Adding upstream version 124.0.1.upstream/124.0.1

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

106 files changed, 26734 insertions, 0 deletions

diff --git a/xpcom/threads/AbstractThread.cpp b/xpcom/threads/AbstractThread.cpp
new file mode 100644
index 0000000000..61289a6789
--- /dev/null
+++ b/xpcom/threads/AbstractThread.cpp
@@ -0,0 +1,359 @@
+/* -*- 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/AbstractThread.h"
+
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/DelayedRunnable.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MozPromise.h"  // We initialize the MozPromise logging in this file.
+#include "mozilla/ProfilerRunnable.h"
+#include "mozilla/StateWatching.h"  // We initialize the StateWatching logging in this file.
+#include "mozilla/StaticPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/Unused.h"
+#include "nsContentUtils.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsIThreadInternal.h"
+#include "nsServiceManagerUtils.h"
+#include "nsThreadManager.h"
+#include "nsThreadUtils.h"
+#include <memory>
+
+namespace mozilla {
+
+LazyLogModule gMozPromiseLog("MozPromise");
+LazyLogModule gStateWatchingLog("StateWatching");
+
+StaticRefPtr<AbstractThread> sMainThread;
+MOZ_THREAD_LOCAL(AbstractThread*) AbstractThread::sCurrentThreadTLS;
+
+class XPCOMThreadWrapper final : public AbstractThread,
+                                 public nsIThreadObserver,
+                                 public nsIDirectTaskDispatcher {
+ public:
+  XPCOMThreadWrapper(nsIThreadInternal* aThread, bool aRequireTailDispatch,
+                     bool aOnThread)
+      : AbstractThread(aRequireTailDispatch),
+        mThread(aThread),
+        mDirectTaskDispatcher(do_QueryInterface(aThread)),
+        mOnThread(aOnThread) {
+    MOZ_DIAGNOSTIC_ASSERT(mThread && mDirectTaskDispatcher);
+    MOZ_DIAGNOSTIC_ASSERT(!aOnThread || IsCurrentThreadIn());
+    if (aOnThread) {
+      MOZ_ASSERT(!sCurrentThreadTLS.get(),
+                 "There can only be a single XPCOMThreadWrapper available on a "
+                 "thread");
+      // Set the default current thread so that GetCurrent() never returns
+      // nullptr.
+      sCurrentThreadTLS.set(this);
+    }
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+  nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                    DispatchReason aReason = NormalDispatch) override {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    AbstractThread* currentThread;
+    if (aReason != TailDispatch && (currentThread = GetCurrent()) &&
+        RequiresTailDispatch(currentThread) &&
+        currentThread->IsTailDispatcherAvailable()) {
+      return currentThread->TailDispatcher().AddTask(this, r.forget());
+    }
+
+    // At a certain point during shutdown, we stop processing events from the
+    // main thread event queue (this happens long after all _other_ XPCOM
+    // threads have been shut down). However, various bits of subsequent
+    // teardown logic (the media shutdown blocker and the final shutdown cycle
+    // collection) can trigger state watching and state mirroring notifications
+    // that result in dispatch to the main thread. This causes shutdown leaks,
+    // because the |Runner| wrapper below creates a guaranteed cycle
+    // (Thread->EventQueue->Runnable->Thread) until the event is processed. So
+    // if we put the event into a queue that will never be processed, we'll wind
+    // up with a leak.
+    //
+    // We opt to just release the runnable in that case. Ordinarily, this
+    // approach could cause problems for runnables that are only safe to be
+    // released on the target thread (and not the dispatching thread). This is
+    // why XPCOM thread dispatch explicitly leaks the runnable when dispatch
+    // fails, rather than releasing it. But given that this condition only
+    // applies very late in shutdown when only one thread remains operational,
+    // that concern is unlikely to apply.
+    if (gXPCOMMainThreadEventsAreDoomed) {
+      return NS_ERROR_FAILURE;
+    }
+
+    RefPtr<nsIRunnable> runner = new Runner(this, r.forget());
+    return mThread->Dispatch(runner.forget(), NS_DISPATCH_NORMAL);
+  }
+
+  // Prevent a GCC warning about the other overload of Dispatch being hidden.
+  using AbstractThread::Dispatch;
+
+  NS_IMETHOD RegisterShutdownTask(nsITargetShutdownTask* aTask) override {
+    return mThread->RegisterShutdownTask(aTask);
+  }
+
+  NS_IMETHOD UnregisterShutdownTask(nsITargetShutdownTask* aTask) override {
+    return mThread->UnregisterShutdownTask(aTask);
+  }
+
+  bool IsCurrentThreadIn() const override {
+    return mThread->IsOnCurrentThread();
+  }
+
+  TaskDispatcher& TailDispatcher() override {
+    MOZ_ASSERT(IsCurrentThreadIn());
+    MOZ_ASSERT(IsTailDispatcherAvailable());
+    if (!mTailDispatcher) {
+      mTailDispatcher =
+          std::make_unique<AutoTaskDispatcher>(mDirectTaskDispatcher,
+                                               /* aIsTailDispatcher = */ true);
+      mThread->AddObserver(this);
+    }
+
+    return *mTailDispatcher;
+  }
+
+  bool IsTailDispatcherAvailable() override {
+    // Our tail dispatching implementation relies on nsIThreadObserver
+    // callbacks. If we're not doing event processing, it won't work.
+    bool inEventLoop =
+        static_cast<nsThread*>(mThread.get())->RecursionDepth() > 0;
+    return inEventLoop;
+  }
+
+  bool MightHaveTailTasks() override { return !!mTailDispatcher; }
+
+  nsIEventTarget* AsEventTarget() override { return mThread; }
+
+  //-----------------------------------------------------------------------------
+  // nsIThreadObserver
+  //-----------------------------------------------------------------------------
+  NS_IMETHOD OnDispatchedEvent() override { return NS_OK; }
+
+  NS_IMETHOD AfterProcessNextEvent(nsIThreadInternal* thread,
+                                   bool eventWasProcessed) override {
+    // This is the primary case.
+    MaybeFireTailDispatcher();
+    return NS_OK;
+  }
+
+  NS_IMETHOD OnProcessNextEvent(nsIThreadInternal* thread,
+                                bool mayWait) override {
+    // In general, the tail dispatcher is handled at the end of the current in
+    // AfterProcessNextEvent() above. However, if start spinning a nested event
+    // loop, it's generally better to fire the tail dispatcher before the first
+    // nested event, rather than after it. This check handles that case.
+    MaybeFireTailDispatcher();
+    return NS_OK;
+  }
+
+  //-----------------------------------------------------------------------------
+  // nsIDirectTaskDispatcher
+  //-----------------------------------------------------------------------------
+  // Forward calls to nsIDirectTaskDispatcher to the underlying nsThread object.
+  // We can't use the generated NS_FORWARD_NSIDIRECTTASKDISPATCHER macro
+  // as already_AddRefed type must be moved.
+  NS_IMETHOD DispatchDirectTask(already_AddRefed<nsIRunnable> aEvent) override {
+    return mDirectTaskDispatcher->DispatchDirectTask(std::move(aEvent));
+  }
+  NS_IMETHOD DrainDirectTasks() override {
+    return mDirectTaskDispatcher->DrainDirectTasks();
+  }
+  NS_IMETHOD HaveDirectTasks(bool* aResult) override {
+    return mDirectTaskDispatcher->HaveDirectTasks(aResult);
+  }
+
+ private:
+  const RefPtr<nsIThreadInternal> mThread;
+  const nsCOMPtr<nsIDirectTaskDispatcher> mDirectTaskDispatcher;
+  std::unique_ptr<AutoTaskDispatcher> mTailDispatcher;
+  const bool mOnThread;
+
+  ~XPCOMThreadWrapper() {
+    if (mOnThread) {
+      MOZ_DIAGNOSTIC_ASSERT(IsCurrentThreadIn(),
+                            "Must be destroyed on the thread it was created");
+      sCurrentThreadTLS.set(nullptr);
+    }
+  }
+
+  void MaybeFireTailDispatcher() {
+    if (mTailDispatcher) {
+      mTailDispatcher->DrainDirectTasks();
+      mThread->RemoveObserver(this);
+      mTailDispatcher.reset();
+    }
+  }
+
+  class Runner : public Runnable {
+   public:
+    explicit Runner(XPCOMThreadWrapper* aThread,
+                    already_AddRefed<nsIRunnable> aRunnable)
+        : Runnable("XPCOMThreadWrapper::Runner"),
+          mThread(aThread),
+          mRunnable(aRunnable) {}
+
+    NS_IMETHOD Run() override {
+      MOZ_ASSERT(mThread == AbstractThread::GetCurrent());
+      MOZ_ASSERT(mThread->IsCurrentThreadIn());
+      SerialEventTargetGuard guard(mThread);
+      AUTO_PROFILE_FOLLOWING_RUNNABLE(mRunnable);
+      return mRunnable->Run();
+    }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+    NS_IMETHOD GetName(nsACString& aName) override {
+      aName.AssignLiteral("AbstractThread::Runner");
+      if (nsCOMPtr<nsINamed> named = do_QueryInterface(mRunnable)) {
+        nsAutoCString name;
+        named->GetName(name);
+        if (!name.IsEmpty()) {
+          aName.AppendLiteral(" for ");
+          aName.Append(name);
+        }
+      }
+      return NS_OK;
+    }
+#endif
+
+   private:
+    const RefPtr<XPCOMThreadWrapper> mThread;
+    const RefPtr<nsIRunnable> mRunnable;
+  };
+};
+
+NS_IMPL_ISUPPORTS(XPCOMThreadWrapper, nsIThreadObserver,
+                  nsIDirectTaskDispatcher, nsISerialEventTarget, nsIEventTarget)
+
+NS_IMETHODIMP_(bool)
+AbstractThread::IsOnCurrentThreadInfallible() { return IsCurrentThreadIn(); }
+
+NS_IMETHODIMP
+AbstractThread::IsOnCurrentThread(bool* aResult) {
+  *aResult = IsCurrentThreadIn();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+AbstractThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+AbstractThread::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                         uint32_t aFlags) {
+  return Dispatch(std::move(aEvent), NormalDispatch);
+}
+
+NS_IMETHODIMP
+AbstractThread::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                                uint32_t aDelayMs) {
+  nsCOMPtr<nsIRunnable> event = aEvent;
+  NS_ENSURE_TRUE(!!aDelayMs, NS_ERROR_UNEXPECTED);
+
+  RefPtr<DelayedRunnable> r =
+      new DelayedRunnable(do_AddRef(this), event.forget(), aDelayMs);
+  nsresult rv = r->Init();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return Dispatch(r.forget(), NS_DISPATCH_NORMAL);
+}
+
+nsresult AbstractThread::TailDispatchTasksFor(AbstractThread* aThread) {
+  if (MightHaveTailTasks()) {
+    return TailDispatcher().DispatchTasksFor(aThread);
+  }
+
+  return NS_OK;
+}
+
+bool AbstractThread::HasTailTasksFor(AbstractThread* aThread) {
+  if (!MightHaveTailTasks()) {
+    return false;
+  }
+  return TailDispatcher().HasTasksFor(aThread);
+}
+
+bool AbstractThread::RequiresTailDispatch(AbstractThread* aThread) const {
+  MOZ_ASSERT(aThread);
+  // We require tail dispatch if both the source and destination
+  // threads support it.
+  return SupportsTailDispatch() && aThread->SupportsTailDispatch();
+}
+
+bool AbstractThread::RequiresTailDispatchFromCurrentThread() const {
+  AbstractThread* current = GetCurrent();
+  return current && RequiresTailDispatch(current);
+}
+
+AbstractThread* AbstractThread::MainThread() {
+  MOZ_ASSERT(sMainThread);
+  return sMainThread;
+}
+
+void AbstractThread::InitTLS() {
+  if (!sCurrentThreadTLS.init()) {
+    MOZ_CRASH();
+  }
+}
+
+void AbstractThread::InitMainThread() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!sMainThread);
+  nsCOMPtr<nsIThreadInternal> mainThread =
+      do_QueryInterface(nsThreadManager::get().GetMainThreadWeak());
+  MOZ_DIAGNOSTIC_ASSERT(mainThread);
+
+  if (!sCurrentThreadTLS.init()) {
+    MOZ_CRASH();
+  }
+  sMainThread = new XPCOMThreadWrapper(mainThread.get(),
+                                       /* aRequireTailDispatch = */ true,
+                                       true /* onThread */);
+}
+
+void AbstractThread::ShutdownMainThread() {
+  MOZ_ASSERT(NS_IsMainThread());
+  sMainThread = nullptr;
+}
+
+void AbstractThread::DispatchStateChange(
+    already_AddRefed<nsIRunnable> aRunnable) {
+  AbstractThread* currentThread = GetCurrent();
+  MOZ_DIAGNOSTIC_ASSERT(currentThread, "An AbstractThread must exist");
+  if (currentThread->IsTailDispatcherAvailable()) {
+    currentThread->TailDispatcher().AddStateChangeTask(this,
+                                                       std::move(aRunnable));
+  } else {
+    // If the tail dispatcher isn't available, we just avoid sending state
+    // updates.
+    //
+    // This happens, specifically (1) During async shutdown (via the media
+    // shutdown blocker), and (2) During the final shutdown cycle collection.
+    // Both of these trigger changes to various watched and mirrored state.
+    nsCOMPtr<nsIRunnable> neverDispatched = aRunnable;
+  }
+}
+
+/* static */
+void AbstractThread::DispatchDirectTask(
+    already_AddRefed<nsIRunnable> aRunnable) {
+  AbstractThread* currentThread = GetCurrent();
+  MOZ_DIAGNOSTIC_ASSERT(currentThread, "An AbstractThread must exist");
+  if (currentThread->IsTailDispatcherAvailable()) {
+    currentThread->TailDispatcher().AddDirectTask(std::move(aRunnable));
+  } else {
+    // If the tail dispatcher isn't available, we post as a regular task.
+    currentThread->Dispatch(std::move(aRunnable));
+  }
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/AbstractThread.h b/xpcom/threads/AbstractThread.h
new file mode 100644
index 0000000000..b53bcf8ca3
--- /dev/null
+++ b/xpcom/threads/AbstractThread.h
@@ -0,0 +1,129 @@
+/* -*- 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/. */
+
+#if !defined(AbstractThread_h_)
+#  define AbstractThread_h_
+
+#  include "mozilla/AlreadyAddRefed.h"
+#  include "mozilla/ThreadLocal.h"
+#  include "nscore.h"
+#  include "nsISerialEventTarget.h"
+#  include "nsISupports.h"
+
+class nsIEventTarget;
+class nsIRunnable;
+class nsIThread;
+
+namespace mozilla {
+
+class TaskDispatcher;
+
+/*
+ * We often want to run tasks on a target that guarantees that events will never
+ * run in parallel. There are various target types that achieve this - namely
+ * nsIThread and TaskQueue. Note that nsIThreadPool (which implements
+ * nsIEventTarget) does not have this property, so we do not want to use
+ * nsIEventTarget for this purpose. This class encapsulates the specifics of
+ * the structures we might use here and provides a consistent interface.
+ *
+ * At present, the supported AbstractThread implementations are TaskQueue,
+ * AbstractThread::MainThread() and XPCOMThreadWrapper which can wrap any
+ * nsThread.
+ *
+ * The primary use of XPCOMThreadWrapper is to allow any threads to provide
+ * Direct Task dispatching which is similar (but not identical to) the microtask
+ * semantics of JS promises. Instantiating a XPCOMThreadWrapper on the current
+ * nsThread is sufficient to enable direct task dispatching.
+ *
+ * You shouldn't use pointers when comparing AbstractThread or nsIThread to
+ * determine if you are currently on the thread, but instead use the
+ * nsISerialEventTarget::IsOnCurrentThread() method.
+ */
+class AbstractThread : public nsISerialEventTarget {
+ public:
+  // Returns the AbstractThread that the caller is currently running in, or null
+  // if the caller is not running in an AbstractThread.
+  static AbstractThread* GetCurrent() { return sCurrentThreadTLS.get(); }
+
+  AbstractThread(bool aSupportsTailDispatch)
+      : mSupportsTailDispatch(aSupportsTailDispatch) {}
+
+  // We don't use NS_DECL_NSIEVENTTARGET so that we can remove the default
+  // |flags| parameter from Dispatch. Otherwise, a single-argument Dispatch call
+  // would be ambiguous.
+  using nsISerialEventTarget::IsOnCurrentThread;
+  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible(void) override;
+  NS_IMETHOD IsOnCurrentThread(bool* _retval) override;
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event,
+                      uint32_t flags) override;
+  NS_IMETHOD DispatchFromScript(nsIRunnable* event, uint32_t flags) override;
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable> event,
+                             uint32_t delay) override;
+
+  enum DispatchReason { NormalDispatch, TailDispatch };
+  virtual nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                            DispatchReason aReason = NormalDispatch) = 0;
+
+  virtual bool IsCurrentThreadIn() const = 0;
+
+  // Returns a TaskDispatcher that will dispatch its tasks when the currently-
+  // running tasks pops off the stack.
+  //
+  // May only be called when running within the it is invoked up, and only on
+  // threads which support it.
+  virtual TaskDispatcher& TailDispatcher() = 0;
+
+  // Returns true if we have tail tasks scheduled, or if this isn't known.
+  // Returns false if we definitely don't have any tail tasks.
+  virtual bool MightHaveTailTasks() { return true; }
+
+  // Returns true if the tail dispatcher is available. In certain edge cases
+  // like shutdown, it might not be.
+  virtual bool IsTailDispatcherAvailable() { return true; }
+
+  // Helper functions for methods on the tail TasklDispatcher. These check
+  // HasTailTasks to avoid allocating a TailDispatcher if it isn't
+  // needed.
+  nsresult TailDispatchTasksFor(AbstractThread* aThread);
+  bool HasTailTasksFor(AbstractThread* aThread);
+
+  // Returns true if this supports the tail dispatcher.
+  bool SupportsTailDispatch() const { return mSupportsTailDispatch; }
+
+  // Returns true if this thread requires all dispatches originating from
+  // aThread go through the tail dispatcher.
+  bool RequiresTailDispatch(AbstractThread* aThread) const;
+  bool RequiresTailDispatchFromCurrentThread() const;
+
+  virtual nsIEventTarget* AsEventTarget() { MOZ_CRASH("Not an event target!"); }
+
+  // Returns the non-DocGroup version of AbstractThread on the main thread.
+  // A DocGroup-versioned one is available in
+  // DispatcherTrait::AbstractThreadFor(). Note:
+  // DispatcherTrait::AbstractThreadFor() SHALL be used when possible.
+  static AbstractThread* MainThread();
+
+  // Must be called exactly once during startup.
+  static void InitTLS();
+  static void InitMainThread();
+  static void ShutdownMainThread();
+
+  void DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable);
+
+  static void DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable);
+
+ protected:
+  virtual ~AbstractThread() = default;
+  static MOZ_THREAD_LOCAL(AbstractThread*) sCurrentThreadTLS;
+
+  // True if we want to require that every task dispatched from tasks running in
+  // this queue go through our queue's tail dispatcher.
+  const bool mSupportsTailDispatch;
+};
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/BlockingResourceBase.cpp b/xpcom/threads/BlockingResourceBase.cpp
new file mode 100644
index 0000000000..c2ba82e07a
--- /dev/null
+++ b/xpcom/threads/BlockingResourceBase.cpp
@@ -0,0 +1,547 @@
+/* -*- 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/BlockingResourceBase.h"
+
+#ifdef DEBUG
+#  include "prthread.h"
+
+#  ifndef MOZ_CALLSTACK_DISABLED
+#    include "CodeAddressService.h"
+#    include "nsHashKeys.h"
+#    include "mozilla/StackWalk.h"
+#    include "nsTHashtable.h"
+#  endif
+
+#  include "mozilla/Attributes.h"
+#  include "mozilla/CondVar.h"
+#  include "mozilla/DeadlockDetector.h"
+#  include "mozilla/RecursiveMutex.h"
+#  include "mozilla/ReentrantMonitor.h"
+#  include "mozilla/Mutex.h"
+#  include "mozilla/RWLock.h"
+#  include "mozilla/UniquePtr.h"
+
+#  if defined(MOZILLA_INTERNAL_API)
+#    include "mozilla/ProfilerThreadSleep.h"
+#  endif  // MOZILLA_INTERNAL_API
+
+#endif  // ifdef DEBUG
+
+namespace mozilla {
+//
+// BlockingResourceBase implementation
+//
+
+// static members
+const char* const BlockingResourceBase::kResourceTypeName[] = {
+    // needs to be kept in sync with BlockingResourceType
+    "Mutex", "ReentrantMonitor", "CondVar", "RecursiveMutex"};
+
+#ifdef DEBUG
+
+PRCallOnceType BlockingResourceBase::sCallOnce;
+MOZ_THREAD_LOCAL(BlockingResourceBase*)
+BlockingResourceBase::sResourceAcqnChainFront;
+BlockingResourceBase::DDT* BlockingResourceBase::sDeadlockDetector;
+
+void BlockingResourceBase::StackWalkCallback(uint32_t aFrameNumber, void* aPc,
+                                             void* aSp, void* aClosure) {
+#  ifndef MOZ_CALLSTACK_DISABLED
+  AcquisitionState* state = (AcquisitionState*)aClosure;
+  state->ref().AppendElement(aPc);
+#  endif
+}
+
+void BlockingResourceBase::GetStackTrace(AcquisitionState& aState,
+                                         const void* aFirstFramePC) {
+#  ifndef MOZ_CALLSTACK_DISABLED
+  // Clear the array...
+  aState.reset();
+  // ...and create a new one; this also puts the state to 'acquired' status
+  // regardless of whether we obtain a stack trace or not.
+  aState.emplace();
+
+  MozStackWalk(StackWalkCallback, aFirstFramePC, kAcquisitionStateStackSize,
+               aState.ptr());
+#  endif
+}
+
+/**
+ * PrintCycle
+ * Append to |aOut| detailed information about the circular
+ * dependency in |aCycle|.  Returns true if it *appears* that this
+ * cycle may represent an imminent deadlock, but this is merely a
+ * heuristic; the value returned may be a false positive or false
+ * negative.
+ *
+ * *NOT* thread safe.  Calls |Print()|.
+ *
+ * FIXME bug 456272 hack alert: because we can't write call
+ * contexts into strings, all info is written to stderr, but only
+ * some info is written into |aOut|
+ */
+static bool PrintCycle(
+    const BlockingResourceBase::DDT::ResourceAcquisitionArray& aCycle,
+    nsACString& aOut) {
+  NS_ASSERTION(aCycle.Length() > 1, "need > 1 element for cycle!");
+
+  bool maybeImminent = true;
+
+  fputs("=== Cyclical dependency starts at\n", stderr);
+  aOut += "Cyclical dependency starts at\n";
+
+  const BlockingResourceBase::DDT::ResourceAcquisitionArray::value_type res =
+      aCycle.ElementAt(0);
+  maybeImminent &= res->Print(aOut);
+
+  BlockingResourceBase::DDT::ResourceAcquisitionArray::index_type i;
+  BlockingResourceBase::DDT::ResourceAcquisitionArray::size_type len =
+      aCycle.Length();
+  const BlockingResourceBase::DDT::ResourceAcquisitionArray::value_type* it =
+      1 + aCycle.Elements();
+  for (i = 1; i < len - 1; ++i, ++it) {
+    fputs("\n--- Next dependency:\n", stderr);
+    aOut += "\nNext dependency:\n";
+
+    maybeImminent &= (*it)->Print(aOut);
+  }
+
+  fputs("\n=== Cycle completed at\n", stderr);
+  aOut += "Cycle completed at\n";
+  (*it)->Print(aOut);
+
+  return maybeImminent;
+}
+
+bool BlockingResourceBase::Print(nsACString& aOut) const {
+  fprintf(stderr, "--- %s : %s", kResourceTypeName[mType], mName);
+  aOut += BlockingResourceBase::kResourceTypeName[mType];
+  aOut += " : ";
+  aOut += mName;
+
+  bool acquired = IsAcquired();
+
+  if (acquired) {
+    fputs(" (currently acquired)\n", stderr);
+    aOut += " (currently acquired)\n";
+  }
+
+  fputs(" calling context\n", stderr);
+#  ifdef MOZ_CALLSTACK_DISABLED
+  fputs("  [stack trace unavailable]\n", stderr);
+#  else
+  const AcquisitionState& state = acquired ? mAcquired : mFirstSeen;
+
+  CodeAddressService<> addressService;
+
+  for (uint32_t i = 0; i < state.ref().Length(); i++) {
+    const size_t kMaxLength = 1024;
+    char buffer[kMaxLength];
+    addressService.GetLocation(i + 1, state.ref()[i], buffer, kMaxLength);
+    const char* fmt = "    %s\n";
+    aOut.AppendLiteral("    ");
+    aOut.Append(buffer);
+    aOut.AppendLiteral("\n");
+    fprintf(stderr, fmt, buffer);
+  }
+
+#  endif
+
+  return acquired;
+}
+
+BlockingResourceBase::BlockingResourceBase(
+    const char* aName, BlockingResourceBase::BlockingResourceType aType)
+    : mName(aName),
+      mType(aType)
+#  ifdef MOZ_CALLSTACK_DISABLED
+      ,
+      mAcquired(false)
+#  else
+      ,
+      mAcquired()
+#  endif
+{
+  MOZ_ASSERT(mName, "Name must be nonnull");
+  // PR_CallOnce guaranatees that InitStatics is called in a
+  // thread-safe way
+  if (PR_SUCCESS != PR_CallOnce(&sCallOnce, InitStatics)) {
+    MOZ_CRASH("can't initialize blocking resource static members");
+  }
+
+  mChainPrev = 0;
+  sDeadlockDetector->Add(this);
+}
+
+BlockingResourceBase::~BlockingResourceBase() {
+  // we don't check for really obviously bad things like freeing
+  // Mutexes while they're still locked.  it is assumed that the
+  // base class, or its underlying primitive, will check for such
+  // stupid mistakes.
+  mChainPrev = 0;  // racy only for stupidly buggy client code
+  if (sDeadlockDetector) {
+    sDeadlockDetector->Remove(this);
+  }
+}
+
+size_t BlockingResourceBase::SizeOfDeadlockDetector(
+    MallocSizeOf aMallocSizeOf) {
+  return sDeadlockDetector
+             ? sDeadlockDetector->SizeOfIncludingThis(aMallocSizeOf)
+             : 0;
+}
+
+PRStatus BlockingResourceBase::InitStatics() {
+  MOZ_ASSERT(sResourceAcqnChainFront.init());
+  sDeadlockDetector = new DDT();
+  if (!sDeadlockDetector) {
+    MOZ_CRASH("can't allocate deadlock detector");
+  }
+  return PR_SUCCESS;
+}
+
+void BlockingResourceBase::Shutdown() {
+  delete sDeadlockDetector;
+  sDeadlockDetector = 0;
+}
+
+MOZ_NEVER_INLINE void BlockingResourceBase::CheckAcquire() {
+  if (mType == eCondVar) {
+    MOZ_ASSERT_UNREACHABLE(
+        "FIXME bug 456272: annots. to allow CheckAcquire()ing condvars");
+    return;
+  }
+
+  BlockingResourceBase* chainFront = ResourceChainFront();
+  mozilla::UniquePtr<DDT::ResourceAcquisitionArray> cycle(
+      sDeadlockDetector->CheckAcquisition(chainFront ? chainFront : 0, this));
+  if (!cycle) {
+    return;
+  }
+
+#  ifndef MOZ_CALLSTACK_DISABLED
+  // Update the current stack before printing.
+  GetStackTrace(mAcquired, CallerPC());
+#  endif
+
+  fputs("###!!! ERROR: Potential deadlock detected:\n", stderr);
+  nsAutoCString out("Potential deadlock detected:\n");
+  bool maybeImminent = PrintCycle(*cycle, out);
+
+  if (maybeImminent) {
+    fputs("\n###!!! Deadlock may happen NOW!\n\n", stderr);
+    out.AppendLiteral("\n###!!! Deadlock may happen NOW!\n\n");
+  } else {
+    fputs("\nDeadlock may happen for some other execution\n\n", stderr);
+    out.AppendLiteral("\nDeadlock may happen for some other execution\n\n");
+  }
+
+  // Only error out if we think a deadlock is imminent.
+  if (maybeImminent) {
+    NS_ERROR(out.get());
+  } else {
+    NS_WARNING(out.get());
+  }
+}
+
+MOZ_NEVER_INLINE void BlockingResourceBase::Acquire() {
+  if (mType == eCondVar) {
+    MOZ_ASSERT_UNREACHABLE(
+        "FIXME bug 456272: annots. to allow Acquire()ing condvars");
+    return;
+  }
+  NS_ASSERTION(!IsAcquired(), "reacquiring already acquired resource");
+
+  ResourceChainAppend(ResourceChainFront());
+
+#  ifdef MOZ_CALLSTACK_DISABLED
+  mAcquired = true;
+#  else
+  // Take a stack snapshot.
+  GetStackTrace(mAcquired, CallerPC());
+  MOZ_ASSERT(IsAcquired());
+
+  if (!mFirstSeen) {
+    mFirstSeen = mAcquired.map(
+        [](AcquisitionState::ValueType& state) { return state.Clone(); });
+  }
+#  endif
+}
+
+void BlockingResourceBase::Release() {
+  if (mType == eCondVar) {
+    MOZ_ASSERT_UNREACHABLE(
+        "FIXME bug 456272: annots. to allow Release()ing condvars");
+    return;
+  }
+
+  BlockingResourceBase* chainFront = ResourceChainFront();
+  NS_ASSERTION(chainFront && IsAcquired(),
+               "Release()ing something that hasn't been Acquire()ed");
+
+  if (chainFront == this) {
+    ResourceChainRemove();
+  } else {
+    // remove this resource from wherever it lives in the chain
+    // we walk backwards in order of acquisition:
+    //  (1)  ...node<-prev<-curr...
+    //              /     /
+    //  (2)  ...prev<-curr...
+    BlockingResourceBase* curr = chainFront;
+    BlockingResourceBase* prev = nullptr;
+    while (curr && (prev = curr->mChainPrev) && (prev != this)) {
+      curr = prev;
+    }
+    if (prev == this) {
+      curr->mChainPrev = prev->mChainPrev;
+    }
+  }
+
+  ClearAcquisitionState();
+}
+
+//
+// Debug implementation of (OffTheBooks)Mutex
+void OffTheBooksMutex::Lock() {
+  CheckAcquire();
+  this->lock();
+  mOwningThread = PR_GetCurrentThread();
+  Acquire();
+}
+
+bool OffTheBooksMutex::TryLock() {
+  bool locked = this->tryLock();
+  if (locked) {
+    mOwningThread = PR_GetCurrentThread();
+    Acquire();
+  }
+  return locked;
+}
+
+void OffTheBooksMutex::Unlock() {
+  Release();
+  mOwningThread = nullptr;
+  this->unlock();
+}
+
+void OffTheBooksMutex::AssertCurrentThreadOwns() const {
+  MOZ_ASSERT(IsAcquired() && mOwningThread == PR_GetCurrentThread());
+}
+
+//
+// Debug implementation of RWLock
+//
+
+bool RWLock::TryReadLock() {
+  bool locked = this->detail::RWLockImpl::tryReadLock();
+  MOZ_ASSERT_IF(locked, mOwningThread == nullptr);
+  return locked;
+}
+
+void RWLock::ReadLock() {
+  // All we want to ensure here is that we're not attempting to acquire the
+  // read lock while this thread is holding the write lock.
+  CheckAcquire();
+  this->detail::RWLockImpl::readLock();
+  MOZ_ASSERT(mOwningThread == nullptr);
+}
+
+void RWLock::ReadUnlock() {
+  MOZ_ASSERT(mOwningThread == nullptr);
+  this->detail::RWLockImpl::readUnlock();
+}
+
+bool RWLock::TryWriteLock() {
+  bool locked = this->detail::RWLockImpl::tryWriteLock();
+  if (locked) {
+    mOwningThread = PR_GetCurrentThread();
+    Acquire();
+  }
+  return locked;
+}
+
+void RWLock::WriteLock() {
+  CheckAcquire();
+  this->detail::RWLockImpl::writeLock();
+  mOwningThread = PR_GetCurrentThread();
+  Acquire();
+}
+
+void RWLock::WriteUnlock() {
+  Release();
+  mOwningThread = nullptr;
+  this->detail::RWLockImpl::writeUnlock();
+}
+
+//
+// Debug implementation of ReentrantMonitor
+void ReentrantMonitor::Enter() {
+  BlockingResourceBase* chainFront = ResourceChainFront();
+
+  // the code below implements monitor reentrancy semantics
+
+  if (this == chainFront) {
+    // immediately re-entered the monitor: acceptable
+    PR_EnterMonitor(mReentrantMonitor);
+    ++mEntryCount;
+    return;
+  }
+
+  // this is sort of a hack around not recording the thread that
+  // owns this monitor
+  if (chainFront) {
+    for (BlockingResourceBase* br = ResourceChainPrev(chainFront); br;
+         br = ResourceChainPrev(br)) {
+      if (br == this) {
+        NS_WARNING(
+            "Re-entering ReentrantMonitor after acquiring other resources.");
+
+        // show the caller why this is potentially bad
+        CheckAcquire();
+
+        PR_EnterMonitor(mReentrantMonitor);
+        ++mEntryCount;
+        return;
+      }
+    }
+  }
+
+  CheckAcquire();
+  PR_EnterMonitor(mReentrantMonitor);
+  NS_ASSERTION(mEntryCount == 0, "ReentrantMonitor isn't free!");
+  Acquire();  // protected by mReentrantMonitor
+  mEntryCount = 1;
+}
+
+void ReentrantMonitor::Exit() {
+  if (--mEntryCount == 0) {
+    Release();  // protected by mReentrantMonitor
+  }
+  PRStatus status = PR_ExitMonitor(mReentrantMonitor);
+  NS_ASSERTION(PR_SUCCESS == status, "bad ReentrantMonitor::Exit()");
+}
+
+nsresult ReentrantMonitor::Wait(PRIntervalTime aInterval) {
+  AssertCurrentThreadIn();
+
+  // save monitor state and reset it to empty
+  int32_t savedEntryCount = mEntryCount;
+  AcquisitionState savedAcquisitionState = TakeAcquisitionState();
+  BlockingResourceBase* savedChainPrev = mChainPrev;
+  mEntryCount = 0;
+  mChainPrev = 0;
+
+  nsresult rv;
+  {
+#  if defined(MOZILLA_INTERNAL_API)
+    AUTO_PROFILER_THREAD_SLEEP;
+#  endif
+    // give up the monitor until we're back from Wait()
+    rv = PR_Wait(mReentrantMonitor, aInterval) == PR_SUCCESS ? NS_OK
+                                                             : NS_ERROR_FAILURE;
+  }
+
+  // restore saved state
+  mEntryCount = savedEntryCount;
+  SetAcquisitionState(std::move(savedAcquisitionState));
+  mChainPrev = savedChainPrev;
+
+  return rv;
+}
+
+//
+// Debug implementation of RecursiveMutex
+void RecursiveMutex::Lock() {
+  BlockingResourceBase* chainFront = ResourceChainFront();
+
+  // the code below implements mutex reentrancy semantics
+
+  if (this == chainFront) {
+    // immediately re-entered the mutex: acceptable
+    LockInternal();
+    ++mEntryCount;
+    return;
+  }
+
+  // this is sort of a hack around not recording the thread that
+  // owns this monitor
+  if (chainFront) {
+    for (BlockingResourceBase* br = ResourceChainPrev(chainFront); br;
+         br = ResourceChainPrev(br)) {
+      if (br == this) {
+        NS_WARNING(
+            "Re-entering RecursiveMutex after acquiring other resources.");
+
+        // show the caller why this is potentially bad
+        CheckAcquire();
+
+        LockInternal();
+        ++mEntryCount;
+        return;
+      }
+    }
+  }
+
+  CheckAcquire();
+  LockInternal();
+  NS_ASSERTION(mEntryCount == 0, "RecursiveMutex isn't free!");
+  Acquire();  // protected by us
+  mOwningThread = PR_GetCurrentThread();
+  mEntryCount = 1;
+}
+
+void RecursiveMutex::Unlock() {
+  if (--mEntryCount == 0) {
+    Release();  // protected by us
+    mOwningThread = nullptr;
+  }
+  UnlockInternal();
+}
+
+void RecursiveMutex::AssertCurrentThreadIn() const {
+  MOZ_ASSERT(IsAcquired() && mOwningThread == PR_GetCurrentThread());
+}
+
+//
+// Debug implementation of CondVar
+void OffTheBooksCondVar::Wait() {
+  // Forward to the timed version of OffTheBooksCondVar::Wait to avoid code
+  // duplication.
+  CVStatus status = Wait(TimeDuration::Forever());
+  MOZ_ASSERT(status == CVStatus::NoTimeout);
+}
+
+CVStatus OffTheBooksCondVar::Wait(TimeDuration aDuration) {
+  AssertCurrentThreadOwnsMutex();
+
+  // save mutex state and reset to empty
+  AcquisitionState savedAcquisitionState = mLock->TakeAcquisitionState();
+  BlockingResourceBase* savedChainPrev = mLock->mChainPrev;
+  PRThread* savedOwningThread = mLock->mOwningThread;
+  mLock->mChainPrev = 0;
+  mLock->mOwningThread = nullptr;
+
+  // give up mutex until we're back from Wait()
+  CVStatus status;
+  {
+#  if defined(MOZILLA_INTERNAL_API)
+    AUTO_PROFILER_THREAD_SLEEP;
+#  endif
+    status = mImpl.wait_for(*mLock, aDuration);
+  }
+
+  // restore saved state
+  mLock->SetAcquisitionState(std::move(savedAcquisitionState));
+  mLock->mChainPrev = savedChainPrev;
+  mLock->mOwningThread = savedOwningThread;
+
+  return status;
+}
+
+#endif  // ifdef DEBUG
+
+}  // namespace mozilla
diff --git a/xpcom/threads/BlockingResourceBase.h b/xpcom/threads/BlockingResourceBase.h
new file mode 100644
index 0000000000..8bb7a78f6f
--- /dev/null
+++ b/xpcom/threads/BlockingResourceBase.h
@@ -0,0 +1,339 @@
+/* -*- 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_BlockingResourceBase_h
+#define mozilla_BlockingResourceBase_h
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/ThreadLocal.h"
+#include "mozilla/Attributes.h"
+
+#include "nscore.h"
+#include "nsDebug.h"
+
+#include "prtypes.h"
+
+#ifdef DEBUG
+
+// NB: Comment this out to enable callstack tracking.
