helper_type::SafeElementAt;
+
+  // A special value that is used to indicate an invalid or unknown index
+  // into the array.
+  static const index_type NoIndex = index_type(-1);
+
+  using base_type::Length;
+
+  //
+  // Finalization method
+  //
+
+  ~nsTArray_Impl() {
+    if (!base_type::IsEmpty()) {
+      ClearAndRetainStorage();
+    }
+    // mHdr cleanup will be handled by base destructor
+  }
+
+  //
+  // Initialization methods
+  //
+
+  nsTArray_Impl() = default;
+
+  // Initialize this array and pre-allocate some number of elements.
+  explicit nsTArray_Impl(size_type aCapacity) { SetCapacity(aCapacity); }
+
+  // Initialize this array with an r-value.
+  // Allow different types of allocators, since the allocator doesn't matter.
+  template <typename Allocator>
+  explicit nsTArray_Impl(nsTArray_Impl<E, Allocator>&& aOther) noexcept {
+    // We cannot be a (Copyable)AutoTArray because that overrides this ctor.
+    MOZ_ASSERT(!this->IsAutoArray());
+
+    // This does not use SwapArrayElements because that's unnecessarily complex.
+    this->MoveConstructNonAutoArray(aOther, sizeof(value_type),
+                                    MOZ_ALIGNOF(value_type));
+  }
+
+  // The array's copy-constructor performs a 'deep' copy of the given array.
+  // @param aOther The array object to copy.
+  //
+  // It's very important that we declare this method as taking |const
+  // self_type&| as opposed to taking |const nsTArray_Impl<E, OtherAlloc>| for
+  // an arbitrary OtherAlloc.
+  //
+  // If we don't declare a constructor taking |const self_type&|, C++ generates
+  // a copy-constructor for this class which merely copies the object's
+  // members, which is obviously wrong.
+  //
+  // You can pass an nsTArray_Impl<E, OtherAlloc> to this method because
+  // nsTArray_Impl<E, X> can be cast to const nsTArray_Impl<E, Y>&.  So the
+  // effect on the API is the same as if we'd declared this method as taking
+  // |const nsTArray_Impl<E, OtherAlloc>&|.
+  nsTArray_Impl(const nsTArray_Impl&) = default;
+
+  // Allow converting to a const array with a different kind of allocator,
+  // Since the allocator doesn't matter for const arrays
+  template <typename Allocator>
+  [[nodiscard]] operator const nsTArray_Impl<E, Allocator>&() const& {
+    return *reinterpret_cast<const nsTArray_Impl<E, Allocator>*>(this);
+  }
+  // And we have to do this for our subclasses too
+  [[nodiscard]] operator const nsTArray<E>&() const& {
+    return *reinterpret_cast<const nsTArray<E>*>(this);
+  }
+  [[nodiscard]] operator const FallibleTArray<E>&() const& {
+    return *reinterpret_cast<const FallibleTArray<E>*>(this);
+  }
+
+  // The array's assignment operator performs a 'deep' copy of the given
+  // array.  It is optimized to reuse existing storage if possible.
+  // @param aOther The array object to copy.
+  nsTArray_Impl& operator=(const nsTArray_Impl&) = default;
+
+  // The array's move assignment operator steals the underlying data from
+  // the other array.
+  // @param other  The array object to move from.
+  self_type& operator=(self_type&& aOther) {
+    if (this != &aOther) {
+      Clear();
+      this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    }
+    return *this;
+  }
+
+  // Return true if this array has the same length and the same
+  // elements as |aOther|.
+  template <typename Allocator>
+  [[nodiscard]] bool operator==(
+      const nsTArray_Impl<E, Allocator>& aOther) const {
+    size_type len = Length();
+    if (len != aOther.Length()) {
+      return false;
+    }
+
+    // XXX std::equal would be as fast or faster here
+    for (index_type i = 0; i < len; ++i) {
+      if (!(operator[](i) == aOther[i])) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  // Return true if this array does not have the same length and the same
+  // elements as |aOther|.
+  [[nodiscard]] bool operator!=(const self_type& aOther) const {
+    return !operator==(aOther);
+  }
+
+  // If Alloc == FallibleAlloc, ReplaceElementsAt might fail, without a way to
+  // signal this to the caller, so we disallow copying via operator=. Callers
+  // should use ReplaceElementsAt with a fallible argument instead, and check
+  // the result.
+  template <typename Allocator,
+            typename = std::enable_if_t<std::is_same_v<Alloc, InfallibleAlloc>,
+                                        Allocator>>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    AssignInternal<InfallibleAlloc>(aOther.Elements(), aOther.Length());
+    return *this;
+  }
+
+  template <typename Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    Clear();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    return *this;
+  }
+
+  // @return The amount of memory used by this nsTArray_Impl, excluding
+  // sizeof(*this). If you want to measure anything hanging off the array, you
+  // must iterate over the elements and measure them individually; hence the
+  // "Shallow" prefix.
+  [[nodiscard]] size_t ShallowSizeOfExcludingThis(
+      mozilla::MallocSizeOf aMallocSizeOf) const {
+    if (this->UsesAutoArrayBuffer() || this->HasEmptyHeader()) {
+      return 0;
+    }
+    return aMallocSizeOf(this->Hdr());
+  }
+
+  // @return The amount of memory used by this nsTArray_Impl, including
+  // sizeof(*this). If you want to measure anything hanging off the array, you
+  // must iterate over the elements and measure them individually; hence the
+  // "Shallow" prefix.
+  [[nodiscard]] size_t ShallowSizeOfIncludingThis(
+      mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  //
+  // Accessor methods
+  //
+
+  // This method provides direct access to the array elements.
+  // @return A pointer to the first element of the array.  If the array is
+  // empty, then this pointer must not be dereferenced.
+  [[nodiscard]] value_type* Elements() MOZ_NONNULL_RETURN {
+    return reinterpret_cast<value_type*>(Hdr() + 1);
+  }
+
+  // This method provides direct, readonly access to the array elements.
+  // @return A pointer to the first element of the array.  If the array is
+  // empty, then this pointer must not be dereferenced.
+  [[nodiscard]] const value_type* Elements() const MOZ_NONNULL_RETURN {
+    return reinterpret_cast<const value_type*>(Hdr() + 1);
+  }
+
+  // This method provides direct access to an element of the array. The given
+  // index must be within the array bounds.
+  // @param aIndex The index of an element in the array.
+  // @return A reference to the i'th element of the array.
+  [[nodiscard]] value_type& ElementAt(index_type aIndex) {
+    if (MOZ_UNLIKELY(aIndex >= Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+    }
+    return Elements()[aIndex];
+  }
+
+  // This method provides direct, readonly access to an element of the array
+  // The given index must be within the array bounds.
+  // @param aIndex The index of an element in the array.
+  // @return A const reference to the i'th element of the array.
+  [[nodiscard]] const value_type& ElementAt(index_type aIndex) const {
+    if (MOZ_UNLIKELY(aIndex >= Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+    }
+    return Elements()[aIndex];
+  }
+
+  // This method provides direct access to an element of the array in a bounds
+  // safe manner. If the requested index is out of bounds the provided default
+  // value is returned.
+  // @param aIndex The index of an element in the array.
+  // @param aDef   The value to return if the index is out of bounds.
+  [[nodiscard]] value_type& SafeElementAt(index_type aIndex, value_type& aDef) {
+    return aIndex < Length() ? Elements()[aIndex] : aDef;
+  }
+
+  // This method provides direct access to an element of the array in a bounds
+  // safe manner. If the requested index is out of bounds the provided default
+  // value is returned.
+  // @param aIndex The index of an element in the array.
+  // @param aDef   The value to return if the index is out of bounds.
+  [[nodiscard]] const value_type& SafeElementAt(index_type aIndex,
+                                                const value_type& aDef) const {
+    return aIndex < Length() ? Elements()[aIndex] : aDef;
+  }
+
+  // Shorthand for ElementAt(aIndex)
+  [[nodiscard]] value_type& operator[](index_type aIndex) {
+    return ElementAt(aIndex);
+  }
+
+  // Shorthand for ElementAt(aIndex)
+  [[nodiscard]] const value_type& operator[](index_type aIndex) const {
+    return ElementAt(aIndex);
+  }
+
+  // Shorthand for ElementAt(length - 1)
+  [[nodiscard]] value_type& LastElement() { return ElementAt(Length() - 1); }
+
+  // Shorthand for ElementAt(length - 1)
+  [[nodiscard]] const value_type& LastElement() const {
+    return ElementAt(Length() - 1);
+  }
+
+  // Shorthand for SafeElementAt(length - 1, def)
+  [[nodiscard]] value_type& SafeLastElement(value_type& aDef) {
+    return SafeElementAt(Length() - 1, aDef);
+  }
+
+  // Shorthand for SafeElementAt(length - 1, def)
+  [[nodiscard]] const value_type& SafeLastElement(
+      const value_type& aDef) const {
+    return SafeElementAt(Length() - 1, aDef);
+  }
+
+  // Methods for range-based for loops.
+  [[nodiscard]] iterator begin() { return iterator(*this, 0); }
+  [[nodiscard]] const_iterator begin() const {
+    return const_iterator(*this, 0);
+  }
+  [[nodiscard]] const_iterator cbegin() const { return begin(); }
+  [[nodiscard]] iterator end() { return iterator(*this, Length()); }
+  [[nodiscard]] const_iterator end() const {
+    return const_iterator(*this, Length());
+  }
+  [[nodiscard]] const_iterator cend() const { return end(); }
+
+  // Methods for reverse iterating.
+  [[nodiscard]] reverse_iterator rbegin() { return reverse_iterator(end()); }
+  [[nodiscard]] const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+  [[nodiscard]] const_reverse_iterator crbegin() const { return rbegin(); }
+  [[nodiscard]] reverse_iterator rend() { return reverse_iterator(begin()); }
+  [[nodiscard]] const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+  [[nodiscard]] const_reverse_iterator crend() const { return rend(); }
+
+  // Span integration
+
+  [[nodiscard]] operator mozilla::Span<value_type>() {
+    return mozilla::Span<value_type>(Elements(), Length());
+  }
+
+  [[nodiscard]] operator mozilla::Span<const value_type>() const {
+    return mozilla::Span<const value_type>(Elements(), Length());
+  }
+
+  //
+  // Search methods
+  //
+
+  // This method searches for the first element in this array that is equal
+  // to the given element.
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       true if the element was found.
+  template <class Item, class Comparator>
+  [[nodiscard]] bool Contains(const Item& aItem,
+                              const Comparator& aComp) const {
+    return ApplyIf(
+        aItem, 0, aComp, []() { return true; }, []() { return false; });
+  }
+
+  // Like Contains(), but assumes a sorted array.
+  template <class Item, class Comparator>
+  [[nodiscard]] bool ContainsSorted(const Item& aItem,
+                                    const Comparator& aComp) const {
+    return BinaryIndexOf(aItem, aComp) != NoIndex;
+  }
+
+  // This method searches for the first element in this array that is equal
+  // to the given element.  This method assumes that 'operator==' is defined
+  // for value_type.
+  // @param aItem  The item to search for.
+  // @return       true if the element was found.
+  template <class Item>
+  [[nodiscard]] bool Contains(const Item& aItem) const {
+    return Contains(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // Like Contains(), but assumes a sorted array.
+  template <class Item>
+  [[nodiscard]] bool ContainsSorted(const Item& aItem) const {
+    return BinaryIndexOf(aItem) != NoIndex;
+  }
+
+  // This method searches for the offset of the first element in this
+  // array that is equal to the given element.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type IndexOf(const Item& aItem, index_type aStart,
+                                   const Comparator& aComp) const {
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    const value_type* iter = Elements() + aStart;
+    const value_type* iend = Elements() + Length();
+    for (; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return index_type(iter - Elements());
+      }
+    }
+    return NoIndex;
+  }
+
+  // This method searches for the offset of the first element in this
+  // array that is equal to the given element.  This method assumes
+  // that 'operator==' is defined for value_type.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type IndexOf(const Item& aItem,
+                                   index_type aStart = 0) const {
+    return IndexOf(aItem, aStart, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method searches for the offset of the last element in this
+  // array that is equal to the given element.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.  If greater than or equal to the
+  //               length of the array, then the entire array is searched.
+  // @param aComp  The Comparator used to determine element equality.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type LastIndexOf(const Item& aItem, index_type aStart,
+                                       const Comparator& aComp) const {
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_type endOffset = aStart >= Length() ? Length() : aStart + 1;
+    const value_type* iend = Elements() - 1;
+    const value_type* iter = iend + endOffset;
+    for (; iter != iend; --iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return index_type(iter - Elements());
+      }
+    }
+    return NoIndex;
+  }
+
+  // This method searches for the offset of the last element in this
+  // array that is equal to the given element.  This method assumes
+  // that 'operator==' is defined for value_type.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.  If greater than or equal to the
+  //               length of the array, then the entire array is searched.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type LastIndexOf(const Item& aItem,
+                                       index_type aStart = NoIndex) const {
+    return LastIndexOf(aItem, aStart, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method searches for the offset for the element in this array
+  // that is equal to the given element. The array is assumed to be sorted.
+  // If there is more than one equivalent element, there is no guarantee
+  // on which one will be returned.
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type BinaryIndexOf(const Item& aItem,
+                                         const Comparator& aComp) const {
+    using mozilla::BinarySearchIf;
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_t index;
+    bool found = BinarySearchIf(
+        Elements(), 0, Length(),
+        // Note: We pass the Compare() args here in reverse order and negate the
+        // results for compatibility reasons. Some existing callers use Equals()
+        // functions with first arguments which match aElement but not aItem, or
+        // second arguments that match aItem but not aElement. To accommodate
+        // those callers, we preserve the argument order of the older version of
+        // this API. These callers, however, should be fixed, and this special
+        // case removed.
+        [&](const value_type& aElement) {
+          return -comp.Compare(aElement, aItem);
+        },
+        &index);
+    return found ? index : NoIndex;
+  }
+
+  // This method searches for the offset for the element in this array
+  // that is equal to the given element. The array is assumed to be sorted.
+  // This method assumes that 'operator==' and 'operator<' are defined.
+  // @param aItem  The item to search for.
+  // @return       The index of the found element or NoIndex if not found.
+  template <class Item>
+  [[nodiscard]] index_type BinaryIndexOf(const Item& aItem) const {
+    return BinaryIndexOf(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  //
+  // Mutation methods
+  //
+ private:
+  template <typename ActualAlloc, class Item>
+  typename ActualAlloc::ResultType AssignInternal(const Item* aArray,
+                                                  size_type aArrayLen);
+
+ public:
+  template <class Allocator, typename ActualAlloc = Alloc>
+  [[nodiscard]] typename ActualAlloc::ResultType Assign(
+      const nsTArray_Impl<E, Allocator>& aOther) {
+    return AssignInternal<ActualAlloc>(aOther.Elements(), aOther.Length());
+  }
+
+  template <class Allocator>
+  [[nodiscard]] bool Assign(const nsTArray_Impl<E, Allocator>& aOther,
+                            const mozilla::fallible_t&) {
+    return Assign<Allocator, FallibleAlloc>(aOther);
+  }
+
+  template <class Allocator>
+  void Assign(nsTArray_Impl<E, Allocator>&& aOther) {
+    Clear();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  // This method call the destructor on each element of the array, empties it,
+  // but does not shrink the array's capacity.
+  // See also SetLengthAndRetainStorage.
+  // Make sure to call Compact() if needed to avoid keeping a huge array
+  // around.
+  void ClearAndRetainStorage() {
+    if (this->HasEmptyHeader()) {
+      return;
+    }
+
+    DestructRange(0, Length());
+    base_type::mHdr->mLength = 0;
+  }
+
+  // This method modifies the length of the array, but unlike SetLength
+  // it doesn't deallocate/reallocate the current internal storage.
+  // The new length MUST be shorter than or equal to the current capacity.
+  // If the new length is larger than the existing length of the array,
+  // then new elements will be constructed using value_type's default
+  // constructor.  If shorter, elements will be destructed and removed.
+  // See also ClearAndRetainStorage.
+  // @param aNewLen  The desired length of this array.
+  void SetLengthAndRetainStorage(size_type aNewLen) {
+    MOZ_ASSERT(aNewLen <= base_type::Capacity());
+    size_type oldLen = Length();
+    if (aNewLen > oldLen) {
+      /// XXX(Bug 1631367) SetLengthAndRetainStorage should be disabled for
+      /// FallibleTArray.
+      InsertElementsAtInternal<InfallibleAlloc>(oldLen, aNewLen - oldLen);
+      return;
+    }
+    if (aNewLen < oldLen) {
+      DestructRange(aNewLen, oldLen - aNewLen);
+      base_type::mHdr->mLength = aNewLen;
+    }
+  }
+
+  // This method replaces a range of elements in this array.
+  // @param aStart    The starting index of the elements to replace.
+  // @param aCount    The number of elements to replace.  This may be zero to
+  //                  insert elements without removing any existing elements.
+  // @param aArray    The values to copy into this array.  Must be non-null,
+  //                  and these elements must not already exist in the array
+  //                  being modified.
+  // @param aArrayLen The number of values to copy into this array.
+  // @return          A pointer to the new elements in the array, or null if
+  //                  the operation failed due to insufficient memory.
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* ReplaceElementsAtInternal(index_type aStart, size_type aCount,
+                                        const Item* aArray,
+                                        size_type aArrayLen);
+
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const Item* aArray,
+                                              size_type aArrayLen,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aArray,
+                                                    aArrayLen);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const nsTArray<Item>& aArray,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aArray);
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              mozilla::Span<Item> aSpan,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aSpan);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  [[nodiscard]] value_type* ReplaceElementsAt(index_type aStart,
+                                              size_type aCount,
+                                              const Item& aItem,
+                                              const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aStart, aCount, aItem);
+  }
+
+  // A variation on the ReplaceElementsAt method defined above.
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementAt(index_type aIndex,
+                                                 Item&& aItem) {
+    value_type* const elem = &ElementAt(aIndex);
+    elem_traits::Destruct(elem);
+    elem_traits::Construct(elem, std::forward<Item>(aItem));
+    return mozilla::WrapNotNullUnchecked(elem);
+  }
+
+  // InsertElementsAt is ReplaceElementsAt with 0 elements to replace.
+  // XXX Provide a proper documentation of InsertElementsAt.
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             const Item* aArray,
+                                             size_type aArrayLen,
+                                             const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aIndex, 0, aArray,
+                                                    aArrayLen);
+  }
+
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* InsertElementsAt(
+      index_type aIndex, const nsTArray_Impl<Item, Allocator>& aArray,
+      const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(
+        aIndex, 0, aArray.Elements(), aArray.Length());
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             mozilla::Span<Item> aSpan,
+                                             const mozilla::fallible_t&) {
+    return ReplaceElementsAtInternal<FallibleAlloc>(aIndex, 0, aSpan.Elements(),
+                                                    aSpan.Length());
+  }
+
+ private:
+  template <typename ActualAlloc>
+  value_type* InsertElementAtInternal(index_type aIndex);
+
+  // Insert a new element without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly inserted element, or null on OOM.
+ public:
+  [[nodiscard]] value_type* InsertElementAt(index_type aIndex,
+                                            const mozilla::fallible_t&) {
+    return InsertElementAtInternal<FallibleAlloc>(aIndex);
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* InsertElementAtInternal(index_type aIndex, Item&& aItem);
+
+  // Insert a new element, move constructing if possible.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementAt(index_type aIndex, Item&& aItem,
+                                            const mozilla::fallible_t&) {
+    return InsertElementAtInternal<FallibleAlloc>(aIndex,
+                                                  std::forward<Item>(aItem));
+  }
+
+  // Reconstruct the element at the given index, and return a pointer to the
+  // reconstructed element.  This will destroy the existing element and
+  // default-construct a new one, giving you a state much like what single-arg
+  // InsertElementAt(), or no-arg AppendElement() does, but without changing the
+  // length of the array.
+  //
+  // array[idx] = value_type()
+  //
+  // would accomplish the same thing as long as value_type has the appropriate
+  // moving operator=, but some types don't for various reasons.
+  mozilla::NotNull<value_type*> ReconstructElementAt(index_type aIndex) {
+    value_type* elem = &ElementAt(aIndex);
+    elem_traits::Destruct(elem);
+    elem_traits::Construct(elem);
+    return mozilla::WrapNotNullUnchecked(elem);
+  }
+
+  // This method searches for the smallest index of an element that is strictly
+  // greater than |aItem|. If |aItem| is inserted at this index, the array will
+  // remain sorted and |aItem| would come after all elements that are equal to
+  // it. If |aItem| is greater than or equal to all elements in the array, the
+  // array length is returned.
+  //
+  // Note that consumers who want to know whether there are existing items equal
+  // to |aItem| in the array can just check that the return value here is > 0
+  // and indexing into the previous slot gives something equal to |aItem|.
+  //
+  //
+  // @param aItem  The item to search for.
+  // @param aComp  The Comparator used.
+  // @return        The index of greatest element <= to |aItem|
+  // @precondition The array is sorted
+  template <class Item, class Comparator>
+  [[nodiscard]] index_type IndexOfFirstElementGt(
+      const Item& aItem, const Comparator& aComp) const {
+    using mozilla::BinarySearchIf;
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    size_t index;
+    BinarySearchIf(
+        Elements(), 0, Length(),
+        [&](const value_type& aElement) {
+          return comp.Compare(aElement, aItem) <= 0 ? 1 : -1;
+        },
+        &index);
+    return index;
+  }
+
+  // A variation on the IndexOfFirstElementGt method defined above.
+  template <class Item>
+  [[nodiscard]] index_type IndexOfFirstElementGt(const Item& aItem) const {
+    return IndexOfFirstElementGt(aItem,
+                                 nsDefaultComparator<value_type, Item>());
+  }
+
+ private:
+  template <typename ActualAlloc, class Item, class Comparator>
+  value_type* InsertElementSortedInternal(Item&& aItem,
+                                          const Comparator& aComp) {
+    index_type index = IndexOfFirstElementGt<Item, Comparator>(aItem, aComp);
+    return InsertElementAtInternal<ActualAlloc>(index,
+                                                std::forward<Item>(aItem));
+  }
+
+  // Inserts |aItem| at such an index to guarantee that if the array
+  // was previously sorted, it will remain sorted after this
+  // insertion.
+ public:
+  template <class Item, class Comparator>
+  [[nodiscard]] value_type* InsertElementSorted(Item&& aItem,
+                                                const Comparator& aComp,
+                                                const mozilla::fallible_t&) {
+    return InsertElementSortedInternal<FallibleAlloc>(std::forward<Item>(aItem),
+                                                      aComp);
+  }
+
+  // A variation on the InsertElementSorted method defined above.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementSorted(Item&& aItem,
+                                                const mozilla::fallible_t&) {
+    return InsertElementSortedInternal<FallibleAlloc>(
+        std::forward<Item>(aItem), nsDefaultComparator<value_type, Item>{});
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* AppendElementsInternal(const Item* aArray, size_type aArrayLen);
+
+  // This method appends elements to the end of this array.
+  // @param aArray    The elements to append to this array.
+  // @param aArrayLen The number of elements to append to this array.
+  // @return          A pointer to the new elements in the array, or null if
+  //                  the operation failed due to insufficient memory.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* AppendElements(const Item* aArray,
+                                           size_type aArrayLen,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aArray, aArrayLen);
+  }
+
+  template <class Item>
+  [[nodiscard]] value_type* AppendElements(mozilla::Span<Item> aSpan,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aSpan.Elements(),
+                                                 aSpan.Length());
+  }
+
+  // A variation on the AppendElements method defined above.
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* AppendElements(
+      const nsTArray_Impl<Item, Allocator>& aArray,
+      const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aArray.Elements(),
+                                                 aArray.Length());
+  }
+
+ private:
+  template <typename ActualAlloc, class Item, class Allocator>
+  value_type* AppendElementsInternal(nsTArray_Impl<Item, Allocator>&& aArray);
+
+  // Move all elements from another array to the end of this array.
+  // @return A pointer to the newly appended elements, or null on OOM.
+ public:
+  template <class Item, class Allocator>
+  [[nodiscard]] value_type* AppendElements(
+      nsTArray_Impl<Item, Allocator>&& aArray, const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(std::move(aArray));
+  }
+
+  // Append a new element, constructed in place from the provided arguments.
+ protected:
+  template <typename ActualAlloc, class... Args>
+  value_type* EmplaceBackInternal(Args&&... aItem);
+
+ public:
+  template <class... Args>
+  [[nodiscard]] value_type* EmplaceBack(const mozilla::fallible_t&,
+                                        Args&&... aArgs) {
+    return EmplaceBackInternal<FallibleAlloc, Args...>(
+        std::forward<Args>(aArgs)...);
+  }
+
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* AppendElementInternal(Item&& aItem);
+
+  // Append a new element, move constructing if possible.
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* AppendElement(Item&& aItem,
+                                          const mozilla::fallible_t&) {
+    return AppendElementInternal<FallibleAlloc>(std::forward<Item>(aItem));
+  }
+
+ private:
+  template <typename ActualAlloc>
+  value_type* AppendElementsInternal(size_type aCount) {
+    if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+            Length(), aCount, sizeof(value_type)))) {
+      return nullptr;
+    }
+    value_type* elems = Elements() + Length();
+    size_type i;
+    for (i = 0; i < aCount; ++i) {
+      elem_traits::Construct(elems + i);
+    }
+    this->IncrementLength(aCount);
+    return elems;
+  }
+
+  // Append new elements without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly appended elements, or null on OOM.
+ public:
+  [[nodiscard]] value_type* AppendElements(size_type aCount,
+                                           const mozilla::fallible_t&) {
+    return AppendElementsInternal<FallibleAlloc>(aCount);
+  }
+
+ private:
+  // Append a new element without copy-constructing. This is useful to avoid
+  // temporaries.
+  // @return A pointer to the newly appended element, or null on OOM.
+ public:
+  [[nodiscard]] value_type* AppendElement(const mozilla::fallible_t&) {
+    return AppendElements(1, mozilla::fallible);
+  }
+
+  // This method removes a single element from this array, like
+  // std::vector::erase.
+  // @param pos to the element to remove
+  const_iterator RemoveElementAt(const_iterator pos) {
+    MOZ_ASSERT(pos.GetArray() == this);
+
+    RemoveElementAt(pos.GetIndex());
+    return pos;
+  }
+
+  // This method removes a range of elements from this array, like
+  // std::vector::erase.
+  // @param first iterator to the first of elements to remove
+  // @param last iterator to the last of elements to remove
+  const_iterator RemoveElementsRange(const_iterator first,
+                                     const_iterator last) {
+    MOZ_ASSERT(first.GetArray() == this);
+    MOZ_ASSERT(last.GetArray() == this);
+    MOZ_ASSERT(last.GetIndex() >= first.GetIndex());
+
+    RemoveElementsAt(first.GetIndex(), last.GetIndex() - first.GetIndex());
+    return first;
+  }
+
+  // This method removes a range of elements from this array.
+  // @param aStart The starting index of the elements to remove.
+  // @param aCount The number of elements to remove.
+  void RemoveElementsAt(index_type aStart, size_type aCount);
+
+ private:
+  // Remove a range of elements from this array, but do not check that
+  // the range is in bounds.
+  // @param aStart The starting index of the elements to remove.
+  // @param aCount The number of elements to remove.
+  void RemoveElementsAtUnsafe(index_type aStart, size_type aCount);
+
+ public:
+  // A variation on the RemoveElementsAt method defined above.
+  void RemoveElementAt(index_type aIndex) { RemoveElementsAt(aIndex, 1); }
+
+  // A variation on RemoveElementAt that removes the last element.
+  void RemoveLastElement() { RemoveLastElements(1); }
+
+  // A variation on RemoveElementsAt that removes the last 'aCount' elements.
+  void RemoveLastElements(const size_type aCount) {
+    // This assertion is redundant, but produces a better error message than the
+    // release assertion within TruncateLength.
+    MOZ_ASSERT(aCount <= Length());
+    TruncateLength(Length() - aCount);
+  }
+
+  // Removes the last element of the array and returns a copy of it.
+  [[nodiscard]] value_type PopLastElement() {
+    // This function intentionally does not call ElementsAt and calls
+    // TruncateLengthUnsafe directly to avoid multiple release checks for
+    // non-emptiness.
+    // This debug assertion is redundant, but produces a better error message
+    // than the release assertion below.
+    MOZ_ASSERT(!base_type::IsEmpty());
+    const size_type oldLen = Length();
+    if (MOZ_UNLIKELY(0 == oldLen)) {
+      mozilla::detail::InvalidArrayIndex_CRASH(1, 0);
+    }
+    value_type elem = std::move(Elements()[oldLen - 1]);
+    TruncateLengthUnsafe(oldLen - 1);
+    return elem;
+  }
+
+  // This method performs index-based removals from an array without preserving
+  // the order of the array. This is useful if you are using the array as a
+  // set-like data structure.
+  //
+  // These removals are efficient, as they move as few elements as possible. At
+  // most N elements, where N is the number of removed elements, will have to
+  // be relocated.
+  //
+  // ## Examples
+  //
+  // When removing an element from the end of the array, it can be removed in
+  // place, by destroying it and decrementing the length.
+  //
+  // [ 1, 2, 3 ] => [ 1, 2 ]
+  //         ^
+  //
+  // When removing any other single element, it is removed by swapping it with
+  // the last element, and then decrementing the length as before.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ]  => [ 1, 6, 3, 4, 5 ]
+  //      ^
+  //
+  // This method also supports efficiently removing a range of elements. If they
+  // are at the end, then they can all be removed like in the one element case.
+  //
+  // [ 1, 2, 3, 4, 5, 6 ] => [ 1, 2 ]
+  //         ^--------^
+  //
+  // If more elements are removed than exist after the removed section, the
+  // remaining elements will be shifted down like in a normal removal.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 2, 7, 8 ]
+  //         ^--------^
+  //
+  // And if fewer elements are removed than exist after the removed section,
+  // elements will be moved from the end of the array to fill the vacated space.
+  //
+  // [ 1, 2, 3, 4, 5, 6, 7, 8 ] => [ 1, 7, 8, 4, 5, 6 ]
+  //      ^--^
+  //
+  // @param aStart The starting index of the elements to remove. @param aCount
+  // The number of elements to remove.
+  void UnorderedRemoveElementsAt(index_type aStart, size_type aCount);
+
+  // A variation on the UnorderedRemoveElementsAt method defined above to remove
+  // a single element. This operation is sometimes called `SwapRemove`.
+  //
+  // This method is O(1), but does not preserve the order of the elements.
+  void UnorderedRemoveElementAt(index_type aIndex) {
+    UnorderedRemoveElementsAt(aIndex, 1);
+  }
+
+  void Clear() {
+    ClearAndRetainStorage();
+    base_type::ShrinkCapacityToZero(sizeof(value_type),
+                                    MOZ_ALIGNOF(value_type));
+  }
+
+  // This method removes elements based on the return value of the
+  // callback function aPredicate. If the function returns true for
+  // an element, the element is removed. aPredicate will be called
+  // for each element in order. It is not safe to access the array
+  // inside aPredicate.
+  //
+  // Returns the number of elements removed.
+  template <typename Predicate>
+  size_type RemoveElementsBy(Predicate aPredicate);
+
+  // This helper function combines IndexOf with RemoveElementAt to "search
+  // and destroy" the first element that is equal to the given element.
+  // @param aItem The item to search for.
+  // @param aComp The Comparator used to determine element equality.
+  // @return true if the element was found
+  template <class Item, class Comparator>
+  bool RemoveElement(const Item& aItem, const Comparator& aComp) {
+    index_type i = IndexOf(aItem, 0, aComp);
+    if (i == NoIndex) {
+      return false;
+    }
+
+    RemoveElementsAtUnsafe(i, 1);
+    return true;
+  }
+
+  // A variation on the RemoveElement method defined above that assumes
+  // that 'operator==' is defined for value_type.
+  template <class Item>
+  bool RemoveElement(const Item& aItem) {
+    return RemoveElement(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This helper function combines IndexOfFirstElementGt with
+  // RemoveElementAt to "search and destroy" the last element that
+  // is equal to the given element.
+  // @param aItem The item to search for.
+  // @param aComp The Comparator used to determine element equality.
+  // @return true if the element was found
+  template <class Item, class Comparator>
+  bool RemoveElementSorted(const Item& aItem, const Comparator& aComp) {
+    index_type index = IndexOfFirstElementGt(aItem, aComp);
+    if (index > 0 && aComp.Equals(ElementAt(index - 1), aItem)) {
+      RemoveElementsAtUnsafe(index - 1, 1);
+      return true;
+    }
+    return false;
+  }
+
+  // A variation on the RemoveElementSorted method defined above.
+  template <class Item>
+  bool RemoveElementSorted(const Item& aItem) {
+    return RemoveElementSorted(aItem, nsDefaultComparator<value_type, Item>());
+  }
+
+  // This method causes the elements contained in this array and the given
+  // array to be swapped.
+  template <class Allocator>
+  void SwapElements(nsTArray_Impl<E, Allocator>& aOther) {
+    // The only case this might fail were if someone called this with a
+    // AutoTArray upcast to nsTArray_Impl, under the conditions mentioned in the
+    // overload for AutoTArray below.
+    this->template SwapArrayElements<InfallibleAlloc>(
+        aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <size_t N>
+  void SwapElements(AutoTArray<E, N>& aOther) {
+    // Allocation might fail if Alloc==FallibleAlloc and
+    // Allocator==InfallibleAlloc and aOther uses auto storage. Allow this for
+    // small inline sizes, and crash in the rare case of a small OOM error.
+    static_assert(!std::is_same_v<Alloc, FallibleAlloc> ||
+                  sizeof(E) * N <= 1024);
+    this->template SwapArrayElements<InfallibleAlloc>(
+        aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <class Allocator>
+  [[nodiscard]] auto SwapElements(nsTArray_Impl<E, Allocator>& aOther,
+                                  const mozilla::fallible_t&) {
+    // Allocation might fail if Alloc==FallibleAlloc and
+    // Allocator==InfallibleAlloc and aOther uses auto storage.
+    return FallibleAlloc::Result(
+        this->template SwapArrayElements<FallibleAlloc>(
+            aOther, sizeof(value_type), MOZ_ALIGNOF(value_type)));
+  }
+
+ private:
+  // Used by ApplyIf functions to invoke a callable that takes either:
+  // - Nothing: F(void)
+  // - Index only: F(size_t)
+  // - Reference to element only: F(maybe-const value_type&)
+  // - Both index and reference: F(size_t, maybe-const value_type&)
+  // `value_type` must be const when called from const method.
+  template <typename T, typename Param0, typename Param1>
+  struct InvokeWithIndexAndOrReferenceHelper {
+    static constexpr bool valid = false;
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, void, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T&) {
+      return f();
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T&) {
+      return f(i);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, T&, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T& e) {
+      return f(e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, const T&, void> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t, T& e) {
+      return f(e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, T&> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T& e) {
+      return f(i, e);
+    }
+  };
+  template <typename T>
+  struct InvokeWithIndexAndOrReferenceHelper<T, size_t, const T&> {
+    static constexpr bool valid = true;
+    template <typename F>
+    static auto Invoke(F&& f, size_t i, T& e) {
+      return f(i, e);
+    }
+  };
+  template <typename T, typename F>
+  static auto InvokeWithIndexAndOrReference(F&& f, size_t i, T& e) {
+    using Invoker = InvokeWithIndexAndOrReferenceHelper<
+        T, typename mozilla::FunctionTypeTraits<F>::template ParameterType<0>,
+        typename mozilla::FunctionTypeTraits<F>::template ParameterType<1>>;
+    static_assert(Invoker::valid,
+                  "ApplyIf's Function parameters must match either: (void), "
+                  "(size_t), (maybe-const value_type&), or "
+                  "(size_t, maybe-const value_type&)");
+    return Invoker::Invoke(std::forward<F>(f), i, e);
+  }
+
+ public:
+  // 'Apply' family of methods.
