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diff --git a/xpcom/ds/nsCOMArray.h b/xpcom/ds/nsCOMArray.h
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+/* -*- 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;
+
+  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);
+  }
+
+ protected:
+  // the actual storage
+  nsTArray<nsISupports*> mArray;
+
+ private:
+  // 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);
+  }
+
+  using TComparatorFunc = int (*)(T*, T*);
+
+  // The default sort function uses nsTArray::Sort.
+  // Note that the order of equal items is unstable with this.
+  void Sort(TComparatorFunc aFunc) {
+    mArray.Sort(
+        [aFunc](nsISupports* const& aLeft, nsISupports* const& aRight) -> int {
+          return aFunc(static_cast<T*>(aLeft), static_cast<T*>(aRight));
+        });
+  }
+
+  // Sort with a stable algorithm, uses nsTArray::StableSort.
+  void StableSort(TComparatorFunc aFunc) {
+    mArray.StableSort(
+        [aFunc](nsISupports* const& aLeft, nsISupports* const& aRight) -> int {
+          return aFunc(static_cast<T*>(aLeft), static_cast<T*>(aRight));
+        });
+  }
+
+  // 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