+#  define MOZ_CALLSTACK_DISABLED
+
+#  include "prinit.h"
+
+#  ifndef MOZ_CALLSTACK_DISABLED
+#    include "mozilla/Maybe.h"
+#    include "nsTArray.h"
+#  endif
+
+#endif
+
+//
+// This header is not meant to be included by client code.
+//
+
+namespace mozilla {
+
+#ifdef DEBUG
+template <class T>
+class DeadlockDetector;
+#endif
+
+/**
+ * BlockingResourceBase
+ * Base class of resources that might block clients trying to acquire them.
+ * Does debugging and deadlock detection in DEBUG builds.
+ **/
+class BlockingResourceBase {
+ public:
+  // Needs to be kept in sync with kResourceTypeNames.
+  enum BlockingResourceType {
+    eMutex,
+    eReentrantMonitor,
+    eCondVar,
+    eRecursiveMutex
+  };
+
+  /**
+   * kResourceTypeName
+   * Human-readable version of BlockingResourceType enum.
+   */
+  static const char* const kResourceTypeName[];
+
+#ifdef DEBUG
+
+  static size_t SizeOfDeadlockDetector(MallocSizeOf aMallocSizeOf);
+
+  /**
+   * Print
+   * Write a description of this blocking resource to |aOut|.  If
+   * the resource appears to be currently acquired, the current
+   * acquisition context is printed and true is returned.
+   * Otherwise, we print the context from |aFirstSeen|, the
+   * first acquisition from which the code calling |Print()|
+   * became interested in us, and return false.
+   *
+   * *NOT* thread safe.  Reads |mAcquisitionContext| without
+   * synchronization, but this will not cause correctness
+   * problems.
+   *
+   * FIXME bug 456272: hack alert: because we can't write call
+   * contexts into strings, all info is written to stderr, but
+   * only some info is written into |aOut|
+   */
+  bool Print(nsACString& aOut) const;
+
+  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    // NB: |mName| is not reported as it's expected to be a static string.
+    //     If we switch to a nsString it should be added to the tally.
+    //     |mChainPrev| is not reported because its memory is not owned.
+    size_t n = aMallocSizeOf(this);
+    return n;
+  }
+
+  // ``DDT'' = ``Deadlock Detector Type''
+  typedef DeadlockDetector<BlockingResourceBase> DDT;
+
+ protected:
+#  ifdef MOZ_CALLSTACK_DISABLED
+  typedef bool AcquisitionState;
+#  else
+  // Using maybe to use emplacement as the acquisition state flag; we may not
+  // always get a stack trace because of possible stack walk suppression or
+  // errors, hence can't use !IsEmpty() on the array itself as indication.
+  static size_t const kAcquisitionStateStackSize = 24;
+  typedef Maybe<AutoTArray<void*, kAcquisitionStateStackSize> >
+      AcquisitionState;
+#  endif
+
+  /**
+   * BlockingResourceBase
+   * Initialize this blocking resource.  Also hooks the resource into
+   * instrumentation code.
+   *
+   * Thread safe.
+   *
+   * @param aName A meaningful, unique name that can be used in
+   *              error messages, et al.
+   * @param aType The specific type of |this|, if any.
+   **/
+  BlockingResourceBase(const char* aName, BlockingResourceType aType);
+
+  ~BlockingResourceBase();
+
+  /**
+   * CheckAcquire
+   *
+   * Thread safe.
+   **/
+  void CheckAcquire();
+
+  /**
+   * Acquire
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   **/
+  void Acquire();  // NS_NEEDS_RESOURCE(this)
+
+  /**
+   * Release
+   * Remove this resource from the current thread's acquisition chain.
+   * The resource does not have to be at the front of the chain, although
+   * it is confusing to release resources in a different order than they
+   * are acquired.  This generates a warning.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   **/
+  void Release();  // NS_NEEDS_RESOURCE(this)
+
+  /**
+   * ResourceChainFront
+   *
+   * Thread safe.
+   *
+   * @return the front of the resource acquisition chain, i.e., the last
+   *         resource acquired.
+   */
+  static BlockingResourceBase* ResourceChainFront() {
+    return sResourceAcqnChainFront.get();
+  }
+
+  /**
+   * ResourceChainPrev
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  static BlockingResourceBase* ResourceChainPrev(
+      const BlockingResourceBase* aResource) {
+    return aResource->mChainPrev;
+  }  // NS_NEEDS_RESOURCE(this)
+
+  /**
+   * ResourceChainAppend
+   * Set |this| to the front of the resource acquisition chain, and link
+   * |this| to |aPrev|.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  void ResourceChainAppend(BlockingResourceBase* aPrev) {
+    mChainPrev = aPrev;
+    sResourceAcqnChainFront.set(this);
+  }  // NS_NEEDS_RESOURCE(this)
+
+  /**
+   * ResourceChainRemove
+   * Remove |this| from the front of the resource acquisition chain.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  void ResourceChainRemove() {
+    NS_ASSERTION(this == ResourceChainFront(), "not at chain front");
+    sResourceAcqnChainFront.set(mChainPrev);
+  }  // NS_NEEDS_RESOURCE(this)
+
+  /**
+   * TakeAcquisitionState
+   * Return whether or not this resource was acquired and mark the resource
+   * as not acquired for subsequent uses.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  AcquisitionState TakeAcquisitionState() {
+#  ifdef MOZ_CALLSTACK_DISABLED
+    bool acquired = mAcquired;
+    ClearAcquisitionState();
+    return acquired;
+#  else
+    return mAcquired.take();
+#  endif
+  }
+
+  /**
+   * SetAcquisitionState
+   * Set whether or not this resource was acquired.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  void SetAcquisitionState(AcquisitionState&& aAcquisitionState) {
+    mAcquired = std::move(aAcquisitionState);
+  }
+
+  /**
+   * ClearAcquisitionState
+   * Indicate this resource is not acquired.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  void ClearAcquisitionState() {
+#  ifdef MOZ_CALLSTACK_DISABLED
+    mAcquired = false;
+#  else
+    mAcquired.reset();
+#  endif
+  }
+
+  /**
+   * IsAcquired
+   * Indicates if this resource is acquired.
+   *
+   * *NOT* thread safe.  Requires ownership of underlying resource.
+   */
+  bool IsAcquired() const { return (bool)mAcquired; }
+
+  /**
+   * mChainPrev
+   * A series of resource acquisitions creates a chain of orders.  This
+   * chain is implemented as a linked list; |mChainPrev| points to the
+   * resource most recently Acquire()'d before this one.
+   **/
+  BlockingResourceBase* mChainPrev;
+
+ private:
+  /**
+   * mName
+   * A descriptive name for this resource.  Used in error
+   * messages etc.
+   */
+  const char* mName;
+
+  /**
+   * mType
+   * The more specific type of this resource.  Used to implement
+   * special semantics (e.g., reentrancy of monitors).
+   **/
+  BlockingResourceType mType;
+
+  /**
+   * mAcquired
+   * Indicates if this resource is currently acquired.
+   */
+  AcquisitionState mAcquired;
+
+#  ifndef MOZ_CALLSTACK_DISABLED
+  /**
+   * mFirstSeen
+   * Inidicates where this resource was first acquired.
+   */
+  AcquisitionState mFirstSeen;
+#  endif
+
+  /**
+   * sCallOnce
+   * Ensures static members are initialized only once, and in a
+   * thread-safe way.
+   */
+  static PRCallOnceType sCallOnce;
+
+  /**
+   * Thread-private pointer to the front of each thread's resource
+   * acquisition chain.
+   */
+  static MOZ_THREAD_LOCAL(BlockingResourceBase*) sResourceAcqnChainFront;
+
+  /**
+   * sDeadlockDetector
+   * Does as named.
+   */
+  static DDT* sDeadlockDetector;
+
+  /**
+   * InitStatics
+   * Inititialize static members of BlockingResourceBase that can't
+   * be statically initialized.
+   *
+   * *NOT* thread safe.
+   */
+  static PRStatus InitStatics();
+
+  /**
+   * Shutdown
+   * Free static members.
+   *
+   * *NOT* thread safe.
+   */
+  static void Shutdown();
+
+  static void StackWalkCallback(uint32_t aFrameNumber, void* aPc, void* aSp,
+                                void* aClosure);
+  static void GetStackTrace(AcquisitionState& aState,
+                            const void* aFirstFramePC);
+
+#  ifdef MOZILLA_INTERNAL_API
+  // so it can call BlockingResourceBase::Shutdown()
+  friend void LogTerm();
+#  endif  // ifdef MOZILLA_INTERNAL_API
+
+#else  // non-DEBUG implementation
+
+  BlockingResourceBase(const char* aName, BlockingResourceType aType) {}
+
+  ~BlockingResourceBase() {}
+
+#endif
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_BlockingResourceBase_h
diff --git a/xpcom/threads/CPUUsageWatcher.cpp b/xpcom/threads/CPUUsageWatcher.cpp
new file mode 100644
index 0000000000..66579b6f44
--- /dev/null
+++ b/xpcom/threads/CPUUsageWatcher.cpp
@@ -0,0 +1,253 @@
+/* -*- 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/CPUUsageWatcher.h"
+#include "mozilla/Try.h"
+
+#include "prsystem.h"
+
+#ifdef XP_MACOSX
+#  include <sys/resource.h>
+#  include <mach/clock.h>
+#  include <mach/mach_host.h>
+#endif
+
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+#  include "mozilla/BackgroundHangMonitor.h"
+#endif
+
+namespace mozilla {
+
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+
+// Even if the machine only has one processor, tolerate up to 50%
+// external CPU usage.
+static const float kTolerableExternalCPUUsageFloor = 0.5f;
+
+struct CPUStats {
+  // The average CPU usage time, which can be summed across all cores in the
+  // system, or averaged between them. Whichever it is, it needs to be in the
+  // same units as updateTime.
+  uint64_t usageTime;
+  // A monotonically increasing value in the same units as usageTime, which can
+  // be used to determine the percentage of active vs idle time
+  uint64_t updateTime;
+};
+
+#  ifdef XP_MACOSX
+
+static const uint64_t kMicrosecondsPerSecond = 1000000LL;
+static const uint64_t kNanosecondsPerMicrosecond = 1000LL;
+
+static uint64_t GetMicroseconds(timeval time) {
+  return ((uint64_t)time.tv_sec) * kMicrosecondsPerSecond +
+         (uint64_t)time.tv_usec;
+}
+
+static uint64_t GetMicroseconds(mach_timespec_t time) {
+  return ((uint64_t)time.tv_sec) * kMicrosecondsPerSecond +
+         ((uint64_t)time.tv_nsec) / kNanosecondsPerMicrosecond;
+}
+
+static Result<CPUStats, CPUUsageWatcherError> GetProcessCPUStats(
+    int32_t numCPUs) {
+  CPUStats result = {};
+  rusage usage;
+  int32_t rusageResult = getrusage(RUSAGE_SELF, &usage);
+  if (rusageResult == -1) {
+    return Err(GetProcessTimesError);
+  }
+  result.usageTime =
+      GetMicroseconds(usage.ru_utime) + GetMicroseconds(usage.ru_stime);
+
+  clock_serv_t realtimeClock;
+  kern_return_t errorResult =
+      host_get_clock_service(mach_host_self(), REALTIME_CLOCK, &realtimeClock);
+  if (errorResult != KERN_SUCCESS) {
+    return Err(GetProcessTimesError);
+  }
+  mach_timespec_t time;
+  errorResult = clock_get_time(realtimeClock, &time);
+  if (errorResult != KERN_SUCCESS) {
+    return Err(GetProcessTimesError);
+  }
+  result.updateTime = GetMicroseconds(time);
+
+  // getrusage will give us the sum of the values across all
+  // of our cores. Divide by the number of CPUs to get an average.
+  result.usageTime /= numCPUs;
+  return result;
+}
+
+static Result<CPUStats, CPUUsageWatcherError> GetGlobalCPUStats() {
+  CPUStats result = {};
+  host_cpu_load_info_data_t loadInfo;
+  mach_msg_type_number_t loadInfoCount = HOST_CPU_LOAD_INFO_COUNT;
+  kern_return_t statsResult =
+      host_statistics(mach_host_self(), HOST_CPU_LOAD_INFO,
+                      (host_info_t)&loadInfo, &loadInfoCount);
+  if (statsResult != KERN_SUCCESS) {
+    return Err(HostStatisticsError);
+  }
+
+  result.usageTime = loadInfo.cpu_ticks[CPU_STATE_USER] +
+                     loadInfo.cpu_ticks[CPU_STATE_NICE] +
+                     loadInfo.cpu_ticks[CPU_STATE_SYSTEM];
+  result.updateTime = result.usageTime + loadInfo.cpu_ticks[CPU_STATE_IDLE];
+  return result;
+}
+
+#  endif  // XP_MACOSX
+
+#  ifdef XP_WIN
+
+// A FILETIME represents the number of 100-nanosecond ticks since 1/1/1601 UTC
+uint64_t FiletimeToInteger(FILETIME filetime) {
+  return ((uint64_t)filetime.dwLowDateTime) | (uint64_t)filetime.dwHighDateTime
+                                                  << 32;
+}
+
+Result<CPUStats, CPUUsageWatcherError> GetProcessCPUStats(int32_t numCPUs) {
+  CPUStats result = {};
+  FILETIME creationFiletime;
+  FILETIME exitFiletime;
+  FILETIME kernelFiletime;
+  FILETIME userFiletime;
+  bool success = GetProcessTimes(GetCurrentProcess(), &creationFiletime,
+                                 &exitFiletime, &kernelFiletime, &userFiletime);
+  if (!success) {
+    return Err(GetProcessTimesError);
+  }
+
+  result.usageTime =
+      FiletimeToInteger(kernelFiletime) + FiletimeToInteger(userFiletime);
+
+  FILETIME nowFiletime;
+  GetSystemTimeAsFileTime(&nowFiletime);
+  result.updateTime = FiletimeToInteger(nowFiletime);
+
+  result.usageTime /= numCPUs;
+
+  return result;
+}
+
+Result<CPUStats, CPUUsageWatcherError> GetGlobalCPUStats() {
+  CPUStats result = {};
+  FILETIME idleFiletime;
+  FILETIME kernelFiletime;
+  FILETIME userFiletime;
+  bool success = GetSystemTimes(&idleFiletime, &kernelFiletime, &userFiletime);
+
+  if (!success) {
+    return Err(GetSystemTimesError);
+  }
+
+  result.usageTime =
+      FiletimeToInteger(kernelFiletime) + FiletimeToInteger(userFiletime);
+  result.updateTime = result.usageTime + FiletimeToInteger(idleFiletime);
+
+  return result;
+}
+
+#  endif  // XP_WIN
+
+Result<Ok, CPUUsageWatcherError> CPUUsageWatcher::Init() {
+  mNumCPUs = PR_GetNumberOfProcessors();
+  if (mNumCPUs <= 0) {
+    mExternalUsageThreshold = 1.0f;
+    return Err(GetNumberOfProcessorsError);
+  }
+  mExternalUsageThreshold =
+      std::max(1.0f - 1.0f / (float)mNumCPUs, kTolerableExternalCPUUsageFloor);
+
+  CPUStats processTimes;
+  MOZ_TRY_VAR(processTimes, GetProcessCPUStats(mNumCPUs));
+  mProcessUpdateTime = processTimes.updateTime;
+  mProcessUsageTime = processTimes.usageTime;
+
+  CPUStats globalTimes;
+  MOZ_TRY_VAR(globalTimes, GetGlobalCPUStats());
+  mGlobalUpdateTime = globalTimes.updateTime;
+  mGlobalUsageTime = globalTimes.usageTime;
+
+  mInitialized = true;
+
+  CPUUsageWatcher* self = this;
+  NS_DispatchToMainThread(NS_NewRunnableFunction(
+      "CPUUsageWatcher::Init",
+      [=]() { BackgroundHangMonitor::RegisterAnnotator(*self); }));
+
+  return Ok();
+}
+
+void CPUUsageWatcher::Uninit() {
+  if (mInitialized) {
+    BackgroundHangMonitor::UnregisterAnnotator(*this);
+  }
+  mInitialized = false;
+}
+
+Result<Ok, CPUUsageWatcherError> CPUUsageWatcher::CollectCPUUsage() {
+  if (!mInitialized) {
+    return Ok();
+  }
+
+  mExternalUsageRatio = 0.0f;
+
+  CPUStats processTimes;
+  MOZ_TRY_VAR(processTimes, GetProcessCPUStats(mNumCPUs));
+  CPUStats globalTimes;
+  MOZ_TRY_VAR(globalTimes, GetGlobalCPUStats());
+
+  uint64_t processUsageDelta = processTimes.usageTime - mProcessUsageTime;
+  uint64_t processUpdateDelta = processTimes.updateTime - mProcessUpdateTime;
+  float processUsageNormalized =
+      processUsageDelta > 0
+          ? (float)processUsageDelta / (float)processUpdateDelta
+          : 0.0f;
+
+  uint64_t globalUsageDelta = globalTimes.usageTime - mGlobalUsageTime;
+  uint64_t globalUpdateDelta = globalTimes.updateTime - mGlobalUpdateTime;
+  float globalUsageNormalized =
+      globalUsageDelta > 0 ? (float)globalUsageDelta / (float)globalUpdateDelta
+                           : 0.0f;
+
+  mProcessUsageTime = processTimes.usageTime;
+  mProcessUpdateTime = processTimes.updateTime;
+  mGlobalUsageTime = globalTimes.usageTime;
+  mGlobalUpdateTime = globalTimes.updateTime;
+
+  mExternalUsageRatio =
+      std::max(0.0f, globalUsageNormalized - processUsageNormalized);
+
+  return Ok();
+}
+
+void CPUUsageWatcher::AnnotateHang(BackgroundHangAnnotations& aAnnotations) {
+  if (!mInitialized) {
+    return;
+  }
+
+  if (mExternalUsageRatio > mExternalUsageThreshold) {
+    aAnnotations.AddAnnotation(u"ExternalCPUHigh"_ns, true);
+  }
+}
+
+#else  // !CPU_USAGE_WATCHER_ACTIVE
+
+Result<Ok, CPUUsageWatcherError> CPUUsageWatcher::Init() { return Ok(); }
+
+void CPUUsageWatcher::Uninit() {}
+
+Result<Ok, CPUUsageWatcherError> CPUUsageWatcher::CollectCPUUsage() {
+  return Ok();
+}
+
+void CPUUsageWatcher::AnnotateHang(BackgroundHangAnnotations& aAnnotations) {}
+
+#endif  // CPU_USAGE_WATCHER_ACTIVE
+
+}  // namespace mozilla
diff --git a/xpcom/threads/CPUUsageWatcher.h b/xpcom/threads/CPUUsageWatcher.h
new file mode 100644
index 0000000000..c3a643378a
--- /dev/null
+++ b/xpcom/threads/CPUUsageWatcher.h
@@ -0,0 +1,100 @@
+/* -*- 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_CPUUsageWatcher_h
+#define mozilla_CPUUsageWatcher_h
+
+#include <stdint.h>
+
+#include "mozilla/HangAnnotations.h"
+#include "mozilla/Result.h"
+
+// We only support OSX and Windows, because on Linux we're forced to read
+// from /proc/stat in order to get global CPU values. We would prefer to not
+// eat that cost for this.
+#if defined(NIGHTLY_BUILD) && (defined(XP_WIN) || defined(XP_MACOSX))
+#  define CPU_USAGE_WATCHER_ACTIVE
+#endif
+
+namespace mozilla {
+
+// Start error values at 1 to allow using the UnusedZero Result
+// optimization.
+enum CPUUsageWatcherError : uint8_t {
+  ClockGetTimeError = 1,
+  GetNumberOfProcessorsError,
+  GetProcessTimesError,
+  GetSystemTimesError,
+  HostStatisticsError,
+  ProcStatError,
+};
+
+namespace detail {
+
+template <>
+struct UnusedZero<CPUUsageWatcherError> : UnusedZeroEnum<CPUUsageWatcherError> {
+};
+
+}  // namespace detail
+
+class CPUUsageHangAnnotator : public BackgroundHangAnnotator {
+ public:
+};
+
+class CPUUsageWatcher : public BackgroundHangAnnotator {
+ public:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+  CPUUsageWatcher()
+      : mInitialized(false),
+        mExternalUsageThreshold(0),
+        mExternalUsageRatio(0),
+        mProcessUsageTime(0),
+        mProcessUpdateTime(0),
+        mGlobalUsageTime(0),
+        mGlobalUpdateTime(0),
+        mNumCPUs(0) {}
+#endif
+
+  Result<Ok, CPUUsageWatcherError> Init();
+
+  void Uninit();
+
+  // Updates necessary values to allow AnnotateHang to function. This must be
+  // called on some semi-regular basis, as it will calculate the mean CPU
+  // usage values between now and the last time it was called.
+  Result<Ok, CPUUsageWatcherError> CollectCPUUsage();
+
+  void AnnotateHang(BackgroundHangAnnotations& aAnnotations) final;
+
+ private:
+#ifdef CPU_USAGE_WATCHER_ACTIVE
+  bool mInitialized;
+  // The threshold above which we will mark a hang as occurring under high
+  // external CPU usage conditions
+  float mExternalUsageThreshold;
+  // The CPU usage (0-1) external to our process, averaged between the two
+  // most recent monitor thread runs
+  float mExternalUsageRatio;
+  // The total cumulative CPU usage time by our process as of the last
+  // CollectCPUUsage or Startup
+  uint64_t mProcessUsageTime;
+  // A time value in the same units as mProcessUsageTime used to
+  // determine the ratio of CPU usage time to idle time
+  uint64_t mProcessUpdateTime;
+  // The total cumulative CPU usage time by all processes as of the last
+  // CollectCPUUsage or Startup
+  uint64_t mGlobalUsageTime;
+  // A time value in the same units as mGlobalUsageTime used to
+  // determine the ratio of CPU usage time to idle time
+  uint64_t mGlobalUpdateTime;
+  // The number of virtual cores on our machine
+  uint64_t mNumCPUs;
+#endif
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_CPUUsageWatcher_h
diff --git a/xpcom/threads/CondVar.h b/xpcom/threads/CondVar.h
new file mode 100644
index 0000000000..e427fc2d9e
--- /dev/null
+++ b/xpcom/threads/CondVar.h
@@ -0,0 +1,139 @@
+/* -*- 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_CondVar_h
+#define mozilla_CondVar_h
+
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/PlatformConditionVariable.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/TimeStamp.h"
+
+#if defined(MOZILLA_INTERNAL_API) && !defined(DEBUG)
+#  include "mozilla/ProfilerThreadSleep.h"
+#endif  // defined( MOZILLA_INTERNAL_API) && !defined(DEBUG)
+
+namespace mozilla {
+
+/**
+ * Similarly to OffTheBooksMutex, OffTheBooksCondvar is identical to CondVar,
+ * except that OffTheBooksCondVar doesn't include leak checking.  Sometimes
+ * you want to intentionally "leak" a CondVar until shutdown; in these cases,
+ * OffTheBooksCondVar is for you.
+ */
+class OffTheBooksCondVar : BlockingResourceBase {
+ public:
+  /**
+   * OffTheBooksCondVar
+   *
+   * The CALLER owns |aLock|.
+   *
+   * @param aLock A Mutex to associate with this condition variable.
+   * @param aName A name which can reference this monitor
+   * @returns If failure, nullptr.
+   *          If success, a valid Monitor* which must be destroyed
+   *          by Monitor::DestroyMonitor()
+   **/
+  OffTheBooksCondVar(OffTheBooksMutex& aLock, const char* aName)
+      : BlockingResourceBase(aName, eCondVar), mLock(&aLock) {}
+
+  /**
+   * ~OffTheBooksCondVar
+   * Clean up after this OffTheBooksCondVar, but NOT its associated Mutex.
+   **/
+  ~OffTheBooksCondVar() = default;
+
+  /**
+   * Wait
+   * @see prcvar.h
+   **/
+#ifndef DEBUG
+  void Wait() {
+#  ifdef MOZILLA_INTERNAL_API
+    AUTO_PROFILER_THREAD_SLEEP;
+#  endif  // MOZILLA_INTERNAL_API
+    mImpl.wait(*mLock);
+  }
+
+  CVStatus Wait(TimeDuration aDuration) {
+#  ifdef MOZILLA_INTERNAL_API
+    AUTO_PROFILER_THREAD_SLEEP;
+#  endif  // MOZILLA_INTERNAL_API
+    return mImpl.wait_for(*mLock, aDuration);
+  }
+#else
+  // NOTE: debug impl is in BlockingResourceBase.cpp
+  void Wait();
+  CVStatus Wait(TimeDuration aDuration);
+#endif
+
+  /**
+   * Notify
+   * @see prcvar.h
+   **/
+  void Notify() { mImpl.notify_one(); }
+
+  /**
+   * NotifyAll
+   * @see prcvar.h
+   **/
+  void NotifyAll() { mImpl.notify_all(); }
+
+#ifdef DEBUG
+  /**
+   * AssertCurrentThreadOwnsMutex
+   * @see Mutex::AssertCurrentThreadOwns
+   **/
+  void AssertCurrentThreadOwnsMutex() const MOZ_ASSERT_CAPABILITY(mLock) {
+    mLock->AssertCurrentThreadOwns();
+  }
+
+  /**
+   * AssertNotCurrentThreadOwnsMutex
+   * @see Mutex::AssertNotCurrentThreadOwns
+   **/
+  void AssertNotCurrentThreadOwnsMutex() const MOZ_ASSERT_CAPABILITY(!mLock) {
+    mLock->AssertNotCurrentThreadOwns();
+  }
+
+#else
+  void AssertCurrentThreadOwnsMutex() const MOZ_ASSERT_CAPABILITY(mLock) {}
+  void AssertNotCurrentThreadOwnsMutex() const MOZ_ASSERT_CAPABILITY(!mLock) {}
+
+#endif  // ifdef DEBUG
+
+ private:
+  OffTheBooksCondVar();
+  OffTheBooksCondVar(const OffTheBooksCondVar&) = delete;
+  OffTheBooksCondVar& operator=(const OffTheBooksCondVar&) = delete;
+
+  OffTheBooksMutex* mLock;
+  detail::ConditionVariableImpl mImpl;
+};
+
+/**
+ * CondVar
+ * Vanilla condition variable.  Please don't use this unless you have a
+ * compelling reason --- Monitor provides a simpler API.
+ */
+class CondVar : public OffTheBooksCondVar {
+ public:
+  CondVar(OffTheBooksMutex& aLock, const char* aName)
+      : OffTheBooksCondVar(aLock, aName) {
+    MOZ_COUNT_CTOR(CondVar);
+  }
+
+  MOZ_COUNTED_DTOR(CondVar)
+
+ private:
+  CondVar();
+  CondVar(const CondVar&);
+  CondVar& operator=(const CondVar&);
+};
+
+}  // namespace mozilla
+
+#endif  // ifndef mozilla_CondVar_h
diff --git a/xpcom/threads/DataMutex.h b/xpcom/threads/DataMutex.h
new file mode 100644
index 0000000000..44f0a35762
--- /dev/null
+++ b/xpcom/threads/DataMutex.h
@@ -0,0 +1,130 @@
+/* -*- 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 DataMutex_h__
+#define DataMutex_h__
+
+#include <utility>
+#include "mozilla/Mutex.h"
+#include "mozilla/StaticMutex.h"
+
+namespace mozilla {
+
+// A template to wrap a type with a mutex so that accesses to the type's
+// data are required to take the lock before accessing it. This ensures
+// that a mutex is explicitly associated with the data that it protects,
+// and makes it impossible to access the data without first taking the
+// associated mutex.
+//
+// This is based on Rust's std::sync::Mutex, which operates under the
+// strategy of locking data, rather than code.
+//
+// Examples:
+//
+//    DataMutex<uint32_t> u32DataMutex(1, "u32DataMutex");
+//    auto x = u32DataMutex.Lock();
+//    *x = 4;
+//    assert(*x, 4u);
+//
+//    DataMutex<nsTArray<uint32_t>> arrayDataMutex("arrayDataMutex");
+//    auto a = arrayDataMutex.Lock();
+//    auto& x = a.ref();
+//    x.AppendElement(1u);
+//    assert(x[0], 1u);
+//
+template <typename T, typename MutexType>
+class DataMutexBase {
+ public:
+  template <typename V>
+  class MOZ_STACK_CLASS AutoLockBase {
+   public:
+    V* operator->() const& { return &ref(); }
+    V* operator->() const&& = delete;
+
+    V& operator*() const& { return ref(); }
+    V& operator*() const&& = delete;
+
+    // Like RefPtr, make this act like its underlying raw pointer type
+    // whenever it is used in a context where a raw pointer is expected.
+    operator V*() const& { return &ref(); }
+
+    // Like RefPtr, don't allow implicit conversion of temporary to raw pointer.
+    operator V*() const&& = delete;
+
+    V& ref() const& {
+      MOZ_ASSERT(mOwner);
+      return mOwner->mValue;
+    }
+    V& ref() const&& = delete;
+
+    AutoLockBase(AutoLockBase&& aOther) : mOwner(aOther.mOwner) {
+      aOther.mOwner = nullptr;
+    }
+
+    ~AutoLockBase() {
+      if (mOwner) {
+        mOwner->mMutex.Unlock();
+        mOwner = nullptr;
+      }
+    }
+
+   private:
+    friend class DataMutexBase;
+
+    AutoLockBase(const AutoLockBase& aOther) = delete;
+
+    explicit AutoLockBase(DataMutexBase<T, MutexType>* aDataMutex)
+        : mOwner(aDataMutex) {
+      MOZ_ASSERT(!!mOwner);
+      mOwner->mMutex.Lock();
+    }
+
+    DataMutexBase<T, MutexType>* mOwner;
+  };
+
+  using AutoLock = AutoLockBase<T>;
+  using ConstAutoLock = AutoLockBase<const T>;
+
+  explicit DataMutexBase(const char* aName) : mMutex(aName) {}
+
+  DataMutexBase(T&& aValue, const char* aName)
+      : mMutex(aName), mValue(std::move(aValue)) {}
+
+  AutoLock Lock() { return AutoLock(this); }
+  ConstAutoLock ConstLock() { return ConstAutoLock(this); }
+
+  const MutexType& Mutex() const { return mMutex; }
+
+ private:
+  MutexType mMutex;
+  T mValue;
+};
+
+// Craft a version of StaticMutex that takes a const char* in its ctor.
+// We need this so it works interchangeably with Mutex which requires a const
+// char* aName in its ctor.
+class StaticMutexNameless : public StaticMutex {
+ public:
+  explicit StaticMutexNameless(const char* aName) : StaticMutex() {}
+
+ private:
+  // Disallow copy construction, `=`, `new`, and `delete` like BaseStaticMutex.
+#ifdef DEBUG
+  StaticMutexNameless(StaticMutexNameless& aOther);
+#endif  // DEBUG
+  StaticMutexNameless& operator=(StaticMutexNameless* aRhs);
+  static void* operator new(size_t) noexcept(true);
+  static void operator delete(void*);
+};
+
+template <typename T>
+using DataMutex = DataMutexBase<T, Mutex>;
+template <typename T>
+using StaticDataMutex = DataMutexBase<T, StaticMutexNameless>;
+
+}  // namespace mozilla
+
+#endif  // DataMutex_h__
diff --git a/xpcom/threads/DeadlockDetector.h b/xpcom/threads/DeadlockDetector.h
new file mode 100644
index 0000000000..5c40941328
--- /dev/null
+++ b/xpcom/threads/DeadlockDetector.h
@@ -0,0 +1,359 @@
+/* -*- 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_DeadlockDetector_h
+#define mozilla_DeadlockDetector_h
+
+#include "mozilla/Attributes.h"
+
+#include <stdlib.h>
+
+#include "prlock.h"
+
+#include "nsClassHashtable.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+
+/**
+ * DeadlockDetector
+ *
+ * The following is an approximate description of how the deadlock detector
+ * works.
+ *
+ * The deadlock detector ensures that all blocking resources are
+ * acquired according to a partial order P.  One type of blocking
+ * resource is a lock.  If a lock l1 is acquired (locked) before l2,
+ * then we say that |l1 <_P l2|.  The detector flags an error if two
+ * locks l1 and l2 have an inconsistent ordering in P; that is, if
+ * both |l1 <_P l2| and |l2 <_P l1|.  This is a potential error
+ * because a thread acquiring l1,l2 according to the first order might
+ * race with a thread acquiring them according to the second order.
+ * If this happens under the right conditions, then the acquisitions
+ * will deadlock.
+ *
+ * This deadlock detector doesn't know at compile-time what P is.  So,
+ * it tries to discover the order at run time.  More precisely, it
+ * finds <i>some</i> order P, then tries to find chains of resource
+ * acquisitions that violate P.  An example acquisition sequence, and
+ * the orders they impose, is
+ *   l1.lock()   // current chain: [ l1 ]
+ *               // order: { }
+ *
+ *   l2.lock()   // current chain: [ l1, l2 ]
+ *               // order: { l1 <_P l2 }
+ *
+ *   l3.lock()   // current chain: [ l1, l2, l3 ]
+ *               // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3 }
+ *               // (note: <_P is transitive, so also |l1 <_P l3|)
+ *
+ *   l2.unlock() // current chain: [ l1, l3 ]
+ *               // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3 }
+ *               // (note: it's OK, but weird, that l2 was unlocked out
+ *               //  of order.  we still have l1 <_P l3).
+ *
+ *   l2.lock()   // current chain: [ l1, l3, l2 ]
+ *               // order: { l1 <_P l2, l2 <_P l3, l1 <_P l3,
+ *                                      l3 <_P l2 (!!!) }
+ * BEEP BEEP!  Here the detector will flag a potential error, since
+ * l2 and l3 were used inconsistently (and potentially in ways that
+ * would deadlock).
+ */
+template <typename T>
+class DeadlockDetector {
+ public:
+  typedef nsTArray<const T*> ResourceAcquisitionArray;
+
+ private:
+  struct OrderingEntry;
+  typedef nsTArray<OrderingEntry*> HashEntryArray;
+  typedef typename HashEntryArray::index_type index_type;
+  typedef typename HashEntryArray::size_type size_type;
+  static const index_type NoIndex = HashEntryArray::NoIndex;
+
+  /**
+   * Value type for the ordering table.  Contains the other
+   * resources on which an ordering constraint |key < other|
+   * exists.  The catch is that we also store the calling context at
+   * which the other resource was acquired; this improves the
+   * quality of error messages when potential deadlock is detected.
+   */
+  struct OrderingEntry {
+    explicit OrderingEntry(const T* aResource)
+        : mOrderedLT()  // FIXME bug 456272: set to empirical dep size?
+          ,
+          mExternalRefs(),
+          mResource(aResource) {}
+    ~OrderingEntry() {}
+
+    size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+      size_t n = aMallocSizeOf(this);
+      n += mOrderedLT.ShallowSizeOfExcludingThis(aMallocSizeOf);
+      n += mExternalRefs.ShallowSizeOfExcludingThis(aMallocSizeOf);
+      return n;
+    }
+
+    HashEntryArray mOrderedLT;     // this <_o Other
+    HashEntryArray mExternalRefs;  // hash entries that reference this
+    const T* mResource;
+  };
+
+  // Throwaway RAII lock to make the following code safer.
+  struct PRAutoLock {
+    explicit PRAutoLock(PRLock* aLock) : mLock(aLock) { PR_Lock(mLock); }
+    ~PRAutoLock() { PR_Unlock(mLock); }
+    PRLock* mLock;
+  };
+
+ public:
+  static const uint32_t kDefaultNumBuckets;
+
+  /**
+   * DeadlockDetector
+   * Create a new deadlock detector.
+   *
+   * @param aNumResourcesGuess Guess at approximate number of resources
+   *        that will be checked.
+   */
+  explicit DeadlockDetector(uint32_t aNumResourcesGuess = kDefaultNumBuckets)
+      : mOrdering(aNumResourcesGuess) {
+    mLock = PR_NewLock();
+    if (!mLock) {
+      MOZ_CRASH("couldn't allocate deadlock detector lock");
+    }
+  }
+
+  /**
+   * ~DeadlockDetector
+   *
+   * *NOT* thread safe.
+   */
+  ~DeadlockDetector() { PR_DestroyLock(mLock); }
+
+  size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    size_t n = aMallocSizeOf(this);
+
+    {
+      PRAutoLock _(mLock);
+      n += mOrdering.ShallowSizeOfExcludingThis(aMallocSizeOf);
+      for (const auto& data : mOrdering.Values()) {
+        // NB: Key is accounted for in the entry.
+        n += data->SizeOfIncludingThis(aMallocSizeOf);
+      }
+    }
+
+    return n;
+  }
+
+  /**
+   * Add
+   * Make the deadlock detector aware of |aResource|.
+   *
+   * WARNING: The deadlock detector owns |aResource|.
+   *
+   * Thread safe.
+   *
+   * @param aResource Resource to make deadlock detector aware of.
+   */
+  void Add(const T* aResource) {
+    PRAutoLock _(mLock);
+    mOrdering.InsertOrUpdate(aResource, MakeUnique<OrderingEntry>(aResource));
+  }
+
+  void Remove(const T* aResource) {
+    PRAutoLock _(mLock);
+
+    OrderingEntry* entry = mOrdering.Get(aResource);
+
+    // Iterate the external refs and remove the entry from them.
+    HashEntryArray& refs = entry->mExternalRefs;
+    for (index_type i = 0; i < refs.Length(); i++) {
+      refs[i]->mOrderedLT.RemoveElementSorted(entry);
+    }
+
+    // Iterate orders and remove this entry from their refs.
+    HashEntryArray& orders = entry->mOrderedLT;
+    for (index_type i = 0; i < orders.Length(); i++) {
+      orders[i]->mExternalRefs.RemoveElementSorted(entry);
+    }
+
+    // Now the entry can be safely removed.