+  //
+  // The advantages of using Apply methods with lambdas include:
+  // - Safety of accessing elements from within the call, when the array cannot
+  //   have been modified between the iteration and the subsequent access.
+  // - Avoiding moot conversions: pointer->index during a search, followed by
+  //   index->pointer after the search when accessing the element.
+  // - Embedding your code into the algorithm, giving the compiler more chances
+  //   to optimize.
+
+  // Search for the first element comparing equal to aItem with the given
+  // comparator (`==` by default).
+  // If such an element exists, return the result of evaluating either:
+  // - `aFunction()`
+  // - `aFunction(index_type)`
+  // - `aFunction(maybe-const? value_type&)`
+  // - `aFunction(index_type, maybe-const? value_type&)`
+  // (`aFunction` must have one of the above signatures with these exact types,
+  //  including references; implicit conversions or generic types not allowed.
+  //  If `this` array is const, the referenced `value_type` must be const too;
+  //  otherwise it may be either const or non-const.)
+  // But if the element is not found, return the result of evaluating
+  // `aFunctionElse()`.
+  template <class Item, class Comparator, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, const Comparator& aComp,
+               Function&& aFunction, FunctionElse&& aFunctionElse) const {
+    static_assert(
+        std::is_same_v<
+            typename mozilla::FunctionTypeTraits<Function>::ReturnType,
+            typename mozilla::FunctionTypeTraits<FunctionElse>::ReturnType>,
+        "ApplyIf's `Function` and `FunctionElse` must return the same type.");
+
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    const value_type* const elements = Elements();
+    const value_type* const iend = elements + Length();
+    for (const value_type* iter = elements + aStart; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return InvokeWithIndexAndOrReference<const value_type>(
+            std::forward<Function>(aFunction), iter - elements, *iter);
+      }
+    }
+    return aFunctionElse();
+  }
+  template <class Item, class Comparator, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, const Comparator& aComp,
+               Function&& aFunction, FunctionElse&& aFunctionElse) {
+    static_assert(
+        std::is_same_v<
+            typename mozilla::FunctionTypeTraits<Function>::ReturnType,
+            typename mozilla::FunctionTypeTraits<FunctionElse>::ReturnType>,
+        "ApplyIf's `Function` and `FunctionElse` must return the same type.");
+
+    ::detail::CompareWrapper<Comparator, Item> comp(aComp);
+
+    value_type* const elements = Elements();
+    value_type* const iend = elements + Length();
+    for (value_type* iter = elements + aStart; iter != iend; ++iter) {
+      if (comp.Equals(*iter, aItem)) {
+        return InvokeWithIndexAndOrReference<value_type>(
+            std::forward<Function>(aFunction), iter - elements, *iter);
+      }
+    }
+    return aFunctionElse();
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, Function&& aFunction,
+               FunctionElse&& aFunctionElse) const {
+    return ApplyIf(aItem, aStart, nsDefaultComparator<value_type, Item>(),
+                   std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, index_type aStart, Function&& aFunction,
+               FunctionElse&& aFunctionElse) {
+    return ApplyIf(aItem, aStart, nsDefaultComparator<value_type, Item>(),
+                   std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, Function&& aFunction,
+               FunctionElse&& aFunctionElse) const {
+    return ApplyIf(aItem, 0, std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+  template <class Item, class Function, class FunctionElse>
+  auto ApplyIf(const Item& aItem, Function&& aFunction,
+               FunctionElse&& aFunctionElse) {
+    return ApplyIf(aItem, 0, std::forward<Function>(aFunction),
+                   std::forward<FunctionElse>(aFunctionElse));
+  }
+
+  //
+  // Allocation
+  //
+
+  // This method may increase the capacity of this array object to the
+  // specified amount.  This method may be called in advance of several
+  // AppendElement operations to minimize heap re-allocations.  This method
+  // will not reduce the number of elements in this array.
+  // @param aCapacity The desired capacity of this array.
+  // @return True if the operation succeeded; false if we ran out of memory
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType SetCapacity(size_type aCapacity) {
+    return ActualAlloc::Result(this->template EnsureCapacity<ActualAlloc>(
+        aCapacity, sizeof(value_type)));
+  }
+
+ public:
+  [[nodiscard]] bool SetCapacity(size_type aCapacity,
+                                 const mozilla::fallible_t&) {
+    return SetCapacity<FallibleAlloc>(aCapacity);
+  }
+
+  // This method modifies the length of the array.  If the new length is
+  // larger than the existing length of the array, then new elements will be
+  // constructed using value_type's default constructor.  Otherwise, this call
+  // removes elements from the array (see also RemoveElementsAt).
+  // @param aNewLen The desired length of this array.
+  // @return True if the operation succeeded; false otherwise.
+  // See also TruncateLength for a more efficient variant if the new length is
+  // guaranteed to be smaller than the old.
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType SetLength(size_type aNewLen) {
+    const size_type oldLen = Length();
+    if (aNewLen > oldLen) {
+      return ActualAlloc::ConvertBoolToResultType(
+          InsertElementsAtInternal<ActualAlloc>(oldLen, aNewLen - oldLen) !=
+          nullptr);
+    }
+
+    TruncateLengthUnsafe(aNewLen);
+    return ActualAlloc::ConvertBoolToResultType(true);
+  }
+
+ public:
+  [[nodiscard]] bool SetLength(size_type aNewLen, const mozilla::fallible_t&) {
+    return SetLength<FallibleAlloc>(aNewLen);
+  }
+
+  // This method modifies the length of the array, but may only be
+  // called when the new length is shorter than the old.  It can
+  // therefore be called when value_type has no default constructor,
+  // unlike SetLength.  It removes elements from the array (see also
+  // RemoveElementsAt).
+  // @param aNewLen The desired length of this array.
+  void TruncateLength(size_type aNewLen) {
+    // This assertion is redundant, but produces a better error message than the
+    // release assertion below.
+    MOZ_ASSERT(aNewLen <= Length(), "caller should use SetLength instead");
+
+    if (MOZ_UNLIKELY(aNewLen > Length())) {
+      mozilla::detail::InvalidArrayIndex_CRASH(aNewLen, Length());
+    }
+
+    TruncateLengthUnsafe(aNewLen);
+  }
+
+ private:
+  void TruncateLengthUnsafe(size_type aNewLen) {
+    const size_type oldLen = Length();
+    if (oldLen) {
+      DestructRange(aNewLen, oldLen - aNewLen);
+      base_type::mHdr->mLength = aNewLen;
+    }
+  }
+
+  // This method ensures that the array has length at least the given
+  // length.  If the current length is shorter than the given length,
+  // then new elements will be constructed using value_type's default
+  // constructor.
+  // @param aMinLen The desired minimum length of this array.
+  // @return True if the operation succeeded; false otherwise.
+ protected:
+  template <typename ActualAlloc = Alloc>
+  typename ActualAlloc::ResultType EnsureLengthAtLeast(size_type aMinLen) {
+    size_type oldLen = Length();
+    if (aMinLen > oldLen) {
+      return ActualAlloc::ConvertBoolToResultType(
+          !!InsertElementsAtInternal<ActualAlloc>(oldLen, aMinLen - oldLen));
+    }
+    return ActualAlloc::ConvertBoolToResultType(true);
+  }
+
+ public:
+  [[nodiscard]] bool EnsureLengthAtLeast(size_type aMinLen,
+                                         const mozilla::fallible_t&) {
+    return EnsureLengthAtLeast<FallibleAlloc>(aMinLen);
+  }
+
+  // This method inserts elements into the array, constructing
+  // them using value_type's default constructor.
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of elements to insert
+ private:
+  template <typename ActualAlloc>
+  value_type* InsertElementsAtInternal(index_type aIndex, size_type aCount) {
+    if (!ActualAlloc::Successful(this->template InsertSlotsAt<ActualAlloc>(
+            aIndex, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type)))) {
+      return nullptr;
+    }
+
+    // Initialize the extra array elements
+    value_type* iter = Elements() + aIndex;
+    value_type* iend = iter + aCount;
+    for (; iter != iend; ++iter) {
+      elem_traits::Construct(iter);
+    }
+
+    return Elements() + aIndex;
+  }
+
+ public:
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             size_type aCount,
+                                             const mozilla::fallible_t&) {
+    return InsertElementsAtInternal<FallibleAlloc>(aIndex, aCount);
+  }
+
+  // This method inserts elements into the array, constructing them
+  // value_type's copy constructor (or whatever one-arg constructor
+  // happens to match the Item type).
+  // @param aIndex the place to insert the new elements. This must be no
+  //               greater than the current length of the array.
+  // @param aCount the number of elements to insert.
+  // @param aItem the value to use when constructing the new elements.
+ private:
+  template <typename ActualAlloc, class Item>
+  value_type* InsertElementsAtInternal(index_type aIndex, size_type aCount,
+                                       const Item& aItem);
+
+ public:
+  template <class Item>
+  [[nodiscard]] value_type* InsertElementsAt(index_type aIndex,
+                                             size_type aCount,
+                                             const Item& aItem,
+                                             const mozilla::fallible_t&) {
+    return InsertElementsAt<Item, FallibleAlloc>(aIndex, aCount, aItem);
+  }
+
+  // This method may be called to minimize the memory used by this array.
+  void Compact() {
+    ShrinkCapacity(sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  //
+  // Sorting
+  //
+
+  // This function is meant to be used with the NS_QuickSort function.  It
+  // maps the callback API expected by NS_QuickSort to the Comparator API
+  // used by nsTArray_Impl.  See nsTArray_Impl::Sort.
+  template <class Comparator>
+  static int Compare(const void* aE1, const void* aE2, void* aData) {
+    const Comparator* c = reinterpret_cast<const Comparator*>(aData);
+    const value_type* a = static_cast<const value_type*>(aE1);
+    const value_type* b = static_cast<const value_type*>(aE2);
+    return c->Compare(*a, *b);
+  }
+
+  // This method sorts the elements of the array.  It uses the LessThan
+  // method defined on the given Comparator object to collate elements.
+  // @param aComp The Comparator used to collate elements.
+  template <class Comparator>
+  void Sort(const Comparator& aComp) {
+    ::detail::CompareWrapper<Comparator, value_type> comp(aComp);
+
+    NS_QuickSort(Elements(), Length(), sizeof(value_type),
+                 Compare<decltype(comp)>, &comp);
+  }
+
+  // A variation on the Sort method defined above that assumes that
+  // 'operator<' is defined for value_type.
+  void Sort() { Sort(nsDefaultComparator<value_type, value_type>()); }
+
+  // This method sorts the elements of the array in a stable way (i.e. not
+  // changing the relative order of elements considered equal by the
+  // Comparator).  It uses the LessThan
+  // method defined on the given Comparator object to collate elements.
+  // @param aComp The Comparator used to collate elements.
+  template <class Comparator>
+  void StableSort(const Comparator& aComp) {
+    const ::detail::CompareWrapper<Comparator, value_type> comp(aComp);
+
+    std::stable_sort(Elements(), Elements() + Length(),
+                     [&comp](const auto& lhs, const auto& rhs) {
+                       return comp.LessThan(lhs, rhs);
+                     });
+  }
+
+  // This method reverses the array in place.
+  void Reverse() {
+    value_type* elements = Elements();
+    const size_type len = Length();
+    for (index_type i = 0, iend = len / 2; i < iend; ++i) {
+      std::swap(elements[i], elements[len - i - 1]);
+    }
+  }
+
+ protected:
+  using base_type::Hdr;
+  using base_type::ShrinkCapacity;
+
+  // This method invokes value_type's destructor on a range of elements.
+  // @param aStart The index of the first element to destroy.
+  // @param aCount The number of elements to destroy.
+  void DestructRange(index_type aStart, size_type aCount) {
+    value_type* iter = Elements() + aStart;
+    value_type* iend = iter + aCount;
+    for (; iter != iend; ++iter) {
+      elem_traits::Destruct(iter);
+    }
+  }
+
+  // This method invokes value_type's copy-constructor on a range of elements.
+  // @param aStart  The index of the first element to construct.
+  // @param aCount  The number of elements to construct.
+  // @param aValues The array of elements to copy.
+  template <class Item>
+  void AssignRange(index_type aStart, size_type aCount, const Item* aValues) {
+    AssignRangeAlgorithm<
+        std::is_trivially_copy_constructible_v<Item>,
+        std::is_same_v<Item, value_type>>::implementation(Elements(), aStart,
+                                                          aCount, aValues);
+  }
+};
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AssignInternal(const Item* aArray,
+                                             size_type aArrayLen) ->
+    typename ActualAlloc::ResultType {
+  static_assert(std::is_same_v<ActualAlloc, InfallibleAlloc> ||
+                std::is_same_v<ActualAlloc, FallibleAlloc>);
+
+  if constexpr (std::is_same_v<ActualAlloc, InfallibleAlloc>) {
+    ClearAndRetainStorage();
+  }
+  // Adjust memory allocation up-front to catch errors in the fallible case.
+  // We might relocate the elements to be destroyed unnecessarily. This could be
+  // optimized, but would make things more complicated.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          aArrayLen, sizeof(value_type)))) {
+    return ActualAlloc::ConvertBoolToResultType(false);
+  }
+
+  MOZ_ASSERT_IF(this->HasEmptyHeader(), aArrayLen == 0);
+  if (!this->HasEmptyHeader()) {
+    if constexpr (std::is_same_v<ActualAlloc, FallibleAlloc>) {
+      ClearAndRetainStorage();
+    }
+    AssignRange(0, aArrayLen, aArray);
+    base_type::mHdr->mLength = aArrayLen;
+  }
+
+  return ActualAlloc::ConvertBoolToResultType(true);
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::ReplaceElementsAtInternal(index_type aStart,
+                                                        size_type aCount,
+                                                        const Item* aArray,
+                                                        size_type aArrayLen)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aStart > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+  if (MOZ_UNLIKELY(aCount > Length() - aStart)) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart + aCount, Length());
+  }
+
+  // Adjust memory allocation up-front to catch errors.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + aArrayLen - aCount, sizeof(value_type)))) {
+    return nullptr;
+  }
+  DestructRange(aStart, aCount);
+  this->template ShiftData<ActualAlloc>(
+      aStart, aCount, aArrayLen, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  AssignRange(aStart, aArrayLen, aArray);
+  return Elements() + aStart;
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::RemoveElementsAt(index_type aStart,
+                                               size_type aCount) {
+  MOZ_ASSERT(aCount == 0 || aStart < Length(), "Invalid aStart index");
+
+  mozilla::CheckedInt<index_type> rangeEnd = aStart;
+  rangeEnd += aCount;
+
+  if (MOZ_UNLIKELY(!rangeEnd.isValid() || rangeEnd.value() > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+
+  RemoveElementsAtUnsafe(aStart, aCount);
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::RemoveElementsAtUnsafe(index_type aStart,
+                                                     size_type aCount) {
+  DestructRange(aStart, aCount);
+  this->template ShiftData<InfallibleAlloc>(
+      aStart, aCount, 0, sizeof(value_type), MOZ_ALIGNOF(value_type));
+}
+
+template <typename E, class Alloc>
+void nsTArray_Impl<E, Alloc>::UnorderedRemoveElementsAt(index_type aStart,
+                                                        size_type aCount) {
+  MOZ_ASSERT(aCount == 0 || aStart < Length(), "Invalid aStart index");
+
+  mozilla::CheckedInt<index_type> rangeEnd = aStart;
+  rangeEnd += aCount;
+
+  if (MOZ_UNLIKELY(!rangeEnd.isValid() || rangeEnd.value() > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aStart, Length());
+  }
+
+  // Destroy the elements which are being removed, and then swap elements in to
+  // replace them from the end. See the docs on the declaration of this
+  // function.
+  DestructRange(aStart, aCount);
+  this->template SwapFromEnd<InfallibleAlloc>(
+      aStart, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type));
+}
+
+template <typename E, class Alloc>
+template <typename Predicate>
+auto nsTArray_Impl<E, Alloc>::RemoveElementsBy(Predicate aPredicate)
+    -> size_type {
+  if (this->HasEmptyHeader()) {
+    return 0;
+  }
+
+  index_type j = 0;
+  const index_type len = Length();
+  value_type* const elements = Elements();
+  for (index_type i = 0; i < len; ++i) {
+    const bool result = aPredicate(elements[i]);
+
+    // Check that the array has not been modified by the predicate.
+    MOZ_DIAGNOSTIC_ASSERT(len == base_type::mHdr->mLength &&
+                          elements == Elements());
+
+    if (result) {
+      elem_traits::Destruct(elements + i);
+    } else {
+      if (j < i) {
+        relocation_type::RelocateNonOverlappingRegion(
+            elements + j, elements + i, 1, sizeof(value_type));
+      }
+      ++j;
+    }
+  }
+
+  base_type::mHdr->mLength = j;
+  return len - j;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::InsertElementsAtInternal(index_type aIndex,
+                                                       size_type aCount,
+                                                       const Item& aItem)
+    -> value_type* {
+  if (!ActualAlloc::Successful(this->template InsertSlotsAt<ActualAlloc>(
+          aIndex, aCount, sizeof(value_type), MOZ_ALIGNOF(value_type)))) {
+    return nullptr;
+  }
+
+  // Initialize the extra array elements
+  value_type* iter = Elements() + aIndex;
+  value_type* iend = iter + aCount;
+  for (; iter != iend; ++iter) {
+    elem_traits::Construct(iter, aItem);
+  }
+
+  return Elements() + aIndex;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc>
+auto nsTArray_Impl<E, Alloc>::InsertElementAtInternal(index_type aIndex)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aIndex > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+  }
+
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  this->template ShiftData<ActualAlloc>(aIndex, 0, 1, sizeof(value_type),
+                                        MOZ_ALIGNOF(value_type));
+  value_type* elem = Elements() + aIndex;
+  elem_traits::Construct(elem);
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::InsertElementAtInternal(index_type aIndex,
+                                                      Item&& aItem)
+    -> value_type* {
+  if (MOZ_UNLIKELY(aIndex > Length())) {
+    mozilla::detail::InvalidArrayIndex_CRASH(aIndex, Length());
+  }
+
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  this->template ShiftData<ActualAlloc>(aIndex, 0, 1, sizeof(value_type),
+                                        MOZ_ALIGNOF(value_type));
+  value_type* elem = Elements() + aIndex;
+  elem_traits::Construct(elem, std::forward<Item>(aItem));
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AppendElementsInternal(const Item* aArray,
+                                                     size_type aArrayLen)
+    -> value_type* {
+  if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+          Length(), aArrayLen, sizeof(value_type)))) {
+    return nullptr;
+  }
+  index_type len = Length();
+  AssignRange(len, aArrayLen, aArray);
+  this->IncrementLength(aArrayLen);
+  return Elements() + len;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item, class Allocator>
+auto nsTArray_Impl<E, Alloc>::AppendElementsInternal(
+    nsTArray_Impl<Item, Allocator>&& aArray) -> value_type* {
+  if constexpr (std::is_same_v<Alloc, Allocator>) {
+    MOZ_ASSERT(&aArray != this, "argument must be different aArray");
+  }
+  if (Length() == 0) {
+    // XXX This might still be optimized. If aArray uses auto-storage but we
+    // won't, we might better retain our storage if it's sufficiently large.
+    this->ShrinkCapacityToZero(sizeof(value_type), MOZ_ALIGNOF(value_type));
+    this->MoveInit(aArray, sizeof(value_type), MOZ_ALIGNOF(value_type));
+    return Elements();
+  }
+
+  index_type len = Length();
+  index_type otherLen = aArray.Length();
+  if (!ActualAlloc::Successful(this->template ExtendCapacity<ActualAlloc>(
+          len, otherLen, sizeof(value_type)))) {
+    return nullptr;
+  }
+  relocation_type::RelocateNonOverlappingRegion(
+      Elements() + len, aArray.Elements(), otherLen, sizeof(value_type));
+  this->IncrementLength(otherLen);
+  aArray.template ShiftData<ActualAlloc>(0, otherLen, 0, sizeof(value_type),
+                                         MOZ_ALIGNOF(value_type));
+  return Elements() + len;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class Item>
+auto nsTArray_Impl<E, Alloc>::AppendElementInternal(Item&& aItem)
+    -> value_type* {
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  value_type* elem = Elements() + Length();
+  elem_traits::Construct(elem, std::forward<Item>(aItem));
+  this->mHdr->mLength += 1;
+  return elem;
+}
+
+template <typename E, class Alloc>
+template <typename ActualAlloc, class... Args>
+auto nsTArray_Impl<E, Alloc>::EmplaceBackInternal(Args&&... aArgs)
+    -> value_type* {
+  // Length() + 1 is guaranteed to not overflow, so EnsureCapacity is OK.
+  if (!ActualAlloc::Successful(this->template EnsureCapacity<ActualAlloc>(
+          Length() + 1, sizeof(value_type)))) {
+    return nullptr;
+  }
+  value_type* elem = Elements() + Length();
+  elem_traits::Emplace(elem, std::forward<Args>(aArgs)...);
+  this->mHdr->mLength += 1;
+  return elem;
+}
+
+template <typename E, typename Alloc>
+inline void ImplCycleCollectionUnlink(nsTArray_Impl<E, Alloc>& aField) {
+  aField.Clear();
+}
+
+namespace detail {
+// This is defined in the cpp file to avoid including
+// nsCycleCollectionNoteChild.h in this header file.
+void SetCycleCollectionArrayFlag(uint32_t& aFlags);
+}  // namespace detail
+
+template <typename E, typename Alloc>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsTArray_Impl<E, Alloc>& aField, const char* aName, uint32_t aFlags = 0) {
+  ::detail::SetCycleCollectionArrayFlag(aFlags);
+  size_t length = aField.Length();
+  for (size_t i = 0; i < length; ++i) {
+    ImplCycleCollectionTraverse(aCallback, aField[i], aName, aFlags);
+  }
+}
+
+//
+// nsTArray is an infallible vector class.  See the comment at the top of this
+// file for more details.
+//
+template <class E>
+class nsTArray : public nsTArray_Impl<E, nsTArrayInfallibleAllocator> {
+ public:
+  using InfallibleAlloc = nsTArrayInfallibleAllocator;
+  using base_type = nsTArray_Impl<E, InfallibleAlloc>;
+  using self_type = nsTArray<E>;
+  using typename base_type::index_type;
+  using typename base_type::size_type;
+  using typename base_type::value_type;
+
+  nsTArray() {}
+  explicit nsTArray(size_type aCapacity) : base_type(aCapacity) {}
+  MOZ_IMPLICIT nsTArray(std::initializer_list<E> aIL) {
+    AppendElements(aIL.begin(), aIL.size());
+  }
+
+  template <class Item>
+  nsTArray(const Item* aArray, size_type aArrayLen) {
+    AppendElements(aArray, aArrayLen);
+  }
+
+  template <class Item>
+  explicit nsTArray(mozilla::Span<Item> aSpan) {
+    AppendElements(aSpan);
+  }
+
+  template <class Allocator>
+  explicit nsTArray(const nsTArray_Impl<E, Allocator>& aOther)
+      : base_type(aOther) {}
+  template <class Allocator>
+  MOZ_IMPLICIT nsTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : base_type(std::move(aOther)) {}
+
+  template <class Allocator>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    base_type::operator=(aOther);
+    return *this;
+  }
+  template <class Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    // This is quite complex, since we don't know if we are an AutoTArray. While
+    // AutoTArray overrides this operator=, this might be called on a nsTArray&
+    // bound to an AutoTArray.
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  using base_type::AppendElement;
+  using base_type::AppendElements;
+  using base_type::EmplaceBack;
+  using base_type::EnsureLengthAtLeast;
+  using base_type::InsertElementAt;
+  using base_type::InsertElementsAt;
+  using base_type::InsertElementSorted;
+  using base_type::ReplaceElementsAt;
+  using base_type::SetCapacity;
+  using base_type::SetLength;
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElements(const Item* aArray,
+                                               size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aArray,
+                                                               aArrayLen));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElements(mozilla::Span<Item> aSpan) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aSpan.Elements(),
+                                                               aSpan.Length()));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> AppendElements(
+      const nsTArray_Impl<Item, Allocator>& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(
+            aArray.Elements(), aArray.Length()));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> AppendElements(
+      nsTArray_Impl<Item, Allocator>&& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(
+            std::move(aArray)));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> AppendElement(Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem)));
+  }
+
+  mozilla::NotNull<value_type*> AppendElements(size_type aCount) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(aCount));
+  }
+
+  mozilla::NotNull<value_type*> AppendElement() {
+    return mozilla::WrapNotNullUnchecked(
+        this->template AppendElementsInternal<InfallibleAlloc>(1));
+  }
+
+  self_type Clone() const {
+    self_type result;
+    result.Assign(*this);
+    return result;
+  }
+
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 size_type aCount) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementsAtInternal<InfallibleAlloc>(aIndex,
+                                                                 aCount));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 size_type aCount,
+                                                 const Item& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementsAtInternal<InfallibleAlloc>(aIndex, aCount,
+                                                                 aItem));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 const Item* aArray,
+                                                 size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aArray, aArrayLen));
+  }
+
+  template <class Item, class Allocator>
+  mozilla::NotNull<value_type*> InsertElementsAt(
+      index_type aIndex, const nsTArray_Impl<Item, Allocator>& aArray) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aArray.Elements(), aArray.Length()));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementsAt(index_type aIndex,
+                                                 mozilla::Span<Item> aSpan) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aIndex, 0, aSpan.Elements(), aSpan.Length()));
+  }
+
+  mozilla::NotNull<value_type*> InsertElementAt(index_type aIndex) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementAtInternal<InfallibleAlloc>(aIndex));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementAt(index_type aIndex,
+                                                Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementAtInternal<InfallibleAlloc>(
+            aIndex, std::forward<Item>(aItem)));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  const Item* aArray,
+                                                  size_type aArrayLen) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template ReplaceElementsAtInternal<InfallibleAlloc>(
+            aStart, aCount, aArray, aArrayLen));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(
+      index_type aStart, size_type aCount, const nsTArray<Item>& aArray) {
+    return ReplaceElementsAt(aStart, aCount, aArray.Elements(),
+                             aArray.Length());
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  mozilla::Span<Item> aSpan) {
+    return ReplaceElementsAt(aStart, aCount, aSpan.Elements(), aSpan.Length());
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> ReplaceElementsAt(index_type aStart,
+                                                  size_type aCount,
+                                                  const Item& aItem) {
+    return ReplaceElementsAt(aStart, aCount, &aItem, 1);
+  }
+
+  template <class Item, class Comparator>
+  mozilla::NotNull<value_type*> InsertElementSorted(Item&& aItem,
+                                                    const Comparator& aComp) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementSortedInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem), aComp));
+  }
+
+  template <class Item>
+  mozilla::NotNull<value_type*> InsertElementSorted(Item&& aItem) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template InsertElementSortedInternal<InfallibleAlloc>(
+            std::forward<Item>(aItem),
+            nsDefaultComparator<value_type, Item>{}));
+  }
+
+  template <class... Args>
+  mozilla::NotNull<value_type*> EmplaceBack(Args&&... aArgs) {
+    return mozilla::WrapNotNullUnchecked(
+        this->template EmplaceBackInternal<InfallibleAlloc, Args...>(
+            std::forward<Args>(aArgs)...));
+  }
+};
+
+template <class E>
+class CopyableTArray : public nsTArray<E> {
+ public:
+  using nsTArray<E>::nsTArray;
+
+  CopyableTArray(const CopyableTArray& aOther) : nsTArray<E>() {
+    this->Assign(aOther);
+  }
+  CopyableTArray& operator=(const CopyableTArray& aOther) {
+    if (this != &aOther) {
+      this->Assign(aOther);
+    }
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableTArray(const nsTArray_Impl<E, Allocator>& aOther) {
+    this->Assign(aOther);
+  }
+  template <typename Allocator>
+  CopyableTArray& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    if constexpr (std::is_same_v<Allocator, nsTArrayInfallibleAllocator>) {
+      if (this == &aOther) {
+        return *this;
+      }
+    }
+    this->Assign(aOther);
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : nsTArray<E>{std::move(aOther)} {}
+  template <typename Allocator>
+  CopyableTArray& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    static_cast<nsTArray<E>&>(*this) = std::move(aOther);
+    return *this;
+  }
+
+  CopyableTArray(CopyableTArray&&) = default;
+  CopyableTArray& operator=(CopyableTArray&&) = default;
+};
+
+//
+// FallibleTArray is a fallible vector class.
+//
+template <class E>
+class FallibleTArray : public nsTArray_Impl<E, nsTArrayFallibleAllocator> {
+ public:
+  typedef nsTArray_Impl<E, nsTArrayFallibleAllocator> base_type;
+  typedef FallibleTArray<E> self_type;
+  typedef typename base_type::size_type size_type;
+
+  FallibleTArray() = default;
+  explicit FallibleTArray(size_type aCapacity) : base_type(aCapacity) {}
+
+  template <class Allocator>
+  explicit FallibleTArray(const nsTArray_Impl<E, Allocator>& aOther)
+      : base_type(aOther) {}
+  template <class Allocator>
+  explicit FallibleTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : base_type(std::move(aOther)) {}
+
+  template <class Allocator>
+  self_type& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    base_type::operator=(aOther);
+    return *this;
+  }
+  template <class Allocator>
+  self_type& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+};
+
+//
+// AutoTArray<E, N> is like nsTArray<E>, but with N elements of inline storage.
+// Storing more than N elements is fine, but it will cause a heap allocation.
+//
+template <class E, size_t N>
+class MOZ_NON_MEMMOVABLE AutoTArray : public nsTArray<E> {
+  static_assert(N != 0, "AutoTArray<E, 0> should be specialized");
+
+ public:
+  typedef AutoTArray<E, N> self_type;
+  typedef nsTArray<E> base_type;
+  typedef typename base_type::Header Header;
+  typedef typename base_type::value_type value_type;
+
+  AutoTArray() : mAlign() { Init(); }
+
+  AutoTArray(self_type&& aOther) : nsTArray<E>() {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  explicit AutoTArray(base_type&& aOther) : mAlign() {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  template <typename Allocator>
+  explicit AutoTArray(nsTArray_Impl<value_type, Allocator>&& aOther) {
+    Init();
+    this->MoveInit(aOther, sizeof(value_type), MOZ_ALIGNOF(value_type));
+  }
+
+  MOZ_IMPLICIT AutoTArray(std::initializer_list<E> aIL) : mAlign() {
+    Init();
+    this->AppendElements(aIL.begin(), aIL.size());
+  }
+
+  self_type& operator=(self_type&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  template <typename Allocator>
+  self_type& operator=(nsTArray_Impl<value_type, Allocator>&& aOther) {
+    base_type::operator=(std::move(aOther));
+    return *this;
+  }
+
+  // Intentionally hides nsTArray_Impl::Clone to make clones usually be
+  // AutoTArray as well.
+  self_type Clone() const {
+    self_type result;
+    result.Assign(*this);
+    return result;
+  }
+
+ private:
+  // nsTArray_base casts itself as an nsAutoArrayBase in order to get a pointer
+  // to mAutoBuf.
+  template <class Allocator, class RelocationStrategy>
+  friend class nsTArray_base;
+
+  void Init() {
+    static_assert(MOZ_ALIGNOF(value_type) <= 8,
+                  "can't handle alignments greater than 8, "
+                  "see nsTArray_base::UsesAutoArrayBuffer()");
+    // Temporary work around for VS2012 RC compiler crash
+    Header** phdr = base_type::PtrToHdr();
+    *phdr = reinterpret_cast<Header*>(&mAutoBuf);
+    (*phdr)->mLength = 0;
+    (*phdr)->mCapacity = N;
+    (*phdr)->mIsAutoArray = 1;
+
+    MOZ_ASSERT(base_type::GetAutoArrayBuffer(MOZ_ALIGNOF(value_type)) ==
+                   reinterpret_cast<Header*>(&mAutoBuf),
+               "GetAutoArrayBuffer needs to be fixed");
+  }
+
+  // Declare mAutoBuf aligned to the maximum of the header's alignment and
+  // value_type's alignment.  We need to use a union rather than
+  // MOZ_ALIGNED_DECL because GCC is picky about what goes into
+  // __attribute__((aligned(foo))).
+  union {
+    char mAutoBuf[sizeof(nsTArrayHeader) + N * sizeof(value_type)];
+    // Do the max operation inline to ensure that it is a compile-time constant.
+    mozilla::AlignedElem<(MOZ_ALIGNOF(Header) > MOZ_ALIGNOF(value_type))
+                             ? MOZ_ALIGNOF(Header)
+                             : MOZ_ALIGNOF(value_type)>
+        mAlign;
+  };
+};
+
+//
+// Specialization of AutoTArray<E, N> for the case where N == 0.
+// AutoTArray<E, 0> behaves exactly like nsTArray<E>, but without this
+// specialization, it stores a useless inline header.
+//
+// We do have many AutoTArray<E, 0> objects in memory: about 2,000 per tab as
+// of May 2014. These are typically not explicitly AutoTArray<E, 0> but rather
+// AutoTArray<E, N> for some value N depending on template parameters, in
+// generic code.
+//
+// For that reason, we optimize this case with the below partial specialization,
+// which ensures that AutoTArray<E, 0> is just like nsTArray<E>, without any
+// inline header overhead.
+//
+template <class E>
+class AutoTArray<E, 0> : public nsTArray<E> {
+  using nsTArray<E>::nsTArray;
+};
+
+template <class E, size_t N>
+struct nsTArray_RelocationStrategy<AutoTArray<E, N>> {
+  using Type = nsTArray_RelocateUsingMoveConstructor<AutoTArray<E, N>>;
+};
+
+template <class E, size_t N>
+class CopyableAutoTArray : public AutoTArray<E, N> {
+ public:
+  typedef CopyableAutoTArray<E, N> self_type;
+  using AutoTArray<E, N>::AutoTArray;
+
+  CopyableAutoTArray(const CopyableAutoTArray& aOther) : AutoTArray<E, N>() {
+    this->Assign(aOther);
+  }
+  CopyableAutoTArray& operator=(const CopyableAutoTArray& aOther) {
+    if (this != &aOther) {
+      this->Assign(aOther);
+    }
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableAutoTArray(const nsTArray_Impl<E, Allocator>& aOther) {
+    this->Assign(aOther);
+  }
+  template <typename Allocator>
+  CopyableAutoTArray& operator=(const nsTArray_Impl<E, Allocator>& aOther) {
+    if constexpr (std::is_same_v<Allocator, nsTArrayInfallibleAllocator>) {
+      if (this == &aOther) {
+        return *this;
+      }
+    }
+    this->Assign(aOther);
+    return *this;
+  }
+  template <typename Allocator>
+  MOZ_IMPLICIT CopyableAutoTArray(nsTArray_Impl<E, Allocator>&& aOther)
+      : AutoTArray<E, N>{std::move(aOther)} {}
+  template <typename Allocator>
+  CopyableAutoTArray& operator=(nsTArray_Impl<E, Allocator>&& aOther) {
+    static_cast<AutoTArray<E, N>&>(*this) = std::move(aOther);
+    return *this;
+  }
+
+  // CopyableTArray exists for cases where an explicit Clone is not possible.