+    mOrdering.Remove(aResource);
+  }
+
+  /**
+   * CheckAcquisition This method is called after acquiring |aLast|,
+   * but before trying to acquire |aProposed|.
+   * It determines whether actually trying to acquire |aProposed|
+   * will create problems.  It is OK if |aLast| is nullptr; this is
+   * interpreted as |aProposed| being the thread's first acquisition
+   * of its current chain.
+   *
+   * Iff acquiring |aProposed| may lead to deadlock for some thread
+   * interleaving (including the current one!), the cyclical
+   * dependency from which this was deduced is returned.  Otherwise,
+   * 0 is returned.
+   *
+   * If a potential deadlock is detected and a resource cycle is
+   * returned, it is the *caller's* responsibility to free it.
+   *
+   * Thread safe.
+   *
+   * @param aLast Last resource acquired by calling thread (or 0).
+   * @param aProposed Resource calling thread proposes to acquire.
+   */
+  ResourceAcquisitionArray* CheckAcquisition(const T* aLast,
+                                             const T* aProposed) {
+    if (!aLast) {
+      // don't check if |0 < aProposed|; just vamoose
+      return 0;
+    }
+
+    NS_ASSERTION(aProposed, "null resource");
+    PRAutoLock _(mLock);
+
+    OrderingEntry* proposed = mOrdering.Get(aProposed);
+    NS_ASSERTION(proposed, "missing ordering entry");
+
+    OrderingEntry* current = mOrdering.Get(aLast);
+    NS_ASSERTION(current, "missing ordering entry");
+
+    // this is the crux of the deadlock detector algorithm
+
+    if (current == proposed) {
+      // reflexive deadlock.  fastpath b/c InTransitiveClosure is
+      // not applicable here.
+      ResourceAcquisitionArray* cycle = new ResourceAcquisitionArray();
+      if (!cycle) {
+        MOZ_CRASH("can't allocate dep. cycle array");
+      }
+      cycle->AppendElement(current->mResource);
+      cycle->AppendElement(aProposed);
+      return cycle;
+    }
+    if (InTransitiveClosure(current, proposed)) {
+      // we've already established |aLast < aProposed|.  all is well.
+      return 0;
+    }
+    if (InTransitiveClosure(proposed, current)) {
+      // the order |aProposed < aLast| has been deduced, perhaps
+      // transitively.  we're attempting to violate that
+      // constraint by acquiring resources in the order
+      // |aLast < aProposed|, and thus we may deadlock under the
+      // right conditions.
+      ResourceAcquisitionArray* cycle = GetDeductionChain(proposed, current);
+      // show how acquiring |aProposed| would complete the cycle
+      cycle->AppendElement(aProposed);
+      return cycle;
+    }
+    // |aLast|, |aProposed| are unordered according to our
+    // poset.  this is fine, but we now need to add this
+    // ordering constraint.
+    current->mOrderedLT.InsertElementSorted(proposed);
+    proposed->mExternalRefs.InsertElementSorted(current);
+    return 0;
+  }
+
+  /**
+   * Return true iff |aTarget| is in the transitive closure of |aStart|
+   * over the ordering relation `<_this'.
+   *
+   * @precondition |aStart != aTarget|
+   */
+  bool InTransitiveClosure(const OrderingEntry* aStart,
+                           const OrderingEntry* aTarget) const {
+    // NB: Using a static comparator rather than default constructing one shows
+    //     a 9% improvement in scalability tests on some systems.
+    static nsDefaultComparator<const OrderingEntry*, const OrderingEntry*> comp;
+    if (aStart->mOrderedLT.BinaryIndexOf(aTarget, comp) != NoIndex) {
+      return true;
+    }
+
+    index_type i = 0;
+    size_type len = aStart->mOrderedLT.Length();
+    for (auto it = aStart->mOrderedLT.Elements(); i < len; ++i, ++it) {
+      if (InTransitiveClosure(*it, aTarget)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Return an array of all resource acquisitions
+   *   aStart <_this r1 <_this r2 <_ ... <_ aTarget
+   * from which |aStart <_this aTarget| was deduced, including
+   * |aStart| and |aTarget|.
+   *
+   * Nb: there may be multiple deductions of |aStart <_this
+   * aTarget|.  This function returns the first ordering found by
+   * depth-first search.
+   *
+   * Nb: |InTransitiveClosure| could be replaced by this function.
+   * However, this one is more expensive because we record the DFS
+   * search stack on the heap whereas the other doesn't.
+   *
+   * @precondition |aStart != aTarget|
+   */
+  ResourceAcquisitionArray* GetDeductionChain(const OrderingEntry* aStart,
+                                              const OrderingEntry* aTarget) {
+    ResourceAcquisitionArray* chain = new ResourceAcquisitionArray();
+    if (!chain) {
+      MOZ_CRASH("can't allocate dep. cycle array");
+    }
+    chain->AppendElement(aStart->mResource);
+
+    NS_ASSERTION(GetDeductionChain_Helper(aStart, aTarget, chain),
+                 "GetDeductionChain called when there's no deadlock");
+    return chain;
+  }
+
+  // precondition: |aStart != aTarget|
+  // invariant: |aStart| is the last element in |aChain|
+  bool GetDeductionChain_Helper(const OrderingEntry* aStart,
+                                const OrderingEntry* aTarget,
+                                ResourceAcquisitionArray* aChain) {
+    if (aStart->mOrderedLT.BinaryIndexOf(aTarget) != NoIndex) {
+      aChain->AppendElement(aTarget->mResource);
+      return true;
+    }
+
+    index_type i = 0;
+    size_type len = aStart->mOrderedLT.Length();
+    for (auto it = aStart->mOrderedLT.Elements(); i < len; ++i, ++it) {
+      aChain->AppendElement((*it)->mResource);
+      if (GetDeductionChain_Helper(*it, aTarget, aChain)) {
+        return true;
+      }
+      aChain->RemoveLastElement();
+    }
+    return false;
+  }
+
+  /**
+   * The partial order on resource acquisitions used by the deadlock
+   * detector.
+   */
+  nsClassHashtable<nsPtrHashKey<const T>, OrderingEntry> mOrdering;
+
+  /**
+   * Protects contentious methods.
+   * Nb: can't use mozilla::Mutex since we are used as its deadlock
+   * detector.
+   */
+  PRLock* mLock;
+
+ private:
+  DeadlockDetector(const DeadlockDetector& aDD) = delete;
+  DeadlockDetector& operator=(const DeadlockDetector& aDD) = delete;
+};
+
+template <typename T>
+// FIXME bug 456272: tune based on average workload
+const uint32_t DeadlockDetector<T>::kDefaultNumBuckets = 32;
+
+}  // namespace mozilla
+
+#endif  // ifndef mozilla_DeadlockDetector_h
diff --git a/xpcom/threads/DelayedRunnable.cpp b/xpcom/threads/DelayedRunnable.cpp
new file mode 100644
index 0000000000..a9231442a7
--- /dev/null
+++ b/xpcom/threads/DelayedRunnable.cpp
@@ -0,0 +1,113 @@
+/* -*- 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 "DelayedRunnable.h"
+
+#include "mozilla/ProfilerRunnable.h"
+
+namespace mozilla {
+
+DelayedRunnable::DelayedRunnable(already_AddRefed<nsISerialEventTarget> aTarget,
+                                 already_AddRefed<nsIRunnable> aRunnable,
+                                 uint32_t aDelay)
+    : mozilla::Runnable("DelayedRunnable"),
+      mTarget(aTarget),
+      mDelayedFrom(TimeStamp::NowLoRes()),
+      mDelay(aDelay),
+      mWrappedRunnable(aRunnable) {}
+
+nsresult DelayedRunnable::Init() {
+  MutexAutoLock lock(mMutex);
+  if (!mWrappedRunnable) {
+    MOZ_ASSERT_UNREACHABLE();
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  nsresult rv = mTarget->RegisterShutdownTask(this);
+  if (NS_FAILED(rv)) {
+    MOZ_DIAGNOSTIC_ASSERT(
+        rv == NS_ERROR_UNEXPECTED,
+        "DelayedRunnable target must support RegisterShutdownTask");
+    NS_WARNING("DelayedRunnable init after target is shutdown");
+    return rv;
+  }
+
+  rv = NS_NewTimerWithCallback(getter_AddRefs(mTimer), this, mDelay,
+                               nsITimer::TYPE_ONE_SHOT, mTarget);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    mTarget->UnregisterShutdownTask(this);
+  }
+  return rv;
+}
+
+NS_IMETHODIMP DelayedRunnable::Run() {
+  MOZ_ASSERT(mTarget->IsOnCurrentThread());
+
+  nsCOMPtr<nsIRunnable> runnable;
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mTimer, "Init() must have been called");
+
+    // Already ran?
+    if (!mWrappedRunnable) {
+      return NS_OK;
+    }
+
+    // Are we too early?
+    if ((mozilla::TimeStamp::NowLoRes() - mDelayedFrom).ToMilliseconds() <
+        mDelay) {
+      return NS_OK;  // Let the nsITimer run us.
+    }
+
+    mTimer->Cancel();
+    mTarget->UnregisterShutdownTask(this);
+    runnable = mWrappedRunnable.forget();
+  }
+
+  AUTO_PROFILE_FOLLOWING_RUNNABLE(runnable);
+  return runnable->Run();
+}
+
+NS_IMETHODIMP DelayedRunnable::Notify(nsITimer* aTimer) {
+  MOZ_ASSERT(mTarget->IsOnCurrentThread());
+
+  nsCOMPtr<nsIRunnable> runnable;
+  {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mTimer, "Init() must have been called");
+
+    // We may have already run due to races
+    if (!mWrappedRunnable) {
+      return NS_OK;
+    }
+
+    mTarget->UnregisterShutdownTask(this);
+    runnable = mWrappedRunnable.forget();
+  }
+
+  AUTO_PROFILE_FOLLOWING_RUNNABLE(runnable);
+  return runnable->Run();
+}
+
+void DelayedRunnable::TargetShutdown() {
+  MOZ_ASSERT(mTarget->IsOnCurrentThread());
+
+  // Called at shutdown
+  MutexAutoLock lock(mMutex);
+  if (!mWrappedRunnable) {
+    return;
+  }
+  mWrappedRunnable = nullptr;
+
+  if (mTimer) {
+    mTimer->Cancel();
+  }
+}
+
+NS_IMPL_ISUPPORTS_INHERITED(DelayedRunnable, Runnable, nsITimerCallback,
+                            nsITargetShutdownTask)
+
+}  // namespace mozilla
diff --git a/xpcom/threads/DelayedRunnable.h b/xpcom/threads/DelayedRunnable.h
new file mode 100644
index 0000000000..242278fa5b
--- /dev/null
+++ b/xpcom/threads/DelayedRunnable.h
@@ -0,0 +1,53 @@
+/* -*- 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_THREADS_DELAYEDRUNNABLE_H_
+#define XPCOM_THREADS_DELAYEDRUNNABLE_H_
+
+#include "mozilla/TimeStamp.h"
+#include "nsCOMPtr.h"
+#include "nsIRunnable.h"
+#include "nsITargetShutdownTask.h"
+#include "nsITimer.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+class DelayedRunnable : public Runnable,
+                        public nsITimerCallback,
+                        public nsITargetShutdownTask {
+ public:
+  DelayedRunnable(already_AddRefed<nsISerialEventTarget> aTarget,
+                  already_AddRefed<nsIRunnable> aRunnable, uint32_t aDelay);
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSITIMERCALLBACK
+
+  nsresult Init();
+
+  /**
+   * Called when the target is going away so the runnable can be released safely
+   * on the target thread.
+   */
+  void TargetShutdown() override;
+
+ private:
+  ~DelayedRunnable() = default;
+  nsresult DoRun();
+
+  const nsCOMPtr<nsISerialEventTarget> mTarget;
+  const TimeStamp mDelayedFrom;
+  const uint32_t mDelay;
+
+  mozilla::Mutex mMutex{"DelayedRunnable"};
+  nsCOMPtr<nsIRunnable> mWrappedRunnable;
+  nsCOMPtr<nsITimer> mTimer;
+};
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/EventQueue.cpp b/xpcom/threads/EventQueue.cpp
new file mode 100644
index 0000000000..0decc3ce4c
--- /dev/null
+++ b/xpcom/threads/EventQueue.cpp
@@ -0,0 +1,131 @@
+/* -*- 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/EventQueue.h"
+
+#include "GeckoProfiler.h"
+#include "InputTaskManager.h"
+#include "VsyncTaskManager.h"
+#include "nsIRunnable.h"
+#include "TaskController.h"
+
+using namespace mozilla;
+using namespace mozilla::detail;
+
+template <size_t ItemsPerPage>
+void EventQueueInternal<ItemsPerPage>::PutEvent(
+    already_AddRefed<nsIRunnable>&& aEvent, EventQueuePriority aPriority,
+    const MutexAutoLock& aProofOfLock, mozilla::TimeDuration* aDelay) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+
+  static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_IDLE) ==
+                static_cast<uint32_t>(EventQueuePriority::Idle));
+  static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_NORMAL) ==
+                static_cast<uint32_t>(EventQueuePriority::Normal));
+  static_assert(
+      static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_MEDIUMHIGH) ==
+      static_cast<uint32_t>(EventQueuePriority::MediumHigh));
+  static_assert(
+      static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_INPUT_HIGH) ==
+      static_cast<uint32_t>(EventQueuePriority::InputHigh));
+  static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_VSYNC) ==
+                static_cast<uint32_t>(EventQueuePriority::Vsync));
+  static_assert(
+      static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_RENDER_BLOCKING) ==
+      static_cast<uint32_t>(EventQueuePriority::RenderBlocking));
+  static_assert(static_cast<uint32_t>(nsIRunnablePriority::PRIORITY_CONTROL) ==
+                static_cast<uint32_t>(EventQueuePriority::Control));
+
+  if (mForwardToTC) {
+    TaskController* tc = TaskController::Get();
+
+    TaskManager* manager = nullptr;
+    if (aPriority == EventQueuePriority::InputHigh) {
+      manager = InputTaskManager::Get();
+    } else if (aPriority == EventQueuePriority::DeferredTimers ||
+               aPriority == EventQueuePriority::Idle) {
+      manager = TaskController::Get()->GetIdleTaskManager();
+    } else if (aPriority == EventQueuePriority::Vsync) {
+      manager = VsyncTaskManager::Get();
+    }
+
+    tc->DispatchRunnable(event.forget(), static_cast<uint32_t>(aPriority),
+                         manager);
+    return;
+  }
+
+  if (profiler_thread_is_being_profiled(ThreadProfilingFeatures::Sampling)) {
+    // check to see if the profiler has been enabled since the last PutEvent
+    while (mDispatchTimes.Count() < mQueue.Count()) {
+      mDispatchTimes.Push(TimeStamp());
+    }
+    mDispatchTimes.Push(aDelay ? TimeStamp::Now() - *aDelay : TimeStamp::Now());
+  }
+
+  mQueue.Push(std::move(event));
+}
+
+template <size_t ItemsPerPage>
+already_AddRefed<nsIRunnable> EventQueueInternal<ItemsPerPage>::GetEvent(
+    const MutexAutoLock& aProofOfLock, mozilla::TimeDuration* aLastEventDelay) {
+  if (mQueue.IsEmpty()) {
+    if (aLastEventDelay) {
+      *aLastEventDelay = TimeDuration();
+    }
+    return nullptr;
+  }
+
+  // We always want to clear the dispatch times, even if the profiler is turned
+  // off, because we want to empty the (previously-collected) dispatch times, if
+  // any, from when the profiler was turned on.  We only want to do something
+  // interesting with the dispatch times if the profiler is turned on, though.
+  if (!mDispatchTimes.IsEmpty()) {
+    TimeStamp dispatch_time = mDispatchTimes.Pop();
+    if (profiler_is_active()) {
+      if (!dispatch_time.IsNull()) {
+        if (aLastEventDelay) {
+          *aLastEventDelay = TimeStamp::Now() - dispatch_time;
+        }
+      }
+    }
+  } else if (profiler_is_active()) {
+    if (aLastEventDelay) {
+      // if we just turned on the profiler, we don't have dispatch
+      // times for events already in the queue.
+      *aLastEventDelay = TimeDuration();
+    }
+  }
+
+  nsCOMPtr<nsIRunnable> result = mQueue.Pop();
+  return result.forget();
+}
+
+template <size_t ItemsPerPage>
+bool EventQueueInternal<ItemsPerPage>::IsEmpty(
+    const MutexAutoLock& aProofOfLock) {
+  return mQueue.IsEmpty();
+}
+
+template <size_t ItemsPerPage>
+bool EventQueueInternal<ItemsPerPage>::HasReadyEvent(
+    const MutexAutoLock& aProofOfLock) {
+  return !IsEmpty(aProofOfLock);
+}
+
+template <size_t ItemsPerPage>
+size_t EventQueueInternal<ItemsPerPage>::Count(
+    const MutexAutoLock& aProofOfLock) const {
+  return mQueue.Count();
+}
+
+namespace mozilla {
+template class EventQueueSized<16>;  // Used by ThreadEventQueue
+template class EventQueueSized<64>;  // Used by ThrottledEventQueue
+namespace detail {
+template class EventQueueInternal<16>;  // Used by ThreadEventQueue
+template class EventQueueInternal<64>;  // Used by ThrottledEventQueue
+}  // namespace detail
+}  // namespace mozilla
diff --git a/xpcom/threads/EventQueue.h b/xpcom/threads/EventQueue.h
new file mode 100644
index 0000000000..b7af71be82
--- /dev/null
+++ b/xpcom/threads/EventQueue.h
@@ -0,0 +1,135 @@
+/* -*- 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_EventQueue_h
+#define mozilla_EventQueue_h
+
+#include "mozilla/Mutex.h"
+#include "mozilla/Queue.h"
+#include "mozilla/TimeStamp.h"
+#include "nsCOMPtr.h"
+#include "nsIRunnable.h"
+
+namespace mozilla {
+
+#define EVENT_QUEUE_PRIORITY_LIST(EVENT_PRIORITY) \
+  EVENT_PRIORITY(Idle, 0)                         \
+  EVENT_PRIORITY(DeferredTimers, 1)               \
+  EVENT_PRIORITY(Low, 2)                          \
+  EVENT_PRIORITY(InputLow, 3)                     \
+  EVENT_PRIORITY(Normal, 4)                       \
+  EVENT_PRIORITY(MediumHigh, 5)                   \
+  EVENT_PRIORITY(InputHigh, 6)                    \
+  EVENT_PRIORITY(Vsync, 7)                        \
+  EVENT_PRIORITY(InputHighest, 8)                 \
+  EVENT_PRIORITY(RenderBlocking, 9)               \
+  EVENT_PRIORITY(Control, 10)
+
+enum class EventQueuePriority {
+#define EVENT_PRIORITY(NAME, VALUE) NAME = VALUE,
+  EVENT_QUEUE_PRIORITY_LIST(EVENT_PRIORITY)
+#undef EVENT_PRIORITY
+      Invalid
+};
+
+class IdlePeriodState;
+
+namespace detail {
+
+// EventQueue is our unsynchronized event queue implementation. It is a queue
+// of runnables used for non-main thread, as well as optionally providing
+// forwarding to TaskController.
+//
+// Since EventQueue is unsynchronized, it should be wrapped in an outer
+// SynchronizedEventQueue implementation (like ThreadEventQueue).
+template <size_t ItemsPerPage>
+class EventQueueInternal {
+ public:
+  explicit EventQueueInternal(bool aForwardToTC) : mForwardToTC(aForwardToTC) {}
+
+  // Add an event to the end of the queue. Implementors are free to use
+  // aPriority however they wish.  If the runnable supports
+  // nsIRunnablePriority and the implementing class supports
+  // prioritization, aPriority represents the result of calling
+  // nsIRunnablePriority::GetPriority().  *aDelay is time the event has
+  // already been delayed (used when moving an event from one queue to
+  // another)
+  void PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                EventQueuePriority aPriority, const MutexAutoLock& aProofOfLock,
+                mozilla::TimeDuration* aDelay = nullptr);
+
+  // Get an event from the front of the queue. This should return null if the
+  // queue is non-empty but the event in front is not ready to run.
+  // *aLastEventDelay is the time the event spent in queues before being
+  // retrieved.
+  already_AddRefed<nsIRunnable> GetEvent(
+      const MutexAutoLock& aProofOfLock,
+      mozilla::TimeDuration* aLastEventDelay = nullptr);
+
+  // Returns true if the queue is empty. Implies !HasReadyEvent().
+  bool IsEmpty(const MutexAutoLock& aProofOfLock);
+
+  // Returns true if the queue is non-empty and if the event in front is ready
+  // to run. Implies !IsEmpty(). This should return true iff GetEvent returns a
+  // non-null value.
+  bool HasReadyEvent(const MutexAutoLock& aProofOfLock);
+
+  // Returns the number of events in the queue.
+  size_t Count(const MutexAutoLock& aProofOfLock) const;
+  // For some reason, if we put this in the .cpp file the linker can't find it
+  already_AddRefed<nsIRunnable> PeekEvent(const MutexAutoLock& aProofOfLock) {
+    if (mQueue.IsEmpty()) {
+      return nullptr;
+    }
+
+    nsCOMPtr<nsIRunnable> result = mQueue.FirstElement();
+    return result.forget();
+  }
+
+  void EnableInputEventPrioritization(const MutexAutoLock& aProofOfLock) {}
+  void FlushInputEventPrioritization(const MutexAutoLock& aProofOfLock) {}
+  void SuspendInputEventPrioritization(const MutexAutoLock& aProofOfLock) {}
+  void ResumeInputEventPrioritization(const MutexAutoLock& aProofOfLock) {}
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    size_t size = mQueue.ShallowSizeOfExcludingThis(aMallocSizeOf);
+    size += mDispatchTimes.ShallowSizeOfExcludingThis(aMallocSizeOf);
+    return size;
+  }
+
+ private:
+  mozilla::Queue<nsCOMPtr<nsIRunnable>, ItemsPerPage> mQueue;
+  // This queue is only populated when the profiler is turned on.
+  mozilla::Queue<mozilla::TimeStamp, ItemsPerPage> mDispatchTimes;
+  TimeDuration mLastEventDelay;
+  // This indicates PutEvent forwards runnables to the TaskController. This
+  // should be true for the top level event queue on the main thread.
+  bool mForwardToTC;
+};
+
+}  // namespace detail
+
+class EventQueue final : public mozilla::detail::EventQueueInternal<16> {
+ public:
+  explicit EventQueue(bool aForwardToTC = false)
+      : mozilla::detail::EventQueueInternal<16>(aForwardToTC) {}
+};
+
+template <size_t ItemsPerPage = 16>
+class EventQueueSized final
+    : public mozilla::detail::EventQueueInternal<ItemsPerPage> {
+ public:
+  explicit EventQueueSized(bool aForwardToTC = false)
+      : mozilla::detail::EventQueueInternal<ItemsPerPage>(aForwardToTC) {}
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_EventQueue_h
diff --git a/xpcom/threads/EventTargetCapability.h b/xpcom/threads/EventTargetCapability.h
new file mode 100644
index 0000000000..0cd85a7523
--- /dev/null
+++ b/xpcom/threads/EventTargetCapability.h
@@ -0,0 +1,95 @@
+/* -*- 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_THREADS_EVENTTARGETCAPABILITY_H_
+#define XPCOM_THREADS_EVENTTARGETCAPABILITY_H_
+
+#include "mozilla/ThreadSafety.h"
+#include "nsIEventTarget.h"
+
+namespace mozilla {
+
+// A helper to ensure that data access and function usage only take place on a
+// specific nsIEventTarget.
+//
+// This class works with Clang's thread safety analysis so that static analysis
+// can ensure AssertOnCurrentThread is called before using guarded data and
+// functions.
+//
+// This means using the class is similar to calling
+// `MOZ_ASSERT(mTarget->IsOnCurrentThread())`
+// prior to accessing things we expect to be on `mTarget`. However, using this
+// helper has the added benefit that static analysis will warn you if you
+// fail to assert prior to usage.
+//
+// The following is a basic example of a class using this to ensure
+// a data member is only accessed on a specific target.
+//
+// class SomeMediaHandlerThing {
+//  public:
+//   SomeMediaHandlerThing(nsIEventTarget* aTarget) : mTargetCapability(aTarget)
+//   {}
+//
+//   void UpdateMediaState() {
+//     mTargetCapability.Dispatch(
+//         NS_NewRunnableFunction("UpdateMediaState", [this] {
+//           mTargetCapability.AssertOnCurrentThread();
+//           IncreaseMediaCount();
+//         }));
+//   }
+//
+//  private:
+//   void IncreaseMediaCount() MOZ_REQUIRES(mTargetCapability) { mMediaCount +=
+//   1; }
+//
+//   uint32_t mMediaCount MOZ_GUARDED_BY(mTargetCapability) = 0;
+//   EventTargetCapability<nsIEventTarget> mTargetCapability;
+// };
+//
+// NOTE: If you need a thread-safety capability for specifically the main
+// thread, the static `mozilla::sMainThreadCapability` capability exists, and
+// can be asserted using `AssertIsOnMainThread()`.
+
+template <typename T>
+class MOZ_CAPABILITY("event target") EventTargetCapability final {
+  static_assert(std::is_base_of_v<nsIEventTarget, T>,
+                "T must derive from nsIEventTarget");
+
+ public:
+  explicit EventTargetCapability(T* aTarget) : mTarget(aTarget) {
+    MOZ_ASSERT(mTarget, "mTarget should be non-null");
+  }
+  ~EventTargetCapability() = default;
+
+  EventTargetCapability(const EventTargetCapability&) = default;
+  EventTargetCapability(EventTargetCapability&&) = default;
+  EventTargetCapability& operator=(const EventTargetCapability&) = default;
+  EventTargetCapability& operator=(EventTargetCapability&&) = default;
+
+  void AssertOnCurrentThread() const MOZ_ASSERT_CAPABILITY(this) {
+    MOZ_ASSERT(IsOnCurrentThread());
+  }
+
+  // Allow users to check if we're on the same thread as the event target.
+  bool IsOnCurrentThread() const { return mTarget->IsOnCurrentThread(); }
+
+  // Allow users to get the event target, so classes don't have to store the
+  // target as a separate member to use it.
+  T* GetEventTarget() const { return mTarget; }
+
+  // Helper to simplify dispatching to mTarget.
+  nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                    uint32_t aFlags = NS_DISPATCH_NORMAL) const {
+    return mTarget->Dispatch(std::move(aRunnable), aFlags);
+  }
+
+ private:
+  RefPtr<T> mTarget;
+};
+
+}  // namespace mozilla
+
+#endif  // XPCOM_THREADS_EVENTTARGETCAPABILITY_H_
diff --git a/xpcom/threads/IdlePeriodState.cpp b/xpcom/threads/IdlePeriodState.cpp
new file mode 100644
index 0000000000..ff1a9f4096
--- /dev/null
+++ b/xpcom/threads/IdlePeriodState.cpp
@@ -0,0 +1,257 @@
+/* -*- 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/AppShutdown.h"
+#include "mozilla/IdlePeriodState.h"
+#include "mozilla/StaticPrefs_idle_period.h"
+#include "mozilla/ipc/IdleSchedulerChild.h"
+#include "mozilla/dom/ContentChild.h"
+#include "nsIIdlePeriod.h"
+#include "nsThreadManager.h"
+#include "nsXPCOM.h"
+#include "nsXULAppAPI.h"
+
+static uint64_t sIdleRequestCounter = 0;
+
+namespace mozilla {
+
+IdlePeriodState::IdlePeriodState(already_AddRefed<nsIIdlePeriod>&& aIdlePeriod)
+    : mIdlePeriod(aIdlePeriod) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+}
+
+IdlePeriodState::~IdlePeriodState() {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  if (mIdleScheduler) {
+    mIdleScheduler->Disconnect();
+  }
+}
+
+size_t IdlePeriodState::SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+  size_t n = 0;
+  if (mIdlePeriod) {
+    n += aMallocSizeOf(mIdlePeriod);
+  }
+
+  return n;
+}
+
+void IdlePeriodState::FlagNotIdle() {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+
+  EnsureIsActive();
+  if (mIdleToken && mIdleToken < TimeStamp::Now()) {
+    ClearIdleToken();
+  }
+}
+
+void IdlePeriodState::RanOutOfTasks(const MutexAutoUnlock& aProofOfUnlock) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  MOZ_ASSERT(!mHasPendingEventsPromisedIdleEvent);
+  EnsureIsPaused(aProofOfUnlock);
+  ClearIdleToken();
+}
+
+TimeStamp IdlePeriodState::GetIdleDeadlineInternal(
+    bool aIsPeek, const MutexAutoUnlock& aProofOfUnlock) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+
+  bool shuttingDown;
+  TimeStamp localIdleDeadline =
+      GetLocalIdleDeadline(shuttingDown, aProofOfUnlock);
+  if (!localIdleDeadline) {
+    if (!aIsPeek) {
+      EnsureIsPaused(aProofOfUnlock);
+      ClearIdleToken();
+    }
+    return TimeStamp();
+  }
+
+  TimeStamp idleDeadline =
+      mHasPendingEventsPromisedIdleEvent || shuttingDown
+          ? localIdleDeadline
+          : GetIdleToken(localIdleDeadline, aProofOfUnlock);
+  if (!idleDeadline) {
+    if (!aIsPeek) {
+      EnsureIsPaused(aProofOfUnlock);
+
+      // Don't call ClearIdleToken() here, since we may have a pending
+      // request already.
+      //
+      // RequestIdleToken can do all sorts of IPC stuff that might
+      // take mutexes.  This is one reason why we need the
+      // MutexAutoUnlock reference!
+      RequestIdleToken(localIdleDeadline);
+    }
+    return TimeStamp();
+  }
+
+  if (!aIsPeek) {
+    EnsureIsActive();
+  }
+  return idleDeadline;
+}
+
+TimeStamp IdlePeriodState::GetLocalIdleDeadline(
+    bool& aShuttingDown, const MutexAutoUnlock& aProofOfUnlock) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  // If we are shutting down, we won't honor the idle period, and we will
+  // always process idle runnables.  This will ensure that the idle queue
+  // gets exhausted at shutdown time to prevent intermittently leaking
+  // some runnables inside that queue and even worse potentially leaving
+  // some important cleanup work unfinished.
+  if (AppShutdown::IsInOrBeyond(ShutdownPhase::XPCOMShutdownThreads) ||
+      nsThreadManager::get().GetCurrentThread()->ShuttingDown()) {
+    aShuttingDown = true;
+    return TimeStamp::Now();
+  }
+
+  aShuttingDown = false;
+  TimeStamp idleDeadline;
+  // This GetIdlePeriodHint() call is the reason we need a MutexAutoUnlock here.
+  mIdlePeriod->GetIdlePeriodHint(&idleDeadline);
+
+  // If HasPendingEvents() has been called and it has returned true because of
+  // pending idle events, there is a risk that we may decide here that we aren't
+  // idle and return null, in which case HasPendingEvents() has effectively
+  // lied.  Since we can't go back and fix the past, we have to adjust what we
+  // do here and forcefully pick the idle queue task here.  Note that this means
+  // that we are choosing to run a task from the idle queue when we would
+  // normally decide that we aren't in an idle period, but this can only happen
+  // if we fall out of the idle period in between the call to HasPendingEvents()
+  // and here, which should hopefully be quite rare.  We are effectively
+  // choosing to prioritize the sanity of our API semantics over the optimal
+  // scheduling.
+  if (!mHasPendingEventsPromisedIdleEvent &&
+      (!idleDeadline || idleDeadline < TimeStamp::Now())) {
+    return TimeStamp();
+  }
+  if (mHasPendingEventsPromisedIdleEvent && !idleDeadline) {
+    // If HasPendingEvents() has been called and it has returned true, but we're
+    // no longer in the idle period, we must return a valid timestamp to pretend
+    // that we are still in the idle period.
+    return TimeStamp::Now();
+  }
+  return idleDeadline;
+}
+
+TimeStamp IdlePeriodState::GetIdleToken(TimeStamp aLocalIdlePeriodHint,
+                                        const MutexAutoUnlock& aProofOfUnlock) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+
+  if (!ShouldGetIdleToken()) {
+    // If the process was in background, it may have an idle token, but it can
+    // be cleared now.
+    ClearIdleToken();
+    return aLocalIdlePeriodHint;
+  }
+
+  if (mIdleToken) {
+    TimeStamp now = TimeStamp::Now();
+    if (mIdleToken < now) {
+      ClearIdleToken();
+      return mIdleToken;
+    }
+    return mIdleToken < aLocalIdlePeriodHint ? mIdleToken
+                                             : aLocalIdlePeriodHint;
+  }
+  return TimeStamp();
+}
+
+void IdlePeriodState::RequestIdleToken(TimeStamp aLocalIdlePeriodHint) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  MOZ_ASSERT(!mActive);
+
+  if (!mIdleScheduler && ShouldGetIdleToken()) {
+    // For now cross-process idle scheduler is supported only on the main
+    // threads of the child processes.
+    mIdleScheduler = ipc::IdleSchedulerChild::GetMainThreadIdleScheduler();
+    if (mIdleScheduler) {
+      mIdleScheduler->Init(this);
+    }
+  }
+
+  if (mIdleScheduler && !mIdleRequestId) {
+    TimeStamp now = TimeStamp::Now();
+    if (aLocalIdlePeriodHint <= now) {
+      return;
+    }
+
+    mIdleRequestId = ++sIdleRequestCounter;
+    mIdleScheduler->SendRequestIdleTime(mIdleRequestId,
+                                        aLocalIdlePeriodHint - now);
+  }
+}
+
+void IdlePeriodState::SetIdleToken(uint64_t aId, TimeDuration aDuration) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+
+  // We check the request ID.  It's possible that the server may be granting a
+  // an ealier request that the client has since cancelled and re-requested.
+  if (mIdleRequestId == aId) {
+    mIdleToken = TimeStamp::Now() + aDuration;
+  }
+}
+
+void IdlePeriodState::SetActive() {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  MOZ_ASSERT(!mActive);
+  if (mIdleScheduler) {
+    mIdleScheduler->SetActive();
+  }
+  mActive = true;
+}
+
+void IdlePeriodState::SetPaused(const MutexAutoUnlock& aProofOfUnlock) {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+  MOZ_ASSERT(mActive);
+  if (mIdleScheduler && mIdleScheduler->SetPaused()) {
+    // We may have gotten a free cpu core for running idle tasks.
+    // We don't try to catch the case when there are prioritized processes
+    // running.
+
+    // This SendSchedule call is why we need the MutexAutoUnlock here, because
+    // IPC can do weird things with mutexes.
+    mIdleScheduler->SendSchedule();
+  }
+  mActive = false;
+}
+
+void IdlePeriodState::ClearIdleToken() {
+  MOZ_ASSERT(NS_IsMainThread(),
+             "Why are we touching idle state off the main thread?");
+
+  if (mIdleRequestId) {
+    if (mIdleScheduler) {
+      // This SendIdleTimeUsed call is why we need to not be holding
+      // any locks here, because IPC can do weird things with mutexes.
+      // Ideally we'd have a MutexAutoUnlock& reference here, but some
+      // callers end up here while just not holding any locks at all.
+      mIdleScheduler->SendIdleTimeUsed(mIdleRequestId);
+    }
+    mIdleRequestId = 0;
+    mIdleToken = TimeStamp();
+  }
+}
+
+bool IdlePeriodState::ShouldGetIdleToken() {
+  return StaticPrefs::idle_period_cross_process_scheduling() &&
+         dom::ContentChild::GetSingleton() &&
+         dom::ContentChild::GetSingleton()->GetProcessPriority() <
+             hal::ProcessPriority::PROCESS_PRIORITY_FOREGROUND;
+}
+}  // namespace mozilla
diff --git a/xpcom/threads/IdlePeriodState.h b/xpcom/threads/IdlePeriodState.h
new file mode 100644
index 0000000000..94d6615677
--- /dev/null
+++ b/xpcom/threads/IdlePeriodState.h
@@ -0,0 +1,196 @@
+/* -*- 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_IdlePeriodState_h
+#define mozilla_IdlePeriodState_h
+
+/**
+ * A class for tracking the state of our idle period.  This includes keeping
+ * track of both the state of our process-local idle period estimate and, for
+ * content processes, managing communication with the parent process for
+ * cross-pprocess idle detection.
+ */
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "nsCOMPtr.h"
+
+#include <stdint.h>
+
+class nsIIdlePeriod;
+
+namespace mozilla {
+class TaskManager;
+namespace ipc {
+class IdleSchedulerChild;
+}  // namespace ipc
+
+class IdlePeriodState {
+ public:
+  explicit IdlePeriodState(already_AddRefed<nsIIdlePeriod>&& aIdlePeriod);
+
+  ~IdlePeriodState();
+
+  // Integration with memory reporting.
+  size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
+
+  // Notification that whoever we are tracking idle state for has found a
+  // non-idle task to process.
+  //
+  // Must not be called while holding any locks.
+  void FlagNotIdle();
+
+  // Notification that whoever we are tracking idle state for has no more
+  // tasks (idle or not) to process.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void RanOutOfTasks(const MutexAutoUnlock& aProofOfUnlock);
+
+  // Notification that whoever we are tracking idle state has idle tasks that
+  // they are considering ready to run and that we should keep claiming they are
+  // ready to run until they call ForgetPendingTaskGuarantee().
+  void EnforcePendingTaskGuarantee() {
+    mHasPendingEventsPromisedIdleEvent = true;
+  }
+
+  // Notification that whoever we are tracking idle state for is done with our
+  // "we have an idle event ready to run" guarantee.  When this happens, we can
+  // reset mHasPendingEventsPromisedIdleEvent to false, because we have
+  // fulfilled our contract.
+  void ForgetPendingTaskGuarantee() {
+    mHasPendingEventsPromisedIdleEvent = false;
+  }
+
+  // Update our cached idle deadline so consumers can use it while holding
+  // locks. Consumers must ClearCachedIdleDeadline() once they are done.
+  void UpdateCachedIdleDeadline(const MutexAutoUnlock& aProofOfUnlock) {
+    mCachedIdleDeadline = GetIdleDeadlineInternal(false, aProofOfUnlock);
+  }
+
+  // If we have local idle deadline, but don't have an idle token, this will
+  // request such from the parent process when this is called in a child
+  // process.