+  // These uses should not be mixed, so we delete Clone() here.
+  self_type Clone() const = delete;
+
+  CopyableAutoTArray(CopyableAutoTArray&&) = default;
+  CopyableAutoTArray& operator=(CopyableAutoTArray&&) = default;
+};
+
+namespace mozilla {
+template <typename E, typename ArrayT>
+class nsTArrayBackInserter {
+  ArrayT* mArray;
+
+  class Proxy {
+    ArrayT& mArray;
+
+   public:
+    explicit Proxy(ArrayT& aArray) : mArray{aArray} {}
+
+    template <typename E2>
+    void operator=(E2&& aValue) {
+      mArray.AppendElement(std::forward<E2>(aValue));
+    }
+  };
+
+ public:
+  using iterator_category = std::output_iterator_tag;
+  using value_type = void;
+  using difference_type = void;
+  using pointer = void;
+  using reference = void;
+  explicit nsTArrayBackInserter(ArrayT& aArray) : mArray{&aArray} {}
+
+  // Return a proxy so that nsTArrayBackInserter has the default special member
+  // functions, and the operator= template is defined in Proxy rather than this
+  // class (which otherwise breaks with recent MS STL versions).
+  // See also Bug 1331137, comment 11.
+  Proxy operator*() { return Proxy(*mArray); }
+
+  nsTArrayBackInserter& operator++() { return *this; }
+  nsTArrayBackInserter& operator++(int) { return *this; }
+};
+}  // namespace mozilla
+
+template <typename E>
+auto MakeBackInserter(nsTArray<E>& aArray) {
+  return mozilla::nsTArrayBackInserter<E, nsTArray<E>>{aArray};
+}
+
+// Span integration
+namespace mozilla {
+template <typename E, class Alloc>
+Span(nsTArray_Impl<E, Alloc>&) -> Span<E>;
+
+template <typename E, class Alloc>
+Span(const nsTArray_Impl<E, Alloc>&) -> Span<const E>;
+
+// Provides a view on a nsTArray through which the existing array elements can
+// be accessed in a non-const way, but the array itself cannot be modified, so
+// that references to elements are guaranteed to be stable.
+template <typename E>
+class nsTArrayView {
+ public:
+  using element_type = E;
+  using pointer = element_type*;
+  using reference = element_type&;
+  using index_type = typename Span<element_type>::index_type;
+  using size_type = typename Span<element_type>::index_type;
+
+  explicit nsTArrayView(nsTArray<element_type> aArray)
+      : mArray(std::move(aArray)), mSpan(mArray) {}
+
+  element_type& operator[](index_type aIndex) { return mSpan[aIndex]; }
+
+  const element_type& operator[](index_type aIndex) const {
+    return mSpan[aIndex];
+  }
+
+  size_type Length() const { return mSpan.Length(); }
+
+  auto begin() { return mSpan.begin(); }
+  auto end() { return mSpan.end(); }
+  auto begin() const { return mSpan.begin(); }
+  auto end() const { return mSpan.end(); }
+  auto cbegin() const { return mSpan.cbegin(); }
+  auto cend() const { return mSpan.cend(); }
+
+  Span<element_type> AsSpan() { return mSpan; }
+  Span<const element_type> AsSpan() const { return mSpan; }
+
+ private:
+  nsTArray<element_type> mArray;
+  const Span<element_type> mSpan;
+};
+
+template <typename Range, typename = std::enable_if_t<std::is_same_v<
+                              typename std::iterator_traits<
+                                  typename Range::iterator>::iterator_category,
+                              std::random_access_iterator_tag>>>
+auto RangeSize(const Range& aRange) {
+  // See https://en.cppreference.com/w/cpp/iterator/begin, section 'User-defined
+  // overloads'.
+  using std::begin;
+  using std::end;
+
+  return std::distance(begin(aRange), end(aRange));
+}
+
+/**
+ * Materialize a range as a nsTArray (or a compatible variant, like AutoTArray)
+ * of an explicitly specified type. The array value type must be implicitly
+ * convertible from the range's value type.
+ */
+template <typename Array, typename Range>
+auto ToTArray(const Range& aRange) {
+  using std::begin;
+  using std::end;
+
+  Array res;
+  res.SetCapacity(RangeSize(aRange));
+  std::copy(begin(aRange), end(aRange), MakeBackInserter(res));
+  return res;
+}
+
+/**
+ * Materialize a range as a nsTArray of its (decayed) value type.
+ */
+template <typename Range>
+auto ToArray(const Range& aRange) {
+  return ToTArray<nsTArray<std::decay_t<
+      typename std::iterator_traits<typename Range::iterator>::value_type>>>(
+      aRange);
+}
+
+/**
+ * Appends all elements from a range to an array.
+ */
+template <typename Array, typename Range>
+void AppendToArray(Array& aArray, const Range& aRange) {
+  using std::begin;
+  using std::end;
+
+  aArray.SetCapacity(aArray.Length() + RangeSize(aRange));
+  std::copy(begin(aRange), end(aRange), MakeBackInserter(aArray));
+}
+
+}  // namespace mozilla
+
+// MOZ_DBG support
+
+template <class E, class Alloc>
+std::ostream& operator<<(std::ostream& aOut,
+                         const nsTArray_Impl<E, Alloc>& aTArray) {
+  return aOut << mozilla::Span(aTArray);
+}
+
+// Assert that AutoTArray doesn't have any extra padding inside.
+//
+// It's important that the data stored in this auto array takes up a multiple of
+// 8 bytes; e.g. AutoTArray<uint32_t, 1> wouldn't work.  Since AutoTArray
+// contains a pointer, its size must be a multiple of alignof(void*).  (This is
+// because any type may be placed into an array, and there's no padding between
+// elements of an array.)  The compiler pads the end of the structure to
+// enforce this rule.
+//
+// If we used AutoTArray<uint32_t, 1> below, this assertion would fail on a
+// 64-bit system, where the compiler inserts 4 bytes of padding at the end of
+// the auto array to make its size a multiple of alignof(void*) == 8 bytes.
+
+static_assert(sizeof(AutoTArray<uint32_t, 2>) ==
+                  sizeof(void*) + sizeof(nsTArrayHeader) + sizeof(uint32_t) * 2,
+              "AutoTArray shouldn't contain any extra padding, "
+              "see the comment");
+
+// Definitions of nsTArray_Impl methods
+#include "nsTArray-inl.h"
+
+#endif  // nsTArray_h__
diff --git a/xpcom/ds/nsTArrayForwardDeclare.h b/xpcom/ds/nsTArrayForwardDeclare.h
new file mode 100644
index 0000000000..f888550803
--- /dev/null
+++ b/xpcom/ds/nsTArrayForwardDeclare.h
@@ -0,0 +1,39 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTArrayForwardDeclare_h__
+#define nsTArrayForwardDeclare_h__
+
+//
+// This simple header file contains forward declarations for the TArray family
+// of classes.
+//
+// Including this header is preferable to writing
+//
+//   template<class E> class nsTArray;
+//
+// yourself, since this header gives us flexibility to e.g. change the default
+// template parameters.
+//
+
+#include <stddef.h>
+
+template <class E>
+class nsTArray;
+
+template <class E>
+class FallibleTArray;
+
+template <class E>
+class CopyableTArray;
+
+template <class E, size_t N>
+class AutoTArray;
+
+template <class E, size_t N>
+class CopyableAutoTArray;
+
+#endif
diff --git a/xpcom/ds/nsTHashMap.h b/xpcom/ds/nsTHashMap.h
new file mode 100644
index 0000000000..8357d2ae7b
--- /dev/null
+++ b/xpcom/ds/nsTHashMap.h
@@ -0,0 +1,70 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef XPCOM_DS_NSTHASHMAP_H_
+#define XPCOM_DS_NSTHASHMAP_H_
+
+#include "mozilla/Attributes.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+#include "nsBaseHashtable.h"
+#include "nsCOMPtr.h"
+#include "nsHashKeys.h"
+#include "nsHashtablesFwd.h"
+#include <type_traits>
+
+namespace mozilla::detail {
+template <class KeyType>
+struct nsKeyClass<
+    KeyType, std::enable_if_t<sizeof(KeyType) &&
+                              std::is_base_of_v<PLDHashEntryHdr, KeyType>>> {
+  using type = KeyType;
+};
+
+template <typename KeyType>
+struct nsKeyClass<KeyType*> {
+  using type = nsPtrHashKey<KeyType>;
+};
+
+template <>
+struct nsKeyClass<nsCString> {
+  using type = nsCStringHashKey;
+};
+
+// This uses the case-sensitive hash key class, if you want the
+// case-insensitive hash key, use nsStringCaseInsensitiveHashKey explicitly.
+template <>
+struct nsKeyClass<nsString> {
+  using type = nsStringHashKey;
+};
+
+template <typename KeyType>
+struct nsKeyClass<KeyType, std::enable_if_t<std::is_integral_v<KeyType> ||
+                                            std::is_enum_v<KeyType>>> {
+  using type = nsIntegralHashKey<KeyType>;
+};
+
+template <>
+struct nsKeyClass<nsCOMPtr<nsISupports>> {
+  using type = nsISupportsHashKey;
+};
+
+template <typename T>
+struct nsKeyClass<RefPtr<T>> {
+  using type = nsRefPtrHashKey<T>;
+};
+
+template <>
+struct nsKeyClass<nsID> {
+  using type = nsIDHashKey;
+};
+
+}  // namespace mozilla::detail
+
+// The actual definition of nsTHashMap is in nsHashtablesFwd.h, since it is a
+// type alias.
+
+#endif  // XPCOM_DS_NSTHASHMAP_H_
diff --git a/xpcom/ds/nsTHashSet.h b/xpcom/ds/nsTHashSet.h
new file mode 100644
index 0000000000..4d905299db
--- /dev/null
+++ b/xpcom/ds/nsTHashSet.h
@@ -0,0 +1,193 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef XPCOM_DS_NSTHASHSET_H_
+#define XPCOM_DS_NSTHASHSET_H_
+
+#include "nsHashtablesFwd.h"
+#include "nsTHashMap.h"  // for nsKeyClass
+
+/**
+ * Templated hash set. Don't use this directly, but use nsTHashSet instead
+ * (defined as a type alias in nsHashtablesFwd.h).
+ *
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ */
+template <class KeyClass>
+class nsTBaseHashSet : protected nsTHashtable<KeyClass> {
+  using Base = nsTHashtable<KeyClass>;
+  typedef mozilla::fallible_t fallible_t;
+
+ public:
+  // XXX We have a similar situation here like with DataType/UserDataType in
+  // nsBaseHashtable. It's less problematic here due to the more constrained
+  // interface, but it may still be confusing. KeyType is not the stored key
+  // type, but the one exposed to the user, i.e. as a parameter type, and as the
+  // value type when iterating. It is currently impossible to move-insert a
+  // RefPtr<T>, e.g., since the KeyType is T* in that case.
+  using ValueType = typename KeyClass::KeyType;
+
+  // KeyType is defined just for compatibility with nsTHashMap. For a set, the
+  // key type is conceptually equivalent to the value type.
+  using KeyType = typename KeyClass::KeyType;
+
+  using Base::Base;
+
+  // Query operations.
+
+  using Base::Contains;
+  using Base::GetGeneration;
+  using Base::ShallowSizeOfExcludingThis;
+  using Base::ShallowSizeOfIncludingThis;
+  using Base::SizeOfExcludingThis;
+  using Base::SizeOfIncludingThis;
+
+  /**
+   * Return the number of entries in the table.
+   * @return    number of entries
+   */
+  [[nodiscard]] uint32_t Count() const { return Base::Count(); }
+
+  /**
+   * Return whether the table is empty.
+   * @return    whether empty
+   */
+  [[nodiscard]] bool IsEmpty() const { return Base::IsEmpty(); }
+
+  using iterator = ::detail::nsTHashtableKeyIterator<KeyClass>;
+  using const_iterator = iterator;
+
+  [[nodiscard]] auto begin() const { return Base::Keys().begin(); }
+
+  [[nodiscard]] auto end() const { return Base::Keys().end(); }
+
+  [[nodiscard]] auto cbegin() const { return Base::Keys().cbegin(); }
+
+  [[nodiscard]] auto cend() const { return Base::Keys().cend(); }
+
+  // Mutating operations.
+
+  using Base::Clear;
+  using Base::MarkImmutable;
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This overload is infallible and crashes if memory is exhausted.
+   *
+   * \note For strict consistency with nsTHashtable::EntryHandle method naming,
+   * this should rather be called OrInsert, as it is legal to call it when the
+   * value is already in the set. For simplicity, as we don't have two methods,
+   * we still use "Insert" instead.
+   */
+  void Insert(ValueType aValue) { Base::PutEntry(aValue); }
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This overload is fallible and returns false if memory is exhausted.
+   *
+   * \note See note on infallible overload.
+   */
+  [[nodiscard]] bool Insert(ValueType aValue, const mozilla::fallible_t&) {
+    return Base::PutEntry(aValue, mozilla::fallible);
+  }
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This member function is infallible and crashes if memory is exhausted.
+   *
+   * \return true if the value was actually inserted, false if it was already in
+   * the set.
+   */
+  bool EnsureInserted(ValueType aValue) { return Base::EnsureInserted(aValue); }
+
+  using Base::Remove;
+
+  /**
+   * Removes a value from the set. Has no effect if the value is not in the set.
+   *
+   * \note For strict consistency with nsTHashtable::EntryHandle method naming,
+   * this should rather be called OrRemove, as it is legal to call it when the
+   * value is not in the set. For simplicity, as we don't have two methods,
+   * we still use "Remove" instead.
+   */
+  void Remove(ValueType aValue) { Base::RemoveEntry(aValue); }
+
+  using Base::EnsureRemoved;
+
+  /**
+   * Removes all elements matching a predicate.
+   *
+   * The predicate must be compatible with signature bool (ValueType).
+   */
+  template <typename Pred>
+  void RemoveIf(Pred&& aPred) {
+    for (auto it = cbegin(), end = cend(); it != end; ++it) {
+      if (aPred(static_cast<ValueType>(*it))) {
+        Remove(it);
+      }
+    }
+  }
+};
+
+template <typename KeyClass>
+auto RangeSize(const nsTBaseHashSet<KeyClass>& aRange) {
+  return aRange.Count();
+}
+
+class nsCycleCollectionTraversalCallback;
+
+template <class KeyClass>
+inline void ImplCycleCollectionUnlink(nsTBaseHashSet<KeyClass>& aField) {
+  aField.Clear();
+}
+
+template <class KeyClass>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    const nsTBaseHashSet<KeyClass>& aField, const char* aName,
+    uint32_t aFlags = 0) {
+  for (const auto& entry : aField) {
+    CycleCollectionNoteChild(aCallback, mozilla::detail::PtrGetWeak(entry),
+                             aName, aFlags);
+  }
+}
+
+namespace mozilla {
+template <typename SetT>
+class nsTSetInserter {
+  SetT* mSet;
+
+  class Proxy {
+    SetT& mSet;
+
+   public:
+    explicit Proxy(SetT& aSet) : mSet{aSet} {}
+
+    template <typename E2>
+    void operator=(E2&& aValue) {
+      mSet.Insert(std::forward<E2>(aValue));
+    }
+  };
+
+ public:
+  using iterator_category = std::output_iterator_tag;
+
+  explicit nsTSetInserter(SetT& aSet) : mSet{&aSet} {}
+
+  Proxy operator*() { return Proxy(*mSet); }
+
+  nsTSetInserter& operator++() { return *this; }
+  nsTSetInserter& operator++(int) { return *this; }
+};
+}  // namespace mozilla
+
+template <typename E>
+auto MakeInserter(nsTBaseHashSet<E>& aSet) {
+  return mozilla::nsTSetInserter<nsTBaseHashSet<E>>{aSet};
+}
+
+#endif  // XPCOM_DS_NSTHASHSET_H_
diff --git a/xpcom/ds/nsTHashtable.h b/xpcom/ds/nsTHashtable.h
new file mode 100644
index 0000000000..d4a385551f
--- /dev/null
+++ b/xpcom/ds/nsTHashtable.h
@@ -0,0 +1,966 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// See the comment at the top of mfbt/HashTable.h for a comparison between
+// PLDHashTable and mozilla::HashTable.
+
+#ifndef nsTHashtable_h__
+#define nsTHashtable_h__
+
+#include <iterator>
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "PLDHashTable.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/fallible.h"
+#include "nsPointerHashKeys.h"
+#include "nsTArrayForwardDeclare.h"
+
+template <class EntryType>
+class nsTHashtable;
+
+namespace detail {
+class nsTHashtableIteratorBase {
+ public:
+  using EndIteratorTag = PLDHashTable::Iterator::EndIteratorTag;
+
+  nsTHashtableIteratorBase(nsTHashtableIteratorBase&& aOther) = default;
+
+  nsTHashtableIteratorBase& operator=(nsTHashtableIteratorBase&& aOther) {
+    // User-defined because the move assignment operator is deleted in
+    // PLDHashtable::Iterator.
+    return operator=(static_cast<const nsTHashtableIteratorBase&>(aOther));
+  }
+
+  nsTHashtableIteratorBase(const nsTHashtableIteratorBase& aOther)
+      : mIterator{aOther.mIterator.Clone()} {}
+  nsTHashtableIteratorBase& operator=(const nsTHashtableIteratorBase& aOther) {
+    // Since PLDHashTable::Iterator has no assignment operator, we destroy and
+    // recreate mIterator.
+    mIterator.~Iterator();
+    new (&mIterator) PLDHashTable::Iterator(aOther.mIterator.Clone());
+    return *this;
+  }
+
+  explicit nsTHashtableIteratorBase(PLDHashTable::Iterator aFrom)
+      : mIterator{std::move(aFrom)} {}
+
+  explicit nsTHashtableIteratorBase(const PLDHashTable& aTable)
+      : mIterator{&const_cast<PLDHashTable&>(aTable)} {}
+
+  nsTHashtableIteratorBase(const PLDHashTable& aTable, EndIteratorTag aTag)
+      : mIterator{&const_cast<PLDHashTable&>(aTable), aTag} {}
+
+  bool operator==(const nsTHashtableIteratorBase& aRhs) const {
+    return mIterator == aRhs.mIterator;
+  }
+  bool operator!=(const nsTHashtableIteratorBase& aRhs) const {
+    return !(*this == aRhs);
+  }
+
+ protected:
+  PLDHashTable::Iterator mIterator;
+};
+
+// STL-style iterators to allow the use in range-based for loops, e.g.
+template <typename T>
+class nsTHashtableEntryIterator : public nsTHashtableIteratorBase {
+  friend class nsTHashtable<std::remove_const_t<T>>;
+
+ public:
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = T;
+  using difference_type = int32_t;
+  using pointer = value_type*;
+  using reference = value_type&;
+
+  using iterator_type = nsTHashtableEntryIterator;
+  using const_iterator_type = nsTHashtableEntryIterator<const T>;
+
+  using nsTHashtableIteratorBase::nsTHashtableIteratorBase;
+
+  value_type* operator->() const {
+    return static_cast<value_type*>(mIterator.Get());
+  }
+  value_type& operator*() const {
+    return *static_cast<value_type*>(mIterator.Get());
+  }
+
+  iterator_type& operator++() {
+    mIterator.Next();
+    return *this;
+  }
+  iterator_type operator++(int) {
+    iterator_type it = *this;
+    ++*this;
+    return it;
+  }
+
+  operator const_iterator_type() const {
+    return const_iterator_type{mIterator.Clone()};
+  }
+};
+
+template <typename EntryType>
+class nsTHashtableKeyIterator : public nsTHashtableIteratorBase {
+  friend class nsTHashtable<EntryType>;
+
+ public:
+  using iterator_category = std::forward_iterator_tag;
+  using value_type = const std::decay_t<typename EntryType::KeyType>;
+  using difference_type = int32_t;
+  using pointer = value_type*;
+  using reference = value_type&;
+
+  using iterator_type = nsTHashtableKeyIterator;
+  using const_iterator_type = nsTHashtableKeyIterator;
+
+  using nsTHashtableIteratorBase::nsTHashtableIteratorBase;
+
+  value_type* operator->() const {
+    return &static_cast<const EntryType*>(mIterator.Get())->GetKey();
+  }
+  decltype(auto) operator*() const {
+    return static_cast<const EntryType*>(mIterator.Get())->GetKey();
+  }
+
+  iterator_type& operator++() {
+    mIterator.Next();
+    return *this;
+  }
+  iterator_type operator++(int) {
+    iterator_type it = *this;
+    ++*this;
+    return it;
+  }
+};
+
+template <typename EntryType>
+class nsTHashtableKeyRange {
+ public:
+  using IteratorType = nsTHashtableKeyIterator<EntryType>;
+  using iterator = IteratorType;
+
+  explicit nsTHashtableKeyRange(const PLDHashTable& aHashtable)
+      : mHashtable{aHashtable} {}
+
+  auto begin() const { return IteratorType{mHashtable}; }
+  auto end() const {
+    return IteratorType{mHashtable, typename IteratorType::EndIteratorTag{}};
+  }
+  auto cbegin() const { return begin(); }
+  auto cend() const { return end(); }
+
+  uint32_t Count() const { return mHashtable.EntryCount(); }
+
+ private:
+  const PLDHashTable& mHashtable;
+};
+
+template <typename EntryType>
+auto RangeSize(const ::detail::nsTHashtableKeyRange<EntryType>& aRange) {
+  return aRange.Count();
+}
+
+}  // namespace detail
+
+/**
+ * a base class for templated hashtables.
+ *
+ * Clients will rarely need to use this class directly. Check the derived
+ * classes first, to see if they will meet your needs.
+ *
+ * @param EntryType  the templated entry-type class that is managed by the
+ *   hashtable. <code>EntryType</code> must extend the following declaration,
+ *   and <strong>must not declare any virtual functions or derive from classes
+ *   with virtual functions.</strong>  Any vtable pointer would break the
+ *   PLDHashTable code.
+ *<pre>   class EntryType : public PLDHashEntryHdr
+ *   {
+ *   public: or friend nsTHashtable<EntryType>;
+ *     // KeyType is what we use when Get()ing or Put()ing this entry
+ *     // this should either be a simple datatype (uint32_t, nsISupports*) or
+ *     // a const reference (const nsAString&)
+ *     typedef something KeyType;
+ *     // KeyTypePointer is the pointer-version of KeyType, because
+ *     // PLDHashTable.h requires keys to cast to <code>const void*</code>
+ *     typedef const something* KeyTypePointer;
+ *
+ *     EntryType(KeyTypePointer aKey);
+ *
+ *     // A copy or C++11 Move constructor must be defined, even if
+ *     // AllowMemMove() == true, otherwise you will cause link errors.
+ *     EntryType(const EntryType& aEnt);  // Either this...
+ *     EntryType(EntryType&& aEnt);       // ...or this
+ *
+ *     // the destructor must be defined... or you will cause link errors!
+ *     ~EntryType();
+ *
+ *     // KeyEquals(): does this entry match this key?
+ *     bool KeyEquals(KeyTypePointer aKey) const;
+ *
+ *     // KeyToPointer(): Convert KeyType to KeyTypePointer
+ *     static KeyTypePointer KeyToPointer(KeyType aKey);
+ *
+ *     // HashKey(): calculate the hash number
+ *     static PLDHashNumber HashKey(KeyTypePointer aKey);
+ *
+ *     // ALLOW_MEMMOVE can we move this class with memmove(), or do we have
+ *     // to use the copy constructor?
+ *     enum { ALLOW_MEMMOVE = true/false };
+ *   }</pre>
+ *
+ * @see nsInterfaceHashtable
+ * @see nsClassHashtable
+ * @see nsTHashMap
+ * @author "Benjamin Smedberg <bsmedberg@covad.net>"
+ */
+
+template <class EntryType>
+class MOZ_NEEDS_NO_VTABLE_TYPE nsTHashtable {
+  typedef mozilla::fallible_t fallible_t;
+  static_assert(std::is_pointer_v<typename EntryType::KeyTypePointer>,
+                "KeyTypePointer should be a pointer");
+
+ public:
+  // Separate constructors instead of default aInitLength parameter since
+  // otherwise the default no-arg constructor isn't found.
+  nsTHashtable()
+      : mTable(Ops(), sizeof(EntryType), PLDHashTable::kDefaultInitialLength) {}
+  explicit nsTHashtable(uint32_t aInitLength)
+      : mTable(Ops(), sizeof(EntryType), aInitLength) {}
+
+  /**
+   * destructor, cleans up and deallocates
+   */
+  ~nsTHashtable() = default;
+
+  nsTHashtable(nsTHashtable<EntryType>&& aOther);
+  nsTHashtable<EntryType>& operator=(nsTHashtable<EntryType>&& aOther);
+
+  nsTHashtable(const nsTHashtable<EntryType>&) = delete;
+  nsTHashtable& operator=(const nsTHashtable<EntryType>&) = delete;
+
+  /**
+   * Return the generation number for the table. This increments whenever
+   * the table data items are moved.
+   */
+  uint32_t GetGeneration() const { return mTable.Generation(); }
+
+  /**
+   * KeyType is typedef'ed for ease of use.
+   */
+  typedef typename EntryType::KeyType KeyType;
+
+  /**
+   * KeyTypePointer is typedef'ed for ease of use.
+   */
+  typedef typename EntryType::KeyTypePointer KeyTypePointer;
+
+  /**
+   * Return the number of entries in the table.
+   * @return    number of entries
+   */
+  uint32_t Count() const { return mTable.EntryCount(); }
+
+  /**
+   * Return true if the hashtable is empty.
+   */
+  bool IsEmpty() const { return Count() == 0; }
+
+  /**
+   * Get the entry associated with a key.
+   * @param     aKey the key to retrieve
+   * @return    pointer to the entry class, if the key exists; nullptr if the
+   *            key doesn't exist
+   */
+  EntryType* GetEntry(KeyType aKey) const {
+    return static_cast<EntryType*>(
+        mTable.Search(EntryType::KeyToPointer(aKey)));
+  }
+
+  /**
+   * Return true if an entry for the given key exists, false otherwise.
+   * @param     aKey the key to retrieve
+   * @return    true if the key exists, false if the key doesn't exist
+   */
+  bool Contains(KeyType aKey) const { return !!GetEntry(aKey); }
+
+  /**
+   * Infallibly get the entry associated with a key, or create a new entry,
+   * @param     aKey the key to retrieve
+   * @return    pointer to the entry retrieved; never nullptr
+   */
+  EntryType* PutEntry(KeyType aKey) {
+    // Infallible WithEntryHandle.
+    return WithEntryHandle(
+        aKey, [](auto entryHandle) { return entryHandle.OrInsert(); });
+  }
+
+  /**
+   * Fallibly get the entry associated with a key, or create a new entry,
+   * @param     aKey the key to retrieve
+   * @return    pointer to the entry retrieved; nullptr only if memory can't
+   *            be allocated
+   */
+  [[nodiscard]] EntryType* PutEntry(KeyType aKey, const fallible_t& aFallible) {
+    return WithEntryHandle(aKey, aFallible, [](auto maybeEntryHandle) {
+      return maybeEntryHandle ? maybeEntryHandle->OrInsert() : nullptr;
+    });
+  }
+
+  /**
+   * Get the entry associated with a key, or create a new entry using infallible
+   * allocation and insert that.
+   * @param     aKey the key to retrieve
+   * @param     aEntry will be assigned (if non-null) to the entry that was
+   * found or created
+   * @return    true if a new entry was created, or false if an existing entry
+   *            was found
+   */
+  [[nodiscard]] bool EnsureInserted(KeyType aKey,
+                                    EntryType** aEntry = nullptr) {
+    auto oldCount = Count();
+    EntryType* entry = PutEntry(aKey);
+    if (aEntry) {
+      *aEntry = entry;
+    }
+    return oldCount != Count();
+  }
+
+  /**
+   * Remove the entry associated with a key.
+   * @param     aKey of the entry to remove
+   */
+  void RemoveEntry(KeyType aKey) {
+    mTable.Remove(EntryType::KeyToPointer(aKey));
+  }
+
+  /**
+   * Lookup the entry associated with aKey and remove it if found, otherwise
+   * do nothing.
+   * @param     aKey of the entry to remove
+   * @return    true if an entry was found and removed, or false if no entry
+   *            was found for aKey
+   */
+  bool EnsureRemoved(KeyType aKey) {
+    auto* entry = GetEntry(aKey);
+    if (entry) {
+      RemoveEntry(entry);
+      return true;
+    }
+    return false;
+  }
+
+  /**
+   * Remove the entry associated with a key.
+   * @param aEntry   the entry-pointer to remove (obtained from GetEntry)
+   */
+  void RemoveEntry(EntryType* aEntry) { mTable.RemoveEntry(aEntry); }
+
+  /**
+   * Remove the entry associated with a key, but don't resize the hashtable.
+   * This is a low-level method, and is not recommended unless you know what
+   * you're doing. If you use it, please add a comment explaining why you
+   * didn't use RemoveEntry().
+   * @param aEntry   the entry-pointer to remove (obtained from GetEntry)
+   */
+  void RawRemoveEntry(EntryType* aEntry) { mTable.RawRemove(aEntry); }
+
+ protected:
+  class EntryHandle {
+   public:
+    EntryHandle(EntryHandle&& aOther) = default;
+    ~EntryHandle() = default;
+
+    EntryHandle(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&&) = delete;
+
+    KeyType Key() const { return mKey; }
+
+    bool HasEntry() const { return mEntryHandle.HasEntry(); }
+
+    explicit operator bool() const { return mEntryHandle.operator bool(); }
+
+    EntryType* Entry() { return static_cast<EntryType*>(mEntryHandle.Entry()); }
+
+    void Insert() { InsertInternal(); }
+
+    EntryType* OrInsert() {
+      if (!HasEntry()) {
+        Insert();
+      }
+      return Entry();
+    }
+
+    void Remove() { mEntryHandle.Remove(); }
+
+    void OrRemove() { mEntryHandle.OrRemove(); }
+
+   protected:
+    template <typename... Args>
+    void InsertInternal(Args&&... aArgs) {
+      MOZ_RELEASE_ASSERT(!HasEntry());
+      mEntryHandle.Insert([&](PLDHashEntryHdr* entry) {
+        new (mozilla::KnownNotNull, entry) EntryType(
+            EntryType::KeyToPointer(mKey), std::forward<Args>(aArgs)...);
+      });
+    }
+
+   private:
+    friend class nsTHashtable;
+
+    EntryHandle(KeyType aKey, PLDHashTable::EntryHandle&& aEntryHandle)
+        : mKey(aKey), mEntryHandle(std::move(aEntryHandle)) {}
+
+    KeyType mKey;
+    PLDHashTable::EntryHandle mEntryHandle;
+  };
+
+  template <class F>
+  auto WithEntryHandle(KeyType aKey, F&& aFunc)
+      -> std::invoke_result_t<F, EntryHandle&&> {
+    return this->mTable.WithEntryHandle(
+        EntryType::KeyToPointer(aKey),
+        [&aKey, &aFunc](auto entryHandle) -> decltype(auto) {
+          return std::forward<F>(aFunc)(
+              EntryHandle{aKey, std::move(entryHandle)});
+        });
+  }
+
+  template <class F>
+  auto WithEntryHandle(KeyType aKey, const mozilla::fallible_t& aFallible,
+                       F&& aFunc)
+      -> std::invoke_result_t<F, mozilla::Maybe<EntryHandle>&&> {
+    return this->mTable.WithEntryHandle(
+        EntryType::KeyToPointer(aKey), aFallible,
+        [&aKey, &aFunc](auto maybeEntryHandle) {
+          return std::forward<F>(aFunc)(
+              maybeEntryHandle
+                  ? mozilla::Some(EntryHandle{aKey, maybeEntryHandle.extract()})
+                  : mozilla::Nothing());
+        });
+  }
+
+ public:
+  class ConstIterator {
+   public:
+    explicit ConstIterator(nsTHashtable* aTable)
+        : mBaseIterator(&aTable->mTable) {}
+    ~ConstIterator() = default;
+
+    KeyType Key() const { return Get()->GetKey(); }
+
+    const EntryType* Get() const {
+      return static_cast<const EntryType*>(mBaseIterator.Get());
+    }
+
+    bool Done() const { return mBaseIterator.Done(); }
+    void Next() { mBaseIterator.Next(); }
+
+    ConstIterator() = delete;
+    ConstIterator(const ConstIterator&) = delete;
+    ConstIterator(ConstIterator&& aOther) = delete;
+    ConstIterator& operator=(const ConstIterator&) = delete;
+    ConstIterator& operator=(ConstIterator&&) = delete;
+
+   protected:
+    PLDHashTable::Iterator mBaseIterator;
+  };
+
+  // This is an iterator that also allows entry removal. Example usage:
+  //
+  //   for (auto iter = table.Iter(); !iter.Done(); iter.Next()) {
+  //     Entry* entry = iter.Get();
+  //     // ... do stuff with |entry| ...
+  //     // ... possibly call iter.Remove() once ...
+  //   }
+  //
+  class Iterator final : public ConstIterator {
+   public:
+    using ConstIterator::ConstIterator;
+
+    using ConstIterator::Get;
+
+    EntryType* Get() const {
+      return static_cast<EntryType*>(this->mBaseIterator.Get());
+    }
+
+    void Remove() { this->mBaseIterator.Remove(); }
+  };
+
+  Iterator Iter() { return Iterator(this); }
+
+  ConstIterator ConstIter() const {
+    return ConstIterator(const_cast<nsTHashtable*>(this));
+  }
+
+  using const_iterator = ::detail::nsTHashtableEntryIterator<const EntryType>;
+  using iterator = ::detail::nsTHashtableEntryIterator<EntryType>;
+
+  iterator begin() { return iterator{mTable}; }
+  const_iterator begin() const { return const_iterator{mTable}; }
+  const_iterator cbegin() const { return begin(); }
+  iterator end() {
+    return iterator{mTable, typename iterator::EndIteratorTag{}};
+  }
+  const_iterator end() const {
+    return const_iterator{mTable, typename const_iterator::EndIteratorTag{}};
+  }
+  const_iterator cend() const { return end(); }
+
+  void Remove(const_iterator& aIter) { aIter.mIterator.Remove(); }
+
+  /**
+   * Return a range of the keys (of KeyType). Note this range iterates over the
+   * keys in place, so modifications to the nsTHashtable invalidate the range
+   * while it's iterated, except when calling Remove() with a key iterator
+   * derived from that range.
+   */
+  auto Keys() const {
+    return ::detail::nsTHashtableKeyRange<EntryType>{mTable};
+  }
+
+  /**
+   * Remove an entry from a key range, specified via a key iterator, e.g.
+   *
+   * for (auto it = hash.Keys().begin(), end = hash.Keys().end();
+   *      it != end; * ++it) {
+   *   if (*it > 42) { hash.Remove(it); }
+   * }
+   */
+  void Remove(::detail::nsTHashtableKeyIterator<EntryType>& aIter) {
+    aIter.mIterator.Remove();
+  }
+
+  /**
+   * Remove all entries, return hashtable to "pristine" state. It's
+   * conceptually the same as calling the destructor and then re-calling the
+   * constructor.
+   */
+  void Clear() { mTable.Clear(); }
+
+  /**
+   * Measure the size of the table's entry storage. Does *not* measure anything
+   * hanging off table entries; hence the "Shallow" prefix. To measure that,
+   * either use SizeOfExcludingThis() or iterate manually over the entries,
+   * calling SizeOfExcludingThis() on each one.