+  void RequestIdleDeadlineIfNeeded(const MutexAutoUnlock& aProofOfUnlock) {
+    GetIdleDeadlineInternal(false, aProofOfUnlock);
+  }
+
+  // Reset our cached idle deadline, so we stop allowing idle runnables to run.
+  void ClearCachedIdleDeadline() { mCachedIdleDeadline = TimeStamp(); }
+
+  // Get the current cached idle deadline.  This may return a null timestamp.
+  TimeStamp GetCachedIdleDeadline() { return mCachedIdleDeadline; }
+
+  // Peek our current idle deadline into mCachedIdleDeadline.  This can cause
+  // mCachedIdleDeadline to be a null timestamp (which means we are not idle
+  // right now).  This method does not have any side-effects on our state, apart
+  // from guaranteeing that if it returns non-null then GetDeadlineForIdleTask
+  // will return non-null until ForgetPendingTaskGuarantee() is called, and its
+  // effects on mCachedIdleDeadline.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void CachePeekedIdleDeadline(const MutexAutoUnlock& aProofOfUnlock) {
+    mCachedIdleDeadline = GetIdleDeadlineInternal(true, aProofOfUnlock);
+  }
+
+  void SetIdleToken(uint64_t aId, TimeDuration aDuration);
+
+  bool IsActive() { return mActive; }
+
+ protected:
+  void EnsureIsActive() {
+    if (!mActive) {
+      SetActive();
+    }
+  }
+
+  void EnsureIsPaused(const MutexAutoUnlock& aProofOfUnlock) {
+    if (mActive) {
+      SetPaused(aProofOfUnlock);
+    }
+  }
+
+  // Returns a null TimeStamp if we're not in the idle period.
+  TimeStamp GetLocalIdleDeadline(bool& aShuttingDown,
+                                 const MutexAutoUnlock& aProofOfUnlock);
+
+  // Gets the idle token, which is the end time of the idle period.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  TimeStamp GetIdleToken(TimeStamp aLocalIdlePeriodHint,
+                         const MutexAutoUnlock& aProofOfUnlock);
+
+  // In case of child processes, requests idle time from the cross-process
+  // idle scheduler.
+  void RequestIdleToken(TimeStamp aLocalIdlePeriodHint);
+
+  // Mark that we don't have idle time to use, nor are expecting to get an idle
+  // token from the idle scheduler.  This must be called while not holding any
+  // locks, but some of the callers aren't holding locks to start with, so
+  // consumers just need to make sure they are not holding locks when they call
+  // this.
+  void ClearIdleToken();
+
+  // SetActive should be called when the event queue is running any type of
+  // tasks.
+  void SetActive();
+  // SetPaused should be called once the event queue doesn't have more
+  // tasks to process, or is waiting for the idle token.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void SetPaused(const MutexAutoUnlock& aProofOfUnlock);
+
+  // Get or peek our idle deadline.  When peeking, we generally don't change any
+  // of our internal state.  When getting, we may request an idle token as
+  // needed.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  TimeStamp GetIdleDeadlineInternal(bool aIsPeek,
+                                    const MutexAutoUnlock& aProofOfUnlock);
+
+  // Whether we should be getting an idle token (i.e. are a content process
+  // and are using cross process idle scheduling).
+  bool ShouldGetIdleToken();
+
+  // Set to true if we have claimed we have a ready-to-run idle task when asked.
+  // In that case, we will ensure that we allow at least one task to run when
+  // someone tries to run a task, even if we have run out of idle period at that
+  // point.  This ensures that we never fail to produce a task to run if we
+  // claim we have a task ready to run.
+  bool mHasPendingEventsPromisedIdleEvent = false;
+
+  // mIdlePeriod keeps track of the current idle period. Calling
+  // mIdlePeriod->GetIdlePeriodHint() will give an estimate of when
+  // the current idle period will end.
+  nsCOMPtr<nsIIdlePeriod> mIdlePeriod;
+
+  // If non-null, this timestamp represents the end time of the idle period.  An
+  // idle period starts when we get the idle token from the parent process and
+  // ends when either there are no more things we want to run at idle priority
+  // or mIdleToken < TimeStamp::Now(), so we have reached our idle deadline.
+  TimeStamp mIdleToken;
+
+  // The id of the last idle request to the cross-process idle scheduler.
+  uint64_t mIdleRequestId = 0;
+
+  // If we're in a content process, we use mIdleScheduler to communicate with
+  // the parent process for purposes of cross-process idle tracking.
+  RefPtr<mozilla::ipc::IdleSchedulerChild> mIdleScheduler;
+
+  // Our cached idle deadline.  This is set by UpdateCachedIdleDeadline() and
+  // cleared by ClearCachedIdleDeadline().  Consumers should do the former while
+  // not holding any locks, but may do the latter while holding locks.
+  TimeStamp mCachedIdleDeadline;
+
+  // mActive is true when the PrioritizedEventQueue or TaskController we are
+  // associated with is running tasks.
+  bool mActive = true;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_IdlePeriodState_h
diff --git a/xpcom/threads/IdleTaskRunner.cpp b/xpcom/threads/IdleTaskRunner.cpp
new file mode 100644
index 0000000000..e9bb895dee
--- /dev/null
+++ b/xpcom/threads/IdleTaskRunner.cpp
@@ -0,0 +1,281 @@
+/* -*- 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 "IdleTaskRunner.h"
+#include "mozilla/TaskController.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+
+already_AddRefed<IdleTaskRunner> IdleTaskRunner::Create(
+    const CallbackType& aCallback, const char* aRunnableName,
+    TimeDuration aStartDelay, TimeDuration aMaxDelay,
+    TimeDuration aMinimumUsefulBudget, bool aRepeating,
+    const MayStopProcessingCallbackType& aMayStopProcessing,
+    const RequestInterruptCallbackType& aRequestInterrupt) {
+  if (aMayStopProcessing && aMayStopProcessing()) {
+    return nullptr;
+  }
+
+  RefPtr<IdleTaskRunner> runner = new IdleTaskRunner(
+      aCallback, aRunnableName, aStartDelay, aMaxDelay, aMinimumUsefulBudget,
+      aRepeating, aMayStopProcessing, aRequestInterrupt);
+  runner->Schedule(false);  // Initial scheduling shouldn't use idle dispatch.
+  return runner.forget();
+}
+
+class IdleTaskRunnerTask : public Task {
+ public:
+  explicit IdleTaskRunnerTask(IdleTaskRunner* aRunner)
+      : Task(Kind::MainThreadOnly, EventQueuePriority::Idle),
+        mRunner(aRunner),
+        mRequestInterrupt(aRunner->mRequestInterrupt) {
+    SetManager(TaskController::Get()->GetIdleTaskManager());
+  }
+
+  TaskResult Run() override {
+    if (mRunner) {
+      // IdleTaskRunner::Run can actually trigger the destruction of the
+      // IdleTaskRunner. Make sure it doesn't get destroyed before the method
+      // finished.
+      RefPtr<IdleTaskRunner> runner(mRunner);
+      runner->Run();
+    }
+    return TaskResult::Complete;
+  }
+
+  void SetIdleDeadline(TimeStamp aDeadline) override {
+    if (mRunner) {
+      mRunner->SetIdleDeadline(aDeadline);
+    }
+  }
+
+  void Cancel() { mRunner = nullptr; }
+
+  bool GetName(nsACString& aName) override {
+    if (mRunner) {
+      aName.Assign(mRunner->GetName());
+    } else {
+      aName.Assign("ExpiredIdleTaskRunner");
+    }
+    return true;
+  }
+
+  void RequestInterrupt(uint32_t aInterruptPriority) override {
+    if (mRequestInterrupt) {
+      mRequestInterrupt(aInterruptPriority);
+    }
+  }
+
+ private:
+  IdleTaskRunner* mRunner;
+
+  // Copied here and invoked even if there is no mRunner currently, to avoid
+  // race conditions checking mRunner when an interrupt is requested.
+  IdleTaskRunner::RequestInterruptCallbackType mRequestInterrupt;
+};
+
+IdleTaskRunner::IdleTaskRunner(
+    const CallbackType& aCallback, const char* aRunnableName,
+    TimeDuration aStartDelay, TimeDuration aMaxDelay,
+    TimeDuration aMinimumUsefulBudget, bool aRepeating,
+    const MayStopProcessingCallbackType& aMayStopProcessing,
+    const RequestInterruptCallbackType& aRequestInterrupt)
+    : mCallback(aCallback),
+      mStartTime(TimeStamp::Now() + aStartDelay),
+      mMaxDelay(aMaxDelay),
+      mMinimumUsefulBudget(aMinimumUsefulBudget),
+      mRepeating(aRepeating),
+      mTimerActive(false),
+      mMayStopProcessing(aMayStopProcessing),
+      mRequestInterrupt(aRequestInterrupt),
+      mName(aRunnableName) {}
+
+void IdleTaskRunner::Run() {
+  if (!mCallback) {
+    return;
+  }
+
+  // Deadline is null when called from timer or RunNextCollectorTimer rather
+  // than during idle time.
+  TimeStamp now = TimeStamp::Now();
+
+  // Note that if called from RunNextCollectorTimer, we may not have reached
+  // mStartTime yet. Pretend we are overdue for idle time.
+  bool overdueForIdle = mDeadline.IsNull();
+  bool didRun = false;
+  bool allowIdleDispatch = false;
+
+  if (mTask) {
+    // If we find ourselves here we should usually be running from this task,
+    // but there are exceptions. In any case we're doing the work now and don't
+    // need our task going forward unless we're re-scheduled.
+    nsRefreshDriver::CancelIdleTask(mTask);
+    // Extra safety, make sure a task can never have a dangling ptr.
+    mTask->Cancel();
+    mTask = nullptr;
+  }
+
+  if (overdueForIdle || ((now + mMinimumUsefulBudget) < mDeadline)) {
+    CancelTimer();
+    didRun = mCallback(mDeadline);
+    // If we didn't do meaningful work, don't schedule using immediate
+    // idle dispatch, since that could lead to a loop until the idle
+    // period ends.
+    allowIdleDispatch = didRun;
+  } else if (now >= mDeadline) {
+    allowIdleDispatch = true;
+  }
+
+  if (mCallback && (mRepeating || !didRun)) {
+    Schedule(allowIdleDispatch);
+  } else {
+    mCallback = nullptr;
+  }
+}
+
+static void TimedOut(nsITimer* aTimer, void* aClosure) {
+  RefPtr<IdleTaskRunner> runner = static_cast<IdleTaskRunner*>(aClosure);
+  runner->Run();
+}
+
+void IdleTaskRunner::SetIdleDeadline(mozilla::TimeStamp aDeadline) {
+  mDeadline = aDeadline;
+}
+
+void IdleTaskRunner::SetMinimumUsefulBudget(int64_t aMinimumUsefulBudget) {
+  mMinimumUsefulBudget = TimeDuration::FromMilliseconds(aMinimumUsefulBudget);
+}
+
+void IdleTaskRunner::SetTimer(TimeDuration aDelay, nsIEventTarget* aTarget) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aTarget->IsOnCurrentThread());
+  // aTarget is always the main thread event target provided from
+  // NS_DispatchToCurrentThreadQueue(). We ignore aTarget here to ensure that
+  // CollectorRunner always run specifically the main thread.
+  SetTimerInternal(aDelay);
+}
+
+void IdleTaskRunner::Cancel() {
+  CancelTimer();
+  mTimer = nullptr;
+  mScheduleTimer = nullptr;
+  mCallback = nullptr;
+}
+
+static void ScheduleTimedOut(nsITimer* aTimer, void* aClosure) {
+  RefPtr<IdleTaskRunner> runnable = static_cast<IdleTaskRunner*>(aClosure);
+  runnable->Schedule(true);
+}
+
+void IdleTaskRunner::Schedule(bool aAllowIdleDispatch) {
+  if (!mCallback) {
+    return;
+  }
+
+  if (mMayStopProcessing && mMayStopProcessing()) {
+    Cancel();
+    return;
+  }
+
+  mDeadline = TimeStamp();
+
+  TimeStamp now = TimeStamp::Now();
+
+  if (aAllowIdleDispatch) {
+    SetTimerInternal(mMaxDelay);
+    if (!mTask) {
+      mTask = new IdleTaskRunnerTask(this);
+      RefPtr<Task> task(mTask);
+      TaskController::Get()->AddTask(task.forget());
+    }
+    return;
+  }
+
+  bool useRefreshDriver = false;
+  if (now >= mStartTime) {
+    // Detect whether the refresh driver is ticking by checking if
+    // GetIdleDeadlineHint returns its input parameter.
+    useRefreshDriver =
+        (nsRefreshDriver::GetIdleDeadlineHint(
+             now, nsRefreshDriver::IdleCheck::OnlyThisProcessRefreshDriver) !=
+         now);
+  }
+
+  if (useRefreshDriver) {
+    if (!mTask) {
+      // If a task was already scheduled, no point rescheduling.
+      mTask = new IdleTaskRunnerTask(this);
+      // RefreshDriver is ticking, let it schedule the idle dispatch.
+      nsRefreshDriver::DispatchIdleTaskAfterTickUnlessExists(mTask);
+    }
+    // Ensure we get called at some point, even if RefreshDriver is stopped.
+    SetTimerInternal(mMaxDelay);
+  } else {
+    // RefreshDriver doesn't seem to be running.
+    if (!mScheduleTimer) {
+      mScheduleTimer = NS_NewTimer();
+      if (!mScheduleTimer) {
+        return;
+      }
+    } else {
+      mScheduleTimer->Cancel();
+    }
+    // We weren't allowed to do idle dispatch immediately, do it after a
+    // short timeout. (Or wait for our start time if we haven't started yet.)
+    uint32_t waitToSchedule = 16; /* ms */
+    if (now < mStartTime) {
+      // + 1 to round milliseconds up to be sure to wait until after
+      // mStartTime.
+      waitToSchedule = (mStartTime - now).ToMilliseconds() + 1;
+    }
+    mScheduleTimer->InitWithNamedFuncCallback(
+        ScheduleTimedOut, this, waitToSchedule,
+        nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY, mName);
+  }
+}
+
+IdleTaskRunner::~IdleTaskRunner() { CancelTimer(); }
+
+void IdleTaskRunner::CancelTimer() {
+  if (mTask) {
+    nsRefreshDriver::CancelIdleTask(mTask);
+    mTask->Cancel();
+    mTask = nullptr;
+  }
+  if (mTimer) {
+    mTimer->Cancel();
+  }
+  if (mScheduleTimer) {
+    mScheduleTimer->Cancel();
+  }
+  mTimerActive = false;
+}
+
+void IdleTaskRunner::SetTimerInternal(TimeDuration aDelay) {
+  if (mTimerActive) {
+    return;
+  }
+  ResetTimer(aDelay);
+}
+
+void IdleTaskRunner::ResetTimer(TimeDuration aDelay) {
+  mTimerActive = false;
+
+  if (!mTimer) {
+    mTimer = NS_NewTimer();
+  } else {
+    mTimer->Cancel();
+  }
+
+  if (mTimer) {
+    mTimer->InitWithNamedFuncCallback(TimedOut, this, aDelay.ToMilliseconds(),
+                                      nsITimer::TYPE_ONE_SHOT, mName);
+    mTimerActive = true;
+  }
+}
+
+}  // end of namespace mozilla
diff --git a/xpcom/threads/IdleTaskRunner.h b/xpcom/threads/IdleTaskRunner.h
new file mode 100644
index 0000000000..97a8c0b09e
--- /dev/null
+++ b/xpcom/threads/IdleTaskRunner.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 IdleTaskRunner_h
+#define IdleTaskRunner_h
+
+#include "mozilla/TimeStamp.h"
+#include "nsIEventTarget.h"
+#include "nsISupports.h"
+#include "nsITimer.h"
+#include <functional>
+
+namespace mozilla {
+
+class IdleTaskRunnerTask;
+
+// A general purpose repeating callback runner (it can be configured to a
+// one-time runner, too.) If it is running repeatedly, one has to either
+// explicitly Cancel() the runner or have MayStopProcessing() callback return
+// true to completely remove the runner.
+class IdleTaskRunner {
+ public:
+  friend class IdleTaskRunnerTask;
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(IdleTaskRunner)
+
+  // Return true if some meaningful work was done.
+  using CallbackType = std::function<bool(TimeStamp aDeadline)>;
+
+  // A callback for "stop processing" decision. Return true to
+  // stop processing. This can be an alternative to Cancel() or
+  // work together in different way.
+  using MayStopProcessingCallbackType = std::function<bool()>;
+
+  // A callback to be invoked when an interrupt is requested
+  // (eg during an idle activity when the user presses a key.)
+  // The callback takes an "interrupt priority" value as its
+  // sole parameter.
+  using RequestInterruptCallbackType = std::function<void(uint32_t)>;
+
+ public:
+  // An IdleTaskRunner has (up to) three phases:
+  //
+  //  - (duration aStartDelay) waiting to run (aStartDelay can be zero)
+  //
+  //  - (duration aMaxDelay) attempting to find a long enough amount of idle
+  //    time, at least aMinimumUsefulBudget
+  //
+  //  - overdue for idle time, run as soon as possible
+  //
+  // If aRepeating is true, then aStartDelay applies only to the first run; the
+  // second run will attempt to run in the first idle slice that is long
+  // enough.
+  //
+  // All durations are in milliseconds.
+  //
+  static already_AddRefed<IdleTaskRunner> Create(
+      const CallbackType& aCallback, const char* aRunnableName,
+      TimeDuration aStartDelay, TimeDuration aMaxDelay,
+      TimeDuration aMinimumUsefulBudget, bool aRepeating,
+      const MayStopProcessingCallbackType& aMayStopProcessing,
+      const RequestInterruptCallbackType& aRequestInterrupt = nullptr);
+
+  void Run();
+
+  // (Used by the task triggering code.) Record the end of the current idle
+  // period, or null if not running during idle time.
+  void SetIdleDeadline(mozilla::TimeStamp aDeadline);
+
+  // If the timer is already active, SetTimer doesn't do anything.
+  void SetTimer(TimeDuration aDelay, nsIEventTarget* aTarget);
+
+  void ResetTimer(TimeDuration aDelay);
+
+  // Update the minimum idle time that this callback would be invoked for.
+  void SetMinimumUsefulBudget(int64_t aMinimumUsefulBudget);
+
+  void Cancel();
+
+  void Schedule(bool aAllowIdleDispatch);
+
+  const char* GetName() { return mName; }
+
+ private:
+  explicit IdleTaskRunner(
+      const CallbackType& aCallback, const char* aRunnableName,
+      TimeDuration aStartDelay, TimeDuration aMaxDelay,
+      TimeDuration aMinimumUsefulBudget, bool aRepeating,
+      const MayStopProcessingCallbackType& aMayStopProcessing,
+      const RequestInterruptCallbackType& aRequestInterrupt);
+  ~IdleTaskRunner();
+  void CancelTimer();
+  void SetTimerInternal(TimeDuration aDelay);
+
+  nsCOMPtr<nsITimer> mTimer;
+  nsCOMPtr<nsITimer> mScheduleTimer;
+  CallbackType mCallback;
+
+  // Do not run until this time.
+  const mozilla::TimeStamp mStartTime;
+
+  // Wait this long for idle time before giving up and running a non-idle
+  // callback.
+  TimeDuration mMaxDelay;
+
+  // If running during idle time, the expected end of the current idle period.
+  // The null timestamp when the run is triggered by aMaxDelay instead of idle.
+  TimeStamp mDeadline;
+
+  // The least duration worth calling the callback for during idle time.
+  TimeDuration mMinimumUsefulBudget;
+
+  bool mRepeating;
+  bool mTimerActive;
+  MayStopProcessingCallbackType mMayStopProcessing;
+  RequestInterruptCallbackType mRequestInterrupt;
+  const char* mName;
+  RefPtr<IdleTaskRunnerTask> mTask;
+};
+
+}  // end of namespace mozilla.
+
+#endif
diff --git a/xpcom/threads/InputTaskManager.cpp b/xpcom/threads/InputTaskManager.cpp
new file mode 100644
index 0000000000..42b2814290
--- /dev/null
+++ b/xpcom/threads/InputTaskManager.cpp
@@ -0,0 +1,156 @@
+/* -*- 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 "InputTaskManager.h"
+#include "VsyncTaskManager.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+
+StaticRefPtr<InputTaskManager> InputTaskManager::gInputTaskManager;
+
+void InputTaskManager::EnableInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mInputQueueState == STATE_DISABLED);
+  mInputQueueState = STATE_ENABLED;
+}
+
+void InputTaskManager::FlushInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mInputQueueState == STATE_ENABLED ||
+             mInputQueueState == STATE_SUSPEND);
+  mInputQueueState =
+      mInputQueueState == STATE_ENABLED ? STATE_FLUSHING : STATE_SUSPEND;
+}
+
+void InputTaskManager::SuspendInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mInputQueueState == STATE_ENABLED ||
+             mInputQueueState == STATE_FLUSHING);
+  mInputQueueState = STATE_SUSPEND;
+}
+
+void InputTaskManager::ResumeInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(mInputQueueState == STATE_SUSPEND);
+  mInputQueueState = STATE_ENABLED;
+}
+
+int32_t InputTaskManager::GetPriorityModifierForEventLoopTurn(
+    const MutexAutoLock& aProofOfLock) {
+  // When the state is disabled, the input task that we have is
+  // very likely SuspendInputEventQueue, so here we also use
+  // normal priority as ResumeInputEventQueue, FlushInputEventQueue and
+  // SetInputEventQueueEnabled all uses normal priority, to
+  // ensure the ordering is correct.
+  if (State() == InputTaskManager::STATE_DISABLED) {
+    return static_cast<int32_t>(EventQueuePriority::Normal) -
+           static_cast<int32_t>(EventQueuePriority::InputHigh);
+  }
+
+  return GetPriorityModifierForEventLoopTurnForStrictVsyncAlignment();
+}
+
+void InputTaskManager::WillRunTask() {
+  TaskManager::WillRunTask();
+  mInputPriorityController.WillRunTask();
+}
+
+int32_t
+InputTaskManager::GetPriorityModifierForEventLoopTurnForStrictVsyncAlignment() {
+  MOZ_ASSERT(!IsSuspended());
+
+  size_t inputCount = PendingTaskCount();
+  if (inputCount > 0 &&
+      mInputPriorityController.ShouldUseHighestPriority(this)) {
+    return static_cast<int32_t>(EventQueuePriority::InputHighest) -
+           static_cast<int32_t>(EventQueuePriority::InputHigh);
+  }
+
+  if (State() == STATE_FLUSHING ||
+      nsRefreshDriver::GetNextTickHint().isNothing()) {
+    return 0;
+  }
+
+  return static_cast<int32_t>(EventQueuePriority::InputLow) -
+         static_cast<int32_t>(EventQueuePriority::InputHigh);
+}
+
+InputTaskManager::InputPriorityController::InputPriorityController()
+    : mInputVsyncState(InputVsyncState::NoPendingVsync) {}
+
+bool InputTaskManager::InputPriorityController::ShouldUseHighestPriority(
+    InputTaskManager* aInputTaskManager) {
+  if (mInputVsyncState == InputVsyncState::HasPendingVsync) {
+    return true;
+  }
+
+  if (mInputVsyncState == InputVsyncState::RunVsync) {
+    return false;
+  }
+
+  if (mInputVsyncState == InputVsyncState::NoPendingVsync &&
+      VsyncTaskManager::Get()->PendingTaskCount()) {
+    EnterPendingVsyncState(aInputTaskManager->PendingTaskCount());
+    return true;
+  }
+
+  return false;
+}
+
+void InputTaskManager::InputPriorityController::EnterPendingVsyncState(
+    uint32_t aNumPendingTasks) {
+  MOZ_ASSERT(mInputVsyncState == InputVsyncState::NoPendingVsync);
+
+  mInputVsyncState = InputVsyncState::HasPendingVsync;
+  mMaxInputTasksToRun = aNumPendingTasks;
+  mRunInputStartTime = TimeStamp::Now();
+}
+
+void InputTaskManager::InputPriorityController::WillRunVsync() {
+  if (mInputVsyncState == InputVsyncState::RunVsync ||
+      mInputVsyncState == InputVsyncState::HasPendingVsync) {
+    LeavePendingVsyncState(false);
+  }
+}
+
+void InputTaskManager::InputPriorityController::LeavePendingVsyncState(
+    bool aRunVsync) {
+  if (aRunVsync) {
+    MOZ_ASSERT(mInputVsyncState == InputVsyncState::HasPendingVsync);
+    mInputVsyncState = InputVsyncState::RunVsync;
+  } else {
+    mInputVsyncState = InputVsyncState::NoPendingVsync;
+  }
+
+  mMaxInputTasksToRun = 0;
+}
+
+void InputTaskManager::InputPriorityController::WillRunTask() {
+  switch (mInputVsyncState) {
+    case InputVsyncState::NoPendingVsync:
+      return;
+    case InputVsyncState::HasPendingVsync:
+      MOZ_ASSERT(mMaxInputTasksToRun > 0);
+      --mMaxInputTasksToRun;
+      if (!mMaxInputTasksToRun ||
+          TimeStamp::Now() - mRunInputStartTime >=
+              TimeDuration::FromMilliseconds(
+                  StaticPrefs::dom_input_event_queue_duration_max())) {
+        LeavePendingVsyncState(true);
+      }
+      return;
+    default:
+      MOZ_DIAGNOSTIC_ASSERT(
+          false, "Shouldn't run this input task when we suppose to run vsync");
+      return;
+  }
+}
+
+// static
+void InputTaskManager::Init() { gInputTaskManager = new InputTaskManager(); }
+
+}  // namespace mozilla
diff --git a/xpcom/threads/InputTaskManager.h b/xpcom/threads/InputTaskManager.h
new file mode 100644
index 0000000000..2f920a31ae
--- /dev/null
+++ b/xpcom/threads/InputTaskManager.h
@@ -0,0 +1,141 @@
+/* -*- 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_InputTaskManager_h
+#define mozilla_InputTaskManager_h
+
+#include "nsTArray.h"
+#include "nsXULAppAPI.h"
+#include "TaskController.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/StaticPrefs_dom.h"
+
+namespace mozilla {
+
+class InputTaskManager : public TaskManager {
+ public:
+  int32_t GetPriorityModifierForEventLoopTurn(
+      const MutexAutoLock& aProofOfLock) final;
+  void WillRunTask() final;
+
+  enum InputEventQueueState {
+    STATE_DISABLED,
+    STATE_FLUSHING,
+    STATE_SUSPEND,
+    STATE_ENABLED
+  };
+
+  void EnableInputEventPrioritization();
+  void FlushInputEventPrioritization();
+  void SuspendInputEventPrioritization();
+  void ResumeInputEventPrioritization();
+
+  InputEventQueueState State() { return mInputQueueState; }
+
+  void SetState(InputEventQueueState aState) { mInputQueueState = aState; }
+
+  static InputTaskManager* Get() { return gInputTaskManager.get(); }
+  static void Cleanup() { gInputTaskManager = nullptr; }
+  static void Init();
+
+  bool IsSuspended(const MutexAutoLock& aProofOfLock) override {
+    MOZ_ASSERT(NS_IsMainThread());
+    return mInputQueueState == STATE_SUSPEND || mSuspensionLevel > 0;
+  }
+
+  bool IsSuspended() {
+    MOZ_ASSERT(NS_IsMainThread());
+    return mSuspensionLevel > 0;
+  }
+
+  void IncSuspensionLevel() {
+    MOZ_ASSERT(NS_IsMainThread());
+    ++mSuspensionLevel;
+  }
+
+  void DecSuspensionLevel() {
+    MOZ_ASSERT(NS_IsMainThread());
+    --mSuspensionLevel;
+  }
+
+  static bool CanSuspendInputEvent() {
+    // Ensure it's content process because InputTaskManager only
+    // works in e10s.
+    //
+    // Input tasks will have nullptr as their task manager when the
+    // event queue state is STATE_DISABLED, so we can't suspend
+    // input events.
+    return XRE_IsContentProcess() &&
+           StaticPrefs::dom_input_events_canSuspendInBCG_enabled() &&
+           InputTaskManager::Get()->State() !=
+               InputEventQueueState::STATE_DISABLED;
+  }
+
+  void NotifyVsync() { mInputPriorityController.WillRunVsync(); }
+
+ private:
+  InputTaskManager() : mInputQueueState(STATE_DISABLED) {}
+
+  class InputPriorityController {
+   public:
+    InputPriorityController();
+    // Determines whether we should use the highest input priority for input
+    // tasks
+    bool ShouldUseHighestPriority(InputTaskManager*);
+
+    void WillRunVsync();
+
+    // Gets called when a input task is going to run; If the current
+    // input vsync state is `HasPendingVsync`, determines whether we
+    // should continue running input tasks or leave the `HasPendingVsync` state
+    // based on
+    //    1. Whether we still have time to process input tasks
+    //    2. Whether we have processed the max number of tasks that
+    //    we should process.
+    void WillRunTask();
+
+   private:
+    // Used to represents the relationship between Input and Vsync.
+    //
+    // HasPendingVsync: There are pending vsync tasks and we are using
+    // InputHighest priority for inputs.
+    // NoPendingVsync: No pending vsync tasks and no need to use InputHighest
+    // priority.
+    // RunVsync: Finished running input tasks and the vsync task
+    // should be run.
+    enum class InputVsyncState {
+      HasPendingVsync,
+      NoPendingVsync,
+      RunVsync,
+    };
+
+    void EnterPendingVsyncState(uint32_t aNumPendingTasks);
+    void LeavePendingVsyncState(bool aRunVsync);
+
+    // Stores the number of pending input tasks when we enter the
+    // InputVsyncState::HasPendingVsync state.
+    uint32_t mMaxInputTasksToRun = 0;
+
+    InputVsyncState mInputVsyncState;
+
+    TimeStamp mRunInputStartTime;
+  };
+
+  int32_t GetPriorityModifierForEventLoopTurnForStrictVsyncAlignment();
+
+  Atomic<InputEventQueueState> mInputQueueState;
+
+  static StaticRefPtr<InputTaskManager> gInputTaskManager;
+
+  // Number of BCGs have asked InputTaskManager to suspend input events
+  uint32_t mSuspensionLevel = 0;
+
+  InputPriorityController mInputPriorityController;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_InputTaskManager_h
diff --git a/xpcom/threads/LazyIdleThread.cpp b/xpcom/threads/LazyIdleThread.cpp
new file mode 100644
index 0000000000..4187fcc4ff
--- /dev/null
+++ b/xpcom/threads/LazyIdleThread.cpp
@@ -0,0 +1,126 @@
+/* -*- 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 "LazyIdleThread.h"
+
+#include "nsIObserverService.h"
+#include "nsServiceManagerUtils.h"
+#include "nsThreadUtils.h"
+
+#ifdef DEBUG
+#  define ASSERT_OWNING_THREAD()                           \
+    do {                                                   \
+      MOZ_ASSERT(mOwningEventTarget->IsOnCurrentThread()); \
+    } while (0)
+#else
+#  define ASSERT_OWNING_THREAD() /* nothing */
+#endif
+
+namespace mozilla {
+
+LazyIdleThread::LazyIdleThread(uint32_t aIdleTimeoutMS, const char* aName,
+                               ShutdownMethod aShutdownMethod)
+    : mOwningEventTarget(GetCurrentSerialEventTarget()),
+      mThreadPool(new nsThreadPool()),
+      mTaskQueue(TaskQueue::Create(do_AddRef(mThreadPool), aName)) {
+  // Configure the threadpool to host a single thread. It will be responsible
+  // for managing the thread's lifetime.
+  MOZ_ALWAYS_SUCCEEDS(mThreadPool->SetThreadLimit(1));
+  MOZ_ALWAYS_SUCCEEDS(mThreadPool->SetIdleThreadLimit(1));
+  MOZ_ALWAYS_SUCCEEDS(mThreadPool->SetIdleThreadTimeout(aIdleTimeoutMS));
+  MOZ_ALWAYS_SUCCEEDS(mThreadPool->SetName(nsDependentCString(aName)));
+
+  if (aShutdownMethod == ShutdownMethod::AutomaticShutdown &&
+      NS_IsMainThread()) {
+    if (nsCOMPtr<nsIObserverService> obs =
+            do_GetService(NS_OBSERVERSERVICE_CONTRACTID)) {
+      MOZ_ALWAYS_SUCCEEDS(
+          obs->AddObserver(this, "xpcom-shutdown-threads", false));
+    }
+  }
+}
+
+static void LazyIdleThreadShutdown(nsThreadPool* aThreadPool,
+                                   TaskQueue* aTaskQueue) {
+  aTaskQueue->BeginShutdown();
+  aTaskQueue->AwaitShutdownAndIdle();
+  aThreadPool->Shutdown();
+}
+
+LazyIdleThread::~LazyIdleThread() {
+  if (!mShutdown) {
+    mOwningEventTarget->Dispatch(NS_NewRunnableFunction(
+        "LazyIdleThread::~LazyIdleThread",
+        [threadPool = mThreadPool, taskQueue = mTaskQueue] {
+          LazyIdleThreadShutdown(threadPool, taskQueue);
+        }));
+  }
+}
+
+void LazyIdleThread::Shutdown() {
+  ASSERT_OWNING_THREAD();
+
+  if (!mShutdown) {
+    mShutdown = true;
+    LazyIdleThreadShutdown(mThreadPool, mTaskQueue);
+  }
+}
+
+nsresult LazyIdleThread::SetListener(nsIThreadPoolListener* aListener) {
+  return mThreadPool->SetListener(aListener);
+}
+
+NS_IMPL_ISUPPORTS(LazyIdleThread, nsIEventTarget, nsISerialEventTarget,
+                  nsIObserver)
+
+NS_IMETHODIMP
+LazyIdleThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                         uint32_t aFlags) {
+  return mTaskQueue->Dispatch(std::move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+LazyIdleThread::DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+LazyIdleThread::IsOnCurrentThread(bool* aIsOnCurrentThread) {
+  return mTaskQueue->IsOnCurrentThread(aIsOnCurrentThread);
+}
+
+NS_IMETHODIMP_(bool)
+LazyIdleThread::IsOnCurrentThreadInfallible() {
+  return mTaskQueue->IsOnCurrentThreadInfallible();
+}
+
+NS_IMETHODIMP
+LazyIdleThread::Observe(nsISupports* /* aSubject */, const char* aTopic,
+                        const char16_t* /* aData */) {
+  MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!");
+  MOZ_ASSERT(!strcmp("xpcom-shutdown-threads", aTopic), "Bad topic!");
+
+  Shutdown();
+  return NS_OK;
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
new file mode 100644
index 0000000000..8a720bc69b
--- /dev/null
+++ b/xpcom/threads/LazyIdleThread.h
@@ -0,0 +1,93 @@
+/* -*- 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_lazyidlethread_h__
+#define mozilla_lazyidlethread_h__
+
+#ifndef MOZILLA_INTERNAL_API
+#  error "This header is only usable from within libxul (MOZILLA_INTERNAL_API)."
+#endif
+
+#include "mozilla/TaskQueue.h"
+#include "nsIObserver.h"
+#include "nsThreadPool.h"
+
+namespace mozilla {
+
+/**
+ * This class provides a basic event target that creates its thread lazily and
+ * destroys its thread after a period of inactivity. It may be created and used
+ * on any thread but it may only be shut down from the thread on which it is
+ * created.
+ */
+class LazyIdleThread final : public nsISerialEventTarget, public nsIObserver {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+  NS_DECL_NSIOBSERVER
+
+  /**
+   * If AutomaticShutdown is specified, and the LazyIdleThread is created on the
+   * main thread, Shutdown() will automatically be called during
+   * xpcom-shutdown-threads.
+   */
+  enum ShutdownMethod { AutomaticShutdown = 0, ManualShutdown };
+
+  /**
+   * Create a new LazyIdleThread that will destroy its thread after the given
+   * number of milliseconds.
+   */
+  LazyIdleThread(uint32_t aIdleTimeoutMS, const char* aName,
+                 ShutdownMethod aShutdownMethod = AutomaticShutdown);
+
+  /**
+   * Shuts down the LazyIdleThread, waiting for any pending work to complete.
+   * Must be called from mOwningEventTarget.
+   */
+  void Shutdown();
+
+  /**
+   * Register a nsIThreadPoolListener on the underlying threadpool to track the
+   * thread as it is created/destroyed.
+   */
+  nsresult SetListener(nsIThreadPoolListener* aListener);
+
+ private:
+  /**
+   * Asynchronously shuts down the LazyIdleThread on mOwningEventTarget.
+   */
+  ~LazyIdleThread();
+
+  /**
+   * The thread which created this LazyIdleThread and is responsible for
+   * shutting it down.
+   */
+  const nsCOMPtr<nsISerialEventTarget> mOwningEventTarget;
+
+  /**
+   * The single-thread backing threadpool which provides the actual threads used
+   * by LazyIdleThread, and implements the timeout.
+   */
+  const RefPtr<nsThreadPool> mThreadPool;
+
+  /**
+   * The serial event target providing a `nsISerialEventTarget` implementation
+   * when on the LazyIdleThread.
+   */
+  const RefPtr<TaskQueue> mTaskQueue;
+
+  /**
+   * Only safe to access on the owning thread or in the destructor (as no other
+   * threads have access then). If `true`, means the LazyIdleThread has already
+   * been shut down, so it does not need to be shut down asynchronously from the
+   * destructor.
+   */
+  bool mShutdown = false;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_lazyidlethread_h__
diff --git a/xpcom/threads/LeakRefPtr.h b/xpcom/threads/LeakRefPtr.h
new file mode 100644
index 0000000000..ee260dad7e
--- /dev/null
+++ b/xpcom/threads/LeakRefPtr.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/. */
+
+/* Smart pointer which leaks its owning refcounted object by default. */
+
+#ifndef LeakRefPtr_h
+#define LeakRefPtr_h
+
+#include "mozilla/AlreadyAddRefed.h"
+
+namespace mozilla {
+
+/**
+ * Instance of this class behaves like a raw pointer which leaks the
+ * resource it's owning if not explicitly released.