+   *
+   * @param     aMallocSizeOf the function used to measure heap-allocated blocks
+   * @return    the measured shallow size of the table
+   */
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return mTable.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /**
+   * Like ShallowSizeOfExcludingThis, but includes sizeof(*this).
+   */
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /**
+   * This is a "deep" measurement of the table. To use it, |EntryType| must
+   * define SizeOfExcludingThis, and that method will be called on all live
+   * entries.
+   */
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    size_t n = ShallowSizeOfExcludingThis(aMallocSizeOf);
+    for (auto iter = ConstIter(); !iter.Done(); iter.Next()) {
+      n += (*iter.Get()).SizeOfExcludingThis(aMallocSizeOf);
+    }
+    return n;
+  }
+
+  /**
+   * Like SizeOfExcludingThis, but includes sizeof(*this).
+   */
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /**
+   * Swap the elements in this hashtable with the elements in aOther.
+   */
+  void SwapElements(nsTHashtable<EntryType>& aOther) {
+    MOZ_ASSERT_IF(this->mTable.Ops() && aOther.mTable.Ops(),
+                  this->mTable.Ops() == aOther.mTable.Ops());
+    std::swap(this->mTable, aOther.mTable);
+  }
+
+  /**
+   * Mark the table as constant after initialization.
+   *
+   * This will prevent assertions when a read-only hash is accessed on multiple
+   * threads without synchronization.
+   */
+  void MarkImmutable() { mTable.MarkImmutable(); }
+
+ protected:
+  PLDHashTable mTable;
+
+  static PLDHashNumber s_HashKey(const void* aKey);
+
+  static bool s_MatchEntry(const PLDHashEntryHdr* aEntry, const void* aKey);
+
+  static void s_CopyEntry(PLDHashTable* aTable, const PLDHashEntryHdr* aFrom,
+                          PLDHashEntryHdr* aTo);
+
+  static void s_ClearEntry(PLDHashTable* aTable, PLDHashEntryHdr* aEntry);
+
+ private:
+  // copy constructor, not implemented
+  nsTHashtable(nsTHashtable<EntryType>& aToCopy) = delete;
+
+  /**
+   * Gets the table's ops.
+   */
+  static const PLDHashTableOps* Ops();
+
+  // assignment operator, not implemented
+  nsTHashtable<EntryType>& operator=(nsTHashtable<EntryType>& aToEqual) =
+      delete;
+};
+
+namespace mozilla {
+namespace detail {
+
+// Like PLDHashTable::MoveEntryStub, but specialized for fixed N (i.e. the size
+// of the entries in the hashtable).  Saves a memory read to figure out the size
+// from the table and gives the compiler the opportunity to inline the memcpy.
+//
+// We define this outside of nsTHashtable so only one copy exists for every N,
+// rather than separate copies for every EntryType used with nsTHashtable.
+template <size_t N>
+static void FixedSizeEntryMover(PLDHashTable*, const PLDHashEntryHdr* aFrom,
+                                PLDHashEntryHdr* aTo) {
+  memcpy(aTo, aFrom, N);
+}
+
+}  // namespace detail
+}  // namespace mozilla
+
+//
+// template definitions
+//
+
+template <class EntryType>
+nsTHashtable<EntryType>::nsTHashtable(nsTHashtable<EntryType>&& aOther)
+    : mTable(std::move(aOther.mTable)) {}
+
+template <class EntryType>
+nsTHashtable<EntryType>& nsTHashtable<EntryType>::operator=(
+    nsTHashtable<EntryType>&& aOther) {
+  mTable = std::move(aOther.mTable);
+  return *this;
+}
+
+template <class EntryType>
+/* static */ const PLDHashTableOps* nsTHashtable<EntryType>::Ops() {
+  // If this variable is a global variable, we get strange start-up failures on
+  // WindowsCrtPatch.h (see bug 1166598 comment 20). But putting it inside a
+  // function avoids that problem.
+  static const PLDHashTableOps sOps = {
+      s_HashKey, s_MatchEntry,
+      EntryType::ALLOW_MEMMOVE
+          ? mozilla::detail::FixedSizeEntryMover<sizeof(EntryType)>
+          : s_CopyEntry,
+      // Simplify hashtable clearing in case our entries are trivially
+      // destructible.
+      std::is_trivially_destructible_v<EntryType> ? nullptr : s_ClearEntry,
+      // We don't use a generic initEntry hook because we want to allow
+      // initialization of data members defined in derived classes directly
+      // in the entry constructor (for example when a member can't be default
+      // constructed).
+      nullptr};
+  return &sOps;
+}
+
+// static definitions
+
+template <class EntryType>
+PLDHashNumber nsTHashtable<EntryType>::s_HashKey(const void* aKey) {
+  return EntryType::HashKey(static_cast<KeyTypePointer>(aKey));
+}
+
+template <class EntryType>
+bool nsTHashtable<EntryType>::s_MatchEntry(const PLDHashEntryHdr* aEntry,
+                                           const void* aKey) {
+  return (static_cast<const EntryType*>(aEntry))
+      ->KeyEquals(static_cast<KeyTypePointer>(aKey));
+}
+
+template <class EntryType>
+void nsTHashtable<EntryType>::s_CopyEntry(PLDHashTable* aTable,
+                                          const PLDHashEntryHdr* aFrom,
+                                          PLDHashEntryHdr* aTo) {
+  auto* fromEntry = const_cast<std::remove_const_t<EntryType>*>(
+      static_cast<const EntryType*>(aFrom));
+
+  new (mozilla::KnownNotNull, aTo) EntryType(std::move(*fromEntry));
+
+  fromEntry->~EntryType();
+}
+
+template <class EntryType>
+void nsTHashtable<EntryType>::s_ClearEntry(PLDHashTable* aTable,
+                                           PLDHashEntryHdr* aEntry) {
+  static_cast<EntryType*>(aEntry)->~EntryType();
+}
+
+class nsCycleCollectionTraversalCallback;
+
+template <class EntryType>
+inline void ImplCycleCollectionUnlink(nsTHashtable<EntryType>& aField) {
+  aField.Clear();
+}
+
+template <class EntryType>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsTHashtable<EntryType>& aField, const char* aName, uint32_t aFlags = 0) {
+  for (auto iter = aField.Iter(); !iter.Done(); iter.Next()) {
+    EntryType* entry = iter.Get();
+    ImplCycleCollectionTraverse(aCallback, *entry, aName, aFlags);
+  }
+}
+
+/**
+ * For nsTHashtable with pointer entries, we can have a template specialization
+ * that layers a typed T* interface on top of a common implementation that
+ * works internally with void pointers.  This arrangement saves code size and
+ * might slightly improve performance as well.
+ */
+
+/**
+ * We need a separate entry type class for the inheritance structure of the
+ * nsTHashtable specialization below; nsVoidPtrHashKey is simply typedefed to a
+ * specialization of nsPtrHashKey, and the formulation:
+ *
+ * class nsTHashtable<nsPtrHashKey<T>> :
+ *   protected nsTHashtable<nsPtrHashKey<const void>
+ *
+ * is not going to turn out very well, since we'd wind up with an nsTHashtable
+ * instantiation that is its own base class.
+ */
+namespace detail {
+
+class VoidPtrHashKey : public nsPtrHashKey<const void> {
+  typedef nsPtrHashKey<const void> Base;
+
+ public:
+  explicit VoidPtrHashKey(const void* aKey) : Base(aKey) {}
+};
+
+}  // namespace detail
+
+/**
+ * See the main nsTHashtable documentation for descriptions of this class's
+ * methods.
+ */
+template <typename T>
+class nsTHashtable<nsPtrHashKey<T>>
+    : protected nsTHashtable<::detail::VoidPtrHashKey> {
+  typedef nsTHashtable<::detail::VoidPtrHashKey> Base;
+  typedef nsPtrHashKey<T> EntryType;
+
+  // We play games with reinterpret_cast'ing between these two classes, so
+  // try to ensure that playing said games is reasonable.
+  static_assert(sizeof(nsPtrHashKey<T>) == sizeof(::detail::VoidPtrHashKey),
+                "hash keys must be the same size");
+
+  nsTHashtable(const nsTHashtable& aOther) = delete;
+  nsTHashtable& operator=(const nsTHashtable& aOther) = delete;
+
+ public:
+  nsTHashtable() = default;
+  explicit nsTHashtable(uint32_t aInitLength) : Base(aInitLength) {}
+
+  ~nsTHashtable() = default;
+
+  nsTHashtable(nsTHashtable&&) = default;
+
+  using Base::Clear;
+  using Base::Count;
+  using Base::GetGeneration;
+  using Base::IsEmpty;
+
+  using Base::MarkImmutable;
+  using Base::ShallowSizeOfExcludingThis;
+  using Base::ShallowSizeOfIncludingThis;
+
+  /* Wrapper functions */
+  EntryType* GetEntry(T* aKey) const {
+    return reinterpret_cast<EntryType*>(Base::GetEntry(aKey));
+  }
+
+  bool Contains(T* aKey) const { return Base::Contains(aKey); }
+
+  EntryType* PutEntry(T* aKey) {
+    return reinterpret_cast<EntryType*>(Base::PutEntry(aKey));
+  }
+
+  [[nodiscard]] EntryType* PutEntry(T* aKey,
+                                    const mozilla::fallible_t& aFallible) {
+    return reinterpret_cast<EntryType*>(Base::PutEntry(aKey, aFallible));
+  }
+
+  [[nodiscard]] bool EnsureInserted(T* aKey, EntryType** aEntry = nullptr) {
+    return Base::EnsureInserted(
+        aKey, reinterpret_cast<::detail::VoidPtrHashKey**>(aEntry));
+  }
+
+  void RemoveEntry(T* aKey) { Base::RemoveEntry(aKey); }
+
+  bool EnsureRemoved(T* aKey) { return Base::EnsureRemoved(aKey); }
+
+  void RemoveEntry(EntryType* aEntry) {
+    Base::RemoveEntry(reinterpret_cast<::detail::VoidPtrHashKey*>(aEntry));
+  }
+
+  void RawRemoveEntry(EntryType* aEntry) {
+    Base::RawRemoveEntry(reinterpret_cast<::detail::VoidPtrHashKey*>(aEntry));
+  }
+
+ protected:
+  class EntryHandle : protected Base::EntryHandle {
+   public:
+    using Base = nsTHashtable::Base::EntryHandle;
+
+    EntryHandle(EntryHandle&& aOther) = default;
+    ~EntryHandle() = default;
+
+    EntryHandle(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&) = delete;
+    EntryHandle& operator=(const EntryHandle&&) = delete;
+
+    using Base::Key;
+
+    using Base::HasEntry;
+
+    using Base::operator bool;
+
+    EntryType* Entry() { return reinterpret_cast<EntryType*>(Base::Entry()); }
+
+    using Base::Insert;
+
+    EntryType* OrInsert() {
+      if (!HasEntry()) {
+        Insert();
+      }
+      return Entry();
+    }
+
+    using Base::Remove;
+
+    using Base::OrRemove;
+
+   private:
+    friend class nsTHashtable;
+
+    explicit EntryHandle(Base&& aBase) : Base(std::move(aBase)) {}
+  };
+
+  template <class F>
+  auto WithEntryHandle(KeyType aKey, F aFunc)
+      -> std::invoke_result_t<F, EntryHandle&&> {
+    return Base::WithEntryHandle(aKey, [&aFunc](auto entryHandle) {
+      return aFunc(EntryHandle{std::move(entryHandle)});
+    });
+  }
+
+  template <class F>
+  auto WithEntryHandle(KeyType aKey, const mozilla::fallible_t& aFallible,
+                       F aFunc)
+      -> std::invoke_result_t<F, mozilla::Maybe<EntryHandle>&&> {
+    return Base::WithEntryHandle(
+        aKey, aFallible, [&aFunc](auto maybeEntryHandle) {
+          return aFunc(maybeEntryHandle ? mozilla::Some(EntryHandle{
+                                              maybeEntryHandle.extract()})
+                                        : mozilla::Nothing());
+        });
+  }
+
+ public:
+  class ConstIterator {
+   public:
+    explicit ConstIterator(nsTHashtable* aTable)
+        : mBaseIterator(&aTable->mTable) {}
+    ~ConstIterator() = default;
+
+    KeyType Key() const { return Get()->GetKey(); }
+
+    const EntryType* Get() const {
+      return static_cast<const EntryType*>(mBaseIterator.Get());
+    }
+
+    bool Done() const { return mBaseIterator.Done(); }
+    void Next() { mBaseIterator.Next(); }
+
+    ConstIterator() = delete;
+    ConstIterator(const ConstIterator&) = delete;
+    ConstIterator(ConstIterator&& aOther) = delete;
+    ConstIterator& operator=(const ConstIterator&) = delete;
+    ConstIterator& operator=(ConstIterator&&) = delete;
+
+   protected:
+    PLDHashTable::Iterator mBaseIterator;
+  };
+
+  class Iterator final : public ConstIterator {
+   public:
+    using ConstIterator::ConstIterator;
+
+    using ConstIterator::Get;
+
+    EntryType* Get() const {
+      return static_cast<EntryType*>(this->mBaseIterator.Get());
+    }
+
+    void Remove() { this->mBaseIterator.Remove(); }
+  };
+
+  Iterator Iter() { return Iterator(this); }
+
+  ConstIterator ConstIter() const {
+    return ConstIterator(const_cast<nsTHashtable*>(this));
+  }
+
+  using const_iterator = ::detail::nsTHashtableEntryIterator<const EntryType>;
+  using iterator = ::detail::nsTHashtableEntryIterator<EntryType>;
+
+  iterator begin() { return iterator{mTable}; }
+  const_iterator begin() const { return const_iterator{mTable}; }
+  const_iterator cbegin() const { return begin(); }
+  iterator end() {
+    return iterator{mTable, typename iterator::EndIteratorTag{}};
+  }
+  const_iterator end() const {
+    return const_iterator{mTable, typename const_iterator::EndIteratorTag{}};
+  }
+  const_iterator cend() const { return end(); }
+
+  auto Keys() const {
+    return ::detail::nsTHashtableKeyRange<nsPtrHashKey<T>>{mTable};
+  }
+
+  void Remove(::detail::nsTHashtableKeyIterator<EntryType>& aIter) {
+    aIter.mIterator.Remove();
+  }
+
+  void SwapElements(nsTHashtable& aOther) { Base::SwapElements(aOther); }
+};
+
+#endif  // nsTHashtable_h__
diff --git a/xpcom/ds/nsTObserverArray.cpp b/xpcom/ds/nsTObserverArray.cpp
new file mode 100644
index 0000000000..68211873dd
--- /dev/null
+++ b/xpcom/ds/nsTObserverArray.cpp
@@ -0,0 +1,27 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsTObserverArray.h"
+
+void nsTObserverArray_base::AdjustIterators(index_type aModPos,
+                                            diff_type aAdjustment) {
+  MOZ_ASSERT(aAdjustment == -1 || aAdjustment == 1, "invalid adjustment");
+  Iterator_base* iter = mIterators;
+  while (iter) {
+    if (iter->mPosition > aModPos) {
+      iter->mPosition += aAdjustment;
+    }
+    iter = iter->mNext;
+  }
+}
+
+void nsTObserverArray_base::ClearIterators() {
+  Iterator_base* iter = mIterators;
+  while (iter) {
+    iter->mPosition = 0;
+    iter = iter->mNext;
+  }
+}
diff --git a/xpcom/ds/nsTObserverArray.h b/xpcom/ds/nsTObserverArray.h
new file mode 100644
index 0000000000..4d3e4087e0
--- /dev/null
+++ b/xpcom/ds/nsTObserverArray.h
@@ -0,0 +1,583 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsTObserverArray_h___
+#define nsTObserverArray_h___
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/ReverseIterator.h"
+#include "nsTArray.h"
+#include "nsCycleCollectionNoteChild.h"
+
+/**
+ * An array of observers. Like a normal array, but supports iterators that are
+ * stable even if the array is modified during iteration.
+ * The template parameter T is the observer type the array will hold;
+ * N is the number of built-in storage slots that come with the array.
+ * NOTE: You probably want to use nsTObserverArray, unless you specifically
+ * want built-in storage. See below.
+ * @see nsTObserverArray, nsTArray
+ */
+
+class nsTObserverArray_base {
+ public:
+  typedef size_t index_type;
+  typedef size_t size_type;
+  typedef ptrdiff_t diff_type;
+
+ protected:
+  class Iterator_base {
+   public:
+    Iterator_base(const Iterator_base&) = delete;
+
+   protected:
+    friend class nsTObserverArray_base;
+
+    Iterator_base(index_type aPosition, Iterator_base* aNext)
+        : mPosition(aPosition), mNext(aNext) {}
+
+    // The current position of the iterator. Its exact meaning differs
+    // depending on iterator. See nsTObserverArray<T>::ForwardIterator.
+    index_type mPosition;
+
+    // The next iterator currently iterating the same array
+    Iterator_base* mNext;
+  };
+
+  nsTObserverArray_base() : mIterators(nullptr) {}
+
+  ~nsTObserverArray_base() {
+    NS_ASSERTION(mIterators == nullptr, "iterators outlasting array");
+  }
+
+  /**
+   * Adjusts iterators after an element has been inserted or removed
+   * from the array.
+   * @param aModPos     Position where elements were added or removed.
+   * @param aAdjustment -1 if an element was removed, 1 if an element was
+   *                    added.
+   */
+  void AdjustIterators(index_type aModPos, diff_type aAdjustment);
+
+  /**
+   * Clears iterators when the array is destroyed.
+   */
+  void ClearIterators();
+
+  mutable Iterator_base* mIterators;
+};
+
+template <class T, size_t N>
+class nsAutoTObserverArray : protected nsTObserverArray_base {
+ public:
+  typedef T value_type;
+  typedef nsTArray<T> array_type;
+
+  nsAutoTObserverArray() = default;
+
+  //
+  // Accessor methods
+  //
+
+  // @return The number of elements in the array.
+  size_type Length() const { return mArray.Length(); }
+
+  // @return True if the array is empty or false otherwise.
+  bool IsEmpty() const { return mArray.IsEmpty(); }
+
+  // This method provides direct, readonly access to the array elements.
+  // @return A pointer to the first element of the array.  If the array is
+  // empty, then this pointer must not be dereferenced.
+  const value_type* Elements() const { return mArray.Elements(); }
+  value_type* Elements() { return mArray.Elements(); }
+
+  // This method provides direct access to an element of the array. The given
+  // index must be within the array bounds. If the underlying array may change
+  // during iteration, use an iterator instead of this function.
+  // @param aIndex The index of an element in the array.
+  // @return A reference to the i'th element of the array.
+  value_type& ElementAt(index_type aIndex) { return mArray.ElementAt(aIndex); }
+
+  // Same as above, but readonly.
+  const value_type& ElementAt(index_type aIndex) const {
+    return mArray.ElementAt(aIndex);
+  }
+
+  // This method provides direct access to an element of the array in a bounds
+  // safe manner. If the requested index is out of bounds the provided default
+  // value is returned.
+  // @param aIndex The index of an element in the array.
+  // @param aDef   The value to return if the index is out of bounds.
+  value_type& SafeElementAt(index_type aIndex, value_type& aDef) {
+    return mArray.SafeElementAt(aIndex, aDef);
+  }
+
+  // Same as above, but readonly.
+  const value_type& SafeElementAt(index_type aIndex,
+                                  const value_type& aDef) const {
+    return mArray.SafeElementAt(aIndex, aDef);
+  }
+
+  // No operator[] is provided because the point of this class is to support
+  // allow modifying the array during iteration, and ElementAt() is not safe
+  // in those conditions.
+
+  //
+  // Search methods
+  //
+
+  // This method searches, starting from the beginning of the array,
+  // for the first element in this array that is equal to the given element.
+  // 'operator==' must be defined for value_type.
+  // @param aItem The item to search for.
+  // @return      true if the element was found.
+  template <class Item>
+  bool Contains(const Item& aItem) const {
+    return IndexOf(aItem) != array_type::NoIndex;
+  }
+
+  // This method searches for the offset of the first element in this
+  // array that is equal to the given element.
+  // 'operator==' must be defined for value_type.
+  // @param aItem  The item to search for.
+  // @param aStart The index to start from.
+  // @return The index of the found element or NoIndex if not found.
+  template <class Item>
+  index_type IndexOf(const Item& aItem, index_type aStart = 0) const {
+    return mArray.IndexOf(aItem, aStart);
+  }
+
+  //
+  // Mutation methods
+  //
+
+  // Insert a given element at the given index.
+  // @param aIndex The index at which to insert item.
+  // @param aItem  The item to insert,
+  template <class Item>
+  void InsertElementAt(index_type aIndex, const Item& aItem) {
+    mArray.InsertElementAt(aIndex, aItem);
+    AdjustIterators(aIndex, 1);
+  }
+
+  // Same as above but without copy constructing.
+  // This is useful to avoid temporaries.
+  value_type* InsertElementAt(index_type aIndex) {
+    value_type* item = mArray.InsertElementAt(aIndex);
+    AdjustIterators(aIndex, 1);
+    return item;
+  }
+
+  // Prepend an element to the array unless it already exists in the array.
+  // 'operator==' must be defined for value_type.
+  // @param aItem The item to prepend.
+  template <class Item>
+  void PrependElementUnlessExists(const Item& aItem) {
+    if (!Contains(aItem)) {
+      mArray.InsertElementAt(0, aItem);
+      AdjustIterators(0, 1);
+    }
+  }
+
+  // Append an element to the array.
+  // @param aItem The item to append.
+  template <class Item>
+  void AppendElement(Item&& aItem) {
+    mArray.AppendElement(std::forward<Item>(aItem));
+  }
+
+  // Same as above, but without copy-constructing. This is useful to avoid
+  // temporaries.
+  value_type* AppendElement() { return mArray.AppendElement(); }
+
+  // Append an element to the array unless it already exists in the array.
+  // 'operator==' must be defined for value_type.
+  // @param aItem The item to append.
+  template <class Item>
+  void AppendElementUnlessExists(const Item& aItem) {
+    if (!Contains(aItem)) {
+      mArray.AppendElement(aItem);
+    }
+  }
+
+  // Remove an element from the array.
+  // @param aIndex The index of the item to remove.
+  void RemoveElementAt(index_type aIndex) {
+    NS_ASSERTION(aIndex < mArray.Length(), "invalid index");
+    mArray.RemoveElementAt(aIndex);
+    AdjustIterators(aIndex, -1);
+  }
+
+  // This helper function combines IndexOf with RemoveElementAt to "search
+  // and destroy" the first element that is equal to the given element.
+  // 'operator==' must be defined for value_type.
+  // @param aItem The item to search for.
+  // @return true if the element was found and removed.
+  template <class Item>
+  bool RemoveElement(const Item& aItem) {
+    index_type index = mArray.IndexOf(aItem, 0);
+    if (index == array_type::NoIndex) {
+      return false;
+    }
+
+    mArray.RemoveElementAt(index);
+    AdjustIterators(index, -1);
+    return true;
+  }
+
+  // See nsTArray::RemoveElementsBy. Neither the predicate nor the removal of
+  // elements from the array must have any side effects that modify the array.
+  template <typename Predicate>
+  void NonObservingRemoveElementsBy(Predicate aPredicate) {
+    index_type i = 0;
+    mArray.RemoveElementsBy([&](const value_type& aItem) {
+      if (aPredicate(aItem)) {
+        // This element is going to be removed.
+        AdjustIterators(i, -1);
+        return true;
+      }
+      ++i;
+      return false;
+    });
+  }
+
+  // Removes all observers and collapses all iterators to the beginning of
+  // the array. The result is that forward iterators will see all elements
+  // in the array.
+  void Clear() {
+    mArray.Clear();
+    ClearIterators();
+  }
+
+  // Compact the array to minimize the memory it uses
+  void Compact() { mArray.Compact(); }
+
+  // Returns the number of bytes on the heap taken up by this object, not
+  // including sizeof(*this). If you want to measure anything hanging off the
+  // array, you must iterate over the elements and measure them individually;
+  // hence the "Shallow" prefix.
+  size_t ShallowSizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return mArray.ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  //
+  // Iterators
+  //
+
+  // Base class for iterators. Do not use this directly.
+  class Iterator : public Iterator_base {
+   protected:
+    friend class nsAutoTObserverArray;
+    typedef nsAutoTObserverArray<T, N> array_type;
+
+    Iterator(const Iterator& aOther)
+        : Iterator(aOther.mPosition, aOther.mArray) {}
+
+    Iterator(index_type aPosition, const array_type& aArray)
+        : Iterator_base(aPosition, aArray.mIterators),
+          mArray(const_cast<array_type&>(aArray)) {
+      aArray.mIterators = this;
+    }
+
+    ~Iterator() {
+      NS_ASSERTION(mArray.mIterators == this,
+                   "Iterators must currently be destroyed in opposite order "
+                   "from the construction order. It is suggested that you "
+                   "simply put them on the stack");
+      mArray.mIterators = mNext;
+    }
+
+    // The array we're iterating
+    array_type& mArray;
+  };
+
+  // Iterates the array forward from beginning to end. mPosition points
+  // to the element that will be returned on next call to GetNext.
+  // Elements:
+  // - prepended to the array during iteration *will not* be traversed
+  // - appended during iteration *will* be traversed
+  // - removed during iteration *will not* be traversed.
+  // @see EndLimitedIterator
+  class ForwardIterator : protected Iterator {
+   public:
+    typedef nsAutoTObserverArray<T, N> array_type;
+    typedef Iterator base_type;
+
+    explicit ForwardIterator(const array_type& aArray) : Iterator(0, aArray) {}
+
+    ForwardIterator(const array_type& aArray, index_type aPos)
+        : Iterator(aPos, aArray) {}
+
+    bool operator<(const ForwardIterator& aOther) const {
+      NS_ASSERTION(&this->mArray == &aOther.mArray,
+                   "not iterating the same array");
+      return base_type::mPosition < aOther.mPosition;
+    }
+
+    // Returns true if there are more elements to iterate.
+    // This must precede a call to GetNext(). If false is
+    // returned, GetNext() must not be called.
+    bool HasMore() const {
+      return base_type::mPosition < base_type::mArray.Length();
+    }
+
+    // Returns the next element and steps one step. This must
+    // be preceded by a call to HasMore().
+    // @return The next observer.
+    value_type& GetNext() {
+      NS_ASSERTION(HasMore(), "iterating beyond end of array");
+      return base_type::mArray.Elements()[base_type::mPosition++];
+    }
+
+    // Removes the element at the current iterator position.
+    // (the last element returned from |GetNext()|)
+    // This will not affect the next call to |GetNext()|
+    void Remove() {
+      return base_type::mArray.RemoveElementAt(base_type::mPosition - 1);
+    }
+  };
+
+  // EndLimitedIterator works like ForwardIterator, but will not iterate new
+  // observers appended to the array after the iterator was created.
+  class EndLimitedIterator : protected ForwardIterator {
+   public:
+    typedef nsAutoTObserverArray<T, N> array_type;
+    typedef Iterator base_type;
+
+    explicit EndLimitedIterator(const array_type& aArray)
+        : ForwardIterator(aArray), mEnd(aArray, aArray.Length()) {}
+
+    // Returns true if there are more elements to iterate.
+    // This must precede a call to GetNext(). If false is
+    // returned, GetNext() must not be called.
+    bool HasMore() const { return *this < mEnd; }
+
+    // Returns the next element and steps one step. This must
+    // be preceded by a call to HasMore().
+    // @return The next observer.
+    value_type& GetNext() {
+      NS_ASSERTION(HasMore(), "iterating beyond end of array");
+      return base_type::mArray.Elements()[base_type::mPosition++];
+    }
+
+    // Removes the element at the current iterator position.
+    // (the last element returned from |GetNext()|)
+    // This will not affect the next call to |GetNext()|
+    void Remove() {
+      return base_type::mArray.RemoveElementAt(base_type::mPosition - 1);
+    }
+
+   private:
+    ForwardIterator mEnd;
+  };
+
+  // Iterates the array backward from end to start. mPosition points
+  // to the element that was returned last.
+  // Elements:
+  // - prepended to the array during iteration *will* be traversed,
+  //   unless the iteration already arrived at the first element
+  // - appended during iteration *will not* be traversed
+  // - removed during iteration *will not* be traversed.
+  class BackwardIterator : protected Iterator {
+   public:
+    typedef nsAutoTObserverArray<T, N> array_type;
+    typedef Iterator base_type;
+
+    explicit BackwardIterator(const array_type& aArray)
+        : Iterator(aArray.Length(), aArray) {}
+
+    // Returns true if there are more elements to iterate.
+    // This must precede a call to GetNext(). If false is
+    // returned, GetNext() must not be called.
+    bool HasMore() const { return base_type::mPosition > 0; }
+
+    // Returns the next element and steps one step. This must
+    // be preceded by a call to HasMore().
+    // @return The next observer.
+    value_type& GetNext() {
+      NS_ASSERTION(HasMore(), "iterating beyond start of array");
+      return base_type::mArray.Elements()[--base_type::mPosition];
+    }
+
+    // Removes the element at the current iterator position.
+    // (the last element returned from |GetNext()|)
+    // This will not affect the next call to |GetNext()|
+    void Remove() {
+      return base_type::mArray.RemoveElementAt(base_type::mPosition);
+    }
+  };
+
+  struct EndSentinel {};
+
+  // Internal type, do not use directly, see
+  // ForwardRange()/EndLimitedRange()/BackwardRange().
+  template <typename Iterator, typename U>
+  struct STLIterator {
+    using value_type = std::remove_reference_t<U>;
+
+    explicit STLIterator(const nsAutoTObserverArray<T, N>& aArray)
+        : mIterator{aArray} {
+      operator++();
+    }
+
+    bool operator!=(const EndSentinel&) const {
+      // We are a non-end-sentinel and the other is an end-sentinel, so we are
+      // still valid if mCurrent is valid.
+      return mCurrent;
+    }
+
+    STLIterator& operator++() {
+      mCurrent = mIterator.HasMore() ? &mIterator.GetNext() : nullptr;
+      return *this;
+    }
+
+    value_type* operator->() { return mCurrent; }
+    U& operator*() { return *mCurrent; }
+
+   private:
+    Iterator mIterator;
+    value_type* mCurrent;
+  };
+
+  // Internal type, do not use directly, see
+  // ForwardRange()/EndLimitedRange()/BackwardRange().
+  template <typename Iterator, typename U>
+  class STLIteratorRange {
+   public:
+    using iterator = STLIterator<Iterator, U>;
+
+    explicit STLIteratorRange(const nsAutoTObserverArray<T, N>& aArray)
+        : mArray{aArray} {}
+
+    STLIteratorRange(const STLIteratorRange& aOther) = delete;
+
+    iterator begin() const { return iterator{mArray}; }
+    EndSentinel end() const { return {}; }
+
+   private:
+    const nsAutoTObserverArray<T, N>& mArray;
+  };
+
+  template <typename U>
+  using STLForwardIteratorRange = STLIteratorRange<ForwardIterator, U>;
+
+  template <typename U>
+  using STLEndLimitedIteratorRange = STLIteratorRange<EndLimitedIterator, U>;
+
+  template <typename U>
+  using STLBackwardIteratorRange = STLIteratorRange<BackwardIterator, U>;
+
+  // Constructs a range (usable with range-based for) based on the
+  // ForwardIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto ForwardRange() { return STLForwardIteratorRange<T>{*this}; }
+
+  // Constructs a const range (usable with range-based for) based on the
+  // ForwardIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto ForwardRange() const { return STLForwardIteratorRange<const T>{*this}; }
+
+  // Constructs a range (usable with range-based for) based on the
+  // EndLimitedIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto EndLimitedRange() { return STLEndLimitedIteratorRange<T>{*this}; }
+
+  // Constructs a const range (usable with range-based for) based on the
+  // EndLimitedIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto EndLimitedRange() const {
+    return STLEndLimitedIteratorRange<const T>{*this};
+  }
+
+  // Constructs a range (usable with range-based for) based on the
+  // BackwardIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto BackwardRange() { return STLBackwardIteratorRange<T>{*this}; }
+
+  // Constructs a const range (usable with range-based for) based on the
+  // BackwardIterator semantics. Note that this range does not provide
+  // full-feature STL-style iterators usable with STL-style algorithms.
+  auto BackwardRange() const {
+    return STLBackwardIteratorRange<const T>{*this};
+  }
+
+  // Constructs a const range (usable with range-based for and STL-style
+  // algorithms) based on a non-observing iterator. The array must not be
+  // modified during iteration.
+  auto NonObservingRange() const {
+    return mozilla::detail::IteratorRange<
+        typename AutoTArray<T, N>::const_iterator,
+        typename AutoTArray<T, N>::const_reverse_iterator>{mArray.cbegin(),
+                                                           mArray.cend()};
+  }
+
+ protected:
+  AutoTArray<T, N> mArray;
+};
+
+template <class T>
+class nsTObserverArray : public nsAutoTObserverArray<T, 0> {
+ public:
+  typedef nsAutoTObserverArray<T, 0> base_type;
+  typedef nsTObserverArray_base::size_type size_type;
+
+  //
+  // Initialization methods
+  //
+
+  nsTObserverArray() = default;
+
+  // Initialize this array and pre-allocate some number of elements.
+  explicit nsTObserverArray(size_type aCapacity) {
+    base_type::mArray.SetCapacity(aCapacity);
+  }
+
+  nsTObserverArray Clone() const {
+    auto result = nsTObserverArray{};
+    result.mArray.Assign(this->mArray);
+    return result;
+  }
+};
+
+template <typename T, size_t N>
+auto MakeBackInserter(nsAutoTObserverArray<T, N>& aArray) {
+  return mozilla::nsTArrayBackInserter<T, nsAutoTObserverArray<T, N>>{aArray};
+}
+
+template <typename T, size_t N>
+inline void ImplCycleCollectionUnlink(nsAutoTObserverArray<T, N>& aField) {
+  aField.Clear();
+}
+
+template <typename T, size_t N>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsAutoTObserverArray<T, N>& aField, const char* aName,
+    uint32_t aFlags = 0) {
+  aFlags |= CycleCollectionEdgeNameArrayFlag;
+  size_t length = aField.Length();
+  for (size_t i = 0; i < length; ++i) {
+    ImplCycleCollectionTraverse(aCallback, aField.ElementAt(i), aName, aFlags);
+  }
+}
+
+// Note that this macro only works if the array holds pointers to XPCOM objects.
+#define NS_OBSERVER_ARRAY_NOTIFY_XPCOM_OBSERVERS(array_, func_, params_) \
+  do {                                                                   \
+    for (RefPtr obs_ : (array_).ForwardRange()) {                        \
+      obs_->func_ params_;                                               \
+    }                                                                    \
+  } while (0)
+
+// Note that this macro only works if the array holds pointers to XPCOM objects.
+#define NS_OBSERVER_ARRAY_NOTIFY_OBSERVERS(array_, func_, params_) \
+  do {                                                             \
+    for (auto* obs_ : (array_).ForwardRange()) {                   \
+      obs_->func_ params_;                                         \
+    }                                                              \
+  } while (0)
+
+#endif  // nsTObserverArray_h___
diff --git a/xpcom/ds/nsTPriorityQueue.h b/xpcom/ds/nsTPriorityQueue.h
new file mode 100644
index 0000000000..1fa1cc1e50
--- /dev/null
+++ b/xpcom/ds/nsTPriorityQueue.h
@@ -0,0 +1,147 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef NS_TPRIORITY_QUEUE_H_
+#define NS_TPRIORITY_QUEUE_H_
+
+#include "mozilla/Assertions.h"
+#include "nsTArray.h"
+
+/**
+ * A templatized priority queue data structure that uses an nsTArray to serve as
+ * a binary heap. The default comparator causes this to act like a min-heap.