+ */
+template <class T>
+class LeakRefPtr {
+ public:
+  explicit LeakRefPtr(already_AddRefed<T>&& aPtr) : mRawPtr(aPtr.take()) {}
+
+  explicit operator bool() const { return !!mRawPtr; }
+
+  LeakRefPtr<T>& operator=(already_AddRefed<T>&& aPtr) {
+    mRawPtr = aPtr.take();
+    return *this;
+  }
+
+  T* get() const { return mRawPtr; }
+
+  already_AddRefed<T> take() {
+    T* rawPtr = mRawPtr;
+    mRawPtr = nullptr;
+    return already_AddRefed<T>(rawPtr);
+  }
+
+  void release() { NS_RELEASE(mRawPtr); }
+
+ private:
+  T* MOZ_OWNING_REF mRawPtr;
+};
+
+}  // namespace mozilla
+
+#endif  // LeakRefPtr_h
diff --git a/xpcom/threads/MainThreadIdlePeriod.cpp b/xpcom/threads/MainThreadIdlePeriod.cpp
new file mode 100644
index 0000000000..805626dd67
--- /dev/null
+++ b/xpcom/threads/MainThreadIdlePeriod.cpp
@@ -0,0 +1,77 @@
+/* -*- 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 "MainThreadIdlePeriod.h"
+
+#include "mozilla/Maybe.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/StaticPrefs_idle_period.h"
+#include "mozilla/dom/Document.h"
+#include "VRManagerChild.h"
+#include "nsRefreshDriver.h"
+#include "nsThreadUtils.h"
+
+// The amount of idle time (milliseconds) reserved for a long idle period.
+static const double kLongIdlePeriodMS = 50.0;
+
+// The minimum amount of time (milliseconds) required for an idle period to be
+// scheduled on the main thread. N.B. layout.idle_period.time_limit adds
+// padding at the end of the idle period, which makes the point in time that we
+// expect to become busy again be:
+//   now + idle_period.min + layout.idle_period.time_limit
+// or during page load
+//   now + idle_period.during_page_load.min + layout.idle_period.time_limit
+
+static const uint32_t kMaxTimerThreadBound = 25;  // Number of timers to check.
+
+namespace mozilla {
+
+NS_IMETHODIMP
+MainThreadIdlePeriod::GetIdlePeriodHint(TimeStamp* aIdleDeadline) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aIdleDeadline);
+
+  TimeStamp now = TimeStamp::Now();
+  TimeStamp currentGuess =
+      now + TimeDuration::FromMilliseconds(kLongIdlePeriodMS);
+
+  currentGuess = nsRefreshDriver::GetIdleDeadlineHint(
+      currentGuess, nsRefreshDriver::IdleCheck::AllVsyncListeners);
+  if (XRE_IsContentProcess()) {
+    currentGuess = gfx::VRManagerChild::GetIdleDeadlineHint(currentGuess);
+  }
+  currentGuess = NS_GetTimerDeadlineHintOnCurrentThread(currentGuess,
+                                                        kMaxTimerThreadBound);
+
+  // If the idle period is too small, then just return a null time
+  // to indicate we are busy. Otherwise return the actual deadline.
+  double highRateMultiplier = nsRefreshDriver::HighRateMultiplier();
+  TimeDuration minIdlePeriod = TimeDuration::FromMilliseconds(
+      std::max(highRateMultiplier * StaticPrefs::idle_period_min(), 1.0));
+  bool busySoon = currentGuess.IsNull() ||
+                  (now >= (currentGuess - minIdlePeriod)) ||
+                  currentGuess < mLastIdleDeadline;
+
+  // During page load use higher minimum idle period.
+  if (!busySoon && XRE_IsContentProcess() &&
+      mozilla::dom::Document::HasRecentlyStartedForegroundLoads()) {
+    TimeDuration minIdlePeriod = TimeDuration::FromMilliseconds(std::max(
+        highRateMultiplier * StaticPrefs::idle_period_during_page_load_min(),
+        1.0));
+    busySoon = (now >= (currentGuess - minIdlePeriod));
+  }
+
+  if (!busySoon) {
+    *aIdleDeadline = mLastIdleDeadline = currentGuess;
+  }
+
+  return NS_OK;
+}
+
+/* static */
+float MainThreadIdlePeriod::GetLongIdlePeriod() { return kLongIdlePeriodMS; }
+
+}  // namespace mozilla
diff --git a/xpcom/threads/MainThreadIdlePeriod.h b/xpcom/threads/MainThreadIdlePeriod.h
new file mode 100644
index 0000000000..7e382f6771
--- /dev/null
+++ b/xpcom/threads/MainThreadIdlePeriod.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 mozilla_dom_mainthreadidleperiod_h
+#define mozilla_dom_mainthreadidleperiod_h
+
+#include "mozilla/TimeStamp.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+class MainThreadIdlePeriod final : public IdlePeriod {
+ public:
+  NS_DECL_NSIIDLEPERIOD
+
+  MainThreadIdlePeriod() : mLastIdleDeadline(TimeStamp::Now()) {}
+
+  static float GetLongIdlePeriod();
+
+ private:
+  virtual ~MainThreadIdlePeriod() = default;
+
+  TimeStamp mLastIdleDeadline;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_dom_mainthreadidleperiod_h
diff --git a/xpcom/threads/MainThreadUtils.h b/xpcom/threads/MainThreadUtils.h
new file mode 100644
index 0000000000..152805eb66
--- /dev/null
+++ b/xpcom/threads/MainThreadUtils.h
@@ -0,0 +1,60 @@
+/* -*- 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 MainThreadUtils_h_
+#define MainThreadUtils_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/ThreadSafety.h"
+#include "nscore.h"
+
+class nsIThread;
+
+/**
+ * Get a reference to the main thread.
+ *
+ * @param aResult
+ *   The resulting nsIThread object.
+ */
+extern nsresult NS_GetMainThread(nsIThread** aResult);
+
+#ifdef MOZILLA_INTERNAL_API
+bool NS_IsMainThreadTLSInitialized();
+extern "C" {
+bool NS_IsMainThread();
+}
+
+namespace mozilla {
+
+/**
+ * A dummy static capability for the thread safety analysis which can be
+ * required by functions and members using `MOZ_REQUIRE(sMainThreadCapability)`
+ * and `MOZ_GUARDED_BY(sMainThreadCapability)` and asserted using
+ * `AssertIsOnMainThread()`.
+ *
+ * If you want a thread-safety-analysis capability for a non-main thread,
+ * consider using the `EventTargetCapability` type.
+ */
+class MOZ_CAPABILITY("main thread") MainThreadCapability final {};
+constexpr MainThreadCapability sMainThreadCapability;
+
+#  ifdef DEBUG
+void AssertIsOnMainThread() MOZ_ASSERT_CAPABILITY(sMainThreadCapability);
+#  else
+inline void AssertIsOnMainThread()
+    MOZ_ASSERT_CAPABILITY(sMainThreadCapability) {}
+#  endif
+
+inline void ReleaseAssertIsOnMainThread()
+    MOZ_ASSERT_CAPABILITY(sMainThreadCapability) {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+}
+
+}  // namespace mozilla
+
+#endif
+
+#endif  // MainThreadUtils_h_
diff --git a/xpcom/threads/Monitor.h b/xpcom/threads/Monitor.h
new file mode 100644
index 0000000000..7bb91f9ad7
--- /dev/null
+++ b/xpcom/threads/Monitor.h
@@ -0,0 +1,316 @@
+/* -*- 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_Monitor_h
+#define mozilla_Monitor_h
+
+#include "mozilla/CondVar.h"
+#include "mozilla/Mutex.h"
+
+namespace mozilla {
+
+/**
+ * Monitor provides a *non*-reentrant monitor: *not* a Java-style
+ * monitor.  If your code needs support for reentrancy, use
+ * ReentrantMonitor instead.  (Rarely should reentrancy be needed.)
+ *
+ * Instead of directly calling Monitor methods, it's safer and simpler
+ * to instead use the RAII wrappers MonitorAutoLock and
+ * MonitorAutoUnlock.
+ */
+class MOZ_CAPABILITY("monitor") Monitor {
+ public:
+  explicit Monitor(const char* aName)
+      : mMutex(aName), mCondVar(mMutex, "[Monitor.mCondVar]") {}
+
+  ~Monitor() = default;
+
+  void Lock() MOZ_CAPABILITY_ACQUIRE() { mMutex.Lock(); }
+  [[nodiscard]] bool TryLock() MOZ_TRY_ACQUIRE(true) {
+    return mMutex.TryLock();
+  }
+  void Unlock() MOZ_CAPABILITY_RELEASE() { mMutex.Unlock(); }
+
+  void Wait() MOZ_REQUIRES(this) { mCondVar.Wait(); }
+  CVStatus Wait(TimeDuration aDuration) MOZ_REQUIRES(this) {
+    return mCondVar.Wait(aDuration);
+  }
+
+  void Notify() { mCondVar.Notify(); }
+  void NotifyAll() { mCondVar.NotifyAll(); }
+
+  void AssertCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(this) {
+    mMutex.AssertCurrentThreadOwns();
+  }
+  void AssertNotCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(!this) {
+    mMutex.AssertNotCurrentThreadOwns();
+  }
+
+ private:
+  Monitor() = delete;
+  Monitor(const Monitor&) = delete;
+  Monitor& operator=(const Monitor&) = delete;
+
+  Mutex mMutex;
+  CondVar mCondVar;
+};
+
+/**
+ * MonitorSingleWriter
+ *
+ * Monitor where a single writer exists, so that reads from the same thread
+ * will not generate data races or consistency issues.
+ *
+ * When possible, use MonitorAutoLock/MonitorAutoUnlock to lock/unlock this
+ * monitor within a scope, instead of calling Lock/Unlock directly.
+ *
+ * This requires an object implementing Mutex's SingleWriterLockOwner, so
+ * we can do correct-thread checks.
+ */
+
+class MonitorSingleWriter : public Monitor {
+ public:
+  // aOwner should be the object that contains the mutex, typically.  We
+  // will use that object (which must have a lifetime the same or greater
+  // than this object) to verify that we're running on the correct thread,
+  // typically only in DEBUG builds
+  explicit MonitorSingleWriter(const char* aName, SingleWriterLockOwner* aOwner)
+      : Monitor(aName)
+#ifdef DEBUG
+        ,
+        mOwner(aOwner)
+#endif
+  {
+    MOZ_COUNT_CTOR(MonitorSingleWriter);
+    MOZ_ASSERT(mOwner);
+  }
+
+  MOZ_COUNTED_DTOR(MonitorSingleWriter)
+
+  void AssertOnWritingThread() const MOZ_ASSERT_CAPABILITY(this) {
+    MOZ_ASSERT(mOwner->OnWritingThread());
+  }
+  void AssertOnWritingThreadOrHeld() const MOZ_ASSERT_CAPABILITY(this) {
+#ifdef DEBUG
+    if (!mOwner->OnWritingThread()) {
+      AssertCurrentThreadOwns();
+    }
+#endif
+  }
+
+ private:
+#ifdef DEBUG
+  SingleWriterLockOwner* mOwner MOZ_UNSAFE_REF(
+      "This is normally the object that contains the MonitorSingleWriter, so "
+      "we don't want to hold a reference to ourselves");
+#endif
+
+  MonitorSingleWriter() = delete;
+  MonitorSingleWriter(const MonitorSingleWriter&) = delete;
+  MonitorSingleWriter& operator=(const MonitorSingleWriter&) = delete;
+};
+
+/**
+ * Lock the monitor for the lexical scope instances of this class are
+ * bound to (except for MonitorAutoUnlock in nested scopes).
+ *
+ * The monitor must be unlocked when instances of this class are
+ * created.
+ */
+namespace detail {
+template <typename T>
+class MOZ_SCOPED_CAPABILITY MOZ_STACK_CLASS BaseMonitorAutoLock {
+ public:
+  explicit BaseMonitorAutoLock(T& aMonitor) MOZ_CAPABILITY_ACQUIRE(aMonitor)
+      : mMonitor(&aMonitor) {
+    mMonitor->Lock();
+  }
+
+  ~BaseMonitorAutoLock() MOZ_CAPABILITY_RELEASE() { mMonitor->Unlock(); }
+  // It's very hard to mess up MonitorAutoLock lock(mMonitor); ... lock.Wait().
+  // The only way you can fail to hold the lock when you call lock.Wait() is to
+  // use MonitorAutoUnlock.   For now we'll ignore that case.
+  void Wait() {
+    mMonitor->AssertCurrentThreadOwns();
+    mMonitor->Wait();
+  }
+  CVStatus Wait(TimeDuration aDuration) {
+    mMonitor->AssertCurrentThreadOwns();
+    return mMonitor->Wait(aDuration);
+  }
+
+  void Notify() { mMonitor->Notify(); }
+  void NotifyAll() { mMonitor->NotifyAll(); }
+
+  // Assert that aLock is the monitor passed to the constructor and that the
+  // current thread owns the monitor.  In coding patterns such as:
+  //
+  // void LockedMethod(const BaseAutoLock<T>& aProofOfLock)
+  // {
+  //   aProofOfLock.AssertOwns(mMonitor);
+  //   ...
+  // }
+  //
+  // Without this assertion, it could be that mMonitor is not actually
+  // locked. It's possible to have code like:
+  //
+  // BaseAutoLock lock(someMonitor);
+  // ...
+  // BaseAutoUnlock unlock(someMonitor);
+  // ...
+  // LockedMethod(lock);
+  //
+  // and in such a case, simply asserting that the monitor pointers match is not
+  // sufficient; monitor ownership must be asserted as well.
+  //
+  // Note that if you are going to use the coding pattern presented above, you
+  // should use this method in preference to using AssertCurrentThreadOwns on
+  // the mutex you expected to be held, since this method provides stronger
+  // guarantees.
+  void AssertOwns(const T& aMonitor) const MOZ_ASSERT_CAPABILITY(aMonitor) {
+    MOZ_ASSERT(&aMonitor == mMonitor);
+    mMonitor->AssertCurrentThreadOwns();
+  }
+
+ private:
+  BaseMonitorAutoLock() = delete;
+  BaseMonitorAutoLock(const BaseMonitorAutoLock&) = delete;
+  BaseMonitorAutoLock& operator=(const BaseMonitorAutoLock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  friend class MonitorAutoUnlock;
+
+ protected:
+  T* mMonitor;
+};
+}  // namespace detail
+typedef detail::BaseMonitorAutoLock<Monitor> MonitorAutoLock;
+typedef detail::BaseMonitorAutoLock<MonitorSingleWriter>
+    MonitorSingleWriterAutoLock;
+
+// clang-format off
+// Use if we've done AssertOnWritingThread(), and then later need to take the
+// lock to write to a protected member. Instead of
+//    MutexSingleWriterAutoLock lock(mutex)
+// use
+//    MutexSingleWriterAutoLockOnThread(lock, mutex)
+// clang-format on
+#define MonitorSingleWriterAutoLockOnThread(lock, monitor) \
+  MOZ_PUSH_IGNORE_THREAD_SAFETY                            \
+  MonitorSingleWriterAutoLock lock(monitor);               \
+  MOZ_POP_THREAD_SAFETY
+
+/**
+ * Unlock the monitor for the lexical scope instances of this class
+ * are bound to (except for MonitorAutoLock in nested scopes).
+ *
+ * The monitor must be locked by the current thread when instances of
+ * this class are created.
+ */
+namespace detail {
+template <typename T>
+class MOZ_STACK_CLASS MOZ_SCOPED_CAPABILITY BaseMonitorAutoUnlock {
+ public:
+  explicit BaseMonitorAutoUnlock(T& aMonitor)
+      MOZ_SCOPED_UNLOCK_RELEASE(aMonitor)
+      : mMonitor(&aMonitor) {
+    mMonitor->Unlock();
+  }
+
+  ~BaseMonitorAutoUnlock() MOZ_SCOPED_UNLOCK_REACQUIRE() { mMonitor->Lock(); }
+
+ private:
+  BaseMonitorAutoUnlock() = delete;
+  BaseMonitorAutoUnlock(const BaseMonitorAutoUnlock&) = delete;
+  BaseMonitorAutoUnlock& operator=(const BaseMonitorAutoUnlock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  T* mMonitor;
+};
+}  // namespace detail
+typedef detail::BaseMonitorAutoUnlock<Monitor> MonitorAutoUnlock;
+typedef detail::BaseMonitorAutoUnlock<MonitorSingleWriter>
+    MonitorSingleWriterAutoUnlock;
+
+/**
+ * Lock the monitor for the lexical scope instances of this class are
+ * bound to (except for MonitorAutoUnlock in nested scopes).
+ *
+ * The monitor must be unlocked when instances of this class are
+ * created.
+ */
+class MOZ_SCOPED_CAPABILITY MOZ_STACK_CLASS ReleasableMonitorAutoLock {
+ public:
+  explicit ReleasableMonitorAutoLock(Monitor& aMonitor)
+      MOZ_CAPABILITY_ACQUIRE(aMonitor)
+      : mMonitor(&aMonitor) {
+    mMonitor->Lock();
+    mLocked = true;
+  }
+
+  ~ReleasableMonitorAutoLock() MOZ_CAPABILITY_RELEASE() {
+    if (mLocked) {
+      mMonitor->Unlock();
+    }
+  }
+
+  // See BaseMonitorAutoLock::Wait
+  void Wait() {
+    mMonitor->AssertCurrentThreadOwns();  // someone could have called Unlock()
+    mMonitor->Wait();
+  }
+  CVStatus Wait(TimeDuration aDuration) {
+    mMonitor->AssertCurrentThreadOwns();
+    return mMonitor->Wait(aDuration);
+  }
+
+  void Notify() {
+    MOZ_ASSERT(mLocked);
+    mMonitor->Notify();
+  }
+  void NotifyAll() {
+    MOZ_ASSERT(mLocked);
+    mMonitor->NotifyAll();
+  }
+
+  // Allow dropping the lock prematurely; for example to support something like:
+  // clang-format off
+  // MonitorAutoLock lock(mMonitor);
+  // ...
+  // if (foo) {
+  //   lock.Unlock();
+  //   MethodThatCantBeCalledWithLock()
+  //   return;
+  // }
+  // clang-format on
+  void Unlock() MOZ_CAPABILITY_RELEASE() {
+    MOZ_ASSERT(mLocked);
+    mMonitor->Unlock();
+    mLocked = false;
+  }
+  void Lock() MOZ_CAPABILITY_ACQUIRE() {
+    MOZ_ASSERT(!mLocked);
+    mMonitor->Lock();
+    mLocked = true;
+  }
+  void AssertCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY() {
+    mMonitor->AssertCurrentThreadOwns();
+  }
+
+ private:
+  bool mLocked;
+  Monitor* mMonitor;
+
+  ReleasableMonitorAutoLock() = delete;
+  ReleasableMonitorAutoLock(const ReleasableMonitorAutoLock&) = delete;
+  ReleasableMonitorAutoLock& operator=(const ReleasableMonitorAutoLock&) =
+      delete;
+  static void* operator new(size_t) noexcept(true);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_Monitor_h
diff --git a/xpcom/threads/MozPromise.h b/xpcom/threads/MozPromise.h
new file mode 100644
index 0000000000..f17e085c2f
--- /dev/null
+++ b/xpcom/threads/MozPromise.h
@@ -0,0 +1,1757 @@
+/* -*- 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/. */
+
+#if !defined(MozPromise_h_)
+#  define MozPromise_h_
+
+#  include <type_traits>
+#  include <utility>
+
+#  include "mozilla/ErrorNames.h"
+#  include "mozilla/Logging.h"
+#  include "mozilla/Maybe.h"
+#  include "mozilla/Monitor.h"
+#  include "mozilla/Mutex.h"
+#  include "mozilla/RefPtr.h"
+#  include "mozilla/UniquePtr.h"
+#  include "mozilla/Variant.h"
+#  include "nsIDirectTaskDispatcher.h"
+#  include "nsISerialEventTarget.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+#  ifdef MOZ_WIDGET_ANDROID
+#    include "mozilla/jni/GeckoResultUtils.h"
+#  endif
+
+#  if MOZ_DIAGNOSTIC_ASSERT_ENABLED
+#    define PROMISE_DEBUG
+#  endif
+
+#  ifdef PROMISE_DEBUG
+#    define PROMISE_ASSERT MOZ_RELEASE_ASSERT
+#  else
+#    define PROMISE_ASSERT(...) \
+      do {                      \
+      } while (0)
+#  endif
+
+#  if DEBUG
+#    include "nsPrintfCString.h"
+#  endif
+
+namespace mozilla {
+
+namespace dom {
+class Promise;
+}
+
+extern LazyLogModule gMozPromiseLog;
+
+#  define PROMISE_LOG(x, ...) \
+    MOZ_LOG(gMozPromiseLog, mozilla::LogLevel::Debug, (x, ##__VA_ARGS__))
+
+namespace detail {
+template <typename F>
+struct MethodTraitsHelper : MethodTraitsHelper<decltype(&F::operator())> {};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...)> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) const> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) volatile> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename ThisType, typename Ret, typename... ArgTypes>
+struct MethodTraitsHelper<Ret (ThisType::*)(ArgTypes...) const volatile> {
+  using ReturnType = Ret;
+  static const size_t ArgSize = sizeof...(ArgTypes);
+};
+template <typename T>
+struct MethodTrait : MethodTraitsHelper<std::remove_reference_t<T>> {};
+
+}  // namespace detail
+
+template <typename MethodType>
+using TakesArgument =
+    std::integral_constant<bool, detail::MethodTrait<MethodType>::ArgSize != 0>;
+
+template <typename MethodType, typename TargetType>
+using ReturnTypeIs =
+    std::is_convertible<typename detail::MethodTrait<MethodType>::ReturnType,
+                        TargetType>;
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise;
+
+template <typename Return>
+struct IsMozPromise : std::false_type {};
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+struct IsMozPromise<MozPromise<ResolveValueT, RejectValueT, IsExclusive>>
+    : std::true_type {};
+
+/*
+ * A promise manages an asynchronous request that may or may not be able to be
+ * fulfilled immediately. When an API returns a promise, the consumer may attach
+ * callbacks to be invoked (asynchronously, on a specified thread) when the
+ * request is either completed (resolved) or cannot be completed (rejected).
+ * Whereas JS promise callbacks are dispatched from Microtask checkpoints,
+ * MozPromises resolution/rejection make a normal round-trip through the event
+ * loop, which simplifies their ordering semantics relative to other native
+ * code.
+ *
+ * MozPromises attempt to mirror the spirit of JS Promises to the extent that
+ * is possible (and desirable) in C++. While the intent is that MozPromises
+ * feel familiar to programmers who are accustomed to their JS-implemented
+ * cousin, we don't shy away from imposing restrictions and adding features that
+ * make sense for the use cases we encounter.
+ *
+ * A MozPromise is ThreadSafe, and may be ->Then()ed on any thread. The Then()
+ * call accepts resolve and reject callbacks, and returns a magic object which
+ * will be implicitly converted to a MozPromise::Request or a MozPromise object
+ * depending on how the return value is used. The magic object serves several
+ * purposes for the consumer.
+ *
+ *   (1) When converting to a MozPromise::Request, it allows the caller to
+ *       cancel the delivery of the resolve/reject value if it has not already
+ *       occurred, via Disconnect() (this must be done on the target thread to
+ *       avoid racing).
+ *
+ *   (2) When converting to a MozPromise (which is called a completion promise),
+ *       it allows promise chaining so ->Then() can be called again to attach
+ *       more resolve and reject callbacks. If the resolve/reject callback
+ *       returns a new MozPromise, that promise is chained to the completion
+ *       promise, such that its resolve/reject value will be forwarded along
+ *       when it arrives. If the resolve/reject callback returns void, the
+ *       completion promise is resolved/rejected with the same value that was
+ *       passed to the callback.
+ *
+ * The MozPromise APIs skirt traditional XPCOM convention by returning nsRefPtrs
+ * (rather than already_AddRefed) from various methods. This is done to allow
+ * elegant chaining of calls without cluttering up the code with intermediate
+ * variables, and without introducing separate API variants for callers that
+ * want a return value (from, say, ->Then()) from those that don't.
+ *
+ * When IsExclusive is true, the MozPromise does a release-mode assertion that
+ * there is at most one call to either Then(...) or ChainTo(...).
+ */
+
+class MozPromiseRefcountable {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(MozPromiseRefcountable)
+ protected:
+  virtual ~MozPromiseRefcountable() = default;
+};
+
+class MozPromiseBase : public MozPromiseRefcountable {
+ public:
+  virtual void AssertIsDead() = 0;
+};
+
+template <typename T>
+class MozPromiseHolder;
+template <typename T>
+class MozPromiseRequestHolder;
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise : public MozPromiseBase {
+  static const uint32_t sMagic = 0xcecace11;
+
+  // Return a |T&&| to enable move when IsExclusive is true or
+  // a |const T&| to enforce copy otherwise.
+  template <typename T,
+            typename R = std::conditional_t<IsExclusive, T&&, const T&>>
+  static R MaybeMove(T& aX) {
+    return static_cast<R>(aX);
+  }
+
+ public:
+  typedef ResolveValueT ResolveValueType;
+  typedef RejectValueT RejectValueType;
+  class ResolveOrRejectValue {
+   public:
+    template <typename ResolveValueType_>
+    void SetResolve(ResolveValueType_&& aResolveValue) {
+      MOZ_ASSERT(IsNothing());
+      mValue = Storage(VariantIndex<ResolveIndex>{},
+                       std::forward<ResolveValueType_>(aResolveValue));
+    }
+
+    template <typename RejectValueType_>
+    void SetReject(RejectValueType_&& aRejectValue) {
+      MOZ_ASSERT(IsNothing());
+      mValue = Storage(VariantIndex<RejectIndex>{},
+                       std::forward<RejectValueType_>(aRejectValue));
+    }
+
+    template <typename ResolveValueType_>
+    static ResolveOrRejectValue MakeResolve(ResolveValueType_&& aResolveValue) {
+      ResolveOrRejectValue val;
+      val.SetResolve(std::forward<ResolveValueType_>(aResolveValue));
+      return val;
+    }
+
+    template <typename RejectValueType_>
+    static ResolveOrRejectValue MakeReject(RejectValueType_&& aRejectValue) {
+      ResolveOrRejectValue val;
+      val.SetReject(std::forward<RejectValueType_>(aRejectValue));
+      return val;
+    }
+
+    bool IsResolve() const { return mValue.template is<ResolveIndex>(); }
+    bool IsReject() const { return mValue.template is<RejectIndex>(); }
+    bool IsNothing() const { return mValue.template is<NothingIndex>(); }
+
+    const ResolveValueType& ResolveValue() const {
+      return mValue.template as<ResolveIndex>();
+    }
+    ResolveValueType& ResolveValue() {
+      return mValue.template as<ResolveIndex>();
+    }
+    const RejectValueType& RejectValue() const {
+      return mValue.template as<RejectIndex>();
+    }
+    RejectValueType& RejectValue() { return mValue.template as<RejectIndex>(); }
+
+   private:
+    enum { NothingIndex, ResolveIndex, RejectIndex };
+    using Storage = Variant<Nothing, ResolveValueType, RejectValueType>;
+    Storage mValue = Storage(VariantIndex<NothingIndex>{});
+  };
+
+ protected:
+  // MozPromise is the public type, and never constructed directly. Construct
+  // a MozPromise::Private, defined below.
+  MozPromise(const char* aCreationSite, bool aIsCompletionPromise)
+      : mCreationSite(aCreationSite),
+        mMutex("MozPromise Mutex"),
+        mHaveRequest(false),
+        mIsCompletionPromise(aIsCompletionPromise)
+#  ifdef PROMISE_DEBUG
+        ,
+        mMagic4(&mMutex)
+#  endif
+  {
+    PROMISE_LOG("%s creating MozPromise (%p)", mCreationSite, this);
+  }
+
+ public:
+  // MozPromise::Private allows us to separate the public interface (upon which
+  // consumers of the promise may invoke methods like Then()) from the private
+  // interface (upon which the creator of the promise may invoke Resolve() or
+  // Reject()). APIs should create and store a MozPromise::Private (usually
+  // via a MozPromiseHolder), and return a MozPromise to consumers.
+  //
+  // NB: We can include the definition of this class inline once B2G ICS is
+  // gone.
+  class Private;
+
+  template <typename ResolveValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndResolve(
+      ResolveValueType_&& aResolveValue, const char* aResolveSite) {
+    static_assert(std::is_convertible_v<ResolveValueType_, ResolveValueT>,
+                  "Resolve() argument must be implicitly convertible to "
+                  "MozPromise's ResolveValueT");
+    RefPtr<typename MozPromise::Private> p =
+        new MozPromise::Private(aResolveSite);
+    p->Resolve(std::forward<ResolveValueType_>(aResolveValue), aResolveSite);
+    return p;
+  }
+
+  template <typename RejectValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndReject(
+      RejectValueType_&& aRejectValue, const char* aRejectSite) {
+    static_assert(std::is_convertible_v<RejectValueType_, RejectValueT>,
+                  "Reject() argument must be implicitly convertible to "
+                  "MozPromise's RejectValueT");
+    RefPtr<typename MozPromise::Private> p =
+        new MozPromise::Private(aRejectSite);
+    p->Reject(std::forward<RejectValueType_>(aRejectValue), aRejectSite);
+    return p;
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  [[nodiscard]] static RefPtr<MozPromise> CreateAndResolveOrReject(
+      ResolveOrRejectValueType_&& aValue, const char* aSite) {
+    RefPtr<typename MozPromise::Private> p = new MozPromise::Private(aSite);
+    p->ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue), aSite);
+    return p;
+  }
+
+  typedef MozPromise<CopyableTArray<ResolveValueType>, RejectValueType,
+                     IsExclusive>
+      AllPromiseType;
+
+  typedef MozPromise<CopyableTArray<ResolveOrRejectValue>, bool, IsExclusive>
+      AllSettledPromiseType;
+
+ private:
+  class AllPromiseHolder : public MozPromiseRefcountable {
+   public:
+    explicit AllPromiseHolder(size_t aDependentPromises)
+        : mPromise(new typename AllPromiseType::Private(__func__)),
+          mOutstandingPromises(aDependentPromises) {
+      MOZ_ASSERT(aDependentPromises > 0);
+      mResolveValues.SetLength(aDependentPromises);
+    }
+
+    template <typename ResolveValueType_>
+    void Resolve(size_t aIndex, ResolveValueType_&& aResolveValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mResolveValues[aIndex].emplace(
+          std::forward<ResolveValueType_>(aResolveValue));
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveValueType> resolveValues;
+        resolveValues.SetCapacity(mResolveValues.Length());
+        for (auto&& resolveValue : mResolveValues) {
+          resolveValues.AppendElement(std::move(resolveValue.ref()));
+        }
+
+        mPromise->Resolve(std::move(resolveValues), __func__);
+        mPromise = nullptr;
+        mResolveValues.Clear();
+      }
+    }
+
+    template <typename RejectValueType_>
+    void Reject(RejectValueType_&& aRejectValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mPromise->Reject(std::forward<RejectValueType_>(aRejectValue), __func__);
+      mPromise = nullptr;
+      mResolveValues.Clear();
+    }
+
+    AllPromiseType* Promise() { return mPromise; }
+
+   private:
+    nsTArray<Maybe<ResolveValueType>> mResolveValues;
+    RefPtr<typename AllPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+
+  // Trying to pass ResolveOrRejectValue by value fails static analysis checks,
+  // so we need to use either a const& or an rvalue reference, depending on
+  // whether IsExclusive is true or not.
+  typedef std::conditional_t<IsExclusive, ResolveOrRejectValue&&,
+                             const ResolveOrRejectValue&>
+      ResolveOrRejectValueParam;
+
+  typedef std::conditional_t<IsExclusive, ResolveValueType&&,
+                             const ResolveValueType&>
+      ResolveValueTypeParam;
+
+  typedef std::conditional_t<IsExclusive, RejectValueType&&,
+                             const RejectValueType&>
+      RejectValueTypeParam;
+
+  class AllSettledPromiseHolder : public MozPromiseRefcountable {
+   public:
+    explicit AllSettledPromiseHolder(size_t aDependentPromises)
+        : mPromise(new typename AllSettledPromiseType::Private(__func__)),
+          mOutstandingPromises(aDependentPromises) {
+      MOZ_ASSERT(aDependentPromises > 0);
+      mValues.SetLength(aDependentPromises);
+    }
+
+    void Settle(size_t aIndex, ResolveOrRejectValueParam aValue) {
+      if (!mPromise) {
+        // Already rejected.
+        return;
+      }
+
+      mValues[aIndex].emplace(MaybeMove(aValue));
+      if (--mOutstandingPromises == 0) {
+        nsTArray<ResolveOrRejectValue> values;
+        values.SetCapacity(mValues.Length());
+        for (auto&& value : mValues) {
+          values.AppendElement(std::move(value.ref()));
+        }
+
+        mPromise->Resolve(std::move(values), __func__);
+        mPromise = nullptr;
+        mValues.Clear();
+      }
+    }
+
+    AllSettledPromiseType* Promise() { return mPromise; }
+
+   private:
+    nsTArray<Maybe<ResolveOrRejectValue>> mValues;
+    RefPtr<typename AllSettledPromiseType::Private> mPromise;
+    size_t mOutstandingPromises;
+  };
+
+ public:
+  [[nodiscard]] static RefPtr<AllPromiseType> All(
+      nsISerialEventTarget* aProcessingTarget,
+      nsTArray<RefPtr<MozPromise>>& aPromises) {
+    if (aPromises.Length() == 0) {
+      return AllPromiseType::CreateAndResolve(
+          CopyableTArray<ResolveValueType>(), __func__);
+    }
+
+    RefPtr<AllPromiseHolder> holder = new AllPromiseHolder(aPromises.Length());
+    RefPtr<AllPromiseType> promise = holder->Promise();
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(
+          aProcessingTarget, __func__,
+          [holder, i](ResolveValueTypeParam aResolveValue) -> void {
+            holder->Resolve(i, MaybeMove(aResolveValue));
+          },
+          [holder](RejectValueTypeParam aRejectValue) -> void {
+            holder->Reject(MaybeMove(aRejectValue));
+          });
+    }
+    return promise;
+  }
+
+  [[nodiscard]] static RefPtr<AllSettledPromiseType> AllSettled(
+      nsISerialEventTarget* aProcessingTarget,
+      nsTArray<RefPtr<MozPromise>>& aPromises) {
+    if (aPromises.Length() == 0) {
+      return AllSettledPromiseType::CreateAndResolve(
+          CopyableTArray<ResolveOrRejectValue>(), __func__);
+    }
+
+    RefPtr<AllSettledPromiseHolder> holder =
+        new AllSettledPromiseHolder(aPromises.Length());
+    RefPtr<AllSettledPromiseType> promise = holder->Promise();
+    for (size_t i = 0; i < aPromises.Length(); ++i) {
+      aPromises[i]->Then(aProcessingTarget, __func__,
+                         [holder, i](ResolveOrRejectValueParam aValue) -> void {
+                           holder->Settle(i, MaybeMove(aValue));
+                         });
+    }
+    return promise;
+  }
+
+  class Request : public MozPromiseRefcountable {
+   public:
+    virtual void Disconnect() = 0;
+
+   protected:
+    Request() : mComplete(false), mDisconnected(false) {}
+    virtual ~Request() = default;
+
+    bool mComplete;
+    bool mDisconnected;
+  };
+
+ protected:
+  /*
+   * A ThenValue tracks a single consumer waiting on the promise. When a
+   * consumer invokes promise->Then(...), a ThenValue is created. Once the
+   * Promise is resolved or rejected, a {Resolve,Reject}Runnable is dispatched,
+   * which invokes the resolve/reject method and then deletes the ThenValue.
+   */
+  class ThenValueBase : public Request {
+    friend class MozPromise;
+    static const uint32_t sMagic = 0xfadece11;
+
+   public:
+    class ResolveOrRejectRunnable final
+        : public PrioritizableCancelableRunnable {
+     public:
+      ResolveOrRejectRunnable(ThenValueBase* aThenValue, MozPromise* aPromise)
+          : PrioritizableCancelableRunnable(
+                aPromise->mPriority,
+                "MozPromise::ThenValueBase::ResolveOrRejectRunnable"),
+            mThenValue(aThenValue),
+            mPromise(aPromise) {
+        MOZ_DIAGNOSTIC_ASSERT(!mPromise->IsPending());
+      }
+
+      ~ResolveOrRejectRunnable() {
+        if (mThenValue) {
+          mThenValue->AssertIsDead();
+        }
+      }
+
+      NS_IMETHOD Run() override {
+        PROMISE_LOG("ResolveOrRejectRunnable::Run() [this=%p]", this);
+        mThenValue->DoResolveOrReject(mPromise->Value());
+        mThenValue = nullptr;
+        mPromise = nullptr;
+        return NS_OK;
+      }
+
+      nsresult Cancel() override { return Run(); }
+
+     private:
+      RefPtr<ThenValueBase> mThenValue;
+      RefPtr<MozPromise> mPromise;
+    };
+
+    ThenValueBase(nsISerialEventTarget* aResponseTarget, const char* aCallSite)
+        : mResponseTarget(aResponseTarget), mCallSite(aCallSite) {
+      MOZ_ASSERT(aResponseTarget);
+    }
+
+#  ifdef PROMISE_DEBUG
+    ~ThenValueBase() {
+      mMagic1 = 0;
+      mMagic2 = 0;
+    }
+#  endif
+
+    void AssertIsDead() {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      // We want to assert that this ThenValues is dead - that is to say, that
+      // there are no consumers waiting for the result. In the case of a normal
+      // ThenValue, we check that it has been disconnected, which is the way
+      // that the consumer signals that it no longer wishes to hear about the
+      // result. If this ThenValue has a completion promise (which is mutually
+      // exclusive with being disconnectable), we recursively assert that every
+      // ThenValue associated with the completion promise is dead.