+ * Only the LessThan method of the comparator is used.
+ */
+template <class T, class Compare = nsDefaultComparator<T, T>>
+class nsTPriorityQueue {
+ public:
+  typedef typename nsTArray<T>::size_type size_type;
+
+  /**
+   * Default constructor also creates a comparator object using the default
+   * constructor for type Compare.
+   */
+  nsTPriorityQueue() : mCompare(Compare()) {}
+
+  /**
+   * Constructor to allow a specific instance of a comparator object to be
+   * used.
+   */
+  explicit nsTPriorityQueue(const Compare& aComp) : mCompare(aComp) {}
+
+  nsTPriorityQueue(nsTPriorityQueue&&) = default;
+  nsTPriorityQueue& operator=(nsTPriorityQueue&&) = default;
+
+  /**
+   * @return True if the queue is empty or false otherwise.
+   */
+  bool IsEmpty() const { return mElements.IsEmpty(); }
+
+  /**
+   * @return The number of elements in the queue.
+   */
+  size_type Length() const { return mElements.Length(); }
+
+  /**
+   * @return The topmost element in the queue without changing the queue. This
+   * is the element 'a' such that there is no other element 'b' in the queue for
+   * which Compare(b, a) returns true. (Since this container does not check
+   * for duplicate entries there may exist 'b' for which Compare(a, b) returns
+   * false.)
+   */
+  const T& Top() const {
+    MOZ_ASSERT(!mElements.IsEmpty(), "Empty queue");
+    return mElements[0];
+  }
+
+  /**
+   * Adds an element to the queue
+   * @param aElement The element to add
+   * @return true on success, false on out of memory.
+   */
+  void Push(T&& aElement) {
+    mElements.AppendElement(std::move(aElement));
+
+    // Sift up
+    size_type i = mElements.Length() - 1;
+    while (i) {
+      size_type parent = (size_type)((i - 1) / 2);
+      if (mCompare.LessThan(mElements[parent], mElements[i])) {
+        break;
+      }
+      std::swap(mElements[i], mElements[parent]);
+      i = parent;
+    }
+  }
+
+  /**
+   * Removes and returns the top-most element from the queue.
+   * @return The topmost element, that is, the element 'a' such that there is no
+   * other element 'b' in the queue for which Compare(b, a) returns true.
+   * @see Top()
+   */
+  T Pop() {
+    MOZ_ASSERT(!mElements.IsEmpty(), "Empty queue");
+    T pop = std::move(mElements[0]);
+
+    const size_type newLength = mElements.Length() - 1;
+    if (newLength == 0) {
+      mElements.Clear();
+      return pop;
+    }
+
+    // Move last to front
+    mElements[0] = mElements.PopLastElement();
+
+    // Sift down
+    size_type i = 0;
+    while (2 * i + 1 < newLength) {
+      size_type swap = i;
+      size_type l_child = 2 * i + 1;
+      if (mCompare.LessThan(mElements[l_child], mElements[i])) {
+        swap = l_child;
+      }
+      size_type r_child = l_child + 1;
+      if (r_child < newLength &&
+          mCompare.LessThan(mElements[r_child], mElements[swap])) {
+        swap = r_child;
+      }
+      if (swap == i) {
+        break;
+      }
+      std::swap(mElements[i], mElements[swap]);
+      i = swap;
+    }
+
+    return pop;
+  }
+
+  /**
+   * Removes all elements from the queue.
+   */
+  void Clear() { mElements.Clear(); }
+
+  /**
+   * Provides readonly access to the queue elements as an array. Generally this
+   * should be avoided but may be needed in some situations such as when the
+   * elements contained in the queue need to be enumerated for cycle-collection.
+   * @return A pointer to the first element of the array.  If the array is
+   * empty, then this pointer must not be dereferenced.
+   */
+  const T* Elements() const { return mElements.Elements(); }
+
+  nsTPriorityQueue Clone() const {
+    auto res = nsTPriorityQueue{mCompare};
+    res.mElements = mElements.Clone();
+    return res;
+  }
+
+ protected:
+  nsTArray<T> mElements;
+  Compare mCompare;  // Comparator object
+};
+
+#endif  // NS_TPRIORITY_QUEUE_H_
diff --git a/xpcom/ds/nsVariant.cpp b/xpcom/ds/nsVariant.cpp
new file mode 100644
index 0000000000..3ccfc7fa31
--- /dev/null
+++ b/xpcom/ds/nsVariant.cpp
@@ -0,0 +1,1881 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsVariant.h"
+#include "prprf.h"
+#include "prdtoa.h"
+#include <math.h>
+#include "nsCycleCollectionParticipant.h"
+#include "xptinfo.h"
+#include "nsReadableUtils.h"
+#include "nsString.h"
+#include "nsCRTGlue.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/Printf.h"
+
+/***************************************************************************/
+// Helpers for static convert functions...
+
+static nsresult String2Double(const char* aString, double* aResult) {
+  char* next;
+  double value = PR_strtod(aString, &next);
+  if (next == aString) {
+    return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+  *aResult = value;
+  return NS_OK;
+}
+
+static nsresult AString2Double(const nsAString& aString, double* aResult) {
+  char* pChars = ToNewCString(aString, mozilla::fallible);
+  if (!pChars) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  nsresult rv = String2Double(pChars, aResult);
+  free(pChars);
+  return rv;
+}
+
+static nsresult AUTF8String2Double(const nsAUTF8String& aString,
+                                   double* aResult) {
+  return String2Double(PromiseFlatUTF8String(aString).get(), aResult);
+}
+
+static nsresult ACString2Double(const nsACString& aString, double* aResult) {
+  return String2Double(PromiseFlatCString(aString).get(), aResult);
+}
+
+// Fills aOutData with double, uint32_t, or int32_t.
+// Returns NS_OK, an error code, or a non-NS_OK success code
+nsresult nsDiscriminatedUnion::ToManageableNumber(
+    nsDiscriminatedUnion* aOutData) const {
+  nsresult rv;
+
+  switch (mType) {
+    // This group results in a int32_t...
+
+#define CASE__NUMBER_INT32(type_, member_)      \
+  case nsIDataType::type_:                      \
+    aOutData->u.mInt32Value = u.member_;        \
+    aOutData->mType = nsIDataType::VTYPE_INT32; \
+    return NS_OK;
+
+    CASE__NUMBER_INT32(VTYPE_INT8, mInt8Value)
+    CASE__NUMBER_INT32(VTYPE_INT16, mInt16Value)
+    CASE__NUMBER_INT32(VTYPE_INT32, mInt32Value)
+    CASE__NUMBER_INT32(VTYPE_UINT8, mUint8Value)
+    CASE__NUMBER_INT32(VTYPE_UINT16, mUint16Value)
+    CASE__NUMBER_INT32(VTYPE_BOOL, mBoolValue)
+    CASE__NUMBER_INT32(VTYPE_CHAR, mCharValue)
+    CASE__NUMBER_INT32(VTYPE_WCHAR, mWCharValue)
+
+#undef CASE__NUMBER_INT32
+
+      // This group results in a uint32_t...
+
+    case nsIDataType::VTYPE_UINT32:
+      aOutData->u.mUint32Value = u.mUint32Value;
+      aOutData->mType = nsIDataType::VTYPE_UINT32;
+      return NS_OK;
+
+      // This group results in a double...
+
+    case nsIDataType::VTYPE_INT64:
+    case nsIDataType::VTYPE_UINT64:
+      // XXX Need boundary checking here.
+      // We may need to return NS_SUCCESS_LOSS_OF_INSIGNIFICANT_DATA
+      aOutData->u.mDoubleValue = double(u.mInt64Value);
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_FLOAT:
+      aOutData->u.mDoubleValue = u.mFloatValue;
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_DOUBLE:
+      aOutData->u.mDoubleValue = u.mDoubleValue;
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      rv = String2Double(u.str.mStringValue, &aOutData->u.mDoubleValue);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_ASTRING:
+      rv = AString2Double(*u.mAStringValue, &aOutData->u.mDoubleValue);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_UTF8STRING:
+      rv = AUTF8String2Double(*u.mUTF8StringValue, &aOutData->u.mDoubleValue);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_CSTRING:
+      rv = ACString2Double(*u.mCStringValue, &aOutData->u.mDoubleValue);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      rv = AString2Double(nsDependentString(u.wstr.mWStringValue),
+                          &aOutData->u.mDoubleValue);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      aOutData->mType = nsIDataType::VTYPE_DOUBLE;
+      return NS_OK;
+
+      // This group fails...
+
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_ID:
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+    case nsIDataType::VTYPE_ARRAY:
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_EMPTY:
+    default:
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+}
+
+/***************************************************************************/
+// Array helpers...
+
+void nsDiscriminatedUnion::FreeArray() {
+  NS_ASSERTION(mType == nsIDataType::VTYPE_ARRAY, "bad FreeArray call");
+  NS_ASSERTION(u.array.mArrayValue, "bad array");
+  NS_ASSERTION(u.array.mArrayCount, "bad array count");
+
+#define CASE__FREE_ARRAY_PTR(type_, ctype_)                 \
+  case nsIDataType::type_: {                                \
+    ctype_** p = (ctype_**)u.array.mArrayValue;             \
+    for (uint32_t i = u.array.mArrayCount; i > 0; p++, i--) \
+      if (*p) free((char*)*p);                              \
+    break;                                                  \
+  }
+
+#define CASE__FREE_ARRAY_IFACE(type_, ctype_)               \
+  case nsIDataType::type_: {                                \
+    ctype_** p = (ctype_**)u.array.mArrayValue;             \
+    for (uint32_t i = u.array.mArrayCount; i > 0; p++, i--) \
+      if (*p) (*p)->Release();                              \
+    break;                                                  \
+  }
+
+  switch (u.array.mArrayType) {
+    case nsIDataType::VTYPE_INT8:
+    case nsIDataType::VTYPE_INT16:
+    case nsIDataType::VTYPE_INT32:
+    case nsIDataType::VTYPE_INT64:
+    case nsIDataType::VTYPE_UINT8:
+    case nsIDataType::VTYPE_UINT16:
+    case nsIDataType::VTYPE_UINT32:
+    case nsIDataType::VTYPE_UINT64:
+    case nsIDataType::VTYPE_FLOAT:
+    case nsIDataType::VTYPE_DOUBLE:
+    case nsIDataType::VTYPE_BOOL:
+    case nsIDataType::VTYPE_CHAR:
+    case nsIDataType::VTYPE_WCHAR:
+      break;
+
+      // XXX We ASSUME that "array of nsID" means "array of pointers to nsID".
+      CASE__FREE_ARRAY_PTR(VTYPE_ID, nsID)
+      CASE__FREE_ARRAY_PTR(VTYPE_CHAR_STR, char)
+      CASE__FREE_ARRAY_PTR(VTYPE_WCHAR_STR, char16_t)
+      CASE__FREE_ARRAY_IFACE(VTYPE_INTERFACE, nsISupports)
+      CASE__FREE_ARRAY_IFACE(VTYPE_INTERFACE_IS, nsISupports)
+
+    // The rest are illegal.
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_UTF8STRING:
+    case nsIDataType::VTYPE_CSTRING:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+    case nsIDataType::VTYPE_ARRAY:
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_EMPTY:
+    default:
+      NS_ERROR("bad type in array!");
+      break;
+  }
+
+  // Free the array memory.
+  free((char*)u.array.mArrayValue);
+
+#undef CASE__FREE_ARRAY_PTR
+#undef CASE__FREE_ARRAY_IFACE
+}
+
+static nsresult CloneArray(uint16_t aInType, const nsIID* aInIID,
+                           uint32_t aInCount, void* aInValue,
+                           uint16_t* aOutType, nsIID* aOutIID,
+                           uint32_t* aOutCount, void** aOutValue) {
+  NS_ASSERTION(aInCount, "bad param");
+  NS_ASSERTION(aInValue, "bad param");
+  NS_ASSERTION(aOutType, "bad param");
+  NS_ASSERTION(aOutCount, "bad param");
+  NS_ASSERTION(aOutValue, "bad param");
+
+  uint32_t i;
+
+  // First we figure out the size of the elements for the new u.array.
+
+  size_t elementSize;
+  size_t allocSize;
+
+  switch (aInType) {
+    case nsIDataType::VTYPE_INT8:
+      elementSize = sizeof(int8_t);
+      break;
+    case nsIDataType::VTYPE_INT16:
+      elementSize = sizeof(int16_t);
+      break;
+    case nsIDataType::VTYPE_INT32:
+      elementSize = sizeof(int32_t);
+      break;
+    case nsIDataType::VTYPE_INT64:
+      elementSize = sizeof(int64_t);
+      break;
+    case nsIDataType::VTYPE_UINT8:
+      elementSize = sizeof(uint8_t);
+      break;
+    case nsIDataType::VTYPE_UINT16:
+      elementSize = sizeof(uint16_t);
+      break;
+    case nsIDataType::VTYPE_UINT32:
+      elementSize = sizeof(uint32_t);
+      break;
+    case nsIDataType::VTYPE_UINT64:
+      elementSize = sizeof(uint64_t);
+      break;
+    case nsIDataType::VTYPE_FLOAT:
+      elementSize = sizeof(float);
+      break;
+    case nsIDataType::VTYPE_DOUBLE:
+      elementSize = sizeof(double);
+      break;
+    case nsIDataType::VTYPE_BOOL:
+      elementSize = sizeof(bool);
+      break;
+    case nsIDataType::VTYPE_CHAR:
+      elementSize = sizeof(char);
+      break;
+    case nsIDataType::VTYPE_WCHAR:
+      elementSize = sizeof(char16_t);
+      break;
+
+    // XXX We ASSUME that "array of nsID" means "array of pointers to nsID".
+    case nsIDataType::VTYPE_ID:
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      elementSize = sizeof(void*);
+      break;
+
+    // The rest are illegal.
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_UTF8STRING:
+    case nsIDataType::VTYPE_CSTRING:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_ARRAY:
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_EMPTY:
+    default:
+      NS_ERROR("bad type in array!");
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+
+  // Alloc the u.array.
+
+  allocSize = aInCount * elementSize;
+  *aOutValue = moz_xmalloc(allocSize);
+
+  // Clone the elements.
+
+  switch (aInType) {
+    case nsIDataType::VTYPE_INT8:
+    case nsIDataType::VTYPE_INT16:
+    case nsIDataType::VTYPE_INT32:
+    case nsIDataType::VTYPE_INT64:
+    case nsIDataType::VTYPE_UINT8:
+    case nsIDataType::VTYPE_UINT16:
+    case nsIDataType::VTYPE_UINT32:
+    case nsIDataType::VTYPE_UINT64:
+    case nsIDataType::VTYPE_FLOAT:
+    case nsIDataType::VTYPE_DOUBLE:
+    case nsIDataType::VTYPE_BOOL:
+    case nsIDataType::VTYPE_CHAR:
+    case nsIDataType::VTYPE_WCHAR:
+      memcpy(*aOutValue, aInValue, allocSize);
+      break;
+
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      if (aOutIID) {
+        *aOutIID = *aInIID;
+      }
+      [[fallthrough]];
+
+    case nsIDataType::VTYPE_INTERFACE: {
+      memcpy(*aOutValue, aInValue, allocSize);
+
+      nsISupports** p = (nsISupports**)*aOutValue;
+      for (i = aInCount; i > 0; ++p, --i)
+        if (*p) {
+          (*p)->AddRef();
+        }
+      break;
+    }
+
+    // XXX We ASSUME that "array of nsID" means "array of pointers to nsID".
+    case nsIDataType::VTYPE_ID: {
+      nsID** inp = (nsID**)aInValue;
+      nsID** outp = (nsID**)*aOutValue;
+      for (i = aInCount; i > 0; --i) {
+        nsID* idp = *(inp++);
+        if (idp) {
+          *(outp++) = idp->Clone();
+        } else {
+          *(outp++) = nullptr;
+        }
+      }
+      break;
+    }
+
+    case nsIDataType::VTYPE_CHAR_STR: {
+      char** inp = (char**)aInValue;
+      char** outp = (char**)*aOutValue;
+      for (i = aInCount; i > 0; i--) {
+        char* str = *(inp++);
+        if (str) {
+          *(outp++) = moz_xstrdup(str);
+        } else {
+          *(outp++) = nullptr;
+        }
+      }
+      break;
+    }
+
+    case nsIDataType::VTYPE_WCHAR_STR: {
+      char16_t** inp = (char16_t**)aInValue;
+      char16_t** outp = (char16_t**)*aOutValue;
+      for (i = aInCount; i > 0; i--) {
+        char16_t* str = *(inp++);
+        if (str) {
+          *(outp++) = NS_xstrdup(str);
+        } else {
+          *(outp++) = nullptr;
+        }
+      }
+      break;
+    }
+
+    // The rest are illegal.
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_ARRAY:
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_EMPTY:
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_UTF8STRING:
+    case nsIDataType::VTYPE_CSTRING:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+    default:
+      NS_ERROR("bad type in array!");
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+
+  *aOutType = aInType;
+  *aOutCount = aInCount;
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+#define TRIVIAL_DATA_CONVERTER(type_, member_, retval_) \
+  if (mType == nsIDataType::type_) {                    \
+    *retval_ = u.member_;                               \
+    return NS_OK;                                       \
+  }
+
+#define NUMERIC_CONVERSION_METHOD_BEGIN(type_, Ctype_, name_)                 \
+  nsresult nsDiscriminatedUnion::ConvertTo##name_(Ctype_* aResult) const {    \
+    TRIVIAL_DATA_CONVERTER(type_, m##name_##Value, aResult)                   \
+    nsDiscriminatedUnion tempData;                                            \
+    nsresult rv = ToManageableNumber(&tempData);                              \
+    /*                                                                     */ \
+    /* NOTE: rv may indicate a success code that we want to preserve       */ \
+    /* For the final return. So all the return cases below should return   */ \
+    /* this rv when indicating success.                                    */ \
+    /*                                                                     */ \
+    if (NS_FAILED(rv)) return rv;                                             \
+    switch (tempData.mType) {
+#define CASE__NUMERIC_CONVERSION_INT32_JUST_CAST(Ctype_) \
+  case nsIDataType::VTYPE_INT32:                         \
+    *aResult = (Ctype_)tempData.u.mInt32Value;           \
+    return rv;
+
+#define CASE__NUMERIC_CONVERSION_INT32_MIN_MAX(Ctype_, min_, max_) \
+  case nsIDataType::VTYPE_INT32: {                                 \
+    int32_t value = tempData.u.mInt32Value;                        \
+    if (value < (min_) || value > (max_)) {                        \
+      return NS_ERROR_LOSS_OF_SIGNIFICANT_DATA;                    \
+    }                                                              \
+    *aResult = (Ctype_)value;                                      \
+    return rv;                                                     \
+  }
+
+#define CASE__NUMERIC_CONVERSION_UINT32_JUST_CAST(Ctype_) \
+  case nsIDataType::VTYPE_UINT32:                         \
+    *aResult = (Ctype_)tempData.u.mUint32Value;           \
+    return rv;
+
+#define CASE__NUMERIC_CONVERSION_UINT32_MAX(Ctype_, max_)         \
+  case nsIDataType::VTYPE_UINT32: {                               \
+    uint32_t value = tempData.u.mUint32Value;                     \
+    if (value > (max_)) return NS_ERROR_LOSS_OF_SIGNIFICANT_DATA; \
+    *aResult = (Ctype_)value;                                     \
+    return rv;                                                    \
+  }
+
+#define CASE__NUMERIC_CONVERSION_DOUBLE_JUST_CAST(Ctype_) \
+  case nsIDataType::VTYPE_DOUBLE:                         \
+    *aResult = (Ctype_)tempData.u.mDoubleValue;           \
+    return rv;
+
+#define CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX_INT(Ctype_, min_, max_)       \
+  case nsIDataType::VTYPE_DOUBLE: {                                           \
+    double value = tempData.u.mDoubleValue;                                   \
+    if (mozilla::IsNaN(value) || value < (min_) || value > (max_)) {          \
+      return NS_ERROR_LOSS_OF_SIGNIFICANT_DATA;                               \
+    }                                                                         \
+    *aResult = (Ctype_)value;                                                 \
+    return (0.0 == fmod(value, 1.0)) ? rv                                     \
+                                     : NS_SUCCESS_LOSS_OF_INSIGNIFICANT_DATA; \
+  }
+
+#define CASES__NUMERIC_CONVERSION_NORMAL(Ctype_, min_, max_) \
+  CASE__NUMERIC_CONVERSION_INT32_MIN_MAX(Ctype_, min_, max_) \
+  CASE__NUMERIC_CONVERSION_UINT32_MAX(Ctype_, max_)          \
+  CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX_INT(Ctype_, min_, max_)
+
+#define NUMERIC_CONVERSION_METHOD_END                      \
+  default:                                                 \
+    NS_ERROR("bad type returned from ToManageableNumber"); \
+    return NS_ERROR_CANNOT_CONVERT_DATA;                   \
+    }                                                      \
+    }
+
+#define NUMERIC_CONVERSION_METHOD_NORMAL(type_, Ctype_, name_, min_, max_) \
+  NUMERIC_CONVERSION_METHOD_BEGIN(type_, Ctype_, name_)                    \
+  CASES__NUMERIC_CONVERSION_NORMAL(Ctype_, min_, max_)                     \
+  NUMERIC_CONVERSION_METHOD_END
+
+/***************************************************************************/
+// These expand into full public methods...
+
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_INT8, uint8_t, Int8, (-127 - 1), 127)
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_INT16, int16_t, Int16, (-32767 - 1),
+                                 32767)
+
+NUMERIC_CONVERSION_METHOD_BEGIN(VTYPE_INT32, int32_t, Int32)
+CASE__NUMERIC_CONVERSION_INT32_JUST_CAST(int32_t)
+CASE__NUMERIC_CONVERSION_UINT32_MAX(int32_t, 2147483647)
+CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX_INT(int32_t, (-2147483647 - 1),
+                                            2147483647)
+NUMERIC_CONVERSION_METHOD_END
+
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_UINT8, uint8_t, Uint8, 0, 255)
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_UINT16, uint16_t, Uint16, 0, 65535)
+
+NUMERIC_CONVERSION_METHOD_BEGIN(VTYPE_UINT32, uint32_t, Uint32)
+CASE__NUMERIC_CONVERSION_INT32_MIN_MAX(uint32_t, 0, 2147483647)
+CASE__NUMERIC_CONVERSION_UINT32_JUST_CAST(uint32_t)
+CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX_INT(uint32_t, 0, 4294967295U)
+NUMERIC_CONVERSION_METHOD_END
+
+// XXX toFloat conversions need to be fixed!
+NUMERIC_CONVERSION_METHOD_BEGIN(VTYPE_FLOAT, float, Float)
+CASE__NUMERIC_CONVERSION_INT32_JUST_CAST(float)
+CASE__NUMERIC_CONVERSION_UINT32_JUST_CAST(float)
+CASE__NUMERIC_CONVERSION_DOUBLE_JUST_CAST(float)
+NUMERIC_CONVERSION_METHOD_END
+
+NUMERIC_CONVERSION_METHOD_BEGIN(VTYPE_DOUBLE, double, Double)
+CASE__NUMERIC_CONVERSION_INT32_JUST_CAST(double)
+CASE__NUMERIC_CONVERSION_UINT32_JUST_CAST(double)
+CASE__NUMERIC_CONVERSION_DOUBLE_JUST_CAST(double)
+NUMERIC_CONVERSION_METHOD_END
+
+// XXX toChar conversions need to be fixed!
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_CHAR, char, Char, CHAR_MIN, CHAR_MAX)
+
+// XXX toWChar conversions need to be fixed!
+NUMERIC_CONVERSION_METHOD_NORMAL(VTYPE_WCHAR, char16_t, WChar, 0,
+                                 std::numeric_limits<char16_t>::max())
+
+#undef NUMERIC_CONVERSION_METHOD_BEGIN
+#undef CASE__NUMERIC_CONVERSION_INT32_JUST_CAST
+#undef CASE__NUMERIC_CONVERSION_INT32_MIN_MAX
+#undef CASE__NUMERIC_CONVERSION_UINT32_JUST_CAST
+#undef CASE__NUMERIC_CONVERSION_UINT32_MIN_MAX
+#undef CASE__NUMERIC_CONVERSION_DOUBLE_JUST_CAST
+#undef CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX
+#undef CASE__NUMERIC_CONVERSION_DOUBLE_MIN_MAX_INT
+#undef CASES__NUMERIC_CONVERSION_NORMAL
+#undef NUMERIC_CONVERSION_METHOD_END
+#undef NUMERIC_CONVERSION_METHOD_NORMAL
+
+/***************************************************************************/
+
+// Just leverage a numeric converter for bool (but restrict the values).
+// XXX Is this really what we want to do?
+
+nsresult nsDiscriminatedUnion::ConvertToBool(bool* aResult) const {
+  TRIVIAL_DATA_CONVERTER(VTYPE_BOOL, mBoolValue, aResult)
+
+  double val;
+  nsresult rv = ConvertToDouble(&val);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  // NaN is falsy in JS, so we might as well make it false here.
+  if (mozilla::IsNaN(val)) {
+    *aResult = false;
+  } else {
+    *aResult = 0.0 != val;
+  }
+  return rv;
+}
+
+/***************************************************************************/
+
+nsresult nsDiscriminatedUnion::ConvertToInt64(int64_t* aResult) const {
+  TRIVIAL_DATA_CONVERTER(VTYPE_INT64, mInt64Value, aResult)
+  TRIVIAL_DATA_CONVERTER(VTYPE_UINT64, mUint64Value, aResult)
+
+  nsDiscriminatedUnion tempData;
+  nsresult rv = ToManageableNumber(&tempData);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  switch (tempData.mType) {
+    case nsIDataType::VTYPE_INT32:
+      *aResult = tempData.u.mInt32Value;
+      return rv;
+    case nsIDataType::VTYPE_UINT32:
+      *aResult = tempData.u.mUint32Value;
+      return rv;
+    case nsIDataType::VTYPE_DOUBLE: {
+      double value = tempData.u.mDoubleValue;
+      if (mozilla::IsNaN(value)) {
+        return NS_ERROR_LOSS_OF_SIGNIFICANT_DATA;
+      }
+      // XXX should check for data loss here!
+      *aResult = value;
+      return rv;
+    }
+    default:
+      NS_ERROR("bad type returned from ToManageableNumber");
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+}
+
+nsresult nsDiscriminatedUnion::ConvertToUint64(uint64_t* aResult) const {
+  return ConvertToInt64((int64_t*)aResult);
+}
+
+/***************************************************************************/
+
+bool nsDiscriminatedUnion::String2ID(nsID* aPid) const {
+  nsAutoString tempString;
+  nsAString* pString;
+
+  switch (mType) {
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      return aPid->Parse(u.str.mStringValue);
+    case nsIDataType::VTYPE_CSTRING:
+      return aPid->Parse(PromiseFlatCString(*u.mCStringValue).get());
+    case nsIDataType::VTYPE_UTF8STRING:
+      return aPid->Parse(PromiseFlatUTF8String(*u.mUTF8StringValue).get());
+    case nsIDataType::VTYPE_ASTRING:
+      pString = u.mAStringValue;
+      break;
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      tempString.Assign(u.wstr.mWStringValue);
+      pString = &tempString;
+      break;
+    default:
+      NS_ERROR("bad type in call to String2ID");
+      return false;
+  }
+
+  char* pChars = ToNewCString(*pString, mozilla::fallible);
+  if (!pChars) {
+    return false;
+  }
+  bool result = aPid->Parse(pChars);
+  free(pChars);
+  return result;
+}
+
+nsresult nsDiscriminatedUnion::ConvertToID(nsID* aResult) const {
+  nsID id;
+
+  switch (mType) {
+    case nsIDataType::VTYPE_ID:
+      *aResult = u.mIDValue;
+      return NS_OK;
+    case nsIDataType::VTYPE_INTERFACE:
+      *aResult = NS_GET_IID(nsISupports);
+      return NS_OK;
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      *aResult = u.iface.mInterfaceID;
+      return NS_OK;
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_UTF8STRING:
+    case nsIDataType::VTYPE_CSTRING:
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      if (!String2ID(&id)) {
+        return NS_ERROR_CANNOT_CONVERT_DATA;
+      }
+      *aResult = id;
+      return NS_OK;
+    default:
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+}
+
+/***************************************************************************/
+
+nsresult nsDiscriminatedUnion::ToString(nsACString& aOutString) const {
+  mozilla::SmprintfPointer pptr;
+
+  switch (mType) {
+    // all the stuff we don't handle...
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_UTF8STRING:
+    case nsIDataType::VTYPE_CSTRING:
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+    case nsIDataType::VTYPE_WCHAR:
+      NS_ERROR("ToString being called for a string type - screwy logic!");
+      [[fallthrough]];
+
+      // XXX We might want stringified versions of these... ???
+
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_EMPTY:
+      aOutString.SetIsVoid(true);
+      return NS_OK;
+
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_ARRAY:
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+    default:
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+
+      // nsID has its own text formatter.
+
+    case nsIDataType::VTYPE_ID: {
+      char* ptr = u.mIDValue.ToString();
+      if (!ptr) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+      aOutString.Assign(ptr);
+      free(ptr);
+      return NS_OK;
+    }
+
+    // Can't use Smprintf for floats, since it's locale-dependent
+#define CASE__APPENDFLOAT_NUMBER(type_, member_) \
+  case nsIDataType::type_: {                     \
+    nsAutoCString str;                           \
+    str.AppendFloat(u.member_);                  \
+    aOutString.Assign(str);                      \
+    return NS_OK;                                \
+  }
+
+      CASE__APPENDFLOAT_NUMBER(VTYPE_FLOAT, mFloatValue)
+      CASE__APPENDFLOAT_NUMBER(VTYPE_DOUBLE, mDoubleValue)
+
+#undef CASE__APPENDFLOAT_NUMBER
+
+      // the rest can be Smprintf'd and use common code.
+
+#define CASE__SMPRINTF_NUMBER(type_, format_, cast_, member_)          \
+  case nsIDataType::type_:                                             \
+    static_assert(sizeof(cast_) >= sizeof(u.member_),                  \
+                  "size of type should be at least as big as member"); \
+    pptr = mozilla::Smprintf(format_, (cast_)u.member_);               \
+    break;
+
+      CASE__SMPRINTF_NUMBER(VTYPE_INT8, "%d", int, mInt8Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_INT16, "%d", int, mInt16Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_INT32, "%d", int, mInt32Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_INT64, "%" PRId64, int64_t, mInt64Value)
+
+      CASE__SMPRINTF_NUMBER(VTYPE_UINT8, "%u", unsigned, mUint8Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_UINT16, "%u", unsigned, mUint16Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_UINT32, "%u", unsigned, mUint32Value)
+      CASE__SMPRINTF_NUMBER(VTYPE_UINT64, "%" PRIu64, int64_t, mUint64Value)
+
+      // XXX Would we rather print "true" / "false" ?
+      CASE__SMPRINTF_NUMBER(VTYPE_BOOL, "%d", int, mBoolValue)
+
+      CASE__SMPRINTF_NUMBER(VTYPE_CHAR, "%c", char, mCharValue)
+
+#undef CASE__SMPRINTF_NUMBER
+  }
+
+  if (!pptr) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  aOutString.Assign(pptr.get());
+  return NS_OK;
+}
+
+nsresult nsDiscriminatedUnion::ConvertToAString(nsAString& aResult) const {
+  switch (mType) {
+    case nsIDataType::VTYPE_ASTRING:
+      aResult.Assign(*u.mAStringValue);
+      return NS_OK;
+    case nsIDataType::VTYPE_CSTRING:
+      CopyASCIItoUTF16(*u.mCStringValue, aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_UTF8STRING:
+      CopyUTF8toUTF16(*u.mUTF8StringValue, aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_CHAR_STR:
+      CopyASCIItoUTF16(mozilla::MakeStringSpan(u.str.mStringValue), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR_STR:
+      aResult.Assign(u.wstr.mWStringValue);
+      return NS_OK;
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      CopyASCIItoUTF16(
+          nsDependentCString(u.str.mStringValue, u.str.mStringLength), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      aResult.Assign(u.wstr.mWStringValue, u.wstr.mWStringLength);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR:
+      aResult.Assign(u.mWCharValue);
+      return NS_OK;
+    default: {
+      nsAutoCString tempCString;
+      nsresult rv = ToString(tempCString);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      CopyASCIItoUTF16(tempCString, aResult);
+      return NS_OK;
+    }
+  }
+}
+
+nsresult nsDiscriminatedUnion::ConvertToACString(nsACString& aResult) const {
+  switch (mType) {
+    case nsIDataType::VTYPE_ASTRING:
+      LossyCopyUTF16toASCII(*u.mAStringValue, aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_CSTRING:
+      aResult.Assign(*u.mCStringValue);
+      return NS_OK;
+    case nsIDataType::VTYPE_UTF8STRING:
+      // XXX This is an extra copy that should be avoided
+      // once Jag lands support for UTF8String and associated
+      // conversion methods.
+      LossyCopyUTF16toASCII(NS_ConvertUTF8toUTF16(*u.mUTF8StringValue),
+                            aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_CHAR_STR:
+      aResult.Assign(*u.str.mStringValue);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR_STR:
+      LossyCopyUTF16toASCII(nsDependentString(u.wstr.mWStringValue), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      aResult.Assign(u.str.mStringValue, u.str.mStringLength);
+      return NS_OK;
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      LossyCopyUTF16toASCII(
+          nsDependentString(u.wstr.mWStringValue, u.wstr.mWStringLength),
+          aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR: {
+      const char16_t* str = &u.mWCharValue;
+      LossyCopyUTF16toASCII(Substring(str, 1), aResult);
+      return NS_OK;
+    }
+    default:
+      return ToString(aResult);
+  }
+}
+
+nsresult nsDiscriminatedUnion::ConvertToAUTF8String(
+    nsAUTF8String& aResult) const {
+  switch (mType) {
+    case nsIDataType::VTYPE_ASTRING:
+      CopyUTF16toUTF8(*u.mAStringValue, aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_CSTRING:
+      // XXX Extra copy, can be removed if we're sure CSTRING can
+      //     only contain ASCII.
+      CopyUTF16toUTF8(NS_ConvertASCIItoUTF16(*u.mCStringValue), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_UTF8STRING:
+      aResult.Assign(*u.mUTF8StringValue);
+      return NS_OK;
+    case nsIDataType::VTYPE_CHAR_STR:
+      // XXX Extra copy, can be removed if we're sure CHAR_STR can
+      //     only contain ASCII.
+      CopyUTF16toUTF8(NS_ConvertASCIItoUTF16(u.str.mStringValue), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR_STR:
+      CopyUTF16toUTF8(mozilla::MakeStringSpan(u.wstr.mWStringValue), aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      // XXX Extra copy, can be removed if we're sure CHAR_STR can
+      //     only contain ASCII.
+      CopyUTF16toUTF8(NS_ConvertASCIItoUTF16(nsDependentCString(
+                          u.str.mStringValue, u.str.mStringLength)),
+                      aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      CopyUTF16toUTF8(
+          nsDependentString(u.wstr.mWStringValue, u.wstr.mWStringLength),
+          aResult);
+      return NS_OK;
+    case nsIDataType::VTYPE_WCHAR: {
+      const char16_t* str = &u.mWCharValue;
+      CopyUTF16toUTF8(Substring(str, 1), aResult);
+      return NS_OK;
+    }
+    default: {
+      nsAutoCString tempCString;
+      nsresult rv = ToString(tempCString);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      // XXX Extra copy, can be removed if we're sure tempCString can
+      //     only contain ASCII.