+      if (MozPromiseBase* p = CompletionPromise()) {
+        p->AssertIsDead();
+      } else {
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+        if (MOZ_UNLIKELY(!Request::mDisconnected)) {
+          MOZ_CRASH_UNSAFE_PRINTF(
+              "MozPromise::ThenValue created from '%s' destroyed without being "
+              "either disconnected, resolved, or rejected (dispatchRv: %s)",
+              mCallSite,
+              mDispatchRv ? GetStaticErrorName(*mDispatchRv)
+                          : "not dispatched");
+        }
+#  endif
+      }
+    }
+
+    void Dispatch(MozPromise* aPromise) {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      aPromise->mMutex.AssertCurrentThreadOwns();
+      MOZ_ASSERT(!aPromise->IsPending());
+
+      nsCOMPtr<nsIRunnable> r = new ResolveOrRejectRunnable(this, aPromise);
+      PROMISE_LOG(
+          "%s Then() call made from %s [Runnable=%p, Promise=%p, ThenValue=%p] "
+          "%s dispatch",
+          aPromise->mValue.IsResolve() ? "Resolving" : "Rejecting", mCallSite,
+          r.get(), aPromise, this,
+          aPromise->mUseSynchronousTaskDispatch ? "synchronous"
+          : aPromise->mUseDirectTaskDispatch    ? "directtask"
+                                                : "normal");
+
+      if (aPromise->mUseSynchronousTaskDispatch &&
+          mResponseTarget->IsOnCurrentThread()) {
+        PROMISE_LOG("ThenValue::Dispatch running task synchronously [this=%p]",
+                    this);
+        r->Run();
+        return;
+      }
+
+      if (aPromise->mUseDirectTaskDispatch &&
+          mResponseTarget->IsOnCurrentThread()) {
+        PROMISE_LOG(
+            "ThenValue::Dispatch dispatch task via direct task queue [this=%p]",
+            this);
+        nsCOMPtr<nsIDirectTaskDispatcher> dispatcher =
+            do_QueryInterface(mResponseTarget);
+        if (dispatcher) {
+          SetDispatchRv(dispatcher->DispatchDirectTask(r.forget()));
+          return;
+        }
+        NS_WARNING(
+            nsPrintfCString(
+                "Direct Task dispatching not available for thread \"%s\"",
+                PR_GetThreadName(PR_GetCurrentThread()))
+                .get());
+        MOZ_DIAGNOSTIC_ASSERT(
+            false,
+            "mResponseTarget must implement nsIDirectTaskDispatcher for direct "
+            "task dispatching");
+      }
+
+      // Promise consumers are allowed to disconnect the Request object and
+      // then shut down the thread or task queue that the promise result would
+      // be dispatched on. So we unfortunately can't assert that promise
+      // dispatch succeeds. :-(
+      // We do record whether or not it succeeds so that if the ThenValueBase is
+      // then destroyed and it was not disconnected, we can include that
+      // information in the assertion message.
+      SetDispatchRv(mResponseTarget->Dispatch(r.forget()));
+    }
+
+    void Disconnect() override {
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsOnCurrentThread());
+      MOZ_DIAGNOSTIC_ASSERT(!Request::mComplete);
+      Request::mDisconnected = true;
+
+      // We could support rejecting the completion promise on disconnection, but
+      // then we'd need to have some sort of default reject value. The use cases
+      // of disconnection and completion promise chaining seem pretty
+      // orthogonal, so let's use assert against it.
+      MOZ_DIAGNOSTIC_ASSERT(!CompletionPromise());
+    }
+
+   protected:
+    virtual MozPromiseBase* CompletionPromise() const = 0;
+    virtual void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) = 0;
+
+    void DoResolveOrReject(ResolveOrRejectValue& aValue) {
+      PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic);
+      MOZ_DIAGNOSTIC_ASSERT(mResponseTarget->IsOnCurrentThread());
+      Request::mComplete = true;
+      if (Request::mDisconnected) {
+        PROMISE_LOG(
+            "ThenValue::DoResolveOrReject disconnected - bailing out [this=%p]",
+            this);
+        return;
+      }
+
+      // Invoke the resolve or reject method.
+      DoResolveOrRejectInternal(aValue);
+    }
+
+    void SetDispatchRv(nsresult aRv) {
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      mDispatchRv = Some(aRv);
+#  endif
+    }
+
+    nsCOMPtr<nsISerialEventTarget>
+        mResponseTarget;  // May be released on any thread.
+#  ifdef PROMISE_DEBUG
+    uint32_t mMagic1 = sMagic;
+#  endif
+    const char* mCallSite;
+#  ifdef PROMISE_DEBUG
+    uint32_t mMagic2 = sMagic;
+#  endif
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    Maybe<nsresult> mDispatchRv;
+#  endif
+  };
+
+  /*
+   * We create two overloads for invoking Resolve/Reject Methods so as to
+   * make the resolve/reject value argument "optional".
+   */
+  template <typename ThisType, typename MethodType, typename ValueType>
+  static std::enable_if_t<TakesArgument<MethodType>::value,
+                          typename detail::MethodTrait<MethodType>::ReturnType>
+  InvokeMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue) {
+    return (aThisVal->*aMethod)(std::forward<ValueType>(aValue));
+  }
+
+  template <typename ThisType, typename MethodType, typename ValueType>
+  static std::enable_if_t<!TakesArgument<MethodType>::value,
+                          typename detail::MethodTrait<MethodType>::ReturnType>
+  InvokeMethod(ThisType* aThisVal, MethodType aMethod, ValueType&& aValue) {
+    return (aThisVal->*aMethod)();
+  }
+
+  // Called when promise chaining is supported.
+  template <bool SupportChaining, typename ThisType, typename MethodType,
+            typename ValueType, typename CompletionPromiseType>
+  static std::enable_if_t<SupportChaining, void> InvokeCallbackMethod(
+      ThisType* aThisVal, MethodType aMethod, ValueType&& aValue,
+      CompletionPromiseType&& aCompletionPromise) {
+    auto p = InvokeMethod(aThisVal, aMethod, std::forward<ValueType>(aValue));
+    if (aCompletionPromise) {
+      p->ChainTo(aCompletionPromise.forget(), "<chained completion promise>");
+    }
+  }
+
+  // Called when promise chaining is not supported.
+  template <bool SupportChaining, typename ThisType, typename MethodType,
+            typename ValueType, typename CompletionPromiseType>
+  static std::enable_if_t<!SupportChaining, void> InvokeCallbackMethod(
+      ThisType* aThisVal, MethodType aMethod, ValueType&& aValue,
+      CompletionPromiseType&& aCompletionPromise) {
+    MOZ_DIAGNOSTIC_ASSERT(
+        !aCompletionPromise,
+        "Can't do promise chaining for a non-promise-returning method.");
+    InvokeMethod(aThisVal, aMethod, std::forward<ValueType>(aValue));
+  }
+
+  template <typename>
+  class ThenCommand;
+
+  template <typename...>
+  class ThenValue;
+
+  template <typename ThisType, typename ResolveMethodType,
+            typename RejectMethodType>
+  class ThenValue<ThisType*, ResolveMethodType, RejectMethodType>
+      : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveMethodType>::ReturnType>::Type;
+    using R2 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<RejectMethodType>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value &&
+                                         std::is_same_v<R1, R2>>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget, ThisType* aThisVal,
+              ResolveMethodType aResolveMethod, RejectMethodType aRejectMethod,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite),
+          mThisVal(aThisVal),
+          mResolveMethod(aResolveMethod),
+          mRejectMethod(aRejectMethod) {}
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Null out our refcounted
+      // this-value now so that it's released predictably on the dispatch
+      // thread.
+      mThisVal = nullptr;
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      if (aValue.IsResolve()) {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mThisVal.get(), mResolveMethod, MaybeMove(aValue.ResolveValue()),
+            std::move(mCompletionPromise));
+      } else {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mThisVal.get(), mRejectMethod, MaybeMove(aValue.RejectValue()),
+            std::move(mCompletionPromise));
+      }
+
+      // Null out mThisVal after invoking the callback so that any references
+      // are released predictably on the dispatch thread. Otherwise, it would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mThisVal = nullptr;
+    }
+
+   private:
+    RefPtr<ThisType>
+        mThisVal;  // Only accessed and refcounted on dispatch thread.
+    ResolveMethodType mResolveMethod;
+    RejectMethodType mRejectMethod;
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  template <typename ThisType, typename ResolveRejectMethodType>
+  class ThenValue<ThisType*, ResolveRejectMethodType> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<typename detail::MethodTrait<
+        ResolveRejectMethodType>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget, ThisType* aThisVal,
+              ResolveRejectMethodType aResolveRejectMethod,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite),
+          mThisVal(aThisVal),
+          mResolveRejectMethod(aResolveRejectMethod) {}
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Null out our refcounted
+      // this-value now so that it's released predictably on the dispatch
+      // thread.
+      mThisVal = nullptr;
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      InvokeCallbackMethod<SupportChaining::value>(
+          mThisVal.get(), mResolveRejectMethod, MaybeMove(aValue),
+          std::move(mCompletionPromise));
+
+      // Null out mThisVal after invoking the callback so that any references
+      // are released predictably on the dispatch thread. Otherwise, it would be
+      // released on whatever thread last drops its reference to the ThenValue,
+      // which may or may not be ok.
+      mThisVal = nullptr;
+    }
+
+   private:
+    RefPtr<ThisType>
+        mThisVal;  // Only accessed and refcounted on dispatch thread.
+    ResolveRejectMethodType mResolveRejectMethod;
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  // NB: We could use std::function here instead of a template if it were
+  // supported. :-(
+  template <typename ResolveFunction, typename RejectFunction>
+  class ThenValue<ResolveFunction, RejectFunction> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveFunction>::ReturnType>::Type;
+    using R2 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<RejectFunction>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value &&
+                                         std::is_same_v<R1, R2>>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget,
+              ResolveFunction&& aResolveFunction,
+              RejectFunction&& aRejectFunction, const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite) {
+      mResolveFunction.emplace(std::move(aResolveFunction));
+      mRejectFunction.emplace(std::move(aRejectFunction));
+    }
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Destroy our callbacks
+      // now so that any references in closures are released predictable on
+      // the dispatch thread.
+      mResolveFunction.reset();
+      mRejectFunction.reset();
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      // Note: The usage of InvokeCallbackMethod here requires that
+      // ResolveFunction/RejectFunction are capture-lambdas (i.e. anonymous
+      // classes with ::operator()), since it allows us to share code more
+      // easily. We could fix this if need be, though it's quite easy to work
+      // around by just capturing something.
+      if (aValue.IsResolve()) {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mResolveFunction.ptr(), &ResolveFunction::operator(),
+            MaybeMove(aValue.ResolveValue()), std::move(mCompletionPromise));
+      } else {
+        InvokeCallbackMethod<SupportChaining::value>(
+            mRejectFunction.ptr(), &RejectFunction::operator(),
+            MaybeMove(aValue.RejectValue()), std::move(mCompletionPromise));
+      }
+
+      // Destroy callbacks after invocation so that any references in closures
+      // are released predictably on the dispatch thread. Otherwise, they would
+      // be released on whatever thread last drops its reference to the
+      // ThenValue, which may or may not be ok.
+      mResolveFunction.reset();
+      mRejectFunction.reset();
+    }
+
+   private:
+    Maybe<ResolveFunction>
+        mResolveFunction;  // Only accessed and deleted on dispatch thread.
+    Maybe<RejectFunction>
+        mRejectFunction;  // Only accessed and deleted on dispatch thread.
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+  template <typename ResolveRejectFunction>
+  class ThenValue<ResolveRejectFunction> : public ThenValueBase {
+    friend class ThenCommand<ThenValue>;
+
+    using R1 = typename RemoveSmartPointer<
+        typename detail::MethodTrait<ResolveRejectFunction>::ReturnType>::Type;
+    using SupportChaining =
+        std::integral_constant<bool, IsMozPromise<R1>::value>;
+
+    // Fall back to MozPromise when promise chaining is not supported to make
+    // code compile.
+    using PromiseType =
+        std::conditional_t<SupportChaining::value, R1, MozPromise>;
+
+   public:
+    ThenValue(nsISerialEventTarget* aResponseTarget,
+              ResolveRejectFunction&& aResolveRejectFunction,
+              const char* aCallSite)
+        : ThenValueBase(aResponseTarget, aCallSite) {
+      mResolveRejectFunction.emplace(std::move(aResolveRejectFunction));
+    }
+
+    void Disconnect() override {
+      ThenValueBase::Disconnect();
+
+      // If a Request has been disconnected, we don't guarantee that the
+      // resolve/reject runnable will be dispatched. Destroy our callbacks
+      // now so that any references in closures are released predictable on
+      // the dispatch thread.
+      mResolveRejectFunction.reset();
+    }
+
+   protected:
+    MozPromiseBase* CompletionPromise() const override {
+      return mCompletionPromise;
+    }
+
+    void DoResolveOrRejectInternal(ResolveOrRejectValue& aValue) override {
+      // Note: The usage of InvokeCallbackMethod here requires that
+      // ResolveRejectFunction is capture-lambdas (i.e. anonymous
+      // classes with ::operator()), since it allows us to share code more
+      // easily. We could fix this if need be, though it's quite easy to work
+      // around by just capturing something.
+      InvokeCallbackMethod<SupportChaining::value>(
+          mResolveRejectFunction.ptr(), &ResolveRejectFunction::operator(),
+          MaybeMove(aValue), std::move(mCompletionPromise));
+
+      // Destroy callbacks after invocation so that any references in closures
+      // are released predictably on the dispatch thread. Otherwise, they would
+      // be released on whatever thread last drops its reference to the
+      // ThenValue, which may or may not be ok.
+      mResolveRejectFunction.reset();
+    }
+
+   private:
+    Maybe<ResolveRejectFunction>
+        mResolveRejectFunction;  // Only accessed and deleted on dispatch
+                                 // thread.
+    RefPtr<typename PromiseType::Private> mCompletionPromise;
+  };
+
+ public:
+  void ThenInternal(already_AddRefed<ThenValueBase> aThenValue,
+                    const char* aCallSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    RefPtr<ThenValueBase> thenValue = aThenValue;
+    MutexAutoLock lock(mMutex);
+    MOZ_DIAGNOSTIC_ASSERT(
+        !IsExclusive || !mHaveRequest,
+        "Using an exclusive promise in a non-exclusive fashion");
+    mHaveRequest = true;
+    PROMISE_LOG("%s invoking Then() [this=%p, aThenValue=%p, isPending=%d]",
+                aCallSite, this, thenValue.get(), (int)IsPending());
+    if (!IsPending()) {
+      thenValue->Dispatch(this);
+    } else {
+      mThenValues.AppendElement(thenValue.forget());
+    }
+  }
+
+ protected:
+  /*
+   * A command object to store all information needed to make a request to
+   * the promise. This allows us to delay the request until further use is
+   * known (whether it is ->Then() again for more promise chaining or ->Track()
+   * to terminate chaining and issue the request).
+   *
+   * This allows a unified syntax for promise chaining and disconnection
+   * and feels more like its JS counterpart.
+   */
+  template <typename ThenValueType>
+  class ThenCommand {
+    // Allow Promise1::ThenCommand to access the private constructor,
+    // Promise2::ThenCommand(ThenCommand&&).
+    template <typename, typename, bool>
+    friend class MozPromise;
+
+    using PromiseType = typename ThenValueType::PromiseType;
+    using Private = typename PromiseType::Private;
+
+    ThenCommand(const char* aCallSite,
+                already_AddRefed<ThenValueType> aThenValue,
+                MozPromise* aReceiver)
+        : mCallSite(aCallSite), mThenValue(aThenValue), mReceiver(aReceiver) {}
+
+    ThenCommand(ThenCommand&& aOther) = default;
+
+   public:
+    ~ThenCommand() {
+      // Issue the request now if the return value of Then() is not used.
+      if (mThenValue) {
+        mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+      }
+    }
+
+    // Allow RefPtr<MozPromise> p = somePromise->Then();
+    //       p->Then(thread1, ...);
+    //       p->Then(thread2, ...);
+    operator RefPtr<PromiseType>() {
+      static_assert(
+          ThenValueType::SupportChaining::value,
+          "The resolve/reject callback needs to return a RefPtr<MozPromise> "
+          "in order to do promise chaining.");
+
+      // mCompletionPromise must be created before ThenInternal() to avoid race.
+      RefPtr<Private> p =
+          new Private("<completion promise>", true /* aIsCompletionPromise */);
+      mThenValue->mCompletionPromise = p;
+      // Note ThenInternal() might nullify mCompletionPromise before return.
+      // So we need to return p instead of mCompletionPromise.
+      mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+      return p;
+    }
+
+    template <typename... Ts>
+    auto Then(Ts&&... aArgs) -> decltype(std::declval<PromiseType>().Then(
+        std::forward<Ts>(aArgs)...)) {
+      return static_cast<RefPtr<PromiseType>>(*this)->Then(
+          std::forward<Ts>(aArgs)...);
+    }
+
+    void Track(MozPromiseRequestHolder<MozPromise>& aRequestHolder) {
+      aRequestHolder.Track(do_AddRef(mThenValue));
+      mReceiver->ThenInternal(mThenValue.forget(), mCallSite);
+    }
+
+    // Allow calling ->Then() again for more promise chaining or ->Track() to
+    // end chaining and track the request for future disconnection.
+    ThenCommand* operator->() { return this; }
+
+   private:
+    const char* mCallSite;
+    RefPtr<ThenValueType> mThenValue;
+    RefPtr<MozPromise> mReceiver;
+  };
+
+ public:
+  template <typename ThisType, typename... Methods,
+            typename ThenValueType = ThenValue<ThisType*, Methods...>,
+            typename ReturnType = ThenCommand<ThenValueType>>
+  ReturnType Then(nsISerialEventTarget* aResponseTarget, const char* aCallSite,
+                  ThisType* aThisVal, Methods... aMethods) {
+    RefPtr<ThenValueType> thenValue =
+        new ThenValueType(aResponseTarget, aThisVal, aMethods..., aCallSite);
+    return ReturnType(aCallSite, thenValue.forget(), this);
+  }
+
+  template <typename... Functions,
+            typename ThenValueType = ThenValue<Functions...>,
+            typename ReturnType = ThenCommand<ThenValueType>>
+  ReturnType Then(nsISerialEventTarget* aResponseTarget, const char* aCallSite,
+                  Functions&&... aFunctions) {
+    RefPtr<ThenValueType> thenValue =
+        new ThenValueType(aResponseTarget, std::move(aFunctions)..., aCallSite);
+    return ReturnType(aCallSite, thenValue.forget(), this);
+  }
+
+  void ChainTo(already_AddRefed<Private> aChainedPromise,
+               const char* aCallSite) {
+    MutexAutoLock lock(mMutex);
+    MOZ_DIAGNOSTIC_ASSERT(
+        !IsExclusive || !mHaveRequest,
+        "Using an exclusive promise in a non-exclusive fashion");
+    mHaveRequest = true;
+    RefPtr<Private> chainedPromise = aChainedPromise;
+    PROMISE_LOG(
+        "%s invoking Chain() [this=%p, chainedPromise=%p, isPending=%d]",
+        aCallSite, this, chainedPromise.get(), (int)IsPending());
+
+    // We want to use the same type of dispatching method with the chained
+    // promises.
+
+    // We need to ensure that the UseSynchronousTaskDispatch branch isn't taken
+    // at compilation time to ensure we're not triggering the static_assert in
+    // UseSynchronousTaskDispatch method. if constexpr (IsExclusive) ensures
+    // that.
+    if (mUseDirectTaskDispatch) {
+      chainedPromise->UseDirectTaskDispatch(aCallSite);
+    } else if constexpr (IsExclusive) {
+      if (mUseSynchronousTaskDispatch) {
+        chainedPromise->UseSynchronousTaskDispatch(aCallSite);
+      }
+    } else {
+      chainedPromise->SetTaskPriority(mPriority, aCallSite);
+    }
+
+    if (!IsPending()) {
+      ForwardTo(chainedPromise);
+    } else {
+      mChainedPromises.AppendElement(chainedPromise);
+    }
+  }
+
+#  ifdef MOZ_WIDGET_ANDROID
+  // Creates a C++ MozPromise from its Java counterpart, GeckoResult.
+  [[nodiscard]] static RefPtr<MozPromise> FromGeckoResult(
+      java::GeckoResult::Param aGeckoResult) {
+    using jni::GeckoResultCallback;
+    RefPtr<Private> p = new Private("GeckoResult Glue", false);
+    auto resolve = GeckoResultCallback::CreateAndAttach<ResolveValueType>(
+        [p](ResolveValueType&& aArg) {
+          p->Resolve(MaybeMove(aArg), __func__);
+        });
+    auto reject = GeckoResultCallback::CreateAndAttach<RejectValueType>(
+        [p](RejectValueType&& aArg) { p->Reject(MaybeMove(aArg), __func__); });
+    aGeckoResult->NativeThen(resolve, reject);
+    return p;
+  }
+#  endif
+
+  // Note we expose the function AssertIsDead() instead of IsDead() since
+  // checking IsDead() is a data race in the situation where the request is not
+  // dead. Therefore we enforce the form |Assert(IsDead())| by exposing
+  // AssertIsDead() only.
+  void AssertIsDead() override {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    for (auto&& then : mThenValues) {
+      then->AssertIsDead();
+    }
+    for (auto&& chained : mChainedPromises) {
+      chained->AssertIsDead();
+    }
+  }
+
+  bool IsResolved() const { return mValue.IsResolve(); }
+
+ protected:
+  bool IsPending() const { return mValue.IsNothing(); }
+
+  ResolveOrRejectValue& Value() {
+    // This method should only be called once the value has stabilized. As
+    // such, we don't need to acquire the lock here.
+    MOZ_DIAGNOSTIC_ASSERT(!IsPending());
+    return mValue;
+  }
+
+  void DispatchAll() {
+    mMutex.AssertCurrentThreadOwns();
+    for (auto&& thenValue : mThenValues) {
+      thenValue->Dispatch(this);
+    }
+    mThenValues.Clear();
+
+    for (auto&& chainedPromise : mChainedPromises) {
+      ForwardTo(chainedPromise);
+    }
+    mChainedPromises.Clear();
+  }
+
+  void ForwardTo(Private* aOther) {
+    MOZ_ASSERT(!IsPending());
+    if (mValue.IsResolve()) {
+      aOther->Resolve(MaybeMove(mValue.ResolveValue()), "<chained promise>");
+    } else {
+      aOther->Reject(MaybeMove(mValue.RejectValue()), "<chained promise>");
+    }
+  }
+
+  virtual ~MozPromise() {
+    PROMISE_LOG("MozPromise::~MozPromise [this=%p]", this);
+    AssertIsDead();
+    // We can't guarantee a completion promise will always be revolved or
+    // rejected since ResolveOrRejectRunnable might not run when dispatch fails.
+    if (!mIsCompletionPromise) {
+      MOZ_ASSERT(!IsPending());
+      MOZ_ASSERT(mThenValues.IsEmpty());
+      MOZ_ASSERT(mChainedPromises.IsEmpty());
+    }
+#  ifdef PROMISE_DEBUG
+    mMagic1 = 0;
+    mMagic2 = 0;
+    mMagic3 = 0;
+    mMagic4 = nullptr;
+#  endif
+  };
+
+  const char* mCreationSite;  // For logging
+  Mutex mMutex MOZ_UNANNOTATED;
+  ResolveOrRejectValue mValue;
+  bool mUseSynchronousTaskDispatch = false;
+  bool mUseDirectTaskDispatch = false;
+  uint32_t mPriority = nsIRunnablePriority::PRIORITY_NORMAL;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic1 = sMagic;
+#  endif
+  // Try shows we never have more than 3 elements when IsExclusive is false.
+  // So '3' is a good value to avoid heap allocation in most cases.
+  AutoTArray<RefPtr<ThenValueBase>, IsExclusive ? 1 : 3> mThenValues;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic2 = sMagic;
+#  endif
+  nsTArray<RefPtr<Private>> mChainedPromises;
+#  ifdef PROMISE_DEBUG
+  uint32_t mMagic3 = sMagic;
+#  endif
+  bool mHaveRequest;
+  const bool mIsCompletionPromise;
+#  ifdef PROMISE_DEBUG
+  void* mMagic4;
+#  endif
+};
+
+template <typename ResolveValueT, typename RejectValueT, bool IsExclusive>
+class MozPromise<ResolveValueT, RejectValueT, IsExclusive>::Private
+    : public MozPromise<ResolveValueT, RejectValueT, IsExclusive> {
+ public:
+  explicit Private(const char* aCreationSite, bool aIsCompletionPromise = false)
+      : MozPromise(aCreationSite, aIsCompletionPromise) {}
+
+  template <typename ResolveValueT_>
+  void Resolve(ResolveValueT_&& aResolveValue, const char* aResolveSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s resolving MozPromise (%p created at %s)", aResolveSite,
+                this, mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aResolveSite, this, mCreationSite);
+      return;
+    }
+    mValue.SetResolve(std::forward<ResolveValueT_>(aResolveValue));
+    DispatchAll();
+  }
+
+  template <typename RejectValueT_>
+  void Reject(RejectValueT_&& aRejectValue, const char* aRejectSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s rejecting MozPromise (%p created at %s)", aRejectSite, this,
+                mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aRejectSite, this, mCreationSite);
+      return;
+    }
+    mValue.SetReject(std::forward<RejectValueT_>(aRejectValue));
+    DispatchAll();
+  }
+
+  template <typename ResolveOrRejectValue_>
+  void ResolveOrReject(ResolveOrRejectValue_&& aValue, const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s resolveOrRejecting MozPromise (%p created at %s)", aSite,
+                this, mCreationSite);
+    if (!IsPending()) {
+      PROMISE_LOG(
+          "%s ignored already resolved or rejected MozPromise (%p created at "
+          "%s)",
+          aSite, this, mCreationSite);
+      return;
+    }
+    mValue = std::forward<ResolveOrRejectValue_>(aValue);
+    DispatchAll();
+  }
+
+  // If the caller and target are both on the same thread, run the the resolve
+  // or reject callback synchronously. Otherwise, the task will be dispatched
+  // via the target Dispatch method.
+  void UseSynchronousTaskDispatch(const char* aSite) {
+    static_assert(
+        IsExclusive,
+        "Synchronous dispatch can only be used with exclusive promises");
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s UseSynchronousTaskDispatch MozPromise (%p created at %s)",
+                aSite, this, mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    mUseSynchronousTaskDispatch = true;
+  }
+
+  // If the caller and target are both on the same thread, run the
+  // resolve/reject callback off the direct task queue instead. This avoids a
+  // full trip to the back of the event queue for each additional asynchronous
+  // step when using MozPromise, and is similar (but not identical to) the
+  // microtask semantics of JS promises.
+  void UseDirectTaskDispatch(const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s UseDirectTaskDispatch MozPromise (%p created at %s)", aSite,
+                this, mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    MOZ_ASSERT(!mUseSynchronousTaskDispatch,
+               "Promise already set for synchronous dispatch");
+    mUseDirectTaskDispatch = true;
+  }
+
+  // If the resolve/reject will be handled on a thread supporting priorities,
+  // one may want to tweak the priority of the task by passing a
+  // nsIRunnablePriority::PRIORITY_* to SetTaskPriority.
+  void SetTaskPriority(uint32_t aPriority, const char* aSite) {
+    PROMISE_ASSERT(mMagic1 == sMagic && mMagic2 == sMagic &&
+                   mMagic3 == sMagic && mMagic4 == &mMutex);
+    MutexAutoLock lock(mMutex);
+    PROMISE_LOG("%s TaskPriority MozPromise (%p created at %s)", aSite, this,
+                mCreationSite);
+    MOZ_ASSERT(IsPending(),
+               "A Promise must not have been already resolved or rejected to "
+               "set dispatch state");
+    MOZ_ASSERT(!mUseSynchronousTaskDispatch,
+               "Promise already set for synchronous dispatch");
+    MOZ_ASSERT(!mUseDirectTaskDispatch,
+               "Promise already set for direct dispatch");
+    mPriority = aPriority;
+  }
+};
+
+// A generic promise type that does the trick for simple use cases.
+typedef MozPromise<bool, nsresult, /* IsExclusive = */ true> GenericPromise;
+
+// A generic, non-exclusive promise type that does the trick for simple use
+// cases.
+typedef MozPromise<bool, nsresult, /* IsExclusive = */ false>
+    GenericNonExclusivePromise;
+
+/*
+ * Class to encapsulate a promise for a particular role. Use this as the member
+ * variable for a class whose method returns a promise.
+ */
+template <typename PromiseType, typename ImplType>
+class MozPromiseHolderBase {
+ public:
+  MozPromiseHolderBase() = default;
+
+  MozPromiseHolderBase(MozPromiseHolderBase&& aOther) = default;
+  MozPromiseHolderBase& operator=(MozPromiseHolderBase&& aOther) = default;
+
+  ~MozPromiseHolderBase() { MOZ_ASSERT(!mPromise); }
+
+  already_AddRefed<PromiseType> Ensure(const char* aMethodName) {
+    static_cast<ImplType*>(this)->Check();
+    if (!mPromise) {
+      mPromise = new (typename PromiseType::Private)(aMethodName);
+    }
+    RefPtr<PromiseType> p = mPromise.get();
+    return p.forget();
+  }
+
+  bool IsEmpty() const {
+    static_cast<const ImplType*>(this)->Check();
+    return !mPromise;
+  }
+
+  already_AddRefed<typename PromiseType::Private> Steal() {
+    static_cast<ImplType*>(this)->Check();
+    return mPromise.forget();
+  }
+
+  template <typename ResolveValueType_>
+  void Resolve(ResolveValueType_&& aResolveValue, const char* aMethodName) {
+    static_assert(std::is_convertible_v<ResolveValueType_,
+                                        typename PromiseType::ResolveValueType>,
+                  "Resolve() argument must be implicitly convertible to "
+                  "MozPromise's ResolveValueT");
+
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->Resolve(std::forward<ResolveValueType_>(aResolveValue),
+                      aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename ResolveValueType_>
+  void ResolveIfExists(ResolveValueType_&& aResolveValue,
+                       const char* aMethodName) {
+    if (!IsEmpty()) {
+      Resolve(std::forward<ResolveValueType_>(aResolveValue), aMethodName);
+    }
+  }
+
+  template <typename RejectValueType_>
+  void Reject(RejectValueType_&& aRejectValue, const char* aMethodName) {
+    static_assert(std::is_convertible_v<RejectValueType_,
+                                        typename PromiseType::RejectValueType>,
+                  "Reject() argument must be implicitly convertible to "
+                  "MozPromise's RejectValueT");
+
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->Reject(std::forward<RejectValueType_>(aRejectValue), aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename RejectValueType_>
+  void RejectIfExists(RejectValueType_&& aRejectValue,
+                      const char* aMethodName) {
+    if (!IsEmpty()) {
+      Reject(std::forward<RejectValueType_>(aRejectValue), aMethodName);
+    }
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  void ResolveOrReject(ResolveOrRejectValueType_&& aValue,
+                       const char* aMethodName) {
+    static_cast<ImplType*>(this)->Check();
+    MOZ_ASSERT(mPromise);
+    mPromise->ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue),
+                              aMethodName);
+    mPromise = nullptr;
+  }
+
+  template <typename ResolveOrRejectValueType_>
+  void ResolveOrRejectIfExists(ResolveOrRejectValueType_&& aValue,
+                               const char* aMethodName) {
+    if (!IsEmpty()) {
+      ResolveOrReject(std::forward<ResolveOrRejectValueType_>(aValue),
+                      aMethodName);
+    }
+  }
+
+  void UseSynchronousTaskDispatch(const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->UseSynchronousTaskDispatch(aSite);
+  }
+
+  void UseDirectTaskDispatch(const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->UseDirectTaskDispatch(aSite);
+  }
+
+  void SetTaskPriority(uint32_t aPriority, const char* aSite) {
+    MOZ_ASSERT(mPromise);
+    mPromise->SetTaskPriority(aPriority, aSite);
+  }
+
+ private:
+  RefPtr<typename PromiseType::Private> mPromise;
+};
+
+template <typename PromiseType>
+class MozPromiseHolder
+    : public MozPromiseHolderBase<PromiseType, MozPromiseHolder<PromiseType>> {
+ public:
+  using MozPromiseHolderBase<
+      PromiseType, MozPromiseHolder<PromiseType>>::MozPromiseHolderBase;
+  static constexpr void Check(){};
+};
+
+template <typename PromiseType>
+class MozMonitoredPromiseHolder
+    : public MozPromiseHolderBase<PromiseType,
+                                  MozMonitoredPromiseHolder<PromiseType>> {
+ public:
+  // Provide a Monitor that should always be held when accessing this instance.
+  explicit MozMonitoredPromiseHolder(Monitor* const aMonitor)
+      : mMonitor(aMonitor) {
+    MOZ_ASSERT(aMonitor);
+  }
+
+  MozMonitoredPromiseHolder(MozMonitoredPromiseHolder&& aOther) = delete;
+  MozMonitoredPromiseHolder& operator=(MozMonitoredPromiseHolder&& aOther) =
+      delete;
+
+  void Check() const { mMonitor->AssertCurrentThreadOwns(); }
+
+ private:
+  Monitor* const mMonitor;
+};
+
+/*
+ * Class to encapsulate a MozPromise::Request reference. Use this as the member
+ * variable for a class waiting on a MozPromise.
+ */
+template <typename PromiseType>
+class MozPromiseRequestHolder {
+ public:
+  MozPromiseRequestHolder() = default;
+  ~MozPromiseRequestHolder() { MOZ_ASSERT(!mRequest); }
+
+  void Track(already_AddRefed<typename PromiseType::Request> aRequest) {
+    MOZ_DIAGNOSTIC_ASSERT(!Exists());
+    mRequest = aRequest;
+  }
+
+  void Complete() {
+    MOZ_DIAGNOSTIC_ASSERT(Exists());
+    mRequest = nullptr;
+  }
+
+  // Disconnects and forgets an outstanding promise. The resolve/reject methods
+  // will never be called.
+  void Disconnect() {
+    MOZ_ASSERT(Exists());
+    mRequest->Disconnect();
+    mRequest = nullptr;
+  }
+
+  void DisconnectIfExists() {
+    if (Exists()) {
+      Disconnect();
+    }
+  }
+
+  bool Exists() const { return !!mRequest; }
+
+ private:
+  RefPtr<typename PromiseType::Request> mRequest;
+};
+
+// Asynchronous Potentially-Cross-Thread Method Calls.
+//
+// This machinery allows callers to schedule a promise-returning function
+// (a method and object, or a function object like a lambda) to be invoked
+// asynchronously on a given thread, while at the same time receiving a
+// promise upon which to invoke Then() immediately. InvokeAsync dispatches a
+// task to invoke the function on the proper thread and also chain the
+// resulting promise to the one that the caller received, so that resolve/
+// reject values are forwarded through.
+
+namespace detail {
+
+// Non-templated base class to allow us to use MOZ_COUNT_{C,D}TOR, which cause
+// assertions when used on templated types.
+class MethodCallBase {
+ public:
+  MOZ_COUNTED_DEFAULT_CTOR(MethodCallBase)
+  MOZ_COUNTED_DTOR_VIRTUAL(MethodCallBase)
+};
+
+template <typename PromiseType, typename MethodType, typename ThisType,
+          typename... Storages>
+class MethodCall : public MethodCallBase {
+ public:
+  template <typename... Args>
+  MethodCall(MethodType aMethod, ThisType* aThisVal, Args&&... aArgs)
+      : mMethod(aMethod),
+        mThisVal(aThisVal),
+        mArgs(std::forward<Args>(aArgs)...) {
+    static_assert(sizeof...(Storages) == sizeof...(Args),
+                  "Storages and Args should have equal sizes");
+  }
+
+  RefPtr<PromiseType> Invoke() { return mArgs.apply(mThisVal.get(), mMethod); }
+
+ private:
+  MethodType mMethod;
+  RefPtr<ThisType> mThisVal;
+  RunnableMethodArguments<Storages...> mArgs;
+};
+
+template <typename PromiseType, typename MethodType, typename ThisType,
+          typename... Storages>
+class ProxyRunnable : public CancelableRunnable {
+ public:
+  ProxyRunnable(
+      typename PromiseType::Private* aProxyPromise,
+      MethodCall<PromiseType, MethodType, ThisType, Storages...>* aMethodCall)
+      : CancelableRunnable("detail::ProxyRunnable"),
+        mProxyPromise(aProxyPromise),
+        mMethodCall(aMethodCall) {}
+
+  NS_IMETHOD Run() override {
+    RefPtr<PromiseType> p = mMethodCall->Invoke();
+    mMethodCall = nullptr;
+    p->ChainTo(mProxyPromise.forget(), "<Proxy Promise>");
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+ private:
+  RefPtr<typename PromiseType::Private> mProxyPromise;
+  UniquePtr<MethodCall<PromiseType, MethodType, ThisType, Storages...>>
+      mMethodCall;
+};
+
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes>
+static RefPtr<PromiseType> InvokeAsyncImpl(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  MOZ_ASSERT(aTarget);
+
+  typedef RefPtr<PromiseType> (ThisType::*MethodType)(ArgTypes...);
+  typedef detail::MethodCall<PromiseType, MethodType, ThisType, Storages...>
+      MethodCallType;
+  typedef detail::ProxyRunnable<PromiseType, MethodType, ThisType, Storages...>
+      ProxyRunnableType;
+
+  MethodCallType* methodCall = new MethodCallType(
+      aMethod, aThisVal, std::forward<ActualArgTypes>(aArgs)...);
+  RefPtr<typename PromiseType::Private> p =
+      new (typename PromiseType::Private)(aCallerName);
+  RefPtr<ProxyRunnableType> r = new ProxyRunnableType(p, methodCall);
+  aTarget->Dispatch(r.forget());
+  return p;
+}
+
+constexpr bool Any() { return false; }
+
+template <typename T1>
+constexpr bool Any(T1 a) {
+  return static_cast<bool>(a);
+}
+
+template <typename T1, typename... Ts>
+constexpr bool Any(T1 a, Ts... aOthers) {
+  return a || Any(aOthers...);
+}
+
+}  // namespace detail
+
+// InvokeAsync with explicitly-specified storages.
+// See ParameterStorage in nsThreadUtils.h for help.