+      CopyUTF16toUTF8(NS_ConvertASCIItoUTF16(tempCString), aResult);
+      return NS_OK;
+    }
+  }
+}
+
+nsresult nsDiscriminatedUnion::ConvertToString(char** aResult) const {
+  uint32_t ignored;
+  return ConvertToStringWithSize(&ignored, aResult);
+}
+
+nsresult nsDiscriminatedUnion::ConvertToWString(char16_t** aResult) const {
+  uint32_t ignored;
+  return ConvertToWStringWithSize(&ignored, aResult);
+}
+
+nsresult nsDiscriminatedUnion::ConvertToStringWithSize(uint32_t* aSize,
+                                                       char** aStr) const {
+  nsAutoString tempString;
+  nsAutoCString tempCString;
+  nsresult rv;
+
+  switch (mType) {
+    case nsIDataType::VTYPE_ASTRING:
+      *aSize = u.mAStringValue->Length();
+      *aStr = ToNewCString(*u.mAStringValue);
+      break;
+    case nsIDataType::VTYPE_CSTRING:
+      *aSize = u.mCStringValue->Length();
+      *aStr = ToNewCString(*u.mCStringValue);
+      break;
+    case nsIDataType::VTYPE_UTF8STRING: {
+      // XXX This is doing 1 extra copy.  Need to fix this
+      // when Jag lands UTF8String
+      // we want:
+      // *aSize = *mUTF8StringValue->Length();
+      // *aStr = ToNewCString(*mUTF8StringValue);
+      // But this will have to do for now.
+      const NS_ConvertUTF8toUTF16 tempString16(*u.mUTF8StringValue);
+      *aSize = tempString16.Length();
+      *aStr = ToNewCString(tempString16);
+      break;
+    }
+    case nsIDataType::VTYPE_CHAR_STR: {
+      nsDependentCString cString(u.str.mStringValue);
+      *aSize = cString.Length();
+      *aStr = ToNewCString(cString);
+      break;
+    }
+    case nsIDataType::VTYPE_WCHAR_STR: {
+      nsDependentString string(u.wstr.mWStringValue);
+      *aSize = string.Length();
+      *aStr = ToNewCString(string);
+      break;
+    }
+    case nsIDataType::VTYPE_STRING_SIZE_IS: {
+      nsDependentCString cString(u.str.mStringValue, u.str.mStringLength);
+      *aSize = cString.Length();
+      *aStr = ToNewCString(cString);
+      break;
+    }
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS: {
+      nsDependentString string(u.wstr.mWStringValue, u.wstr.mWStringLength);
+      *aSize = string.Length();
+      *aStr = ToNewCString(string);
+      break;
+    }
+    case nsIDataType::VTYPE_WCHAR:
+      tempString.Assign(u.mWCharValue);
+      *aSize = tempString.Length();
+      *aStr = ToNewCString(tempString);
+      break;
+    default:
+      rv = ToString(tempCString);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      *aSize = tempCString.Length();
+      *aStr = ToNewCString(tempCString);
+      break;
+  }
+
+  return *aStr ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
+}
+nsresult nsDiscriminatedUnion::ConvertToWStringWithSize(uint32_t* aSize,
+                                                        char16_t** aStr) const {
+  nsAutoString tempString;
+  nsAutoCString tempCString;
+  nsresult rv;
+
+  switch (mType) {
+    case nsIDataType::VTYPE_ASTRING:
+      *aSize = u.mAStringValue->Length();
+      *aStr = ToNewUnicode(*u.mAStringValue);
+      break;
+    case nsIDataType::VTYPE_CSTRING:
+      *aSize = u.mCStringValue->Length();
+      *aStr = ToNewUnicode(*u.mCStringValue);
+      break;
+    case nsIDataType::VTYPE_UTF8STRING: {
+      *aStr = UTF8ToNewUnicode(*u.mUTF8StringValue, aSize);
+      break;
+    }
+    case nsIDataType::VTYPE_CHAR_STR: {
+      nsDependentCString cString(u.str.mStringValue);
+      *aSize = cString.Length();
+      *aStr = ToNewUnicode(cString);
+      break;
+    }
+    case nsIDataType::VTYPE_WCHAR_STR: {
+      nsDependentString string(u.wstr.mWStringValue);
+      *aSize = string.Length();
+      *aStr = ToNewUnicode(string);
+      break;
+    }
+    case nsIDataType::VTYPE_STRING_SIZE_IS: {
+      nsDependentCString cString(u.str.mStringValue, u.str.mStringLength);
+      *aSize = cString.Length();
+      *aStr = ToNewUnicode(cString);
+      break;
+    }
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS: {
+      nsDependentString string(u.wstr.mWStringValue, u.wstr.mWStringLength);
+      *aSize = string.Length();
+      *aStr = ToNewUnicode(string);
+      break;
+    }
+    case nsIDataType::VTYPE_WCHAR:
+      tempString.Assign(u.mWCharValue);
+      *aSize = tempString.Length();
+      *aStr = ToNewUnicode(tempString);
+      break;
+    default:
+      rv = ToString(tempCString);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+      *aSize = tempCString.Length();
+      *aStr = ToNewUnicode(tempCString);
+      break;
+  }
+
+  return *aStr ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
+}
+
+nsresult nsDiscriminatedUnion::ConvertToISupports(nsISupports** aResult) const {
+  switch (mType) {
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      if (u.iface.mInterfaceValue) {
+        return u.iface.mInterfaceValue->QueryInterface(NS_GET_IID(nsISupports),
+                                                       (void**)aResult);
+      } else {
+        *aResult = nullptr;
+        return NS_OK;
+      }
+    default:
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+}
+
+nsresult nsDiscriminatedUnion::ConvertToInterface(nsIID** aIID,
+                                                  void** aInterface) const {
+  const nsIID* piid;
+
+  switch (mType) {
+    case nsIDataType::VTYPE_INTERFACE:
+      piid = &NS_GET_IID(nsISupports);
+      break;
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      piid = &u.iface.mInterfaceID;
+      break;
+    default:
+      return NS_ERROR_CANNOT_CONVERT_DATA;
+  }
+
+  *aIID = piid->Clone();
+
+  if (u.iface.mInterfaceValue) {
+    return u.iface.mInterfaceValue->QueryInterface(*piid, aInterface);
+  }
+
+  *aInterface = nullptr;
+  return NS_OK;
+}
+
+nsresult nsDiscriminatedUnion::ConvertToArray(uint16_t* aType, nsIID* aIID,
+                                              uint32_t* aCount,
+                                              void** aPtr) const {
+  // XXX perhaps we'd like to add support for converting each of the various
+  // types into an array containing one element of that type. We can leverage
+  // CloneArray to do this if we want to support this.
+
+  if (mType == nsIDataType::VTYPE_ARRAY) {
+    return CloneArray(u.array.mArrayType, &u.array.mArrayInterfaceID,
+                      u.array.mArrayCount, u.array.mArrayValue, aType, aIID,
+                      aCount, aPtr);
+  }
+  return NS_ERROR_CANNOT_CONVERT_DATA;
+}
+
+/***************************************************************************/
+// static setter functions...
+
+#define DATA_SETTER_PROLOGUE Cleanup()
+
+#define DATA_SETTER_EPILOGUE(type_) mType = nsIDataType::type_;
+
+#define DATA_SETTER(type_, member_, value_) \
+  DATA_SETTER_PROLOGUE;                     \
+  u.member_ = value_;                       \
+  DATA_SETTER_EPILOGUE(type_)
+
+#define DATA_SETTER_WITH_CAST(type_, member_, cast_, value_) \
+  DATA_SETTER_PROLOGUE;                                      \
+  u.member_ = cast_ value_;                                  \
+  DATA_SETTER_EPILOGUE(type_)
+
+/********************************************/
+
+#define CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(type_) {
+#define CASE__SET_FROM_VARIANT_VTYPE__GETTER(member_, name_) \
+  rv = aValue->GetAs##name_(&(u.member_));
+
+#define CASE__SET_FROM_VARIANT_VTYPE__GETTER_CAST(cast_, member_, name_) \
+  rv = aValue->GetAs##name_(cast_ & (u.member_));
+
+#define CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(type_) \
+  if (NS_SUCCEEDED(rv)) {                            \
+    mType = nsIDataType::type_;                      \
+  }                                                  \
+  break;                                             \
+  }
+
+#define CASE__SET_FROM_VARIANT_TYPE(type_, member_, name_) \
+  case nsIDataType::type_:                                 \
+    CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(type_)           \
+    CASE__SET_FROM_VARIANT_VTYPE__GETTER(member_, name_)   \
+    CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(type_)
+
+#define CASE__SET_FROM_VARIANT_VTYPE_CAST(type_, cast_, member_, name_) \
+  case nsIDataType::type_:                                              \
+    CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(type_)                        \
+    CASE__SET_FROM_VARIANT_VTYPE__GETTER_CAST(cast_, member_, name_)    \
+    CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(type_)
+
+nsresult nsDiscriminatedUnion::SetFromVariant(nsIVariant* aValue) {
+  nsresult rv = NS_OK;
+
+  Cleanup();
+
+  uint16_t type = aValue->GetDataType();
+
+  switch (type) {
+    CASE__SET_FROM_VARIANT_VTYPE_CAST(VTYPE_INT8, (uint8_t*), mInt8Value, Int8)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_INT16, mInt16Value, Int16)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_INT32, mInt32Value, Int32)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_UINT8, mUint8Value, Uint8)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_UINT16, mUint16Value, Uint16)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_UINT32, mUint32Value, Uint32)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_FLOAT, mFloatValue, Float)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_DOUBLE, mDoubleValue, Double)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_BOOL, mBoolValue, Bool)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_CHAR, mCharValue, Char)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_WCHAR, mWCharValue, WChar)
+    CASE__SET_FROM_VARIANT_TYPE(VTYPE_ID, mIDValue, ID)
+
+    case nsIDataType::VTYPE_ASTRING:
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_ASTRING);
+      u.mAStringValue = new nsString();
+
+      rv = aValue->GetAsAString(*u.mAStringValue);
+      if (NS_FAILED(rv)) {
+        delete u.mAStringValue;
+      }
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_ASTRING)
+
+    case nsIDataType::VTYPE_CSTRING:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_CSTRING);
+      u.mCStringValue = new nsCString();
+
+      rv = aValue->GetAsACString(*u.mCStringValue);
+      if (NS_FAILED(rv)) {
+        delete u.mCStringValue;
+      }
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_CSTRING)
+
+    case nsIDataType::VTYPE_UTF8STRING:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_UTF8STRING);
+      u.mUTF8StringValue = new nsUTF8String();
+
+      rv = aValue->GetAsAUTF8String(*u.mUTF8StringValue);
+      if (NS_FAILED(rv)) {
+        delete u.mUTF8StringValue;
+      }
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_UTF8STRING)
+
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_STRING_SIZE_IS);
+      rv = aValue->GetAsStringWithSize(&u.str.mStringLength,
+                                       &u.str.mStringValue);
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_STRING_SIZE_IS)
+
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_INTERFACE_IS);
+      // XXX This iid handling is ugly!
+      nsIID* iid;
+      rv = aValue->GetAsInterface(&iid, (void**)&u.iface.mInterfaceValue);
+      if (NS_SUCCEEDED(rv)) {
+        u.iface.mInterfaceID = *iid;
+        free((char*)iid);
+      }
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_INTERFACE_IS)
+
+    case nsIDataType::VTYPE_ARRAY:
+      CASE__SET_FROM_VARIANT_VTYPE_PROLOGUE(VTYPE_ARRAY);
+      rv = aValue->GetAsArray(&u.array.mArrayType, &u.array.mArrayInterfaceID,
+                              &u.array.mArrayCount, &u.array.mArrayValue);
+      CASE__SET_FROM_VARIANT_VTYPE_EPILOGUE(VTYPE_ARRAY)
+
+    case nsIDataType::VTYPE_VOID:
+      SetToVoid();
+      rv = NS_OK;
+      break;
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+      SetToEmptyArray();
+      rv = NS_OK;
+      break;
+    case nsIDataType::VTYPE_EMPTY:
+      SetToEmpty();
+      rv = NS_OK;
+      break;
+    default:
+      NS_ERROR("bad type in variant!");
+      rv = NS_ERROR_FAILURE;
+      break;
+  }
+  return rv;
+}
+
+void nsDiscriminatedUnion::SetFromInt8(uint8_t aValue) {
+  DATA_SETTER_WITH_CAST(VTYPE_INT8, mInt8Value, (uint8_t), aValue);
+}
+void nsDiscriminatedUnion::SetFromInt16(int16_t aValue) {
+  DATA_SETTER(VTYPE_INT16, mInt16Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromInt32(int32_t aValue) {
+  DATA_SETTER(VTYPE_INT32, mInt32Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromInt64(int64_t aValue) {
+  DATA_SETTER(VTYPE_INT64, mInt64Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromUint8(uint8_t aValue) {
+  DATA_SETTER(VTYPE_UINT8, mUint8Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromUint16(uint16_t aValue) {
+  DATA_SETTER(VTYPE_UINT16, mUint16Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromUint32(uint32_t aValue) {
+  DATA_SETTER(VTYPE_UINT32, mUint32Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromUint64(uint64_t aValue) {
+  DATA_SETTER(VTYPE_UINT64, mUint64Value, aValue);
+}
+void nsDiscriminatedUnion::SetFromFloat(float aValue) {
+  DATA_SETTER(VTYPE_FLOAT, mFloatValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromDouble(double aValue) {
+  DATA_SETTER(VTYPE_DOUBLE, mDoubleValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromBool(bool aValue) {
+  DATA_SETTER(VTYPE_BOOL, mBoolValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromChar(char aValue) {
+  DATA_SETTER(VTYPE_CHAR, mCharValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromWChar(char16_t aValue) {
+  DATA_SETTER(VTYPE_WCHAR, mWCharValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromID(const nsID& aValue) {
+  DATA_SETTER(VTYPE_ID, mIDValue, aValue);
+}
+void nsDiscriminatedUnion::SetFromAString(const nsAString& aValue) {
+  DATA_SETTER_PROLOGUE;
+  u.mAStringValue = new nsString(aValue);
+  DATA_SETTER_EPILOGUE(VTYPE_ASTRING);
+}
+
+void nsDiscriminatedUnion::SetFromACString(const nsACString& aValue) {
+  DATA_SETTER_PROLOGUE;
+  u.mCStringValue = new nsCString(aValue);
+  DATA_SETTER_EPILOGUE(VTYPE_CSTRING);
+}
+
+void nsDiscriminatedUnion::SetFromAUTF8String(const nsAUTF8String& aValue) {
+  DATA_SETTER_PROLOGUE;
+  u.mUTF8StringValue = new nsUTF8String(aValue);
+  DATA_SETTER_EPILOGUE(VTYPE_UTF8STRING);
+}
+
+nsresult nsDiscriminatedUnion::SetFromString(const char* aValue) {
+  DATA_SETTER_PROLOGUE;
+  if (!aValue) {
+    return NS_ERROR_NULL_POINTER;
+  }
+  return SetFromStringWithSize(strlen(aValue), aValue);
+}
+nsresult nsDiscriminatedUnion::SetFromWString(const char16_t* aValue) {
+  DATA_SETTER_PROLOGUE;
+  if (!aValue) {
+    return NS_ERROR_NULL_POINTER;
+  }
+  return SetFromWStringWithSize(NS_strlen(aValue), aValue);
+}
+void nsDiscriminatedUnion::SetFromISupports(nsISupports* aValue) {
+  return SetFromInterface(NS_GET_IID(nsISupports), aValue);
+}
+void nsDiscriminatedUnion::SetFromInterface(const nsIID& aIID,
+                                            nsISupports* aValue) {
+  DATA_SETTER_PROLOGUE;
+  NS_IF_ADDREF(aValue);
+  u.iface.mInterfaceValue = aValue;
+  u.iface.mInterfaceID = aIID;
+  DATA_SETTER_EPILOGUE(VTYPE_INTERFACE_IS);
+}
+nsresult nsDiscriminatedUnion::SetFromArray(uint16_t aType, const nsIID* aIID,
+                                            uint32_t aCount, void* aValue) {
+  DATA_SETTER_PROLOGUE;
+  if (!aValue || !aCount) {
+    return NS_ERROR_NULL_POINTER;
+  }
+
+  nsresult rv = CloneArray(aType, aIID, aCount, aValue, &u.array.mArrayType,
+                           &u.array.mArrayInterfaceID, &u.array.mArrayCount,
+                           &u.array.mArrayValue);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  DATA_SETTER_EPILOGUE(VTYPE_ARRAY);
+  return NS_OK;
+}
+nsresult nsDiscriminatedUnion::SetFromStringWithSize(uint32_t aSize,
+                                                     const char* aValue) {
+  DATA_SETTER_PROLOGUE;
+  if (!aValue) {
+    return NS_ERROR_NULL_POINTER;
+  }
+  u.str.mStringValue = (char*)moz_xmemdup(aValue, (aSize + 1) * sizeof(char));
+  u.str.mStringLength = aSize;
+  DATA_SETTER_EPILOGUE(VTYPE_STRING_SIZE_IS);
+  return NS_OK;
+}
+nsresult nsDiscriminatedUnion::SetFromWStringWithSize(uint32_t aSize,
+                                                      const char16_t* aValue) {
+  DATA_SETTER_PROLOGUE;
+  if (!aValue) {
+    return NS_ERROR_NULL_POINTER;
+  }
+  u.wstr.mWStringValue =
+      (char16_t*)moz_xmemdup(aValue, (aSize + 1) * sizeof(char16_t));
+  u.wstr.mWStringLength = aSize;
+  DATA_SETTER_EPILOGUE(VTYPE_WSTRING_SIZE_IS);
+  return NS_OK;
+}
+void nsDiscriminatedUnion::AllocateWStringWithSize(uint32_t aSize) {
+  DATA_SETTER_PROLOGUE;
+  u.wstr.mWStringValue = (char16_t*)moz_xmalloc((aSize + 1) * sizeof(char16_t));
+  u.wstr.mWStringValue[aSize] = '\0';
+  u.wstr.mWStringLength = aSize;
+  DATA_SETTER_EPILOGUE(VTYPE_WSTRING_SIZE_IS);
+}
+void nsDiscriminatedUnion::SetToVoid() {
+  DATA_SETTER_PROLOGUE;
+  DATA_SETTER_EPILOGUE(VTYPE_VOID);
+}
+void nsDiscriminatedUnion::SetToEmpty() {
+  DATA_SETTER_PROLOGUE;
+  DATA_SETTER_EPILOGUE(VTYPE_EMPTY);
+}
+void nsDiscriminatedUnion::SetToEmptyArray() {
+  DATA_SETTER_PROLOGUE;
+  DATA_SETTER_EPILOGUE(VTYPE_EMPTY_ARRAY);
+}
+
+/***************************************************************************/
+
+void nsDiscriminatedUnion::Cleanup() {
+  switch (mType) {
+    case nsIDataType::VTYPE_INT8:
+    case nsIDataType::VTYPE_INT16:
+    case nsIDataType::VTYPE_INT32:
+    case nsIDataType::VTYPE_INT64:
+    case nsIDataType::VTYPE_UINT8:
+    case nsIDataType::VTYPE_UINT16:
+    case nsIDataType::VTYPE_UINT32:
+    case nsIDataType::VTYPE_UINT64:
+    case nsIDataType::VTYPE_FLOAT:
+    case nsIDataType::VTYPE_DOUBLE:
+    case nsIDataType::VTYPE_BOOL:
+    case nsIDataType::VTYPE_CHAR:
+    case nsIDataType::VTYPE_WCHAR:
+    case nsIDataType::VTYPE_VOID:
+    case nsIDataType::VTYPE_ID:
+      break;
+    case nsIDataType::VTYPE_ASTRING:
+      delete u.mAStringValue;
+      break;
+    case nsIDataType::VTYPE_CSTRING:
+      delete u.mCStringValue;
+      break;
+    case nsIDataType::VTYPE_UTF8STRING:
+      delete u.mUTF8StringValue;
+      break;
+    case nsIDataType::VTYPE_CHAR_STR:
+    case nsIDataType::VTYPE_STRING_SIZE_IS:
+      free((char*)u.str.mStringValue);
+      break;
+    case nsIDataType::VTYPE_WCHAR_STR:
+    case nsIDataType::VTYPE_WSTRING_SIZE_IS:
+      free((char*)u.wstr.mWStringValue);
+      break;
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      NS_IF_RELEASE(u.iface.mInterfaceValue);
+      break;
+    case nsIDataType::VTYPE_ARRAY:
+      FreeArray();
+      break;
+    case nsIDataType::VTYPE_EMPTY_ARRAY:
+    case nsIDataType::VTYPE_EMPTY:
+      break;
+    default:
+      NS_ERROR("bad type in variant!");
+      break;
+  }
+
+  mType = nsIDataType::VTYPE_EMPTY;
+}
+
+void nsDiscriminatedUnion::Traverse(
+    nsCycleCollectionTraversalCallback& aCb) const {
+  switch (mType) {
+    case nsIDataType::VTYPE_INTERFACE:
+    case nsIDataType::VTYPE_INTERFACE_IS:
+      NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "mData");
+      aCb.NoteXPCOMChild(u.iface.mInterfaceValue);
+      break;
+    case nsIDataType::VTYPE_ARRAY:
+      switch (u.array.mArrayType) {
+        case nsIDataType::VTYPE_INTERFACE:
+        case nsIDataType::VTYPE_INTERFACE_IS: {
+          nsISupports** p = (nsISupports**)u.array.mArrayValue;
+          for (uint32_t i = u.array.mArrayCount; i > 0; ++p, --i) {
+            NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "mData[i]");
+            aCb.NoteXPCOMChild(*p);
+          }
+          break;
+        }
+        default:
+          break;
+      }
+      break;
+    default:
+      break;
+  }
+}
+
+/***************************************************************************/
+/***************************************************************************/
+// members...
+
+nsVariantBase::nsVariantBase() : mWritable(true) {}
+
+// For all the data getters we just forward to the static (and sharable)
+// 'ConvertTo' functions.
+
+uint16_t nsVariantBase::GetDataType() { return mData.GetType(); }
+
+NS_IMETHODIMP
+nsVariantBase::GetAsInt8(uint8_t* aResult) {
+  return mData.ConvertToInt8(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsInt16(int16_t* aResult) {
+  return mData.ConvertToInt16(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsInt32(int32_t* aResult) {
+  return mData.ConvertToInt32(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsInt64(int64_t* aResult) {
+  return mData.ConvertToInt64(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsUint8(uint8_t* aResult) {
+  return mData.ConvertToUint8(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsUint16(uint16_t* aResult) {
+  return mData.ConvertToUint16(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsUint32(uint32_t* aResult) {
+  return mData.ConvertToUint32(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsUint64(uint64_t* aResult) {
+  return mData.ConvertToUint64(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsFloat(float* aResult) {
+  return mData.ConvertToFloat(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsDouble(double* aResult) {
+  return mData.ConvertToDouble(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsBool(bool* aResult) { return mData.ConvertToBool(aResult); }
+
+NS_IMETHODIMP
+nsVariantBase::GetAsChar(char* aResult) { return mData.ConvertToChar(aResult); }
+
+NS_IMETHODIMP
+nsVariantBase::GetAsWChar(char16_t* aResult) {
+  return mData.ConvertToWChar(aResult);
+}
+
+NS_IMETHODIMP_(nsresult)
+nsVariantBase::GetAsID(nsID* aResult) { return mData.ConvertToID(aResult); }
+
+NS_IMETHODIMP
+nsVariantBase::GetAsAString(nsAString& aResult) {
+  return mData.ConvertToAString(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsACString(nsACString& aResult) {
+  return mData.ConvertToACString(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsAUTF8String(nsAUTF8String& aResult) {
+  return mData.ConvertToAUTF8String(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsString(char** aResult) {
+  return mData.ConvertToString(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsWString(char16_t** aResult) {
+  return mData.ConvertToWString(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsISupports(nsISupports** aResult) {
+  return mData.ConvertToISupports(aResult);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsJSVal(JS::MutableHandleValue) {
+  // Can only get the jsval from an XPCVariant.
+  return NS_ERROR_CANNOT_CONVERT_DATA;
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsInterface(nsIID** aIID, void** aInterface) {
+  return mData.ConvertToInterface(aIID, aInterface);
+}
+
+NS_IMETHODIMP_(nsresult)
+nsVariantBase::GetAsArray(uint16_t* aType, nsIID* aIID, uint32_t* aCount,
+                          void** aPtr) {
+  return mData.ConvertToArray(aType, aIID, aCount, aPtr);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsStringWithSize(uint32_t* aSize, char** aStr) {
+  return mData.ConvertToStringWithSize(aSize, aStr);
+}
+
+NS_IMETHODIMP
+nsVariantBase::GetAsWStringWithSize(uint32_t* aSize, char16_t** aStr) {
+  return mData.ConvertToWStringWithSize(aSize, aStr);
+}
+
+/***************************************************************************/
+
+NS_IMETHODIMP
+nsVariantBase::GetWritable(bool* aWritable) {
+  *aWritable = mWritable;
+  return NS_OK;
+}
+NS_IMETHODIMP
+nsVariantBase::SetWritable(bool aWritable) {
+  if (!mWritable && aWritable) {
+    return NS_ERROR_FAILURE;
+  }
+  mWritable = aWritable;
+  return NS_OK;
+}
+
+/***************************************************************************/
+
+// For all the data setters we just forward to the static (and sharable)
+// 'SetFrom' functions.
+
+NS_IMETHODIMP
+nsVariantBase::SetAsInt8(uint8_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromInt8(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsInt16(int16_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromInt16(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsInt32(int32_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromInt32(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsInt64(int64_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromInt64(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsUint8(uint8_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromUint8(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsUint16(uint16_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromUint16(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsUint32(uint32_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromUint32(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsUint64(uint64_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromUint64(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsFloat(float aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromFloat(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsDouble(double aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromDouble(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsBool(bool aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromBool(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsChar(char aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromChar(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsWChar(char16_t aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromWChar(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsID(const nsID& aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromID(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsAString(const nsAString& aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromAString(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsACString(const nsACString& aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromACString(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsAUTF8String(const nsAUTF8String& aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromAUTF8String(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsString(const char* aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromString(aValue);
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsWString(const char16_t* aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromWString(aValue);
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsISupports(nsISupports* aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromISupports(aValue);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsInterface(const nsIID& aIID, void* aInterface) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetFromInterface(aIID, (nsISupports*)aInterface);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsArray(uint16_t aType, const nsIID* aIID, uint32_t aCount,
+                          void* aPtr) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromArray(aType, aIID, aCount, aPtr);
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsStringWithSize(uint32_t aSize, const char* aStr) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromStringWithSize(aSize, aStr);
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsWStringWithSize(uint32_t aSize, const char16_t* aStr) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromWStringWithSize(aSize, aStr);
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsVoid() {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetToVoid();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsEmpty() {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetToEmpty();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetAsEmptyArray() {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  mData.SetToEmptyArray();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsVariantBase::SetFromVariant(nsIVariant* aValue) {
+  if (!mWritable) {
+    return NS_ERROR_OBJECT_IS_IMMUTABLE;
+  }
+  return mData.SetFromVariant(aValue);
+}
+
+/* nsVariant implementation */
+
+NS_IMPL_ISUPPORTS(nsVariant, nsIVariant, nsIWritableVariant)
+
+/* nsVariantCC implementation */
+NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(nsVariantCC)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
+  NS_INTERFACE_MAP_ENTRY(nsIVariant)
+  NS_INTERFACE_MAP_ENTRY(nsIWritableVariant)
+NS_INTERFACE_MAP_END
+
+NS_IMPL_CYCLE_COLLECTION_CLASS(nsVariantCC)
+
+NS_IMPL_CYCLE_COLLECTING_ADDREF(nsVariantCC)
+NS_IMPL_CYCLE_COLLECTING_RELEASE(nsVariantCC)
+
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(nsVariantCC)
+  tmp->mData.Traverse(cb);
+NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
+
+NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(nsVariantCC)
+  tmp->mData.Cleanup();
+NS_IMPL_CYCLE_COLLECTION_UNLINK_END
diff --git a/xpcom/ds/nsVariant.h b/xpcom/ds/nsVariant.h
new file mode 100644
index 0000000000..8de1fe4618
--- /dev/null
+++ b/xpcom/ds/nsVariant.h
@@ -0,0 +1,223 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsVariant_h
+#define nsVariant_h
+
+#include "nsIVariant.h"
+#include "nsStringFwd.h"
+#include "mozilla/Attributes.h"
+#include "nsCycleCollectionParticipant.h"
+
+/**
+ * Map the nsAUTF8String, nsUTF8String classes to the nsACString and
+ * nsCString classes respectively for now.  These defines need to be removed
+ * once Jag lands his nsUTF8String implementation.
+ */
+#define nsAUTF8String nsACString
+#define nsUTF8String nsCString
+#define PromiseFlatUTF8String PromiseFlatCString
+
+/**
+ * nsDiscriminatedUnion is a class that nsIVariant implementors can use
+ * to hold the underlying data.
+ */
+
+class nsDiscriminatedUnion {
+ public:
+  nsDiscriminatedUnion() : mType(nsIDataType::VTYPE_EMPTY) {
+    u.mInt8Value = '\0';
+  }
+  nsDiscriminatedUnion(const nsDiscriminatedUnion&) = delete;
+  nsDiscriminatedUnion(nsDiscriminatedUnion&&) = delete;
+
+  ~nsDiscriminatedUnion() { Cleanup(); }
+
+  nsDiscriminatedUnion& operator=(const nsDiscriminatedUnion&) = delete;
+  nsDiscriminatedUnion& operator=(nsDiscriminatedUnion&&) = delete;
+
+  void Cleanup();
+
+  uint16_t GetType() const { return mType; }
+
+  [[nodiscard]] nsresult ConvertToInt8(uint8_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToInt16(int16_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToInt32(int32_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToInt64(int64_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToUint8(uint8_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToUint16(uint16_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToUint32(uint32_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToUint64(uint64_t* aResult) const;
+  [[nodiscard]] nsresult ConvertToFloat(float* aResult) const;
+  [[nodiscard]] nsresult ConvertToDouble(double* aResult) const;
+  [[nodiscard]] nsresult ConvertToBool(bool* aResult) const;
+  [[nodiscard]] nsresult ConvertToChar(char* aResult) const;
+  [[nodiscard]] nsresult ConvertToWChar(char16_t* aResult) const;
+
+  [[nodiscard]] nsresult ConvertToID(nsID* aResult) const;
+
+  [[nodiscard]] nsresult ConvertToAString(nsAString& aResult) const;
+  [[nodiscard]] nsresult ConvertToAUTF8String(nsAUTF8String& aResult) const;
+  [[nodiscard]] nsresult ConvertToACString(nsACString& aResult) const;
+  [[nodiscard]] nsresult ConvertToString(char** aResult) const;
+  [[nodiscard]] nsresult ConvertToWString(char16_t** aResult) const;
+  [[nodiscard]] nsresult ConvertToStringWithSize(uint32_t* aSize,
+                                                 char** aStr) const;
+  [[nodiscard]] nsresult ConvertToWStringWithSize(uint32_t* aSize,
+                                                  char16_t** aStr) const;
+
+  [[nodiscard]] nsresult ConvertToISupports(nsISupports** aResult) const;
+  [[nodiscard]] nsresult ConvertToInterface(nsIID** aIID,
+                                            void** aInterface) const;
+  [[nodiscard]] nsresult ConvertToArray(uint16_t* aType, nsIID* aIID,
+                                        uint32_t* aCount, void** aPtr) const;
+
+  [[nodiscard]] nsresult SetFromVariant(nsIVariant* aValue);
+
+  void SetFromInt8(uint8_t aValue);
+  void SetFromInt16(int16_t aValue);
+  void SetFromInt32(int32_t aValue);
+  void SetFromInt64(int64_t aValue);
+  void SetFromUint8(uint8_t aValue);
+  void SetFromUint16(uint16_t aValue);
+  void SetFromUint32(uint32_t aValue);
+  void SetFromUint64(uint64_t aValue);
+  void SetFromFloat(float aValue);
+  void SetFromDouble(double aValue);
+  void SetFromBool(bool aValue);
+  void SetFromChar(char aValue);
+  void SetFromWChar(char16_t aValue);
+  void SetFromID(const nsID& aValue);
+  void SetFromAString(const nsAString& aValue);
+  void SetFromAUTF8String(const nsAUTF8String& aValue);
+  void SetFromACString(const nsACString& aValue);
+  [[nodiscard]] nsresult SetFromString(const char* aValue);
+  [[nodiscard]] nsresult SetFromWString(const char16_t* aValue);
+  void SetFromISupports(nsISupports* aValue);
+  void SetFromInterface(const nsIID& aIID, nsISupports* aValue);
+  [[nodiscard]] nsresult SetFromArray(uint16_t aType, const nsIID* aIID,
+                                      uint32_t aCount, void* aValue);
+  [[nodiscard]] nsresult SetFromStringWithSize(uint32_t aSize,
+                                               const char* aValue);
+  [[nodiscard]] nsresult SetFromWStringWithSize(uint32_t aSize,
+                                                const char16_t* aValue);
+
+  // Like SetFromWStringWithSize, but leaves the string uninitialized. It does
+  // does write the null-terminator.
+  void AllocateWStringWithSize(uint32_t aSize);
+
+  void SetToVoid();
+  void SetToEmpty();
+  void SetToEmptyArray();
+
+  void Traverse(nsCycleCollectionTraversalCallback& aCb) const;
+
+ private:
+  [[nodiscard]] nsresult ToManageableNumber(
+      nsDiscriminatedUnion* aOutData) const;
+  void FreeArray();
+  [[nodiscard]] bool String2ID(nsID* aPid) const;
+  [[nodiscard]] nsresult ToString(nsACString& aOutString) const;
+
+ public:
+  union {
+    int8_t mInt8Value;
+    int16_t mInt16Value;
+    int32_t mInt32Value;
+    int64_t mInt64Value;
+    uint8_t mUint8Value;
+    uint16_t mUint16Value;
+    uint32_t mUint32Value;
+    uint64_t mUint64Value;
+    float mFloatValue;
+    double mDoubleValue;
+    bool mBoolValue;
+    char mCharValue;
+    char16_t mWCharValue;
+    nsIID mIDValue;
+    nsAString* mAStringValue;
+    nsAUTF8String* mUTF8StringValue;
+    nsACString* mCStringValue;
+    struct {
+      // This is an owning reference that cannot be an nsCOMPtr because
+      // nsDiscriminatedUnion needs to be POD.  AddRef/Release are manually
+      // called on this.
+      nsISupports* MOZ_OWNING_REF mInterfaceValue;
+      nsIID mInterfaceID;
+    } iface;
+    struct {
+      nsIID mArrayInterfaceID;
+      void* mArrayValue;
+      uint32_t mArrayCount;
+      uint16_t mArrayType;
+    } array;
+    struct {
+      char* mStringValue;
+      uint32_t mStringLength;
+    } str;
+    struct {
+      char16_t* mWStringValue;
+      uint32_t mWStringLength;
+    } wstr;
+  } u;
+  uint16_t mType;
+};
+
+/**
+ * nsVariant implements the generic variant support. The xpcom module registers
+ * a factory (see NS_VARIANT_CONTRACTID in nsIVariant.idl) that will create
+ * these objects. They are created 'empty' and 'writable'.
+ *
+ * nsIVariant users won't usually need to see this class.
+ */
+class nsVariantBase : public nsIWritableVariant {
+ public:
+  NS_DECL_NSIVARIANT
+  NS_DECL_NSIWRITABLEVARIANT
+
+  nsVariantBase();
+
+ protected:
+  ~nsVariantBase() = default;
+
+  nsDiscriminatedUnion mData;
+  bool mWritable;
+};
+
+class nsVariant final : public nsVariantBase {
+ public:
+  NS_DECL_ISUPPORTS
+
+  nsVariant() = default;
+
+ private:
+  ~nsVariant() = default;
+};
+
+class nsVariantCC final : public nsVariantBase {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_CLASS(nsVariantCC)
+
+  nsVariantCC() = default;
+
+ private:
+  ~nsVariantCC() = default;
+};
+
+/**
+ * Users of nsIVariant should be using the contractID and not this CID.
+ *  - see NS_VARIANT_CONTRACTID in nsIVariant.idl.
+ */
+
+#define NS_VARIANT_CID                              \
+  { /* 0D6EA1D0-879C-11d5-90EF-0010A4E73D9A */      \
+    0xd6ea1d0, 0x879c, 0x11d5, {                    \
+      0x90, 0xef, 0x0, 0x10, 0xa4, 0xe7, 0x3d, 0x9a \
+    }                                               \
+  }
+
+#endif  // nsVariant_h
diff --git a/xpcom/ds/nsWhitespaceTokenizer.h b/xpcom/ds/nsWhitespaceTokenizer.h
new file mode 100644
index 0000000000..77fea70850
--- /dev/null
+++ b/xpcom/ds/nsWhitespaceTokenizer.h
@@ -0,0 +1,96 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef __nsWhitespaceTokenizer_h
+#define __nsWhitespaceTokenizer_h
+
+#include "mozilla/RangedPtr.h"
+#include "nsDependentSubstring.h"
+#include "nsCRTGlue.h"
+
+template <typename DependentSubstringType, bool IsWhitespace(char16_t)>
+class nsTWhitespaceTokenizer {
+  typedef typename DependentSubstringType::char_type CharType;
+  typedef typename DependentSubstringType::substring_type SubstringType;
+
+ public:
+  explicit nsTWhitespaceTokenizer(const SubstringType& aSource)
+      : mIter(aSource.Data(), aSource.Length()),
+        mEnd(aSource.Data() + aSource.Length(), aSource.Data(),
+             aSource.Length()),
+        mWhitespaceBeforeFirstToken(false),
+        mWhitespaceAfterCurrentToken(false) {
+    while (mIter < mEnd && IsWhitespace(*mIter)) {
+      mWhitespaceBeforeFirstToken = true;
+      ++mIter;
+    }
+  }
+
+  /**
+   * Checks if any more tokens are available.
+   */
+  bool hasMoreTokens() const { return mIter < mEnd; }
+
+  /*
+   * Returns true if there is whitespace prior to the first token.
+   */
+  bool whitespaceBeforeFirstToken() const {
+    return mWhitespaceBeforeFirstToken;
+  }
+
+  /*
+   * Returns true if there is any whitespace after the current token.
+   * This is always true unless we're reading the last token.
+   */
+  bool whitespaceAfterCurrentToken() const {
+    return mWhitespaceAfterCurrentToken;
+  }
+
+  /**
+   * Returns the next token.
+   */
+  const DependentSubstringType nextToken() {
+    const mozilla::RangedPtr<const CharType> tokenStart = mIter;
+    while (mIter < mEnd && !IsWhitespace(*mIter)) {
+      ++mIter;
+    }
+    const mozilla::RangedPtr<const CharType> tokenEnd = mIter;
+    mWhitespaceAfterCurrentToken = false;
+    while (mIter < mEnd && IsWhitespace(*mIter)) {
+      mWhitespaceAfterCurrentToken = true;
+      ++mIter;
+    }
+    return Substring(tokenStart.get(), tokenEnd.get());
+  }
+
+ private:
+  mozilla::RangedPtr<const CharType> mIter;
+  const mozilla::RangedPtr<const CharType> mEnd;
+  bool mWhitespaceBeforeFirstToken;
+  bool mWhitespaceAfterCurrentToken;
+};
+
+template <bool IsWhitespace(char16_t) = NS_IsAsciiWhitespace>
+class nsWhitespaceTokenizerTemplate
+    : public nsTWhitespaceTokenizer<nsDependentSubstring, IsWhitespace> {
+ public:
+  explicit nsWhitespaceTokenizerTemplate(const nsAString& aSource)
+      : nsTWhitespaceTokenizer<nsDependentSubstring, IsWhitespace>(aSource) {}
+};
+
+typedef nsWhitespaceTokenizerTemplate<> nsWhitespaceTokenizer;
+
+template <bool IsWhitespace(char16_t) = NS_IsAsciiWhitespace>
+class nsCWhitespaceTokenizerTemplate
+    : public nsTWhitespaceTokenizer<nsDependentCSubstring, IsWhitespace> {
+ public:
+  explicit nsCWhitespaceTokenizerTemplate(const nsACString& aSource)
+      : nsTWhitespaceTokenizer<nsDependentCSubstring, IsWhitespace>(aSource) {}
+};
+
+typedef nsCWhitespaceTokenizerTemplate<> nsCWhitespaceTokenizer;
+
+#endif /* __nsWhitespaceTokenizer_h */
diff --git a/xpcom/ds/nsWindowsRegKey.cpp b/xpcom/ds/nsWindowsRegKey.cpp
new file mode 100644
index 0000000000..1f9bb58ac6
--- /dev/null
+++ b/xpcom/ds/nsWindowsRegKey.cpp
@@ -0,0 +1,523 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <windows.h>
+#include <shlwapi.h>
+#include <stdlib.h>
+#include "nsWindowsRegKey.h"
+#include "nsString.h"
+#include "nsCOMPtr.h"
+#include "mozilla/Attributes.h"
+
+//-----------------------------------------------------------------------------
+
+// According to MSDN, the following limits apply (in characters excluding room
+// for terminating null character):
+#define MAX_KEY_NAME_LEN 255
+#define MAX_VALUE_NAME_LEN 16383
+
+class nsWindowsRegKey final : public nsIWindowsRegKey {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIWINDOWSREGKEY
+
+  nsWindowsRegKey()
+      : mKey(nullptr), mWatchEvent(nullptr), mWatchRecursive(FALSE) {}
+
+ private:
+  ~nsWindowsRegKey() { Close(); }
+
+  HKEY mKey;
+  HANDLE mWatchEvent;
+  BOOL mWatchRecursive;
+};
+
+NS_IMPL_ISUPPORTS(nsWindowsRegKey, nsIWindowsRegKey)
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetKey(HKEY* aKey) {
+  *aKey = mKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::SetKey(HKEY aKey) {
+  // We do not close the older aKey!
+  StopWatching();
+
+  mKey = aKey;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::Close() {
+  StopWatching();
+
+  if (mKey) {
+    RegCloseKey(mKey);
+    mKey = nullptr;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::Open(uint32_t aRootKey, const nsAString& aPath,
+                      uint32_t aMode) {
+  Close();
+
+  LONG rv =
+      RegOpenKeyExW((HKEY)(intptr_t)aRootKey, PromiseFlatString(aPath).get(), 0,
+                    (REGSAM)aMode, &mKey);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::Create(uint32_t aRootKey, const nsAString& aPath,
+                        uint32_t aMode) {
+  Close();
+
+  DWORD disposition;
+  LONG rv = RegCreateKeyExW(
+      (HKEY)(intptr_t)aRootKey, PromiseFlatString(aPath).get(), 0, nullptr,
+      REG_OPTION_NON_VOLATILE, (REGSAM)aMode, nullptr, &mKey, &disposition);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::OpenChild(const nsAString& aPath, uint32_t aMode,
+                           nsIWindowsRegKey** aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  nsCOMPtr<nsIWindowsRegKey> child = new nsWindowsRegKey();
+
+  nsresult rv = child->Open((uintptr_t)mKey, aPath, aMode);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  child.swap(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::CreateChild(const nsAString& aPath, uint32_t aMode,
+                             nsIWindowsRegKey** aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  nsCOMPtr<nsIWindowsRegKey> child = new nsWindowsRegKey();
+
+  nsresult rv = child->Create((uintptr_t)mKey, aPath, aMode);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  child.swap(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetChildCount(uint32_t* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD numSubKeys;
+  LONG rv =
+      RegQueryInfoKeyW(mKey, nullptr, nullptr, nullptr, &numSubKeys, nullptr,
+                       nullptr, nullptr, nullptr, nullptr, nullptr, nullptr);
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aResult = numSubKeys;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetChildName(uint32_t aIndex, nsAString& aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  FILETIME lastWritten;
+
+  wchar_t nameBuf[MAX_KEY_NAME_LEN + 1];
+  DWORD nameLen = sizeof(nameBuf) / sizeof(nameBuf[0]);
+
+  LONG rv = RegEnumKeyExW(mKey, aIndex, nameBuf, &nameLen, nullptr, nullptr,
+                          nullptr, &lastWritten);
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_NOT_AVAILABLE;  // XXX what's the best error code here?
+  }
+
+  aResult.Assign(nameBuf, nameLen);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::HasChild(const nsAString& aName, bool* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  // Check for the existence of a child key by opening the key with minimal
+  // rights.  Perhaps there is a more efficient way to do this?
+
+  HKEY key;
+  LONG rv = RegOpenKeyExW(mKey, PromiseFlatString(aName).get(), 0,
+                          STANDARD_RIGHTS_READ, &key);
+
+  if ((*aResult = (rv == ERROR_SUCCESS && key))) {
+    RegCloseKey(key);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetValueCount(uint32_t* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD numValues;
+  LONG rv =
+      RegQueryInfoKeyW(mKey, nullptr, nullptr, nullptr, nullptr, nullptr,
+                       nullptr, &numValues, nullptr, nullptr, nullptr, nullptr);
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aResult = numValues;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetValueName(uint32_t aIndex, nsAString& aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  wchar_t nameBuf[MAX_VALUE_NAME_LEN];
+  DWORD nameLen = sizeof(nameBuf) / sizeof(nameBuf[0]);
+
+  LONG rv = RegEnumValueW(mKey, aIndex, nameBuf, &nameLen, nullptr, nullptr,
+                          nullptr, nullptr);
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_NOT_AVAILABLE;  // XXX what's the best error code here?
+  }
+
+  aResult.Assign(nameBuf, nameLen);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::HasValue(const nsAString& aName, bool* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0, nullptr,
+                             nullptr, nullptr);
+
+  *aResult = (rv == ERROR_SUCCESS);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::RemoveChild(const nsAString& aName) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegDeleteKeyW(mKey, PromiseFlatString(aName).get());
+
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::RemoveValue(const nsAString& aName) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegDeleteValueW(mKey, PromiseFlatString(aName).get());
+
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::GetValueType(const nsAString& aName, uint32_t* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0,
+                             (LPDWORD)aResult, nullptr, nullptr);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::ReadStringValue(const nsAString& aName, nsAString& aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD type, size;
+
+  const nsString& flatName = PromiseFlatString(aName);
+
+  LONG rv = RegQueryValueExW(mKey, flatName.get(), 0, &type, nullptr, &size);
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // This must be a string type in order to fetch the value as a string.
+  // We're being a bit forgiving here by allowing types other than REG_SZ.
+  if (type != REG_SZ && type != REG_EXPAND_SZ && type != REG_MULTI_SZ) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // The buffer size must be a multiple of 2.
+  if (size % 2 != 0) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  if (size == 0) {
+    aResult.Truncate();
+    return NS_OK;
+  }
+
+  // |size| may or may not include the terminating null character.
+  DWORD resultLen = size / 2;
+
+  if (!aResult.SetLength(resultLen, mozilla::fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  auto begin = aResult.BeginWriting();
+
+  rv = RegQueryValueExW(mKey, flatName.get(), 0, &type, (LPBYTE)begin, &size);
+
+  if (!aResult.CharAt(resultLen - 1)) {
+    // The string passed to us had a null terminator in the final position.
+    aResult.Truncate(resultLen - 1);
+  }
+
+  // Expand the environment variables if needed
+  if (type == REG_EXPAND_SZ) {
+    const nsString& flatSource = PromiseFlatString(aResult);
+    resultLen = ExpandEnvironmentStringsW(flatSource.get(), nullptr, 0);
+    if (resultLen > 1) {
+      nsAutoString expandedResult;
+      // |resultLen| includes the terminating null character
+      --resultLen;
+      if (!expandedResult.SetLength(resultLen, mozilla::fallible)) {
+        return NS_ERROR_OUT_OF_MEMORY;
+      }
+
+      resultLen = ExpandEnvironmentStringsW(
+          flatSource.get(), expandedResult.get(), resultLen + 1);
+      if (resultLen <= 0) {
+        rv = ERROR_UNKNOWN_FEATURE;
+        aResult.Truncate();
+      } else {
+        rv = ERROR_SUCCESS;
+        aResult = expandedResult;
+      }
+    } else if (resultLen == 1) {
+      // It apparently expands to nothing (just a null terminator).
+      aResult.Truncate();
+    }
+  }
+
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::ReadIntValue(const nsAString& aName, uint32_t* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD size = sizeof(*aResult);
+  LONG rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0, nullptr,
+                             (LPBYTE)aResult, &size);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::ReadInt64Value(const nsAString& aName, uint64_t* aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD size = sizeof(*aResult);
+  LONG rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0, nullptr,
+                             (LPBYTE)aResult, &size);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::ReadBinaryValue(const nsAString& aName, nsACString& aResult) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  DWORD size;
+  LONG rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0, nullptr,
+                             nullptr, &size);
+
+  if (rv != ERROR_SUCCESS) {
+    return NS_ERROR_FAILURE;
+  }
+
+  if (!size) {
+    aResult.Truncate();
+    return NS_OK;
+  }
+
+  if (!aResult.SetLength(size, mozilla::fallible)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  auto begin = aResult.BeginWriting();
+
+  rv = RegQueryValueExW(mKey, PromiseFlatString(aName).get(), 0, nullptr,
+                        (LPBYTE)begin, &size);
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::WriteStringValue(const nsAString& aName,
+                                  const nsAString& aValue) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  // Need to indicate complete size of buffer including null terminator.
+  const nsString& flatValue = PromiseFlatString(aValue);
+
+  LONG rv = RegSetValueExW(mKey, PromiseFlatString(aName).get(), 0, REG_SZ,
+                           (const BYTE*)flatValue.get(),
+                           (flatValue.Length() + 1) * sizeof(char16_t));
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::WriteIntValue(const nsAString& aName, uint32_t aValue) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegSetValueExW(mKey, PromiseFlatString(aName).get(), 0, REG_DWORD,
+                           (const BYTE*)&aValue, sizeof(aValue));
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::WriteInt64Value(const nsAString& aName, uint64_t aValue) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  LONG rv = RegSetValueExW(mKey, PromiseFlatString(aName).get(), 0, REG_QWORD,
+                           (const BYTE*)&aValue, sizeof(aValue));
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::WriteBinaryValue(const nsAString& aName,
+                                  const nsACString& aValue) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  const nsCString& flatValue = PromiseFlatCString(aValue);
+  LONG rv = RegSetValueExW(mKey, PromiseFlatString(aName).get(), 0, REG_BINARY,
+                           (const BYTE*)flatValue.get(), flatValue.Length());
+  return (rv == ERROR_SUCCESS) ? NS_OK : NS_ERROR_FAILURE;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::StartWatching(bool aRecurse) {
+  if (NS_WARN_IF(!mKey)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  if (mWatchEvent) {
+    return NS_OK;
+  }
+
+  mWatchEvent = CreateEventW(nullptr, TRUE, FALSE, nullptr);
+  if (!mWatchEvent) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  DWORD filter = REG_NOTIFY_CHANGE_NAME | REG_NOTIFY_CHANGE_ATTRIBUTES |
+                 REG_NOTIFY_CHANGE_LAST_SET | REG_NOTIFY_CHANGE_SECURITY;
+
+  LONG rv = RegNotifyChangeKeyValue(mKey, aRecurse, filter, mWatchEvent, TRUE);
+  if (rv != ERROR_SUCCESS) {
+    StopWatching();
+    // On older versions of Windows, this call is not implemented, so simply
+    // return NS_OK in those cases and pretend that the watching is happening.
+    return (rv == ERROR_CALL_NOT_IMPLEMENTED) ? NS_OK : NS_ERROR_FAILURE;
+  }
+
+  mWatchRecursive = aRecurse;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::StopWatching() {
+  if (mWatchEvent) {
+    CloseHandle(mWatchEvent);
+    mWatchEvent = nullptr;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::HasChanged(bool* aResult) {
+  if (mWatchEvent && WaitForSingleObject(mWatchEvent, 0) == WAIT_OBJECT_0) {
+    // An event only gets signaled once, then it's done, so we have to set up
+    // another event to watch.
+    StopWatching();
+    StartWatching(mWatchRecursive);
+    *aResult = true;
+  } else {
+    *aResult = false;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsRegKey::IsWatching(bool* aResult) {
+  *aResult = (mWatchEvent != nullptr);
+  return NS_OK;
+}
+
+//-----------------------------------------------------------------------------
+
+void NS_NewWindowsRegKey(nsIWindowsRegKey** aResult) {
+  RefPtr<nsWindowsRegKey> key = new nsWindowsRegKey();
+  key.forget(aResult);
+}
+
+//-----------------------------------------------------------------------------
+
+nsresult nsWindowsRegKeyConstructor(const nsIID& aIID, void** aResult) {
+  nsCOMPtr<nsIWindowsRegKey> key;
+  NS_NewWindowsRegKey(getter_AddRefs(key));
+  return key->QueryInterface(aIID, aResult);
+}
diff --git a/xpcom/ds/nsWindowsRegKey.h b/xpcom/ds/nsWindowsRegKey.h
new file mode 100644
index 0000000000..c8032540ab
--- /dev/null
+++ b/xpcom/ds/nsWindowsRegKey.h
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsWindowsRegKey_h__
+#define nsWindowsRegKey_h__
+
+//-----------------------------------------------------------------------------
+
+#include "nsIWindowsRegKey.h"
+
+/**
+ * This ContractID may be used to instantiate a windows registry key object
+ * via the XPCOM component manager.
+ */
+#define NS_WINDOWSREGKEY_CONTRACTID "@mozilla.org/windows-registry-key;1"
+
+/**
+ * This function may be used to instantiate a windows registry key object prior
+ * to XPCOM being initialized.
+ */
+extern "C" void NS_NewWindowsRegKey(nsIWindowsRegKey** aResult);
+
+//-----------------------------------------------------------------------------
+
+#ifdef IMPL_LIBXUL
+
+// a53bc624-d577-4839-b8ec-bb5040a52ff4
+#  define NS_WINDOWSREGKEY_CID                         \
+    {                                                  \
+      0xa53bc624, 0xd577, 0x4839, {                    \
+        0xb8, 0xec, 0xbb, 0x50, 0x40, 0xa5, 0x2f, 0xf4 \
+      }                                                \
+    }
+
+[[nodiscard]] extern nsresult nsWindowsRegKeyConstructor(const nsIID& aIID,
+                                                         void** aResult);
+
+#endif  // IMPL_LIBXUL
+
+//-----------------------------------------------------------------------------
+
+#endif  // nsWindowsRegKey_h__
diff --git a/xpcom/ds/test/python.ini b/xpcom/ds/test/python.ini
new file mode 100644
index 0000000000..680185c496
--- /dev/null
+++ b/xpcom/ds/test/python.ini
@@ -0,0 +1,4 @@
+[DEFAULT]
+subsuite = xpcom
+
+[test_dafsa.py]
diff --git a/xpcom/ds/test/test_dafsa.py b/xpcom/ds/test/test_dafsa.py
new file mode 100644
index 0000000000..9becdd6c06
--- /dev/null
+++ b/xpcom/ds/test/test_dafsa.py
@@ -0,0 +1,546 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import unittest
+from io import StringIO
+
+import mozunit
+from incremental_dafsa import Dafsa, Node
+
+
+def _node_to_string(node: Node, prefix, buffer, cache):
+    if not node.is_end_node:
+        prefix += (
+            str(ord(node.character)) if ord(node.character) < 10 else node.character
+        )
+    else:
+        prefix += "$"
+    cached = cache.get(id(node))
+    buffer.write("{}{}".format(prefix, "=" if cached else "").strip() + "\n")
+
+    if not cached:
+        cache[id(node)] = node
+        if node:
+            for node in sorted(node.children.values(), key=lambda n: n.character):
+                _node_to_string(node, prefix, buffer, cache)
+
+
+def _dafsa_to_string(dafsa: Dafsa):
+    """Encodes the dafsa into a string notation.
+
+    Each node is printed on its own line with all the nodes that precede it.
+    The end node is designated with the "$" character.
+    If it joins into an existing node, the end of the line is adorned with a "=".
+    Though this doesn't carry information about which other prefix it has joined with,
+    it has seemed to be precise enough for testing.
+
+    For example, with the input data of:
+    * a1
+    * ac1
+    * bc1
+
+    [root] --- a ---- 1 --- [end]
+       |        |   /
+        -- b -- c---
+
+    The output will be:
+    a
+    a1
+    a1$ <- end node was found
+    ac
+    ac1= <- joins with the "a1" prefix
+    b
+    bc= <- joins with the "ac" prefix
+    """
+    buffer = StringIO()
+    cache = {}
+
+    for node in sorted(dafsa.root_node.children.values(), key=lambda n: n.character):
+        _node_to_string(node, "", buffer, cache)
+    return buffer.getvalue().strip()
+
+
+def _to_words(data):
+    return [line.strip() for line in data.strip().split("\n")]
+
+
+def _assert_dafsa(data, expected):
+    words = _to_words(data)
+    dafsa = Dafsa.from_tld_data(words)
+
+    expected = expected.strip()
+    expected = "\n".join([line.strip() for line in expected.split("\n")])
+    as_string = _dafsa_to_string(dafsa)
+    assert as_string == expected
+
+
+class TestDafsa(unittest.TestCase):
+    def test_1(self):
+        _assert_dafsa(
+            """
+        a1
+        ac1
+        acc1
+        bd1
+        bc1
+        bcc1
+        """,
+            """
+        a
+        a1
+        a1$
+        ac
+        ac1=
+        acc
+        acc1=
+        b
+        bc=
+        bd
+        bd1=
+        """,
+        )
+
+    def test_2(self):
+        _assert_dafsa(
+            """
+        ab1
+        b1
+        bb1
+        bbb1
+        """,
+            """
+        a
+        ab
+        ab1
+        ab1$
+        b
+        b1=
+        bb
+        bb1=
+        bbb=
+        """,
+        )
+
+    def test_3(self):
+        _assert_dafsa(
+            """
+        a.ca1
+        a.com1
+        c.corg1
+        b.ca1
+        b.com1
+        b.corg1
+        """,
+            """
+        a
+        a.
+        a.c
+        a.ca
+        a.ca1
+        a.ca1$
+        a.co
+        a.com
+        a.com1=
+        b
+        b.
+        b.c
+        b.ca=
+        b.co
+        b.com=
+        b.cor
+        b.corg
+        b.corg1=
+        c
+        c.
+        c.c
+        c.co
+        c.cor=
+        """,
+        )
+
+    def test_4(self):
+        _assert_dafsa(
+            """
+        acom1
+        bcomcom1
+        acomcom1
+        """,
+            """
+        a
+        ac
+        aco
+        acom
+        acom1
+        acom1$
+        acomc
+        acomco
+        acomcom
+        acomcom1=
+        b
+        bc
+        bco
+        bcom
+        bcomc=
+        """,
+        )
+
+    def test_5(self):
+        _assert_dafsa(
+            """
+        a.d1
+        a.c.d1
+        b.d1
+        b.c.d1
+        """,
+            """
+        a
+        a.
+        a.c
+        a.c.
+        a.c.d
+        a.c.d1
+        a.c.d1$
+        a.d=
+        b
+        b.=
+        """,
+        )
+
+    def test_6(self):
+        _assert_dafsa(
+            """
+        a61
+        a661
+        b61
+        b661
+        """,
+            """
+        a
+        a6
+        a61
+        a61$
+        a66
+        a661=
+        b
+        b6=
+        """,
+        )
+
+    def test_7(self):
+        _assert_dafsa(
+            """
+        a61
+        a6661
+        b61
+        b6661
+        """,
+            """
+        a
+        a6
+        a61
+        a61$
+        a66
+        a666
+        a6661=
+        b
+        b6=
+        """,
+        )
+
+    def test_8(self):
+        _assert_dafsa(
+            """
+        acc1
+        bc1
+        bccc1
+        """,
+            """
+        a
+        ac
+        acc
+        acc1
+        acc1$
+        b
+        bc
+        bc1=
+        bcc=
+        """,
+        )
+
+    def test_9(self):
+        _assert_dafsa(
+            """
+        acc1
+        bc1
+        bcc1
+        """,
+            """
+        a
+        ac
+        acc
+        acc1
+        acc1$
+        b
+        bc
+        bc1=
+        bcc=
+        """,
+        )
+
+    def test_10(self):
+        _assert_dafsa(
+            """
+        acc1
+        cc1
+        cccc1
+        """,
+            """
+        a
+        ac
+        acc
+        acc1
+        acc1$
+        c
+        cc
+        cc1=
+        ccc=
+        """,
+        )
+
+    def test_11(self):
+        _assert_dafsa(
+            """
+        ac1
+        acc1
+        bc1
+        bcc1
+        """,
+            """
+        a
+        ac
+        ac1
+        ac1$
+        acc
+        acc1=
+        b
+        bc=
+        """,
+        )
+
+    def test_12(self):
+        _assert_dafsa(
+            """
+        acd1
+        bcd1
+        bcdd1
+        """,
+            """
+        a
+        ac
+        acd
+        acd1
+        acd1$
+        b
+        bc
+        bcd
+        bcd1=
+        bcdd=
+        """,
+        )
+
+    def test_13(self):
+        _assert_dafsa(
+            """
+        ac1
+        acc1
+        bc1
+        bcc1
+        bccc1
+        """,
+            """
+        a
+        ac
+        ac1
+        ac1$
+        acc
+        acc1=
+        b
+        bc
+        bc1=
+        bcc=
+        """,
+        )
+
+    def test_14(self):
+        _assert_dafsa(
+            """
+        acc1
+        acccc1
+        bcc1
+        bcccc1
+        bcccccc1
+        """,
+            """
+        a
+        ac
+        acc
+        acc1
+        acc1$
+        accc
+        acccc
+        acccc1=
+        b
+        bc
+        bcc
+        bcc1=
+        bccc=
+        """,
+        )
+
+    def test_15(self):
+        _assert_dafsa(
+            """
+        ac1
+        bc1
+        acac1
+        """,
+            """
+        a
+        ac
+        ac1
+        ac1$
+        aca
+        acac
+        acac1=
+        b
+        bc=
+        """,
+        )
+
+    def test_16(self):
+        _assert_dafsa(
+            """
+        bat1
+        t1
+        tbat1
+        """,
+            """
+        b
+        ba
+        bat
+        bat1
+        bat1$
+        t
+        t1=
+        tb=
+        """,
+        )
+
+    def test_17(self):
+        _assert_dafsa(
+            """
+        acow1
+        acat1
+        t1
+        tcat1
+        acatcat1
+        """,
+            """
+        a
+        ac
+        aca
+        acat
+        acat1
+        acat1$
+        acatc
+        acatca
+        acatcat
+        acatcat1=
+        aco
+        acow
+        acow1=
+        t=
+        """,
+        )
+
+    def test_18(self):
+        _assert_dafsa(
+            """
+        bc1
+        abc1
+        abcxyzc1
+        """,
+            """
+        a
+        ab
+        abc
+        abc1
+        abc1$
+        abcx
+        abcxy
+        abcxyz
+        abcxyzc
+        abcxyzc1=
+        b
+        bc=
+        """,
+        )
+
+    def test_19(self):
+        _assert_dafsa(
+            """
+        a.z1
+        a.y1
+        c.z1
+        d.z1
+        d.y1
+        """,
+            """
+        a
+        a.
+        a.y
+        a.y1
+        a.y1$
+        a.z
+        a.z1=
+        c
+        c.
+        c.z=
+        d
+        d.=
+        """,
+        )
+
+    def test_20(self):
+        _assert_dafsa(
+            """
+        acz1
+        acy1
+        accz1
+        acccz1
+        bcz1
+        bcy1
+        bccz1
+        bcccz1
+        """,
+            """
+        a
+        ac
+        acc
+        accc
+        acccz
+        acccz1
+        acccz1$
+        accz=
+        acy
+        acy1=
+        acz=
+        b
+        bc=
+        """,
+        )
+
+
+if __name__ == "__main__":
+    mozunit.main()
diff --git a/xpcom/ds/tools/incremental_dafsa.py b/xpcom/ds/tools/incremental_dafsa.py
new file mode 100644
index 0000000000..1157c26850
--- /dev/null
+++ b/xpcom/ds/tools/incremental_dafsa.py
@@ -0,0 +1,509 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+"""
+Incremental algorithm for creating a "deterministic acyclic finite state
+automaton" (DAFSA). At the time of writing this algorithm, there was existing logic
+that depended on a different format for the DAFSA, so this contains convenience
+functions for converting to a compatible structure. This legacy format is defined
+in make_dafsa.py.
+"""
+
+from typing import Callable, Dict, List, Optional
+
+
+class Node:
+    children: Dict[str, "Node"]
+    parents: Dict[str, List["Node"]]
+    character: str
+    is_root_node: bool
+    is_end_node: bool
+
+    def __init__(self, character, is_root_node=False, is_end_node=False):
+        self.children = {}
+        self.parents = {}
+        self.character = character
+        self.is_root_node = is_root_node
+        self.is_end_node = is_end_node
+
+    def __str__(self):
+        """Produce a helpful string representation of this node.
+
+        This is expected to only be used for debugging.
+        The produced output is:
+
+        "c[def.] <123>"
+         ^ ^      ^
+         | |      Internal python ID of the node (used for de-duping)
+         | |
+         | One possible path through the tree to the end
+         |
+         Current node character
+        """
+
+        if self.is_root_node:
+            return "<root>"
+        elif self.is_end_node:
+            return "<end>"
+
+        first_potential_match = ""
+        node = self
+        while node.children:
+            first_character = next(iter(node.children))
+            if first_character:
+                first_potential_match += first_character
+            node = node.children[first_character]
+
+        return "%s[%s] <%d>" % (self.character, first_potential_match, id(self))
+
+    def add_child(self, child):
+        self.children[child.character] = child
+        child.parents.setdefault(self.character, [])
+        child.parents[self.character].append(self)
+
+    def remove(self):
+        # remove() must only be called when this node has only a single parent, and that
+        # parent doesn't need this child anymore.
+        # The caller is expected to have performed this validation.
+        # (placing asserts here add a non-trivial performance hit)
+
+        # There's only a single parent, so only one list should be in the "parents" map
+        parent_list = next(iter(self.parents.values()))
+        self.remove_parent(parent_list[0])
+        for child in list(self.children.values()):
+            child.remove_parent(self)
+
+    def remove_parent(self, parent_node: "Node"):
+        parent_node.children.pop(self.character)
+        parents_for_character = self.parents[parent_node.character]
+        parents_for_character.remove(parent_node)
+        if not parents_for_character:
+            self.parents.pop(parent_node.character)
+
+    def copy_fork_node(self, fork_node: "Node", child_to_avoid: Optional["Node"]):
+        """Shallow-copy a node's children.
+
+        When adding a new word, sometimes previously-joined suffixes aren't perfect
+        matches any more. When this happens, some nodes need to be "copied" out.
+        For all non-end nodes, there's a child to avoid in the shallow-copy.
+        """
+
+        for child in fork_node.children.values():
+            if child is not child_to_avoid:
+                self.add_child(child)
+
+    def is_fork(self):
+        """Check if this node has multiple parents"""
+
+        if len(self.parents) == 0:
+            return False
+
+        if len(self.parents) > 1:
+            return True
+
+        return len(next(iter(self.parents.values()))) > 1
+
+    def is_replacement_for_prefix_end_node(self, old: "Node"):
+        """Check if this node is a valid replacement for an old end node.
+
+        A node is a valid replacement if it maintains all existing child paths while
+        adding the new child path needed for the new word.
+
+        Args:
+            old: node being replaced
+
+        Returns: True if this node is a valid replacement node.
+        """
+
+        if len(self.children) != len(old.children) + 1:
+            return False
+
+        for character, other_node in old.children.items():
+            this_node = self.children.get(character)
+            if other_node is not this_node:
+                return False
+
+        return True
+
+    def is_replacement_for_prefix_node(self, old: "Node"):
+        """Check if this node is a valid replacement for a non-end node.
+
+        A node is a valid replacement if it:
+        * Has one new child that the old node doesn't
+        * Is missing a child that the old node has
+        * Shares all other children
+
+        Returns: True if this node is a valid replacement node.
+        """
+
+        if len(self.children) != len(old.children):
+            return False
+
+        found_extra_child = False
+
+        for character, other_node in old.children.items():
+            this_node = self.children.get(character)
+            if other_node is not this_node:
+                if found_extra_child:
+                    # Found two children in the old node that aren't in the new one,
+                    # this isn't a valid replacement
+                    return False
+                else:
+                    found_extra_child = True
+
+        return found_extra_child
+
+
+class SuffixCursor:
+    index: int  # Current position of the cursor within the DAFSA.
+    node: Node
+
+    def __init__(self, index, node):
+        self.index = index
+        self.node = node
+
+    def _query(self, character: str, check: Callable[[Node], bool]):
+        for node in self.node.parents.get(character, []):
+            if check(node):
+                self.index -= 1
+                self.node = node
+                return True
+        return False
+
+    def find_single_child(self, character):
+        """Find the next matching suffix node that has a single child.
+
+        Return True if such a node is found."""
+        return self._query(character, lambda node: len(node.children) == 1)
+
+    def find_end_of_prefix_replacement(self, end_of_prefix: Node):
+        """Find the next matching suffix node that replaces the old prefix-end node.
+
+        Return True if such a node is found."""
+        return self._query(
+            end_of_prefix.character,
+            lambda node: node.is_replacement_for_prefix_end_node(end_of_prefix),
+        )
+
+    def find_inside_of_prefix_replacement(self, prefix_node: Node):
+        """Find the next matching suffix node that replaces a node within the prefix.
+
+        Return True if such a node is found."""
+        return self._query(
+            prefix_node.character,
+            lambda node: node.is_replacement_for_prefix_node(prefix_node),
+        )
+
+
+class DafsaAppendStateMachine:
+    """State machine for adding a word to a Dafsa.
+
+    Each state returns a function reference to the "next state". States should be
+    invoked until "None" is returned, in which case the new word has been appended.
+
+    The prefix and suffix indexes are placed according to the currently-known valid
+    value (not the next value being investigated). Additionally, they are 0-indexed
+    against the root node (which sits behind the beginning of the string).
+
+    Let's imagine we're at the following state when adding, for example, the
+    word "mozilla.org":
+
+        mozilla.org
+       ^    ^    ^ ^
+       |    |    | |
+      /     |    | \
+    [root]  |    |  [end] node
+    node    |    \
+            |     suffix
+            \
+             prefix
+
+    In this state, the furthest prefix match we could find was:
+        [root] - m - o - z - i - l
+    The index of the prefix match is "5".
+
+    Additionally, we've been looking for suffix nodes, and we've already found:
+        r - g - [end]
+    The current suffix index is "10".