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes,
+          std::enable_if_t<sizeof...(Storages) != 0, int> = 0>
+static RefPtr<PromiseType> InvokeAsync(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  static_assert(
+      sizeof...(Storages) == sizeof...(ArgTypes),
+      "Provided Storages and method's ArgTypes should have equal sizes");
+  static_assert(sizeof...(Storages) == sizeof...(ActualArgTypes),
+                "Provided Storages and ActualArgTypes should have equal sizes");
+  return detail::InvokeAsyncImpl<Storages...>(
+      aTarget, aThisVal, aCallerName, aMethod,
+      std::forward<ActualArgTypes>(aArgs)...);
+}
+
+// InvokeAsync with no explicitly-specified storages, will copy arguments and
+// then move them out of the runnable into the target method parameters.
+template <typename... Storages, typename PromiseType, typename ThisType,
+          typename... ArgTypes, typename... ActualArgTypes,
+          std::enable_if_t<sizeof...(Storages) == 0, int> = 0>
+static RefPtr<PromiseType> InvokeAsync(
+    nsISerialEventTarget* aTarget, ThisType* aThisVal, const char* aCallerName,
+    RefPtr<PromiseType> (ThisType::*aMethod)(ArgTypes...),
+    ActualArgTypes&&... aArgs) {
+  static_assert(
+      !detail::Any(
+          std::is_pointer_v<std::remove_reference_t<ActualArgTypes>>...),
+      "Cannot pass pointer types through InvokeAsync, Storages must be "
+      "provided");
+  static_assert(sizeof...(ArgTypes) == sizeof...(ActualArgTypes),
+                "Method's ArgTypes and ActualArgTypes should have equal sizes");
+  return detail::InvokeAsyncImpl<
+      StoreCopyPassByRRef<std::decay_t<ActualArgTypes>>...>(
+      aTarget, aThisVal, aCallerName, aMethod,
+      std::forward<ActualArgTypes>(aArgs)...);
+}
+
+namespace detail {
+
+template <typename Function, typename PromiseType>
+class ProxyFunctionRunnable : public CancelableRunnable {
+  using FunctionStorage = std::decay_t<Function>;
+
+ public:
+  template <typename F>
+  ProxyFunctionRunnable(typename PromiseType::Private* aProxyPromise,
+                        F&& aFunction)
+      : CancelableRunnable("detail::ProxyFunctionRunnable"),
+        mProxyPromise(aProxyPromise),
+        mFunction(new FunctionStorage(std::forward<F>(aFunction))) {}
+
+  NS_IMETHOD Run() override {
+    RefPtr<PromiseType> p = (*mFunction)();
+    mFunction = nullptr;
+    p->ChainTo(mProxyPromise.forget(), "<Proxy Promise>");
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+ private:
+  RefPtr<typename PromiseType::Private> mProxyPromise;
+  UniquePtr<FunctionStorage> mFunction;
+};
+
+// Note: The following struct and function are not for public consumption (yet?)
+// as we would prefer all calls to pass on-the-spot lambdas (or at least moved
+// function objects). They could be moved outside of detail if really needed.
+
+// We prefer getting function objects by non-lvalue-ref (to avoid copying them
+// and their captures). This struct is a tag that allows the use of objects
+// through lvalue-refs where necessary.
+struct AllowInvokeAsyncFunctionLVRef {};
+
+// Invoke a function object (e.g., lambda or std/mozilla::function)
+// asynchronously; note that the object will be copied if provided by
+// lvalue-ref. Return a promise that the function should eventually resolve or
+// reject.
+template <typename Function>
+static auto InvokeAsync(nsISerialEventTarget* aTarget, const char* aCallerName,
+                        AllowInvokeAsyncFunctionLVRef, Function&& aFunction)
+    -> decltype(aFunction()) {
+  static_assert(
+      IsRefcountedSmartPointer<decltype(aFunction())>::value &&
+          IsMozPromise<
+              typename RemoveSmartPointer<decltype(aFunction())>::Type>::value,
+      "Function object must return RefPtr<MozPromise>");
+  MOZ_ASSERT(aTarget);
+  typedef typename RemoveSmartPointer<decltype(aFunction())>::Type PromiseType;
+  typedef detail::ProxyFunctionRunnable<Function, PromiseType>
+      ProxyRunnableType;
+
+  auto p = MakeRefPtr<typename PromiseType::Private>(aCallerName);
+  auto r = MakeRefPtr<ProxyRunnableType>(p, std::forward<Function>(aFunction));
+  aTarget->Dispatch(r.forget());
+  return p;
+}
+
+}  // namespace detail
+
+// Invoke a function object (e.g., lambda) asynchronously.
+// Return a promise that the function should eventually resolve or reject.
+template <typename Function>
+static auto InvokeAsync(nsISerialEventTarget* aTarget, const char* aCallerName,
+                        Function&& aFunction) -> decltype(aFunction()) {
+  static_assert(!std::is_lvalue_reference_v<Function>,
+                "Function object must not be passed by lvalue-ref (to avoid "
+                "unplanned copies); Consider move()ing the object.");
+  return detail::InvokeAsync(aTarget, aCallerName,
+                             detail::AllowInvokeAsyncFunctionLVRef(),
+                             std::forward<Function>(aFunction));
+}
+
+#  undef PROMISE_LOG
+#  undef PROMISE_ASSERT
+#  undef PROMISE_DEBUG
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/Mutex.h b/xpcom/threads/Mutex.h
new file mode 100644
index 0000000000..5116f1d7c3
--- /dev/null
+++ b/xpcom/threads/Mutex.h
@@ -0,0 +1,485 @@
+/* -*- 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_Mutex_h
+#define mozilla_Mutex_h
+
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/ThreadSafety.h"
+#include "mozilla/PlatformMutex.h"
+#include "mozilla/Maybe.h"
+#include "nsISupports.h"
+
+//
+// Provides:
+//
+//  - Mutex, a non-recursive mutex
+//  - MutexAutoLock, an RAII class for ensuring that Mutexes are properly
+//    locked and unlocked
+//  - MutexAutoUnlock, complementary sibling to MutexAutoLock
+//
+//  - OffTheBooksMutex, a non-recursive mutex that doesn't do leak checking
+//  - OffTheBooksMutexAuto{Lock,Unlock} - Like MutexAuto{Lock,Unlock}, but for
+//    an OffTheBooksMutex.
+//
+// Using MutexAutoLock/MutexAutoUnlock etc. is MUCH preferred to making bare
+// calls to Lock and Unlock.
+//
+namespace mozilla {
+
+/**
+ * OffTheBooksMutex is identical to Mutex, except that OffTheBooksMutex doesn't
+ * include leak checking.  Sometimes you want to intentionally "leak" a mutex
+ * until shutdown; in these cases, OffTheBooksMutex is for you.
+ */
+class MOZ_CAPABILITY("mutex") OffTheBooksMutex : public detail::MutexImpl,
+                                                 BlockingResourceBase {
+ public:
+  /**
+   * @param aName A name which can reference this lock
+   * @returns If failure, nullptr
+   *          If success, a valid Mutex* which must be destroyed
+   *          by Mutex::DestroyMutex()
+   **/
+  explicit OffTheBooksMutex(const char* aName)
+      : BlockingResourceBase(aName, eMutex)
+#ifdef DEBUG
+        ,
+        mOwningThread(nullptr)
+#endif
+  {
+  }
+
+  ~OffTheBooksMutex() {
+#ifdef DEBUG
+    MOZ_ASSERT(!mOwningThread, "destroying a still-owned lock!");
+#endif
+  }
+
+#ifndef DEBUG
+  /**
+   * Lock this mutex.
+   **/
+  void Lock() MOZ_CAPABILITY_ACQUIRE() { this->lock(); }
+
+  /**
+   * Try to lock this mutex, returning true if we were successful.
+   **/
+  [[nodiscard]] bool TryLock() MOZ_TRY_ACQUIRE(true) { return this->tryLock(); }
+
+  /**
+   * Unlock this mutex.
+   **/
+  void Unlock() MOZ_CAPABILITY_RELEASE() { this->unlock(); }
+
+  /**
+   * Assert that the current thread owns this mutex in debug builds.
+   *
+   * Does nothing in non-debug builds.
+   **/
+  void AssertCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(this) {}
+
+  /**
+   * Assert that the current thread does not own this mutex.
+   *
+   * Note that this function is not implemented for debug builds *and*
+   * non-debug builds due to difficulties in dealing with memory ordering.
+   *
+   * It is therefore mostly useful as documentation.
+   **/
+  void AssertNotCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(!this) {}
+
+#else
+  void Lock() MOZ_CAPABILITY_ACQUIRE();
+
+  [[nodiscard]] bool TryLock() MOZ_TRY_ACQUIRE(true);
+  void Unlock() MOZ_CAPABILITY_RELEASE();
+
+  void AssertCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(this);
+  void AssertNotCurrentThreadOwns() const MOZ_ASSERT_CAPABILITY(!this) {
+    // FIXME bug 476536
+  }
+#endif  // ifndef DEBUG
+
+ private:
+  OffTheBooksMutex() = delete;
+  OffTheBooksMutex(const OffTheBooksMutex&) = delete;
+  OffTheBooksMutex& operator=(const OffTheBooksMutex&) = delete;
+
+  friend class OffTheBooksCondVar;
+
+#ifdef DEBUG
+  PRThread* mOwningThread;
+#endif
+};
+
+/**
+ * Mutex
+ * When possible, use MutexAutoLock/MutexAutoUnlock to lock/unlock this
+ * mutex within a scope, instead of calling Lock/Unlock directly.
+ */
+class Mutex : public OffTheBooksMutex {
+ public:
+  explicit Mutex(const char* aName) : OffTheBooksMutex(aName) {
+    MOZ_COUNT_CTOR(Mutex);
+  }
+
+  MOZ_COUNTED_DTOR(Mutex)
+
+ private:
+  Mutex() = delete;
+  Mutex(const Mutex&) = delete;
+  Mutex& operator=(const Mutex&) = delete;
+};
+
+/**
+ * MutexSingleWriter
+ *
+ * Mutex where a single writer exists, so that reads from the same thread
+ * will not generate data races or consistency issues.
+ *
+ * When possible, use MutexAutoLock/MutexAutoUnlock to lock/unlock this
+ * mutex within a scope, instead of calling Lock/Unlock directly.
+ *
+ * This requires an object implementing Mutex's SingleWriterLockOwner, so
+ * we can do correct-thread checks.
+ */
+// Subclass this in the object owning the mutex
+class SingleWriterLockOwner {
+ public:
+  SingleWriterLockOwner() = default;
+  ~SingleWriterLockOwner() = default;
+
+  virtual bool OnWritingThread() const = 0;
+};
+
+class MutexSingleWriter : public OffTheBooksMutex {
+ public:
+  // aOwner should be the object that contains the mutex, typically.  We
+  // will use that object (which must have a lifetime the same or greater
+  // than this object) to verify that we're running on the correct thread,
+  // typically only in DEBUG builds
+  explicit MutexSingleWriter(const char* aName, SingleWriterLockOwner* aOwner)
+      : OffTheBooksMutex(aName)
+#ifdef DEBUG
+        ,
+        mOwner(aOwner)
+#endif
+  {
+    MOZ_COUNT_CTOR(MutexSingleWriter);
+    MOZ_ASSERT(mOwner);
+  }
+
+  MOZ_COUNTED_DTOR(MutexSingleWriter)
+
+  /**
+   * Statically assert that we're on the only thread that modifies data
+   * guarded by this Mutex.  This allows static checking for the pattern of
+   * having a single thread modify a set of data, and read it (under lock)
+   * on other threads, and reads on the thread that modifies it doesn't
+   * require a lock.  This doesn't solve the issue of some data under the
+   * Mutex following this pattern, and other data under the mutex being
+   * written from multiple threads.
+   *
+   * We could set the writing thread and dynamically check it in debug
+   * builds, but this doesn't.  We could also use thread-safety/capability
+   * system to provide direct thread assertions.
+   **/
+  void AssertOnWritingThread() const MOZ_ASSERT_CAPABILITY(this) {
+    MOZ_ASSERT(mOwner->OnWritingThread());
+  }
+  void AssertOnWritingThreadOrHeld() const MOZ_ASSERT_CAPABILITY(this) {
+#ifdef DEBUG
+    if (!mOwner->OnWritingThread()) {
+      AssertCurrentThreadOwns();
+    }
+#endif
+  }
+
+ private:
+#ifdef DEBUG
+  SingleWriterLockOwner* mOwner MOZ_UNSAFE_REF(
+      "This is normally the object that contains the MonitorSingleWriter, so "
+      "we don't want to hold a reference to ourselves");
+#endif
+
+  MutexSingleWriter() = delete;
+  MutexSingleWriter(const MutexSingleWriter&) = delete;
+  MutexSingleWriter& operator=(const MutexSingleWriter&) = delete;
+};
+
+namespace detail {
+template <typename T>
+class MOZ_RAII BaseAutoUnlock;
+
+/**
+ * MutexAutoLock
+ * Acquires the Mutex when it enters scope, and releases it when it leaves
+ * scope.
+ *
+ * MUCH PREFERRED to bare calls to Mutex.Lock and Unlock.
+ */
+template <typename T>
+class MOZ_RAII MOZ_SCOPED_CAPABILITY BaseAutoLock {
+ public:
+  /**
+   * Constructor
+   * The constructor aquires the given lock.  The destructor
+   * releases the lock.
+   *
+   * @param aLock A valid mozilla::Mutex* returned by
+   *              mozilla::Mutex::NewMutex.
+   **/
+  explicit BaseAutoLock(T aLock) MOZ_CAPABILITY_ACQUIRE(aLock) : mLock(aLock) {
+    mLock.Lock();
+  }
+
+  ~BaseAutoLock(void) MOZ_CAPABILITY_RELEASE() { mLock.Unlock(); }
+
+  // Assert that aLock is the mutex passed to the constructor and that the
+  // current thread owns the mutex.  In coding patterns such as:
+  //
+  // void LockedMethod(const BaseAutoLock<T>& aProofOfLock)
+  // {
+  //   aProofOfLock.AssertOwns(mMutex);
+  //   ...
+  // }
+  //
+  // Without this assertion, it could be that mMutex is not actually
+  // locked. It's possible to have code like:
+  //
+  // BaseAutoLock lock(someMutex);
+  // ...
+  // BaseAutoUnlock unlock(someMutex);
+  // ...
+  // LockedMethod(lock);
+  //
+  // and in such a case, simply asserting that the mutex pointers match is not
+  // sufficient; mutex ownership must be asserted as well.
+  //
+  // Note that if you are going to use the coding pattern presented above, you
+  // should use this method in preference to using AssertCurrentThreadOwns on
+  // the mutex you expected to be held, since this method provides stronger
+  // guarantees.
+  void AssertOwns(const T& aMutex) const MOZ_ASSERT_CAPABILITY(aMutex) {
+    MOZ_ASSERT(&aMutex == &mLock);
+    mLock.AssertCurrentThreadOwns();
+  }
+
+ private:
+  BaseAutoLock() = delete;
+  BaseAutoLock(BaseAutoLock&) = delete;
+  BaseAutoLock& operator=(BaseAutoLock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  friend class BaseAutoUnlock<T>;
+
+  T mLock;
+};
+
+template <typename MutexType>
+BaseAutoLock(MutexType&) -> BaseAutoLock<MutexType&>;
+}  // namespace detail
+
+typedef detail::BaseAutoLock<Mutex&> MutexAutoLock;
+typedef detail::BaseAutoLock<MutexSingleWriter&> MutexSingleWriterAutoLock;
+typedef detail::BaseAutoLock<OffTheBooksMutex&> OffTheBooksMutexAutoLock;
+
+// Specialization of Maybe<*AutoLock> for space efficiency and to silence
+// thread-safety analysis, which cannot track what's going on.
+template <class MutexType>
+class Maybe<detail::BaseAutoLock<MutexType&>> {
+ public:
+  Maybe() : mLock(nullptr) {}
+  ~Maybe() MOZ_NO_THREAD_SAFETY_ANALYSIS {
+    if (mLock) {
+      mLock->Unlock();
+    }
+  }
+
+  constexpr bool isSome() const { return mLock; }
+  constexpr bool isNothing() const { return !mLock; }
+
+  void emplace(MutexType& aMutex) MOZ_NO_THREAD_SAFETY_ANALYSIS {
+    MOZ_RELEASE_ASSERT(!mLock);
+    mLock = &aMutex;
+    mLock->Lock();
+  }
+
+  void reset() MOZ_NO_THREAD_SAFETY_ANALYSIS {
+    if (mLock) {
+      mLock->Unlock();
+      mLock = nullptr;
+    }
+  }
+
+ private:
+  MutexType* mLock;
+};
+
+// Use if we've done AssertOnWritingThread(), and then later need to take the
+// lock to write to a protected member. Instead of
+//    MutexSingleWriterAutoLock lock(mutex)
+// use
+//    MutexSingleWriterAutoLockOnThread(lock, mutex)
+#define MutexSingleWriterAutoLockOnThread(lock, mutex) \
+  MOZ_PUSH_IGNORE_THREAD_SAFETY                        \
+  MutexSingleWriterAutoLock lock(mutex);               \
+  MOZ_POP_THREAD_SAFETY
+
+namespace detail {
+/**
+ * ReleasableMutexAutoLock
+ * Acquires the Mutex when it enters scope, and releases it when it leaves
+ * scope. Allows calling Unlock (and Lock) as an alternative to
+ * MutexAutoUnlock; this can avoid an extra lock/unlock pair.
+ *
+ */
+template <typename T>
+class MOZ_RAII MOZ_SCOPED_CAPABILITY ReleasableBaseAutoLock {
+ public:
+  /**
+   * Constructor
+   * The constructor aquires the given lock.  The destructor
+   * releases the lock.
+   *
+   * @param aLock A valid mozilla::Mutex& returned by
+   *              mozilla::Mutex::NewMutex.
+   **/
+  explicit ReleasableBaseAutoLock(T aLock) MOZ_CAPABILITY_ACQUIRE(aLock)
+      : mLock(aLock) {
+    mLock.Lock();
+    mLocked = true;
+  }
+
+  ~ReleasableBaseAutoLock(void) MOZ_CAPABILITY_RELEASE() {
+    if (mLocked) {
+      Unlock();
+    }
+  }
+
+  void AssertOwns(const T& aMutex) const MOZ_ASSERT_CAPABILITY(aMutex) {
+    MOZ_ASSERT(&aMutex == &mLock);
+    mLock.AssertCurrentThreadOwns();
+  }
+
+  // Allow dropping the lock prematurely; for example to support something like:
+  // clang-format off
+  // MutexAutoLock lock(mMutex);
+  // ...
+  // if (foo) {
+  //   lock.Unlock();
+  //   MethodThatCantBeCalledWithLock()
+  //   return;
+  // }
+  // clang-format on
+  void Unlock() MOZ_CAPABILITY_RELEASE() {
+    MOZ_ASSERT(mLocked);
+    mLock.Unlock();
+    mLocked = false;
+  }
+  void Lock() MOZ_CAPABILITY_ACQUIRE() {
+    MOZ_ASSERT(!mLocked);
+    mLock.Lock();
+    mLocked = true;
+  }
+
+ private:
+  ReleasableBaseAutoLock() = delete;
+  ReleasableBaseAutoLock(ReleasableBaseAutoLock&) = delete;
+  ReleasableBaseAutoLock& operator=(ReleasableBaseAutoLock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  bool mLocked;
+  T mLock;
+};
+
+template <typename MutexType>
+ReleasableBaseAutoLock(MutexType&) -> ReleasableBaseAutoLock<MutexType&>;
+}  // namespace detail
+
+typedef detail::ReleasableBaseAutoLock<Mutex&> ReleasableMutexAutoLock;
+
+namespace detail {
+/**
+ * BaseAutoUnlock
+ * Releases the Mutex when it enters scope, and re-acquires it when it leaves
+ * scope.
+ *
+ * MUCH PREFERRED to bare calls to Mutex.Unlock and Lock.
+ */
+template <typename T>
+class MOZ_RAII MOZ_SCOPED_CAPABILITY BaseAutoUnlock {
+ public:
+  explicit BaseAutoUnlock(T aLock) MOZ_SCOPED_UNLOCK_RELEASE(aLock)
+      : mLock(aLock) {
+    mLock.Unlock();
+  }
+
+  explicit BaseAutoUnlock(BaseAutoLock<T>& aAutoLock)
+      /* MOZ_CAPABILITY_RELEASE(aAutoLock.mLock) */
+      : mLock(aAutoLock.mLock) {
+    NS_ASSERTION(mLock, "null lock");
+    mLock->Unlock();
+  }
+
+  ~BaseAutoUnlock() MOZ_SCOPED_UNLOCK_REACQUIRE() { mLock.Lock(); }
+
+ private:
+  BaseAutoUnlock() = delete;
+  BaseAutoUnlock(BaseAutoUnlock&) = delete;
+  BaseAutoUnlock& operator=(BaseAutoUnlock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  T mLock;
+};
+
+template <typename MutexType>
+BaseAutoUnlock(MutexType&) -> BaseAutoUnlock<MutexType&>;
+}  // namespace detail
+
+typedef detail::BaseAutoUnlock<Mutex&> MutexAutoUnlock;
+typedef detail::BaseAutoUnlock<MutexSingleWriter&> MutexSingleWriterAutoUnlock;
+typedef detail::BaseAutoUnlock<OffTheBooksMutex&> OffTheBooksMutexAutoUnlock;
+
+namespace detail {
+/**
+ * BaseAutoTryLock
+ * Tries to acquire the Mutex when it enters scope, and releases it when it
+ * leaves scope.
+ *
+ * MUCH PREFERRED to bare calls to Mutex.TryLock and Unlock.
+ */
+template <typename T>
+class MOZ_RAII MOZ_SCOPED_CAPABILITY BaseAutoTryLock {
+ public:
+  explicit BaseAutoTryLock(T& aLock) MOZ_CAPABILITY_ACQUIRE(aLock)
+      : mLock(aLock.TryLock() ? &aLock : nullptr) {}
+
+  ~BaseAutoTryLock() MOZ_CAPABILITY_RELEASE() {
+    if (mLock) {
+      mLock->Unlock();
+      mLock = nullptr;
+    }
+  }
+
+  explicit operator bool() const { return mLock; }
+
+ private:
+  BaseAutoTryLock(BaseAutoTryLock&) = delete;
+  BaseAutoTryLock& operator=(BaseAutoTryLock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  T* mLock;
+};
+}  // namespace detail
+
+typedef detail::BaseAutoTryLock<Mutex> MutexAutoTryLock;
+typedef detail::BaseAutoTryLock<OffTheBooksMutex> OffTheBooksMutexAutoTryLock;
+
+}  // namespace mozilla
+
+#endif  // ifndef mozilla_Mutex_h
diff --git a/xpcom/threads/Queue.h b/xpcom/threads/Queue.h
new file mode 100644
index 0000000000..fa36433fdf
--- /dev/null
+++ b/xpcom/threads/Queue.h
@@ -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/. */
+
+#ifndef mozilla_Queue_h
+#define mozilla_Queue_h
+
+#include <utility>
+#include <stdint.h>
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Assertions.h"
+#include "mozalloc.h"
+
+namespace mozilla {
+
+// define to turn on additional (DEBUG) asserts
+// #define EXTRA_ASSERTS 1
+
+// A queue implements a singly linked list of pages, each of which contains some
+// number of elements. Since the queue needs to store a "next" pointer, the
+// actual number of elements per page won't be quite as many as were requested.
+//
+// Each page consists of N entries.  We use the head buffer as a circular buffer
+// if it's the only buffer; if we have more than one buffer when the head is
+// empty we release it.  This avoids occasional freeing and reallocating buffers
+// every N entries.  We'll still allocate and free every N if the normal queue
+// depth is greated than N.  A fancier solution would be to move an empty Head
+// buffer to be an empty tail buffer, freeing if we have multiple empty tails,
+// but that probably isn't worth it.
+//
+// Cases:
+//   a) single buffer, circular
+//      Push: if not full:
+//              Add to tail, bump tail and reset to 0 if at end
+//            full:
+//              Add new page, insert there and set tail to 1
+//      Pop:
+//            take entry and bump head, reset to 0 if at end
+//   b) multiple buffers:
+//      Push: if not full:
+//              Add to tail, bump tail
+//            full:
+//              Add new page, insert there and set tail to 1
+//      Pop:
+//            take entry and bump head, reset to 0 if at end
+//            if buffer is empty, free head buffer and promote next to head
+//
+template <class T, size_t RequestedItemsPerPage = 256>
+class Queue {
+ public:
+  Queue() = default;
+
+  Queue(Queue&& aOther) noexcept
+      : mHead(std::exchange(aOther.mHead, nullptr)),
+        mTail(std::exchange(aOther.mTail, nullptr)),
+        mOffsetHead(std::exchange(aOther.mOffsetHead, 0)),
+        mHeadLength(std::exchange(aOther.mHeadLength, 0)),
+        mTailLength(std::exchange(aOther.mTailLength, 0)) {}
+
+  Queue& operator=(Queue&& aOther) noexcept {
+    Clear();
+
+    mHead = std::exchange(aOther.mHead, nullptr);
+    mTail = std::exchange(aOther.mTail, nullptr);
+    mOffsetHead = std::exchange(aOther.mOffsetHead, 0);
+    mHeadLength = std::exchange(aOther.mHeadLength, 0);
+    mTailLength = std::exchange(aOther.mTailLength, 0);
+    return *this;
+  }
+
+  ~Queue() { Clear(); }
+
+  // Discard all elements form the queue, clearing it to be empty.
+  void Clear() {
+    while (!IsEmpty()) {
+      Pop();
+    }
+    if (mHead) {
+      free(mHead);
+      mHead = nullptr;
+    }
+  }
+
+  T& Push(T&& aElement) {
+#if defined(EXTRA_ASSERTS) && DEBUG
+    size_t original_length = Count();
+#endif
+    if (!mHead) {
+      mHead = NewPage();
+      MOZ_ASSERT(mHead);
+
+      mTail = mHead;
+      T* eltPtr = &mTail->mEvents[0];
+      new (eltPtr) T(std::move(aElement));
+      mOffsetHead = 0;
+      mHeadLength = 1;
+#ifdef EXTRA_ASSERTS
+      MOZ_ASSERT(Count() == original_length + 1);
+#endif
+      return *eltPtr;
+    }
+    if ((mHead == mTail && mHeadLength == ItemsPerPage) ||
+        (mHead != mTail && mTailLength == ItemsPerPage)) {
+      // either we have one (circular) buffer and it's full, or
+      // we have multiple buffers and the last buffer is full
+      Page* page = NewPage();
+      MOZ_ASSERT(page);
+
+      mTail->mNext = page;
+      mTail = page;
+      T* eltPtr = &page->mEvents[0];
+      new (eltPtr) T(std::move(aElement));
+      mTailLength = 1;
+#ifdef EXTRA_ASSERTS
+      MOZ_ASSERT(Count() == original_length + 1);
+#endif
+      return *eltPtr;
+    }
+    if (mHead == mTail) {
+      // we have space in the (single) head buffer
+      uint16_t offset = (mOffsetHead + mHeadLength++) % ItemsPerPage;
+      T* eltPtr = &mTail->mEvents[offset];
+      new (eltPtr) T(std::move(aElement));
+#ifdef EXTRA_ASSERTS
+      MOZ_ASSERT(Count() == original_length + 1);
+#endif
+      return *eltPtr;
+    }
+    // else we have space to insert into last buffer
+    T* eltPtr = &mTail->mEvents[mTailLength++];
+    new (eltPtr) T(std::move(aElement));
+#ifdef EXTRA_ASSERTS
+    MOZ_ASSERT(Count() == original_length + 1);
+#endif
+    return *eltPtr;
+  }
+
+  bool IsEmpty() const {
+    return !mHead || (mHead == mTail && mHeadLength == 0);
+  }
+
+  T Pop() {
+#if defined(EXTRA_ASSERTS) && DEBUG
+    size_t original_length = Count();
+#endif
+    MOZ_ASSERT(!IsEmpty());
+
+    T result = std::move(mHead->mEvents[mOffsetHead]);
+    mHead->mEvents[mOffsetHead].~T();
+    mOffsetHead = (mOffsetHead + 1) % ItemsPerPage;
+    mHeadLength -= 1;
+
+    // Check if mHead points to empty (circular) Page and we have more
+    // pages
+    if (mHead != mTail && mHeadLength == 0) {
+      Page* dead = mHead;
+      mHead = mHead->mNext;
+      free(dead);
+      mOffsetHead = 0;
+      // if there are still >1 pages, the new head is full.
+      if (mHead != mTail) {
+        mHeadLength = ItemsPerPage;
+      } else {
+        mHeadLength = mTailLength;
+        mTailLength = 0;
+      }
+    }
+
+#ifdef EXTRA_ASSERTS
+    MOZ_ASSERT(Count() == original_length - 1);
+#endif
+    return result;
+  }
+
+  T& FirstElement() {
+    MOZ_ASSERT(!IsEmpty());
+    return mHead->mEvents[mOffsetHead];
+  }
+
+  const T& FirstElement() const {
+    MOZ_ASSERT(!IsEmpty());
+    return mHead->mEvents[mOffsetHead];
+  }
+
+  T& LastElement() {
+    MOZ_ASSERT(!IsEmpty());
+    uint16_t offset =
+        mHead == mTail ? mOffsetHead + mHeadLength - 1 : mTailLength - 1;
+    return mTail->mEvents[offset];
+  }
+
+  const T& LastElement() const {
+    MOZ_ASSERT(!IsEmpty());
+    uint16_t offset =
+        mHead == mTail ? mOffsetHead + mHeadLength - 1 : mTailLength - 1;
+    return mTail->mEvents[offset];
+  }
+
+  size_t Count() const {
+    // It is obvious count is 0 when the queue is empty.
+    if (!mHead) {
+      return 0;
+    }
+
+    // Compute full (intermediate) pages; Doesn't count first or last page
+    int count = 0;
+    // 1 buffer will have mHead == mTail; 2 will have mHead->mNext == mTail
+    for (Page* page = mHead; page != mTail && page->mNext != mTail;
+         page = page->mNext) {
+      count += ItemsPerPage;
+    }
+    // add first and last page
+    count += mHeadLength + mTailLength;
+    MOZ_ASSERT(count >= 0);
+
+    return count;
+  }
+
+  size_t ShallowSizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    size_t n = 0;
+    if (mHead) {
+      for (Page* page = mHead; page != mTail; page = page->mNext) {
+        n += aMallocSizeOf(page);
+      }
+    }
+    return n;
+  }
+
+  size_t ShallowSizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + ShallowSizeOfExcludingThis(aMallocSizeOf);
+  }
+
+ private:
+  static_assert(
+      (RequestedItemsPerPage & (RequestedItemsPerPage - 1)) == 0,
+      "RequestedItemsPerPage should be a power of two to avoid heap slop.");
+
+  // Since a Page must also contain a "next" pointer, we use one of the items to
+  // store this pointer. If sizeof(T) > sizeof(Page*), then some space will be
+  // wasted. So be it.
+  static const size_t ItemsPerPage = RequestedItemsPerPage - 1;
+
+  // Page objects are linked together to form a simple deque.
+  struct Page {
+    struct Page* mNext;
+    T mEvents[ItemsPerPage];
+  };
+
+  static Page* NewPage() {
+    return static_cast<Page*>(moz_xcalloc(1, sizeof(Page)));
+  }
+
+  Page* mHead = nullptr;
+  Page* mTail = nullptr;
+
+  uint16_t mOffsetHead = 0;  // Read position in head page
+  uint16_t mHeadLength = 0;  // Number of items in the head page
+  uint16_t mTailLength = 0;  // Number of items in the tail page
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_Queue_h
diff --git a/xpcom/threads/RWLock.cpp b/xpcom/threads/RWLock.cpp
new file mode 100644
index 0000000000..949934c8cc
--- /dev/null
+++ b/xpcom/threads/RWLock.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 "mozilla/RWLock.h"
+
+namespace mozilla {
+
+RWLock::RWLock(const char* aName)
+    : BlockingResourceBase(aName, eMutex)
+#ifdef DEBUG
+      ,
+      mOwningThread(nullptr)
+#endif
+{
+}
+
+#ifdef DEBUG
+bool RWLock::LockedForWritingByCurrentThread() {
+  return mOwningThread == PR_GetCurrentThread();
+}
+#endif
+
+}  // namespace mozilla
+
+#undef NativeHandle
diff --git a/xpcom/threads/RWLock.h b/xpcom/threads/RWLock.h
new file mode 100644
index 0000000000..e03d008631
--- /dev/null
+++ b/xpcom/threads/RWLock.h
@@ -0,0 +1,243 @@
+/* -*- 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/. */
+
+// An interface for read-write locks.
+
+#ifndef mozilla_RWLock_h
+#define mozilla_RWLock_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/PlatformRWLock.h"
+#include "mozilla/ThreadSafety.h"
+
+namespace mozilla {
+
+// A RWLock is similar to a Mutex, but whereas a Mutex permits only a single
+// reader thread or a single writer thread to access a piece of data, a
+// RWLock distinguishes between readers and writers: you may have multiple
+// reader threads concurrently accessing a piece of data or a single writer
+// thread.  This difference should guide your usage of RWLock: if you are not
+// reading the data from multiple threads simultaneously or you are writing
+// to the data roughly as often as read from it, then Mutex will suit your
+// purposes just fine.
+//
+// You should be using the AutoReadLock and AutoWriteLock classes, below,
+// for RAII read locking and write locking, respectively.  If you really must
+// take a read lock manually, call the ReadLock method; to relinquish that
+// read lock, call the ReadUnlock method.  Similarly, WriteLock and WriteUnlock
+// perform the same operations, but for write locks.
+//
+// It is unspecified what happens when a given thread attempts to acquire the
+// same lock in multiple ways; some underlying implementations of RWLock do
+// support acquiring a read lock multiple times on a given thread, but you
+// should not rely on this behavior.
+//
+// It is unspecified whether RWLock gives priority to waiting readers or
+// a waiting writer when unlocking.
+class MOZ_CAPABILITY("rwlock") RWLock : public detail::RWLockImpl,
+                                        public BlockingResourceBase {
+ public:
+  explicit RWLock(const char* aName);
+
+#ifdef DEBUG
+  bool LockedForWritingByCurrentThread();
+  [[nodiscard]] bool TryReadLock() MOZ_SHARED_TRYLOCK_FUNCTION(true);
+  void ReadLock() MOZ_ACQUIRE_SHARED();
+  void ReadUnlock() MOZ_RELEASE_SHARED();
+  [[nodiscard]] bool TryWriteLock() MOZ_TRY_ACQUIRE(true);
+  void WriteLock() MOZ_CAPABILITY_ACQUIRE();
+  void WriteUnlock() MOZ_EXCLUSIVE_RELEASE();
+#else
+  [[nodiscard]] bool TryReadLock() MOZ_SHARED_TRYLOCK_FUNCTION(true) {
+    return detail::RWLockImpl::tryReadLock();
+  }
+  void ReadLock() MOZ_ACQUIRE_SHARED() { detail::RWLockImpl::readLock(); }
+  void ReadUnlock() MOZ_RELEASE_SHARED() { detail::RWLockImpl::readUnlock(); }
+  [[nodiscard]] bool TryWriteLock() MOZ_TRY_ACQUIRE(true) {
+    return detail::RWLockImpl::tryWriteLock();
+  }
+  void WriteLock() MOZ_CAPABILITY_ACQUIRE() { detail::RWLockImpl::writeLock(); }
+  void WriteUnlock() MOZ_EXCLUSIVE_RELEASE() {
+    detail::RWLockImpl::writeUnlock();
+  }
+#endif
+
+ private:
+  RWLock() = delete;
+  RWLock(const RWLock&) = delete;
+  RWLock& operator=(const RWLock&) = delete;
+
+#ifdef DEBUG
+  // We record the owning thread for write locks only.
+  PRThread* mOwningThread;
+#endif
+};
+
+// We only use this once; not sure we can add thread safety attributions here
+template <typename T>
+class MOZ_RAII BaseAutoTryReadLock {
+ public:
+  explicit BaseAutoTryReadLock(T& aLock)
+      : mLock(aLock.TryReadLock() ? &aLock : nullptr) {}
+
+  ~BaseAutoTryReadLock() {
+    if (mLock) {
+      mLock->ReadUnlock();
+    }
+  }
+
+  explicit operator bool() const { return mLock; }
+
+ private:
+  BaseAutoTryReadLock() = delete;
+  BaseAutoTryReadLock(const BaseAutoTryReadLock&) = delete;
+  BaseAutoTryReadLock& operator=(const BaseAutoTryReadLock&) = delete;
+
+  T* mLock;
+};
+
+template <typename T>
+class MOZ_SCOPED_CAPABILITY MOZ_RAII BaseAutoReadLock {
+ public:
+  explicit BaseAutoReadLock(T& aLock) MOZ_ACQUIRE_SHARED(aLock)
+      : mLock(&aLock) {
+    MOZ_ASSERT(mLock, "null lock");
+    mLock->ReadLock();
+  }
+
+  // Not MOZ_RELEASE_SHARED(), which would make sense - apparently this trips
+  // over a bug in clang's static analyzer and it says it expected an
+  // exclusive unlock.
+  ~BaseAutoReadLock() MOZ_RELEASE_GENERIC() { mLock->ReadUnlock(); }
+
+ private:
+  BaseAutoReadLock() = delete;
+  BaseAutoReadLock(const BaseAutoReadLock&) = delete;
+  BaseAutoReadLock& operator=(const BaseAutoReadLock&) = delete;
+
+  T* mLock;
+};
+
+// XXX Mutex attributions?
+template <typename T>
+class MOZ_RAII BaseAutoTryWriteLock {
+ public:
+  explicit BaseAutoTryWriteLock(T& aLock)
+      : mLock(aLock.TryWriteLock() ? &aLock : nullptr) {}
+
+  ~BaseAutoTryWriteLock() {
+    if (mLock) {
+      mLock->WriteUnlock();
+    }
+  }
+
+  explicit operator bool() const { return mLock; }
+
+ private:
+  BaseAutoTryWriteLock() = delete;
+  BaseAutoTryWriteLock(const BaseAutoTryWriteLock&) = delete;
+  BaseAutoTryWriteLock& operator=(const BaseAutoTryWriteLock&) = delete;
+
+  T* mLock;
+};
+
+template <typename T>
+class MOZ_SCOPED_CAPABILITY MOZ_RAII BaseAutoWriteLock final {
+ public:
+  explicit BaseAutoWriteLock(T& aLock) MOZ_CAPABILITY_ACQUIRE(aLock)
+      : mLock(&aLock) {
+    MOZ_ASSERT(mLock, "null lock");
+    mLock->WriteLock();
+  }
+
+  ~BaseAutoWriteLock() MOZ_CAPABILITY_RELEASE() { mLock->WriteUnlock(); }
+
+ private:
+  BaseAutoWriteLock() = delete;
+  BaseAutoWriteLock(const BaseAutoWriteLock&) = delete;
+  BaseAutoWriteLock& operator=(const BaseAutoWriteLock&) = delete;
+
+  T* mLock;
+};
+
+// Read try-lock and unlock a RWLock with RAII semantics.  Much preferred to
+// bare calls to TryReadLock() and ReadUnlock().