+    The next suffix node we'll attempt to find is at index "9".
+    """
+
+    root_node: Node
+    prefix_index: int
+    suffix_cursor: SuffixCursor
+    stack: List[Node]
+    word: str
+    suffix_overlaps_prefix: bool
+    first_fork_index: Optional[int]
+    _state: Callable
+
+    def __init__(self, word, root_node, end_node):
+        self.root_node = root_node
+        self.prefix_index = 0
+        self.suffix_cursor = SuffixCursor(len(word) + 1, end_node)
+        self.stack = [root_node]
+        self.word = word
+        self.suffix_overlaps_prefix = False
+        self.first_fork_index = None
+        self._state = self._find_prefix
+
+    def run(self):
+        """Run this state machine to completion, adding the new word."""
+        while self._state is not None:
+            self._state = self._state()
+
+    def _find_prefix(self):
+        """Find the longest existing prefix that matches the current word."""
+        prefix_node = self.root_node
+        while self.prefix_index < len(self.word):
+            next_character = self.word[self.prefix_index]
+            next_node = prefix_node.children.get(next_character)
+            if not next_node:
+                # We're finished finding the prefix, let's find the longest suffix
+                # match now.
+                return self._find_suffix_nodes_after_prefix
+
+            self.prefix_index += 1
+            prefix_node = next_node
+            self.stack.append(next_node)
+
+            if not self.first_fork_index and next_node.is_fork():
+                self.first_fork_index = self.prefix_index
+
+        # Deja vu, we've appended this string before. Since this string has
+        # already been appended, we don't have to do anything.
+        return None
+
+    def _find_suffix_nodes_after_prefix(self):
+        """Find the chain of suffix nodes for characters after the prefix."""
+        while self.suffix_cursor.index - 1 > self.prefix_index:
+            # To fetch the next character, we need to subtract two from the current
+            # suffix index. This is because:
+            # * The next suffix node is 1 node before our current node (subtract 1)
+            # * The suffix index includes the root node before the beginning of the
+            #   string - it's like the string is 1-indexed (subtract 1 again).
+            next_character = self.word[self.suffix_cursor.index - 2]
+            if not self.suffix_cursor.find_single_child(next_character):
+                return self._add_new_nodes
+
+            if self.suffix_cursor.node is self.stack[-1]:
+                # The suffix match is overlapping with the prefix! This can happen in
+                # cases like:
+                # * "ab"
+                # * "abb"
+                # The suffix cursor is at the same node as the prefix match, but they're
+                # at different positions in the word.
+                #
+                # [root] - a - b - [end]
+                #              ^
+                #             / \
+                #            /   \
+                #      prefix     suffix
+                #            \    /
+                #             \  /
+                #              VV
+                #            "abb"
+                if not self.first_fork_index:
+                    # There hasn't been a fork, so our prefix isn't shared. So, we
+                    # can mark this node as a fork, since the repetition means
+                    # that there's two paths that are now using this node
+                    self.first_fork_index = self.prefix_index
+                    return self._add_new_nodes
+
+                # Removes the link between the unique part of the prefix and the
+                # shared part of the prefix.
+                self.stack[self.first_fork_index].remove_parent(
+                    self.stack[self.first_fork_index - 1]
+                )
+                self.suffix_overlaps_prefix = True
+
+        if self.first_fork_index is None:
+            return self._find_next_suffix_nodes
+        elif self.suffix_cursor.index - 1 == self.first_fork_index:
+            return self._find_next_suffix_node_at_prefix_end_at_fork
+        else:
+            return self._find_next_suffix_node_at_prefix_end_after_fork
+
+    def _find_next_suffix_node_at_prefix_end_at_fork(self):
+        """Find the next suffix node that replaces the end of the prefix.
+
+        In this state, the prefix_end node is the same as the first fork node.
+        Therefore, if a match can be found, the old prefix node can't be entirely
+        deleted since it's used elsewhere. Instead, just the link between our
+        unique prefix and the end of the fork is removed.
+        """
+        existing_node = self.stack[self.prefix_index]
+        if not self.suffix_cursor.find_end_of_prefix_replacement(existing_node):
+            return self._add_new_nodes
+
+        self.prefix_index -= 1
+        self.first_fork_index = None
+
+        if not self.suffix_overlaps_prefix:
+            existing_node.remove_parent(self.stack[self.prefix_index])
+        else:
+            # When the suffix overlaps the prefix, the old "parent link" was removed
+            # earlier in the "find_suffix_nodes_after_prefix" step.
+            self.suffix_overlaps_prefix = False
+
+        return self._find_next_suffix_nodes
+
+    def _find_next_suffix_node_at_prefix_end_after_fork(self):
+        """Find the next suffix node that replaces the end of the prefix.
+
+        In this state, the prefix_end node is after the first fork node.
+        Therefore, even if a match is found, we don't want to modify the replaced
+        prefix node since an unrelated word chain uses it.
+        """
+        existing_node = self.stack[self.prefix_index]
+        if not self.suffix_cursor.find_end_of_prefix_replacement(existing_node):
+            return self._add_new_nodes
+
+        self.prefix_index -= 1
+        if self.prefix_index == self.first_fork_index:
+            return self._find_next_suffix_node_within_prefix_at_fork
+        else:
+            return self._find_next_suffix_nodes_within_prefix_after_fork
+
+    def _find_next_suffix_node_within_prefix_at_fork(self):
+        """Find the next suffix node within a prefix.
+
+        In this state, we've already worked our way back and found nodes in the suffix
+        to replace prefix nodes after the fork node. We have now reached the fork node,
+        and if we find a replacement for it, then we can remove the link between it
+        and our then-unique prefix and clear the fork status.
+        """
+        existing_node = self.stack[self.prefix_index]
+        if not self.suffix_cursor.find_inside_of_prefix_replacement(existing_node):
+            return self._add_new_nodes
+
+        self.prefix_index -= 1
+        self.first_fork_index = None
+
+        if not self.suffix_overlaps_prefix:
+            existing_node.remove_parent(self.stack[self.prefix_index])
+        else:
+            # When the suffix overlaps the prefix, the old "parent link" was removed
+            # earlier in the "find_suffix_nodes_after_prefix" step.
+            self.suffix_overlaps_prefix = False
+
+        return self._find_next_suffix_nodes
+
+    def _find_next_suffix_nodes_within_prefix_after_fork(self):
+        """Find the next suffix nodes within a prefix.
+
+        Finds suffix nodes to replace prefix nodes, but doesn't modify the prefix
+        nodes since they're after a fork (so, we're sharing prefix nodes with
+        other words and can't modify them).
+        """
+        while True:
+            existing_node = self.stack[self.prefix_index]
+            if not self.suffix_cursor.find_inside_of_prefix_replacement(existing_node):
+                return self._add_new_nodes
+
+            self.prefix_index -= 1
+            if self.prefix_index == self.first_fork_index:
+                return self._find_next_suffix_node_within_prefix_at_fork
+
+    def _find_next_suffix_nodes(self):
+        """Find all remaining suffix nodes in the chain.
+
+        In this state, there's no (longer) any fork, so there's no other words
+        using our current prefix. Therefore, as we find replacement nodes as we
+        work our way backwards, we can remove the now-unused prefix nodes.
+        """
+        while True:
+            existing_node = self.stack[self.prefix_index]
+            if not self.suffix_cursor.find_end_of_prefix_replacement(existing_node):
+                return self._add_new_nodes
+
+            # This prefix node is wholly replaced by the new suffix node, so it can
+            # be deleted.
+            existing_node.remove()
+            self.prefix_index -= 1
+
+    def _add_new_nodes(self):
+        """Adds new nodes to support the new word.
+
+        Duplicates forked nodes to make room for new links, adds new nodes for new
+        characters, and splices the prefix to the suffix to finish embedding the new
+        word into the DAFSA.
+        """
+        if self.first_fork_index is not None:
+            front_node = _duplicate_fork_nodes(
+                self.stack,
+                self.first_fork_index,
+                self.prefix_index,
+                # if suffix_overlaps_parent, the parent link was removed
+                # earlier in the word-adding process.
+                remove_parent_link=not self.suffix_overlaps_prefix,
+            )
+        else:
+            front_node = self.stack[self.prefix_index]
+
+        new_text = self.word[self.prefix_index : self.suffix_cursor.index - 1]
+        for character in new_text:
+            new_node = Node(character)
+            front_node.add_child(new_node)
+            front_node = new_node
+
+        front_node.add_child(self.suffix_cursor.node)
+        return None  # Done!
+
+
+def _duplicate_fork_nodes(stack, fork_index, prefix_index, remove_parent_link=True):
+    parent_node = stack[fork_index - 1]
+    if remove_parent_link:
+        # remove link to old chain that we're going to be copying
+        stack[fork_index].remove_parent(parent_node)
+
+    for index in range(fork_index, prefix_index + 1):
+        fork_node = stack[index]
+        replacement_node = Node(fork_node.character)
+        child_to_avoid = None
+        if index < len(stack) - 1:
+            # We're going to be manually replacing the next node in the stack,
+            # so don't connect it as a child.
+            child_to_avoid = stack[index + 1]
+
+        replacement_node.copy_fork_node(fork_node, child_to_avoid)
+        parent_node.add_child(replacement_node)
+        parent_node = replacement_node
+
+    return parent_node
+
+
+class Dafsa:
+    root_node: Node
+    end_node: Node
+
+    def __init__(self):
+        self.root_node = Node(None, is_root_node=True)
+        self.end_node = Node(None, is_end_node=True)
+
+    @classmethod
+    def from_tld_data(cls, lines):
+        """Create a dafsa for TLD data.
+
+        TLD data has a domain and a "type" enum. The source data encodes the type as a
+        text number, but the dafsa-consuming code assumes that the type is a raw byte
+        number (e.g.: "1" => 0x01).
+
+        This function acts as a helper, processing this TLD detail before creating a
+        standard dafsa.
+        """
+
+        dafsa = cls()
+        for i, word in enumerate(lines):
+            domain_number = word[-1]
+            # Convert type from string to byte representation
+            raw_domain_number = chr(ord(domain_number) & 0x0F)
+
+            word = "%s%s" % (word[:-1], raw_domain_number)
+            dafsa.append(word)
+        return dafsa
+
+    def append(self, word):
+        state_machine = DafsaAppendStateMachine(word, self.root_node, self.end_node)
+        state_machine.run()
diff --git a/xpcom/ds/tools/make_dafsa.py b/xpcom/ds/tools/make_dafsa.py
new file mode 100644
index 0000000000..a9cedf78a8
--- /dev/null
+++ b/xpcom/ds/tools/make_dafsa.py
@@ -0,0 +1,372 @@
+#!/usr/bin/env python
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""
+A Deterministic acyclic finite state automaton (DAFSA) is a compact
+representation of an unordered word list (dictionary).
+
+http://en.wikipedia.org/wiki/Deterministic_acyclic_finite_state_automaton
+
+This python program converts a list of strings to a byte array in C++.
+This python program fetches strings and return values from a gperf file
+and generates a C++ file with a byte array representing graph that can be
+used as a memory efficient replacement for the perfect hash table.
+
+The input strings are assumed to consist of printable 7-bit ASCII characters
+and the return values are assumed to be one digit integers.
+
+In this program a DAFSA is a diamond shaped graph starting at a common
+root node and ending at a common end node. All internal nodes contain
+a character and each word is represented by the characters in one path from
+the root node to the end node.
+
+The order of the operations is crucial since lookups will be performed
+starting from the source with no backtracking. Thus a node must have at
+most one child with a label starting by the same character. The output
+is also arranged so that all jumps are to increasing addresses, thus forward
+in memory.
+
+The generated output has suffix free decoding so that the sign of leading
+bits in a link (a reference to a child node) indicate if it has a size of one,
+two or three bytes and if it is the last outgoing link from the actual node.
+A node label is terminated by a byte with the leading bit set.
+
+The generated byte array can described by the following BNF:
+
+<byte> ::= < 8-bit value in range [0x00-0xFF] >
+
+<char> ::= < printable 7-bit ASCII character, byte in range [0x20-0x7F] >
+<end_char> ::= < char + 0x80, byte in range [0xA0-0xFF] >
+<return value> ::= < value + 0x80, byte in range [0x80-0x8F] >
+
+<offset1> ::= < byte in range [0x00-0x3F] >
+<offset2> ::= < byte in range [0x40-0x5F] >
+<offset3> ::= < byte in range [0x60-0x7F] >
+
+<end_offset1> ::= < byte in range [0x80-0xBF] >
+<end_offset2> ::= < byte in range [0xC0-0xDF] >
+<end_offset3> ::= < byte in range [0xE0-0xFF] >
+
+<prefix> ::= <char>
+
+<label> ::= <end_char>
+          | <char> <label>
+
+<end_label> ::= <return_value>
+          | <char> <end_label>
+
+<offset> ::= <offset1>
+           | <offset2> <byte>
+           | <offset3> <byte> <byte>
+
+<end_offset> ::= <end_offset1>
+               | <end_offset2> <byte>
+               | <end_offset3> <byte> <byte>
+
+<offsets> ::= <end_offset>
+            | <offset> <offsets>
+
+<source> ::= <offsets>
+
+<node> ::= <label> <offsets>
+         | <prefix> <node>
+         | <end_label>
+
+<dafsa> ::= <source>
+          | <dafsa> <node>
+
+Decoding:
+
+<char> -> printable 7-bit ASCII character
+<end_char> & 0x7F -> printable 7-bit ASCII character
+<return value> & 0x0F -> integer
+<offset1 & 0x3F> -> integer
+((<offset2> & 0x1F>) << 8) + <byte> -> integer
+((<offset3> & 0x1F>) << 16) + (<byte> << 8) + <byte> -> integer
+
+end_offset1, end_offset2 and and_offset3 are decoded same as offset1,
+offset2 and offset3 respectively.
+
+The first offset in a list of offsets is the distance in bytes between the
+offset itself and the first child node. Subsequent offsets are the distance
+between previous child node and next child node. Thus each offset links a node
+to a child node. The distance is always counted between start addresses, i.e.
+first byte in decoded offset or first byte in child node.
+
+Example 1:
+
+%%
+aa, 1
+a, 2
+%%
+
+The input is first parsed to a list of words:
+["aa1", "a2"]
+
+This produces the following graph:
+[root] --- a --- 0x02 --- [end]
+            |              /
+             |           /
+              - a --- 0x01
+
+A C++ representation of the compressed graph is generated:
+
+const unsigned char dafsa[7] = {
+  0x81, 0xE1, 0x02, 0x81, 0x82, 0x61, 0x81,
+};
+
+The bytes in the generated array has the following meaning:
+
+ 0: 0x81 <end_offset1>  child at position 0 + (0x81 & 0x3F) -> jump to 1
+
+ 1: 0xE1 <end_char>     label character (0xE1 & 0x7F) -> match "a"
+ 2: 0x02 <offset1>      child at position 2 + (0x02 & 0x3F) -> jump to 4
+
+ 3: 0x81 <end_offset1>  child at position 4 + (0x81 & 0x3F) -> jump to 5
+ 4: 0x82 <return_value> 0x82 & 0x0F -> return 2
+
+ 5: 0x61 <char>         label character 0x61 -> match "a"
+ 6: 0x81 <return_value> 0x81 & 0x0F -> return 1
+
+Example 2:
+
+%%
+aa, 1
+bbb, 2
+baa, 1
+%%
+
+The input is first parsed to a list of words:
+["aa1", "bbb2", "baa1"]
+
+This produces the following graph:
+[root] --- a --- a --- 0x01 --- [end]
+ |       /           /         /
+  |    /           /         /
+   - b --- b --- b --- 0x02
+
+A C++ representation of the compressed graph is generated:
+
+const unsigned char dafsa[11] = {
+  0x02, 0x83, 0xE2, 0x02, 0x83, 0x61, 0x61, 0x81, 0x62, 0x62, 0x82,
+};
+
+The bytes in the generated array has the following meaning:
+
+ 0: 0x02 <offset1>      child at position 0 + (0x02 & 0x3F) -> jump to 2
+ 1: 0x83 <end_offset1>  child at position 2 + (0x83 & 0x3F) -> jump to 5
+
+ 2: 0xE2 <end_char>     label character (0xE2 & 0x7F) -> match "b"
+ 3: 0x02 <offset1>      child at position 3 + (0x02 & 0x3F) -> jump to 5
+ 4: 0x83 <end_offset1>  child at position 5 + (0x83 & 0x3F) -> jump to 8
+
+ 5: 0x61 <char>         label character 0x61 -> match "a"
+ 6: 0x61 <char>         label character 0x61 -> match "a"
+ 7: 0x81 <return_value> 0x81 & 0x0F -> return 1
+
+ 8: 0x62 <char>         label character 0x62 -> match "b"
+ 9: 0x62 <char>         label character 0x62 -> match "b"
+10: 0x82 <return_value> 0x82 & 0x0F -> return 2
+"""
+import struct
+import sys
+
+from incremental_dafsa import Dafsa, Node
+
+
+class InputError(Exception):
+    """Exception raised for errors in the input file."""
+
+
+def top_sort(dafsa: Dafsa):
+    """Generates list of nodes in topological sort order."""
+    incoming = {}
+
+    def count_incoming(node: Node):
+        """Counts incoming references."""
+        if not node.is_end_node:
+            if id(node) not in incoming:
+                incoming[id(node)] = 1
+                for child in node.children.values():
+                    count_incoming(child)
+            else:
+                incoming[id(node)] += 1
+
+    for node in dafsa.root_node.children.values():
+        count_incoming(node)
+
+    for node in dafsa.root_node.children.values():
+        incoming[id(node)] -= 1
+
+    waiting = [
+        node for node in dafsa.root_node.children.values() if incoming[id(node)] == 0
+    ]
+    nodes = []
+
+    while waiting:
+        node = waiting.pop()
+        assert incoming[id(node)] == 0
+        nodes.append(node)
+        for child in node.children.values():
+            if not child.is_end_node:
+                incoming[id(child)] -= 1
+                if incoming[id(child)] == 0:
+                    waiting.append(child)
+    return nodes
+
+
+def encode_links(node: Node, offsets, current):
+    """Encodes a list of children as one, two or three byte offsets."""
+    if next(iter(node.children.values())).is_end_node:
+        # This is an <end_label> node and no links follow such nodes
+        return []
+    guess = 3 * len(node.children)
+    assert node.children
+
+    children = sorted(node.children.values(), key=lambda x: -offsets[id(x)])
+    while True:
+        offset = current + guess
+        buf = []
+        for child in children:
+            last = len(buf)
+            distance = offset - offsets[id(child)]
+            assert distance > 0 and distance < (1 << 21)
+
+            if distance < (1 << 6):
+                # A 6-bit offset: "s0xxxxxx"
+                buf.append(distance)
+            elif distance < (1 << 13):
+                # A 13-bit offset: "s10xxxxxxxxxxxxx"
+                buf.append(0x40 | (distance >> 8))
+                buf.append(distance & 0xFF)
+            else:
+                # A 21-bit offset: "s11xxxxxxxxxxxxxxxxxxxxx"
+                buf.append(0x60 | (distance >> 16))
+                buf.append((distance >> 8) & 0xFF)
+                buf.append(distance & 0xFF)
+            # Distance in first link is relative to following record.
+            # Distance in other links are relative to previous link.
+            offset -= distance
+        if len(buf) == guess:
+            break
+        guess = len(buf)
+    # Set most significant bit to mark end of links in this node.
+    buf[last] |= 1 << 7
+    buf.reverse()
+    return buf
+
+
+def encode_prefix(label):
+    """Encodes a node label as a list of bytes without a trailing high byte.
+
+    This method encodes a node if there is exactly one child  and the
+    child follows immediately after so that no jump is needed. This label
+    will then be a prefix to the label in the child node.
+    """
+    assert label
+    return [ord(c) for c in reversed(label)]
+
+
+def encode_label(label):
+    """Encodes a node label as a list of bytes with a trailing high byte >0x80."""
+    buf = encode_prefix(label)
+    # Set most significant bit to mark end of label in this node.
+    buf[0] |= 1 << 7
+    return buf
+
+
+def encode(dafsa: Dafsa):
+    """Encodes a DAFSA to a list of bytes"""
+    output = []
+    offsets = {}
+
+    for node in reversed(top_sort(dafsa)):
+        if (
+            len(node.children) == 1
+            and not next(iter(node.children.values())).is_end_node
+            and (offsets[id(next(iter(node.children.values())))] == len(output))
+        ):
+            output.extend(encode_prefix(node.character))
+        else:
+            output.extend(encode_links(node, offsets, len(output)))
+            output.extend(encode_label(node.character))
+        offsets[id(node)] = len(output)
+
+    output.extend(encode_links(dafsa.root_node, offsets, len(output)))
+    output.reverse()
+    return output
+
+
+def to_cxx(data, preamble=None):
+    """Generates C++ code from a list of encoded bytes."""
+    text = "/* This file is generated. DO NOT EDIT!\n\n"
+    text += "The byte array encodes a dictionary of strings and values. See "
+    text += "make_dafsa.py for documentation."
+    text += "*/\n\n"
+
+    if preamble:
+        text += preamble
+        text += "\n\n"
+
+    text += "const unsigned char kDafsa[%s] = {\n" % len(data)
+    for i in range(0, len(data), 12):
+        text += "  "
+        text += ", ".join("0x%02x" % byte for byte in data[i : i + 12])
+        text += ",\n"
+    text += "};\n"
+    return text
+
+
+def words_to_cxx(words, preamble=None):
+    """Generates C++ code from a word list"""
+    dafsa = Dafsa.from_tld_data(words)
+    return to_cxx(encode(dafsa), preamble)
+
+
+def words_to_bin(words):
+    """Generates bytes from a word list"""
+    dafsa = Dafsa.from_tld_data(words)
+    data = encode(dafsa)
+    return struct.pack("%dB" % len(data), *data)
+
+
+def parse_gperf(infile):
+    """Parses gperf file and extract strings and return code"""
+    lines = [line.strip() for line in infile]
+
+    # Extract the preamble.
+    first_delimeter = lines.index("%%")
+    preamble = "\n".join(lines[0:first_delimeter])
+
+    # Extract strings after the first '%%' and before the second '%%'.
+    begin = first_delimeter + 1
+    end = lines.index("%%", begin)
+    lines = lines[begin:end]
+    for line in lines:
+        if line[-3:-1] != ", ":
+            raise InputError('Expected "domainname, <digit>", found "%s"' % line)
+        # Technically the DAFSA format could support return values in range [0-31],
+        # but the values below are the only with a defined meaning.
+        if line[-1] not in "0124":
+            raise InputError(
+                'Expected value to be one of {0,1,2,4}, found "%s"' % line[-1]
+            )
+    return (preamble, [line[:-3] + line[-1] for line in lines])
+
+
+def main(outfile, infile):
+    with open(infile, "r") as infile:
+        preamble, words = parse_gperf(infile)
+        outfile.write(words_to_cxx(words, preamble))
+    return 0
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print("usage: %s infile outfile" % sys.argv[0])
+        sys.exit(1)
+
+    with open(sys.argv[2], "w") as outfile:
+        sys.exit(main(outfile, sys.argv[1]))
diff --git a/xpcom/ds/tools/perfecthash.py b/xpcom/ds/tools/perfecthash.py
new file mode 100644
index 0000000000..b4e964a125
--- /dev/null
+++ b/xpcom/ds/tools/perfecthash.py
@@ -0,0 +1,371 @@
+# perfecthash.py - Helper for generating perfect hash functions
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+"""
+A perfect hash function (PHF) is a function which maps distinct elements from a
+source set to a sequence of integers with no collisions.
+
+PHFs generated by this module use a 32-bit FNV hash to index an intermediate
+table. The value from that table is then used to run a second 32-bit FNV hash to
+get a final index.
+
+The algorithm works by starting with the largest set of conflicts, and testing
+intermediate table values until no conflicts are generated, allowing efficient
+index lookup at runtime. (see also: http://stevehanov.ca/blog/index.php?id=119)
+
+NOTE: This perfect hash generator was designed to be simple, easy to follow, and
+maintainable, rather than being as optimized as possible, due to the relatively
+small dataset it was designed for. In the future we may want to optimize further.
+"""
+
+import textwrap
+from collections import namedtuple
+
+import six
+from mozbuild.util import ensure_bytes
+
+
+# Iteration over bytestrings works differently in Python 2 and 3; this function
+# captures the two possibilities. Returns an 'int' given the output of iterating
+# through a bytestring regardless of the input.
+def _ord(c):
+    if six.PY3:
+        return c
+    return ord(c)
+
+
+class PerfectHash(object):
+    """PerfectHash objects represent a computed perfect hash function, which
+    can be generated at compile time to provide highly efficient and compact
+    static HashTables.
+
+    Consumers must provide an intermediate table size to store the generated
+    hash function. Larger tables will generate more quickly, while smaller
+    tables will consume less space on disk."""
+
+    # 32-bit FNV offset basis and prime value.
+    # NOTE: Must match values in |PerfectHash.h|
+    FNV_OFFSET_BASIS = 0x811C9DC5
+    FNV_PRIME = 16777619
+    U32_MAX = 0xFFFFFFFF
+
+    # Bucket of entries which map into a given intermediate index.
+    Bucket = namedtuple("Bucket", "index entries")
+
+    def __init__(self, entries, size, validate=True, key=lambda e: e[0]):
+        """Create a new PerfectHash
+
+        @param entries  set of entries to generate a PHF for
+        @param size     size of the PHF intermediate table
+        @param validate test the generated hash function after generation
+        @param key      function to get 'memoryview'-compatible key for an
+                        entry. defaults to extracting the first element of an
+                        entry 2-tuple.
+        """
+
+        assert 0 < len(entries) < self.U32_MAX, "bad # of entries!"
+        self._key = key
+
+        # Allocate the intermediate table and keys.
+        self.table = [0] * size
+        self.entries = [None] * len(entries)
+
+        # A set of Bucket values. Each bucket contains the index into the
+        # intermediate table, and a list of Entries which map into that bucket.
+        buckets = [self.Bucket(idx, []) for idx in range(size)]
+
+        # Determine which input strings map to which buckets in the table.
+        for entry in entries:
+            assert entry is not None, "None entry in entries"
+            assert self.key(entry).itemsize == 1, "non-byte key elements"
+            buckets[self._hash(self.key(entry)) % size].entries.append(entry)
+
+        # Sort the buckets such that the largest one comes first.
+        buckets.sort(key=lambda b: len(b.entries), reverse=True)
+
+        for bucket in buckets:
+            # Once we've reached an empty bucket, we're done.
+            if len(bucket.entries) == 0:
+                break
+
+            # Try values for the basis until we find one with no conflicts.
+            idx = 0
+            basis = 1
+            slots = []
+            while idx < len(bucket.entries):
+                slot = self._hash(self.key(bucket.entries[idx]), basis) % len(
+                    self.entries
+                )
+                if self.entries[slot] is not None or slot in slots:
+                    # There was a conflict, try the next basis.
+                    basis += 1
+                    idx = 0
+                    del slots[:]
+                    assert basis < self.U32_MAX, "table too small"
+                else:
+                    slots.append(slot)
+                    idx += 1
+
+            assert basis < self.U32_MAX
+
+            # We've found a basis which doesn't conflict
+            self.table[bucket.index] = basis
+            for slot, entry in zip(slots, bucket.entries):
+                self.entries[slot] = entry
+
+        # Validate that looking up each key succeeds
+        if validate:
+            for entry in entries:
+                assert self.get_entry(self.key(entry)), "get_entry(%s)" % repr(entry)
+
+    @classmethod
+    def _hash(cls, key, basis=FNV_OFFSET_BASIS):
+        """A basic FNV-based hash function. key is the memoryview to hash.
+        32-bit FNV is used for indexing into the first table, and the value
+        stored in that table is used as the offset basis for indexing into the
+        values table."""
+        for byte in memoryview(ensure_bytes(key)):
+            obyte = _ord(byte)
+            basis ^= obyte  # xor-in the byte
+            basis *= cls.FNV_PRIME  # Multiply by the FNV prime
+            basis &= cls.U32_MAX  # clamp to 32-bits
+        return basis
+
+    def key(self, entry):
+        return memoryview(ensure_bytes(self._key(entry)))
+
+    def get_raw_index(self, key):
+        """Determine the index in self.entries without validating"""
+        mid = self.table[self._hash(key) % len(self.table)]
+        return self._hash(key, mid) % len(self.entries)
+
+    def get_index(self, key):
+        """Given a key, determine the index in self.entries"""
+        idx = self.get_raw_index(key)
+        if memoryview(ensure_bytes(key)) != self.key(self.entries[idx]):
+            return None
+        return idx
+
+    def get_entry(self, key):
+        """Given a key, get the corresponding entry"""
+        idx = self.get_index(key)
+        if idx is None:
+            return None
+        return self.entries[idx]
+
+    @staticmethod
+    def _indent(text, amount=1):
+        return text.replace("\n", "\n" + ("  " * amount))
+
+    def codegen(self, name, entry_type):
+        """Create a helper for codegening PHF logic"""
+        return CGHelper(self, name, entry_type)
+
+    def cxx_codegen(
+        self,
+        name,
+        entry_type,
+        lower_entry,
+        entries_name=None,
+        return_type=None,
+        return_entry="return entry;",
+        key_type="const char*",
+        key_bytes="aKey",
+        key_length="strlen(aKey)",
+    ):
+        """Generate complete C++ code for a get_entry-style method.
+
+        @param name         Name for the entry getter function.
+        @param entry_type   C++ type of each entry in static memory.
+        @param lower_entry  Function called with each entry to convert it to a
+                            C++ literal of type 'entry_type'.
+
+        # Optional Keyword Parameters:
+        @param entries_name Name for the generated entry table.
+
+        @param return_type  C++ return type, default: 'const entry_type&'
+        @param return_entry Override the default behaviour for returning the
+                            found entry. 'entry' is a reference to the found
+                            entry, and 'aKey' is the lookup key. 'return_entry'
+                            can be used for additional checks, e.g. for keys
+                            not in the table.
+
+        @param key_type     C++ key type, default: 'const char*'
+        @param key_bytes    'const char*' expression to get bytes for 'aKey'
+        @param key_length   'size_t' expression to get length of 'aKey'"""
+
+        if entries_name is None:
+            entries_name = "s%sEntries" % name
+
+        cg = self.codegen(entries_name, entry_type)
+        entries = cg.gen_entries(lower_entry)
+        getter = cg.gen_getter(
+            name, return_type, return_entry, key_type, key_bytes, key_length
+        )
+
+        return "%s\n\n%s" % (entries, getter)
+
+
+class CGHelper(object):
+    """Helper object for generating C++ code for a PerfectHash.
+    Created using PerfectHash.codegen()."""
+
+    def __init__(self, phf, entries_name, entry_type):
+        self.phf = phf
+        self.entries_name = entries_name
+        self.entry_type = entry_type
+
+    @staticmethod
+    def _indent(text, amount=1):
+        return text.replace("\n", "\n" + ("  " * amount))
+
+    def basis_ty(self):
+        """Determine how big of an integer is needed for bases table."""
+        max_base = max(self.phf.table)
+        if max_base <= 0xFF:
+            return "uint8_t"
+        elif max_base <= 0xFFFF:
+            return "uint16_t"
+        return "uint32_t"
+
+    def basis_table(self, name="BASES"):
+        """Generate code for a static basis table"""
+        bases = ""
+        for idx, base in enumerate(self.phf.table):
+            if idx and idx % 16 == 0:  # 16 bases per line
+                bases += "\n  "
+            bases += "%4d," % base
+        return (
+            textwrap.dedent(
+                """\
+            static const %s %s[] = {
+              %s
+            };
+            """
+            )
+            % (self.basis_ty(), name, bases)
+        )
+
+    def gen_entries(self, lower_entry):
+        """Generate code for an entries table"""
+        entries = self._indent(
+            ",\n".join(lower_entry(entry).rstrip() for entry in self.phf.entries)
+        )
+        return (
+            textwrap.dedent(
+                """\
+            const %s %s[] = {
+              %s
+            };
+            """
+            )
+            % (self.entry_type, self.entries_name, entries)
+        )
+
+    def gen_getter(
+        self,
+        name,
+        return_type=None,
+        return_entry="return entry;",
+        key_type="const char*",
+        key_bytes="aKey",
+        key_length="strlen(aKey)",
+    ):
+        """Generate the code for a C++ getter.
+
+        @param name         Name for the entry getter function.
+        @param return_type  C++ return type, default: 'const entry_type&'
+        @param return_entry Override the default behaviour for returning the
+                            found entry. 'entry' is a reference to the found
+                            entry, and 'aKey' is the lookup key. 'return_entry'
+                            can be used for additional checks, e.g. for keys
+                            not in the table.
+
+        @param key_type     C++ key type, default: 'const char*'
+        @param key_bytes    'const char*' expression to get bytes for 'aKey'
+        @param key_length   'size_t' expression to get length of 'aKey'"""
+
+        if return_type is None:
+            return_type = "const %s&" % self.entry_type
+
+        return textwrap.dedent(
+            """
+            %(return_type)s
+            %(name)s(%(key_type)s aKey)
+            {
+              %(basis_table)s
+
+              const char* bytes = %(key_bytes)s;
+              size_t length = %(key_length)s;
+              auto& entry = mozilla::perfecthash::Lookup(bytes, length, BASES,
+                                                         %(entries_name)s);
+              %(return_entry)s
+            }
+            """
+        ) % {
+            "name": name,
+            "basis_table": self._indent(self.basis_table()),
+            "entries_name": self.entries_name,
+            "return_type": return_type,
+            "return_entry": self._indent(return_entry),
+            "key_type": key_type,
+            "key_bytes": key_bytes,
+            "key_length": key_length,
+        }
+
+    def gen_jslinearstr_getter(
+        self, name, return_type=None, return_entry="return entry;"
+    ):
+        """Generate code for a specialized getter taking JSLinearStrings.
+        This getter avoids copying the JS string, but only supports ASCII keys.
+
+        @param name         Name for the entry getter function.
+        @param return_type  C++ return type, default: 'const entry_type&'
+        @param return_entry Override the default behaviour for returning the
+                            found entry. 'entry' is a reference to the found
+                            entry, and 'aKey' is the lookup key. 'return_entry'
+                            can be used for additional checks, e.g. for keys
+                            not in the table."""
+
+        assert all(
+            _ord(b) <= 0x7F for e in self.phf.entries for b in self.phf.key(e)
+        ), "non-ASCII key"
+
+        if return_type is None:
+            return_type = "const %s&" % self.entry_type
+
+        return textwrap.dedent(
+            """
+            %(return_type)s
+            %(name)s(JSLinearString* aKey)
+            {
+              %(basis_table)s
+
+              size_t length = JS::GetLinearStringLength(aKey);
+
+              JS::AutoCheckCannotGC nogc;
+              if (JS::LinearStringHasLatin1Chars(aKey)) {
+                auto& entry = mozilla::perfecthash::Lookup(
+                  JS::GetLatin1LinearStringChars(nogc, aKey),
+                  length, BASES, %(entries_name)s);
+
+                %(return_entry)s
+              } else {
+                auto& entry = mozilla::perfecthash::Lookup(
+                  JS::GetTwoByteLinearStringChars(nogc, aKey),
+                  length, BASES, %(entries_name)s);
+
+                %(return_entry)s
+              }
+            }
+            """
+        ) % {
+            "name": name,
+            "basis_table": self._indent(self.basis_table()),
+            "entries_name": self.entries_name,
+            "return_type": return_type,
+            "return_entry": self._indent(return_entry, 2),
+        }