+typedef BaseAutoTryReadLock<RWLock> AutoTryReadLock;
+
+// Read lock and unlock a RWLock with RAII semantics.  Much preferred to bare
+// calls to ReadLock() and ReadUnlock().
+typedef BaseAutoReadLock<RWLock> AutoReadLock;
+
+// Write try-lock and unlock a RWLock with RAII semantics.  Much preferred to
+// bare calls to TryWriteLock() and WriteUnlock().
+typedef BaseAutoTryWriteLock<RWLock> AutoTryWriteLock;
+
+// Write lock and unlock a RWLock with RAII semantics.  Much preferred to bare
+// calls to WriteLock() and WriteUnlock().
+typedef BaseAutoWriteLock<RWLock> AutoWriteLock;
+
+class MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS MOZ_CAPABILITY("rwlock")
+    StaticRWLock {
+ public:
+  // In debug builds, check that mLock is initialized for us as we expect by
+  // the compiler.  In non-debug builds, don't declare a constructor so that
+  // the compiler can see that the constructor is trivial.
+#ifdef DEBUG
+  StaticRWLock() { MOZ_ASSERT(!mLock); }
+#endif
+
+  [[nodiscard]] bool TryReadLock() MOZ_SHARED_TRYLOCK_FUNCTION(true) {
+    return Lock()->TryReadLock();
+  }
+  void ReadLock() MOZ_ACQUIRE_SHARED() { Lock()->ReadLock(); }
+  void ReadUnlock() MOZ_RELEASE_SHARED() { Lock()->ReadUnlock(); }
+  [[nodiscard]] bool TryWriteLock() MOZ_TRY_ACQUIRE(true) {
+    return Lock()->TryWriteLock();
+  }
+  void WriteLock() MOZ_CAPABILITY_ACQUIRE() { Lock()->WriteLock(); }
+  void WriteUnlock() MOZ_EXCLUSIVE_RELEASE() { Lock()->WriteUnlock(); }
+
+ private:
+  [[nodiscard]] RWLock* Lock() MOZ_RETURN_CAPABILITY(*mLock) {
+    if (mLock) {
+      return mLock;
+    }
+
+    RWLock* lock = new RWLock("StaticRWLock");
+    if (!mLock.compareExchange(nullptr, lock)) {
+      delete lock;
+    }
+
+    return mLock;
+  }
+
+  Atomic<RWLock*> mLock;
+
+  // Disallow copy constructor, but only in debug mode.  We only define
+  // a default constructor in debug mode (see above); if we declared
+  // this constructor always, the compiler wouldn't generate a trivial
+  // default constructor for us in non-debug mode.
+#ifdef DEBUG
+  StaticRWLock(const StaticRWLock& aOther);
+#endif
+
+  // Disallow these operators.
+  StaticRWLock& operator=(StaticRWLock* aRhs) = delete;
+  static void* operator new(size_t) noexcept(true) = delete;
+  static void operator delete(void*) = delete;
+};
+
+typedef BaseAutoTryReadLock<StaticRWLock> StaticAutoTryReadLock;
+typedef BaseAutoReadLock<StaticRWLock> StaticAutoReadLock;
+typedef BaseAutoTryWriteLock<StaticRWLock> StaticAutoTryWriteLock;
+typedef BaseAutoWriteLock<StaticRWLock> StaticAutoWriteLock;
+
+}  // namespace mozilla
+
+#endif  // mozilla_RWLock_h
diff --git a/xpcom/threads/RecursiveMutex.cpp b/xpcom/threads/RecursiveMutex.cpp
new file mode 100644
index 0000000000..7c45052c07
--- /dev/null
+++ b/xpcom/threads/RecursiveMutex.cpp
@@ -0,0 +1,85 @@
+/* -*- 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/RecursiveMutex.h"
+
+#ifdef XP_WIN
+#  include <windows.h>
+
+#  define NativeHandle(m) (reinterpret_cast<CRITICAL_SECTION*>(&m))
+#endif
+
+namespace mozilla {
+
+RecursiveMutex::RecursiveMutex(const char* aName)
+    : BlockingResourceBase(aName, eRecursiveMutex)
+#ifdef DEBUG
+      ,
+      mOwningThread(nullptr),
+      mEntryCount(0)
+#endif
+{
+#ifdef XP_WIN
+  // This number was adapted from NSPR.
+  static const DWORD sLockSpinCount = 100;
+
+#  if defined(RELEASE_OR_BETA)
+  // Vista and later automatically allocate and subsequently leak a debug info
+  // object for each critical section that we allocate unless we tell the
+  // system not to do that.
+  DWORD flags = CRITICAL_SECTION_NO_DEBUG_INFO;
+#  else
+  DWORD flags = 0;
+#  endif
+  BOOL r =
+      InitializeCriticalSectionEx(NativeHandle(mMutex), sLockSpinCount, flags);
+  MOZ_RELEASE_ASSERT(r);
+#else
+  pthread_mutexattr_t attr;
+
+  MOZ_RELEASE_ASSERT(pthread_mutexattr_init(&attr) == 0,
+                     "pthread_mutexattr_init failed");
+
+  MOZ_RELEASE_ASSERT(
+      pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE) == 0,
+      "pthread_mutexattr_settype failed");
+
+  MOZ_RELEASE_ASSERT(pthread_mutex_init(&mMutex, &attr) == 0,
+                     "pthread_mutex_init failed");
+
+  MOZ_RELEASE_ASSERT(pthread_mutexattr_destroy(&attr) == 0,
+                     "pthread_mutexattr_destroy failed");
+#endif
+}
+
+RecursiveMutex::~RecursiveMutex() {
+#ifdef XP_WIN
+  DeleteCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_destroy(&mMutex) == 0,
+                     "pthread_mutex_destroy failed");
+#endif
+}
+
+void RecursiveMutex::LockInternal() {
+#ifdef XP_WIN
+  EnterCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_lock(&mMutex) == 0,
+                     "pthread_mutex_lock failed");
+#endif
+}
+
+void RecursiveMutex::UnlockInternal() {
+#ifdef XP_WIN
+  LeaveCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_unlock(&mMutex) == 0,
+                     "pthread_mutex_unlock failed");
+#endif
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/RecursiveMutex.h b/xpcom/threads/RecursiveMutex.h
new file mode 100644
index 0000000000..dde21c9a35
--- /dev/null
+++ b/xpcom/threads/RecursiveMutex.h
@@ -0,0 +1,120 @@
+/* -*- 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/. */
+
+// A lock that can be acquired multiple times on the same thread.
+
+#ifndef mozilla_RecursiveMutex_h
+#define mozilla_RecursiveMutex_h
+
+#include "mozilla/ThreadSafety.h"
+#include "mozilla/BlockingResourceBase.h"
+
+#ifndef XP_WIN
+#  include <pthread.h>
+#endif
+
+namespace mozilla {
+
+class MOZ_CAPABILITY("recursive mutex") RecursiveMutex
+    : public BlockingResourceBase {
+ public:
+  explicit RecursiveMutex(const char* aName);
+  ~RecursiveMutex();
+
+#ifdef DEBUG
+  void Lock() MOZ_CAPABILITY_ACQUIRE();
+  void Unlock() MOZ_CAPABILITY_RELEASE();
+#else
+  void Lock() MOZ_CAPABILITY_ACQUIRE() { LockInternal(); }
+  void Unlock() MOZ_CAPABILITY_RELEASE() { UnlockInternal(); }
+#endif
+
+#ifdef DEBUG
+  /**
+   * AssertCurrentThreadIn
+   **/
+  void AssertCurrentThreadIn() const MOZ_ASSERT_CAPABILITY(this);
+  /**
+   * AssertNotCurrentThreadIn
+   **/
+  void AssertNotCurrentThreadIn() const MOZ_EXCLUDES(this) {
+    // Not currently implemented. See bug 476536 for discussion.
+  }
+#else
+  void AssertCurrentThreadIn() const MOZ_ASSERT_CAPABILITY(this) {}
+  void AssertNotCurrentThreadIn() const MOZ_EXCLUDES(this) {}
+#endif
+
+ private:
+  RecursiveMutex() = delete;
+  RecursiveMutex(const RecursiveMutex&) = delete;
+  RecursiveMutex& operator=(const RecursiveMutex&) = delete;
+
+  void LockInternal();
+  void UnlockInternal();
+
+#ifdef DEBUG
+  PRThread* mOwningThread;
+  size_t mEntryCount;
+#endif
+
+#if !defined(XP_WIN)
+  pthread_mutex_t mMutex;
+#else
+  // We eschew including windows.h and using CRITICAL_SECTION here so that files
+  // including us don't also pull in windows.h.  Just use a type that's big
+  // enough for CRITICAL_SECTION, and we'll fix it up later.
+  void* mMutex[6];
+#endif
+};
+
+class MOZ_RAII MOZ_SCOPED_CAPABILITY RecursiveMutexAutoLock {
+ public:
+  explicit RecursiveMutexAutoLock(RecursiveMutex& aRecursiveMutex)
+      MOZ_CAPABILITY_ACQUIRE(aRecursiveMutex)
+      : mRecursiveMutex(&aRecursiveMutex) {
+    NS_ASSERTION(mRecursiveMutex, "null mutex");
+    mRecursiveMutex->Lock();
+  }
+
+  ~RecursiveMutexAutoLock(void) MOZ_CAPABILITY_RELEASE() {
+    mRecursiveMutex->Unlock();
+  }
+
+ private:
+  RecursiveMutexAutoLock() = delete;
+  RecursiveMutexAutoLock(const RecursiveMutexAutoLock&) = delete;
+  RecursiveMutexAutoLock& operator=(const RecursiveMutexAutoLock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  mozilla::RecursiveMutex* mRecursiveMutex;
+};
+
+class MOZ_RAII MOZ_SCOPED_CAPABILITY RecursiveMutexAutoUnlock {
+ public:
+  explicit RecursiveMutexAutoUnlock(RecursiveMutex& aRecursiveMutex)
+      MOZ_SCOPED_UNLOCK_RELEASE(aRecursiveMutex)
+      : mRecursiveMutex(&aRecursiveMutex) {
+    NS_ASSERTION(mRecursiveMutex, "null mutex");
+    mRecursiveMutex->Unlock();
+  }
+
+  ~RecursiveMutexAutoUnlock(void) MOZ_SCOPED_UNLOCK_REACQUIRE() {
+    mRecursiveMutex->Lock();
+  }
+
+ private:
+  RecursiveMutexAutoUnlock() = delete;
+  RecursiveMutexAutoUnlock(const RecursiveMutexAutoUnlock&) = delete;
+  RecursiveMutexAutoUnlock& operator=(const RecursiveMutexAutoUnlock&) = delete;
+  static void* operator new(size_t) noexcept(true);
+
+  mozilla::RecursiveMutex* mRecursiveMutex;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_RecursiveMutex_h
diff --git a/xpcom/threads/ReentrantMonitor.h b/xpcom/threads/ReentrantMonitor.h
new file mode 100644
index 0000000000..09debad577
--- /dev/null
+++ b/xpcom/threads/ReentrantMonitor.h
@@ -0,0 +1,251 @@
+/* -*- 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_ReentrantMonitor_h
+#define mozilla_ReentrantMonitor_h
+
+#include "prmon.h"
+
+#if defined(MOZILLA_INTERNAL_API) && !defined(DEBUG)
+#  include "mozilla/ProfilerThreadSleep.h"
+#endif  // defined( MOZILLA_INTERNAL_API) && !defined(DEBUG)
+
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/ThreadSafety.h"
+#include "nsISupports.h"
+//
+// Provides:
+//
+//  - ReentrantMonitor, a Java-like monitor
+//  - ReentrantMonitorAutoEnter, an RAII class for ensuring that
+//    ReentrantMonitors are properly entered and exited
+//
+// Using ReentrantMonitorAutoEnter is MUCH preferred to making bare calls to
+// ReentrantMonitor.Enter and Exit.
+//
+namespace mozilla {
+
+/**
+ * ReentrantMonitor
+ * Java-like monitor.
+ * When possible, use ReentrantMonitorAutoEnter to hold this monitor within a
+ * scope, instead of calling Enter/Exit directly.
+ **/
+class MOZ_CAPABILITY("reentrant monitor") ReentrantMonitor
+    : BlockingResourceBase {
+ public:
+  /**
+   * ReentrantMonitor
+   * @param aName A name which can reference this monitor
+   */
+  explicit ReentrantMonitor(const char* aName)
+      : BlockingResourceBase(aName, eReentrantMonitor)
+#ifdef DEBUG
+        ,
+        mEntryCount(0)
+#endif
+  {
+    MOZ_COUNT_CTOR(ReentrantMonitor);
+    mReentrantMonitor = PR_NewMonitor();
+    if (!mReentrantMonitor) {
+      MOZ_CRASH("Can't allocate mozilla::ReentrantMonitor");
+    }
+  }
+
+  /**
+   * ~ReentrantMonitor
+   **/
+  ~ReentrantMonitor() {
+    NS_ASSERTION(mReentrantMonitor,
+                 "improperly constructed ReentrantMonitor or double free");
+    PR_DestroyMonitor(mReentrantMonitor);
+    mReentrantMonitor = 0;
+    MOZ_COUNT_DTOR(ReentrantMonitor);
+  }
+
+#ifndef DEBUG
+  /**
+   * Enter
+   * @see prmon.h
+   **/
+  void Enter() MOZ_CAPABILITY_ACQUIRE() { PR_EnterMonitor(mReentrantMonitor); }
+
+  /**
+   * Exit
+   * @see prmon.h
+   **/
+  void Exit() MOZ_CAPABILITY_RELEASE() { PR_ExitMonitor(mReentrantMonitor); }
+
+  /**
+   * Wait
+   * @see prmon.h
+   **/
+  nsresult Wait(PRIntervalTime aInterval = PR_INTERVAL_NO_TIMEOUT) {
+    PR_ASSERT_CURRENT_THREAD_IN_MONITOR(mReentrantMonitor);
+#  ifdef MOZILLA_INTERNAL_API
+    AUTO_PROFILER_THREAD_SLEEP;
+#  endif  // MOZILLA_INTERNAL_API
+    return PR_Wait(mReentrantMonitor, aInterval) == PR_SUCCESS
+               ? NS_OK
+               : NS_ERROR_FAILURE;
+  }
+
+#else  // ifndef DEBUG
+  void Enter() MOZ_CAPABILITY_ACQUIRE();
+  void Exit() MOZ_CAPABILITY_RELEASE();
+  nsresult Wait(PRIntervalTime aInterval = PR_INTERVAL_NO_TIMEOUT);
+
+#endif  // ifndef DEBUG
+
+  /**
+   * Notify
+   * @see prmon.h
+   **/
+  nsresult Notify() {
+    return PR_Notify(mReentrantMonitor) == PR_SUCCESS ? NS_OK
+                                                      : NS_ERROR_FAILURE;
+  }
+
+  /**
+   * NotifyAll
+   * @see prmon.h
+   **/
+  nsresult NotifyAll() {
+    return PR_NotifyAll(mReentrantMonitor) == PR_SUCCESS ? NS_OK
+                                                         : NS_ERROR_FAILURE;
+  }
+
+#ifdef DEBUG
+  /**
+   * AssertCurrentThreadIn
+   * @see prmon.h
+   **/
+  void AssertCurrentThreadIn() MOZ_ASSERT_CAPABILITY(this) {
+    PR_ASSERT_CURRENT_THREAD_IN_MONITOR(mReentrantMonitor);
+  }
+
+  /**
+   * AssertNotCurrentThreadIn
+   * @see prmon.h
+   **/
+  void AssertNotCurrentThreadIn() MOZ_ASSERT_CAPABILITY(!this) {
+    // FIXME bug 476536
+  }
+
+#else
+  void AssertCurrentThreadIn() MOZ_ASSERT_CAPABILITY(this) {}
+  void AssertNotCurrentThreadIn() MOZ_ASSERT_CAPABILITY(!this) {}
+
+#endif  // ifdef DEBUG
+
+ private:
+  ReentrantMonitor();
+  ReentrantMonitor(const ReentrantMonitor&);
+  ReentrantMonitor& operator=(const ReentrantMonitor&);
+
+  PRMonitor* mReentrantMonitor;
+#ifdef DEBUG
+  int32_t mEntryCount;
+#endif
+};
+
+/**
+ * ReentrantMonitorAutoEnter
+ * Enters the ReentrantMonitor when it enters scope, and exits it when
+ * it leaves scope.
+ *
+ * MUCH PREFERRED to bare calls to ReentrantMonitor.Enter and Exit.
+ */
+class MOZ_SCOPED_CAPABILITY MOZ_STACK_CLASS ReentrantMonitorAutoEnter {
+ public:
+  /**
+   * Constructor
+   * The constructor aquires the given lock.  The destructor
+   * releases the lock.
+   *
+   * @param aReentrantMonitor A valid mozilla::ReentrantMonitor*.
+   **/
+  explicit ReentrantMonitorAutoEnter(
+      mozilla::ReentrantMonitor& aReentrantMonitor)
+      MOZ_CAPABILITY_ACQUIRE(aReentrantMonitor)
+      : mReentrantMonitor(&aReentrantMonitor) {
+    NS_ASSERTION(mReentrantMonitor, "null monitor");
+    mReentrantMonitor->Enter();
+  }
+
+  ~ReentrantMonitorAutoEnter(void) MOZ_CAPABILITY_RELEASE() {
+    mReentrantMonitor->Exit();
+  }
+
+  nsresult Wait(PRIntervalTime aInterval = PR_INTERVAL_NO_TIMEOUT) {
+    return mReentrantMonitor->Wait(aInterval);
+  }
+
+  nsresult Notify() { return mReentrantMonitor->Notify(); }
+  nsresult NotifyAll() { return mReentrantMonitor->NotifyAll(); }
+
+ private:
+  ReentrantMonitorAutoEnter();
+  ReentrantMonitorAutoEnter(const ReentrantMonitorAutoEnter&);
+  ReentrantMonitorAutoEnter& operator=(const ReentrantMonitorAutoEnter&);
+  static void* operator new(size_t) noexcept(true);
+
+  friend class ReentrantMonitorAutoExit;
+
+  mozilla::ReentrantMonitor* mReentrantMonitor;
+};
+
+/**
+ * ReentrantMonitorAutoExit
+ * Exit the ReentrantMonitor when it enters scope, and enters it when it leaves
+ * scope.
+ *
+ * MUCH PREFERRED to bare calls to ReentrantMonitor.Exit and Enter.
+ */
+class MOZ_SCOPED_CAPABILITY MOZ_STACK_CLASS ReentrantMonitorAutoExit {
+ public:
+  /**
+   * Constructor
+   * The constructor releases the given lock.  The destructor
+   * acquires the lock. The lock must be held before constructing
+   * this object!
+   *
+   * @param aReentrantMonitor A valid mozilla::ReentrantMonitor*. It
+   *                 must be already locked.
+   **/
+  explicit ReentrantMonitorAutoExit(ReentrantMonitor& aReentrantMonitor)
+      MOZ_EXCLUSIVE_RELEASE(aReentrantMonitor)
+      : mReentrantMonitor(&aReentrantMonitor) {
+    NS_ASSERTION(mReentrantMonitor, "null monitor");
+    mReentrantMonitor->AssertCurrentThreadIn();
+    mReentrantMonitor->Exit();
+  }
+
+  explicit ReentrantMonitorAutoExit(
+      ReentrantMonitorAutoEnter& aReentrantMonitorAutoEnter)
+      MOZ_EXCLUSIVE_RELEASE(aReentrantMonitorAutoEnter.mReentrantMonitor)
+      : mReentrantMonitor(aReentrantMonitorAutoEnter.mReentrantMonitor) {
+    NS_ASSERTION(mReentrantMonitor, "null monitor");
+    mReentrantMonitor->AssertCurrentThreadIn();
+    mReentrantMonitor->Exit();
+  }
+
+  ~ReentrantMonitorAutoExit(void) MOZ_EXCLUSIVE_RELEASE() {
+    mReentrantMonitor->Enter();
+  }
+
+ private:
+  ReentrantMonitorAutoExit();
+  ReentrantMonitorAutoExit(const ReentrantMonitorAutoExit&);
+  ReentrantMonitorAutoExit& operator=(const ReentrantMonitorAutoExit&);
+  static void* operator new(size_t) noexcept(true);
+
+  ReentrantMonitor* mReentrantMonitor;
+};
+
+}  // namespace mozilla
+
+#endif  // ifndef mozilla_ReentrantMonitor_h
diff --git a/xpcom/threads/SchedulerGroup.cpp b/xpcom/threads/SchedulerGroup.cpp
new file mode 100644
index 0000000000..f69ce07003
--- /dev/null
+++ b/xpcom/threads/SchedulerGroup.cpp
@@ -0,0 +1,20 @@
+/* -*- 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/SchedulerGroup.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+/* static */
+nsresult SchedulerGroup::Dispatch(already_AddRefed<nsIRunnable>&& aRunnable) {
+  if (NS_IsMainThread()) {
+    return NS_DispatchToCurrentThread(std::move(aRunnable));
+  }
+  return NS_DispatchToMainThread(std::move(aRunnable));
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/SchedulerGroup.h b/xpcom/threads/SchedulerGroup.h
new file mode 100644
index 0000000000..09f0aac26a
--- /dev/null
+++ b/xpcom/threads/SchedulerGroup.h
@@ -0,0 +1,26 @@
+/* -*- 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_SchedulerGroup_h
+#define mozilla_SchedulerGroup_h
+
+#include "nscore.h"
+#include "mozilla/AlreadyAddRefed.h"
+
+class nsIEventTarget;
+class nsIRunnable;
+class nsISerialEventTarget;
+
+namespace mozilla {
+
+class SchedulerGroup {
+ public:
+  static nsresult Dispatch(already_AddRefed<nsIRunnable>&& aRunnable);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SchedulerGroup_h
diff --git a/xpcom/threads/SharedThreadPool.cpp b/xpcom/threads/SharedThreadPool.cpp
new file mode 100644
index 0000000000..a2de1c4495
--- /dev/null
+++ b/xpcom/threads/SharedThreadPool.cpp
@@ -0,0 +1,221 @@
+/* -*- 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/SharedThreadPool.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/ReentrantMonitor.h"
+#include "mozilla/Services.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/StaticPtr.h"
+#include "nsTHashMap.h"
+#include "nsXPCOMCIDInternal.h"
+#include "nsComponentManagerUtils.h"
+#include "nsIObserver.h"
+#include "nsIObserverService.h"
+#include "nsIThreadManager.h"
+#include "nsThreadPool.h"
+
+namespace mozilla {
+
+// Created and destroyed on the main thread.
+static StaticAutoPtr<ReentrantMonitor> sMonitor;
+
+// Hashtable, maps thread pool name to SharedThreadPool instance.
+// Modified only on the main thread.
+static StaticAutoPtr<nsTHashMap<nsCStringHashKey, SharedThreadPool*>> sPools;
+
+static already_AddRefed<nsIThreadPool> CreateThreadPool(const nsCString& aName);
+
+class SharedThreadPoolShutdownObserver : public nsIObserver {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIOBSERVER
+ protected:
+  virtual ~SharedThreadPoolShutdownObserver() = default;
+};
+
+NS_IMPL_ISUPPORTS(SharedThreadPoolShutdownObserver, nsIObserver, nsISupports)
+
+NS_IMETHODIMP
+SharedThreadPoolShutdownObserver::Observe(nsISupports* aSubject,
+                                          const char* aTopic,
+                                          const char16_t* aData) {
+  MOZ_RELEASE_ASSERT(!strcmp(aTopic, "xpcom-shutdown-threads"));
+#ifdef EARLY_BETA_OR_EARLIER
+  {
+    ReentrantMonitorAutoEnter mon(*sMonitor);
+    if (!sPools->IsEmpty()) {
+      nsAutoCString str;
+      for (const auto& key : sPools->Keys()) {
+        str.AppendPrintf("\"%s\" ", nsAutoCString(key).get());
+      }
+      printf_stderr(
+          "SharedThreadPool in xpcom-shutdown-threads. Waiting for "
+          "pools %s\n",
+          str.get());
+    }
+  }
+#endif
+  SharedThreadPool::SpinUntilEmpty();
+  sMonitor = nullptr;
+  sPools = nullptr;
+  return NS_OK;
+}
+
+void SharedThreadPool::InitStatics() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!sMonitor && !sPools);
+  sMonitor = new ReentrantMonitor("SharedThreadPool");
+  sPools = new nsTHashMap<nsCStringHashKey, SharedThreadPool*>();
+  nsCOMPtr<nsIObserverService> obsService =
+      mozilla::services::GetObserverService();
+  nsCOMPtr<nsIObserver> obs = new SharedThreadPoolShutdownObserver();
+  obsService->AddObserver(obs, "xpcom-shutdown-threads", false);
+}
+
+/* static */
+bool SharedThreadPool::IsEmpty() {
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  return !sPools->Count();
+}
+
+/* static */
+void SharedThreadPool::SpinUntilEmpty() {
+  MOZ_ASSERT(NS_IsMainThread());
+  SpinEventLoopUntil("SharedThreadPool::SpinUntilEmpty"_ns, []() -> bool {
+    sMonitor->AssertNotCurrentThreadIn();
+    return IsEmpty();
+  });
+}
+
+already_AddRefed<SharedThreadPool> SharedThreadPool::Get(
+    const nsCString& aName, uint32_t aThreadLimit) {
+  MOZ_ASSERT(sMonitor && sPools);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  RefPtr<SharedThreadPool> pool;
+
+  return sPools->WithEntryHandle(
+      aName, [&](auto&& entry) -> already_AddRefed<SharedThreadPool> {
+        if (entry) {
+          pool = entry.Data();
+          if (NS_FAILED(pool->EnsureThreadLimitIsAtLeast(aThreadLimit))) {
+            NS_WARNING("Failed to set limits on thread pool");
+          }
+        } else {
+          nsCOMPtr<nsIThreadPool> threadPool(CreateThreadPool(aName));
+          if (NS_WARN_IF(!threadPool)) {
+            sPools->Remove(aName);  // XXX entry.Remove()
+            return nullptr;
+          }
+          pool = new SharedThreadPool(aName, threadPool);
+
+          // Set the thread and idle limits. Note that we don't rely on the
+          // EnsureThreadLimitIsAtLeast() call below, as the default thread
+          // limit is 4, and if aThreadLimit is less than 4 we'll end up with a
+          // pool with 4 threads rather than what we expected; so we'll have
+          // unexpected behaviour.
+          nsresult rv = pool->SetThreadLimit(aThreadLimit);
+          if (NS_WARN_IF(NS_FAILED(rv))) {
+            sPools->Remove(aName);  // XXX entry.Remove()
+            return nullptr;
+          }
+
+          rv = pool->SetIdleThreadLimit(aThreadLimit);
+          if (NS_WARN_IF(NS_FAILED(rv))) {
+            sPools->Remove(aName);  // XXX entry.Remove()
+            return nullptr;
+          }
+
+          entry.Insert(pool.get());
+        }
+
+        return pool.forget();
+      });
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::AddRef(void) {
+  MOZ_ASSERT(sMonitor);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
+  nsrefcnt count = ++mRefCnt;
+  NS_LOG_ADDREF(this, count, "SharedThreadPool", sizeof(*this));
+  return count;
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType) SharedThreadPool::Release(void) {
+  MOZ_ASSERT(sMonitor);
+  ReentrantMonitorAutoEnter mon(*sMonitor);
+  nsrefcnt count = --mRefCnt;
+  NS_LOG_RELEASE(this, count, "SharedThreadPool");
+  if (count) {
+    return count;
+  }
+
+  // Remove SharedThreadPool from table of pools.
+  sPools->Remove(mName);
+  MOZ_ASSERT(!sPools->Get(mName));
+
+  // Dispatch an event to the main thread to call Shutdown() on
+  // the nsIThreadPool. The Runnable here will add a refcount to the pool,
+  // and when the Runnable releases the nsIThreadPool it will be deleted.
+  NS_DispatchToMainThread(NewRunnableMethod("nsIThreadPool::Shutdown", mPool,
+                                            &nsIThreadPool::Shutdown));
+
+  // Stabilize refcount, so that if something in the dtor QIs, it won't explode.
+  mRefCnt = 1;
+  delete this;
+  return 0;
+}
+
+NS_IMPL_QUERY_INTERFACE(SharedThreadPool, nsIThreadPool, nsIEventTarget)
+
+SharedThreadPool::SharedThreadPool(const nsCString& aName, nsIThreadPool* aPool)
+    : mName(aName), mPool(aPool), mRefCnt(0) {}
+
+SharedThreadPool::~SharedThreadPool() = default;
+
+nsresult SharedThreadPool::EnsureThreadLimitIsAtLeast(uint32_t aLimit) {
+  // We limit the number of threads that we use. Note that we
+  // set the thread limit to the same as the idle limit so that we're not
+  // constantly creating and destroying threads (see Bug 881954). When the
+  // thread pool threads shutdown they dispatch an event to the main thread
+  // to call nsIThread::Shutdown(), and if we're very busy that can take a
+  // while to run, and we end up with dozens of extra threads. Note that
+  // threads that are idle for 60 seconds are shutdown naturally.
+  uint32_t existingLimit = 0;
+  nsresult rv;
+
+  rv = mPool->GetThreadLimit(&existingLimit);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (aLimit > existingLimit) {
+    rv = mPool->SetThreadLimit(aLimit);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  rv = mPool->GetIdleThreadLimit(&existingLimit);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (aLimit > existingLimit) {
+    rv = mPool->SetIdleThreadLimit(aLimit);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return NS_OK;
+}
+
+static already_AddRefed<nsIThreadPool> CreateThreadPool(
+    const nsCString& aName) {
+  nsCOMPtr<nsIThreadPool> pool = new nsThreadPool();
+
+  nsresult rv = pool->SetName(aName);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+
+  rv = pool->SetThreadStackSize(nsIThreadManager::kThreadPoolStackSize);
+  NS_ENSURE_SUCCESS(rv, nullptr);
+
+  return pool.forget();
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/SharedThreadPool.h b/xpcom/threads/SharedThreadPool.h
new file mode 100644
index 0000000000..f2c7068051
--- /dev/null
+++ b/xpcom/threads/SharedThreadPool.h
@@ -0,0 +1,132 @@
+/* -*- 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 SharedThreadPool_h_
+#define SharedThreadPool_h_
+
+#include <utility>
+#include <type_traits>
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/RefCountType.h"
+#include "nsCOMPtr.h"
+#include "nsID.h"
+#include "nsIThreadPool.h"
+#include "nsString.h"
+#include "nscore.h"
+
+class nsIRunnable;
+
+namespace mozilla {
+
+// Wrapper that makes an nsIThreadPool a singleton, and provides a
+// consistent threadsafe interface to get instances. Callers simply get a
+// SharedThreadPool by the name of its nsIThreadPool. All get requests of
+// the same name get the same SharedThreadPool. Users must store a reference
+// to the pool, and when the last reference to a SharedThreadPool is dropped
+// the pool is shutdown and deleted. Users aren't required to manually
+// shutdown the pool, and can release references on any thread. This can make
+// it significantly easier to use thread pools, because the caller doesn't need
+// to worry about joining and tearing it down.
+//
+// On Windows all threads in the pool have MSCOM initialized with
+// COINIT_MULTITHREADED. Note that not all users of MSCOM use this mode see [1],
+// and mixing MSCOM objects between the two is terrible for performance, and can
+// cause some functions to fail. So be careful when using Win32 APIs on a
+// SharedThreadPool, and avoid sharing objects if at all possible.
+//
+// [1]
+// https://searchfox.org/mozilla-central/search?q=coinitialize&redirect=false
+class SharedThreadPool : public nsIThreadPool {
+ public:
+  // Gets (possibly creating) the shared thread pool singleton instance with
+  // thread pool named aName.
+  static already_AddRefed<SharedThreadPool> Get(const nsCString& aName,
+                                                uint32_t aThreadLimit = 4);
+
+  // We implement custom threadsafe AddRef/Release pair, that destroys the
+  // the shared pool singleton when the refcount drops to 0. The addref/release
+  // are implemented using locking, so it's not recommended that you use them
+  // in a tight loop.
+  NS_IMETHOD QueryInterface(REFNSIID aIID, void** aInstancePtr) override;
+  NS_IMETHOD_(MozExternalRefCountType) AddRef(void) override;
+  NS_IMETHOD_(MozExternalRefCountType) Release(void) override;
+  using HasThreadSafeRefCnt = std::true_type;
+
+  // Forward behaviour to wrapped thread pool implementation.
+  NS_FORWARD_SAFE_NSITHREADPOOL(mPool);
+
+  // Call this when dispatching from an event on the same
+  // threadpool that is about to complete. We should not create a new thread
+  // in that case since a thread is about to become idle.
+  nsresult DispatchFromEndOfTaskInThisPool(nsIRunnable* event) {
+    return Dispatch(event, NS_DISPATCH_AT_END);
+  }
+
+  NS_IMETHOD DispatchFromScript(nsIRunnable* event, uint32_t flags) override {
+    return Dispatch(event, flags);
+  }
+
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> event,
+                      uint32_t flags = NS_DISPATCH_NORMAL) override {
+    return !mPool ? NS_ERROR_NULL_POINTER
+                  : mPool->Dispatch(std::move(event), flags);
+  }
+
+  NS_IMETHOD DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  using nsIEventTarget::Dispatch;
+
+  NS_IMETHOD RegisterShutdownTask(nsITargetShutdownTask* task) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD UnregisterShutdownTask(nsITargetShutdownTask* task) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD IsOnCurrentThread(bool* _retval) override {
+    return !mPool ? NS_ERROR_NULL_POINTER : mPool->IsOnCurrentThread(_retval);
+  }
+
+  NS_IMETHOD_(bool) IsOnCurrentThreadInfallible() override {
+    return mPool && mPool->IsOnCurrentThread();
+  }
+
+  // Creates necessary statics. Called once at startup.
+  static void InitStatics();
+
+  // Spins the event loop until all thread pools are shutdown.
+  // *Must* be called on the main thread.
+  static void SpinUntilEmpty();
+
+ private:
+  // Returns whether there are no pools in existence at the moment.
+  static bool IsEmpty();
+
+  // Creates a singleton SharedThreadPool wrapper around aPool.
+  // aName is the name of the aPool, and is used to lookup the
+  // SharedThreadPool in the hash table of all created pools.
+  SharedThreadPool(const nsCString& aName, nsIThreadPool* aPool);
+  virtual ~SharedThreadPool();
+
+  nsresult EnsureThreadLimitIsAtLeast(uint32_t aThreadLimit);
+
+  // Name of mPool.
+  const nsCString mName;
+
+  // Thread pool being wrapped.
+  nsCOMPtr<nsIThreadPool> mPool;
+
+  // Refcount. We implement custom ref counting so that the thread pool is
+  // shutdown in a threadsafe manner and singletonness is preserved.
+  nsrefcnt mRefCnt;
+};
+
+}  // namespace mozilla
+
+#endif  // SharedThreadPool_h_
diff --git a/xpcom/threads/SpinEventLoopUntil.h b/xpcom/threads/SpinEventLoopUntil.h
new file mode 100644
index 0000000000..a281177268
--- /dev/null
+++ b/xpcom/threads/SpinEventLoopUntil.h
@@ -0,0 +1,191 @@
+/* -*- 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_threads_SpinEventLoopUntil_h__
+#define xpcom_threads_SpinEventLoopUntil_h__
+
+#include "MainThreadUtils.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/StaticMutex.h"
+#include "nsString.h"
+#include "nsThreadUtils.h"
+#include "xpcpublic.h"
+
+class nsIThread;
+
+// A wrapper for nested event loops.
+//
+// This function is intended to make code more obvious (do you remember
+// what NS_ProcessNextEvent(nullptr, true) means?) and slightly more
+// efficient, as people often pass nullptr or NS_GetCurrentThread to
+// NS_ProcessNextEvent, which results in needless querying of the current
+// thread every time through the loop.
+//
+// You should use this function in preference to NS_ProcessNextEvent inside
+// a loop unless one of the following is true:
+//
+// * You need to pass `false` to NS_ProcessNextEvent; or
+// * You need to do unusual things around the call to NS_ProcessNextEvent,
+//   such as unlocking mutexes that you are holding.
+//
+// If you *do* need to call NS_ProcessNextEvent manually, please do call
+// NS_GetCurrentThread() outside of your loop and pass the returned pointer
+// into NS_ProcessNextEvent for a tiny efficiency win.
+namespace mozilla {
+
+// You should normally not need to deal with this template parameter.  If
+// you enjoy esoteric event loop details, read on.
+//
+// If you specify that NS_ProcessNextEvent wait for an event, it is possible
+// for NS_ProcessNextEvent to return false, i.e. to indicate that an event
+// was not processed.  This can only happen when the thread has been shut
+// down by another thread, but is still attempting to process events outside
+// of a nested event loop.
+//
+// This behavior is admittedly strange.  The scenario it deals with is the
+// following:
+//
+// * The current thread has been shut down by some owner thread.
+// * The current thread is spinning an event loop waiting for some condition
+//   to become true.
+// * Said condition is actually being fulfilled by another thread, so there
+//   are timing issues in play.
+//
+// Thus, there is a small window where the current thread's event loop
+// spinning can check the condition, find it false, and call
+// NS_ProcessNextEvent to wait for another event.  But we don't actually
+// want it to wait indefinitely, because there might not be any other events
+// in the event loop, and the current thread can't accept dispatched events
+// because it's being shut down.  Thus, actually blocking would hang the
+// thread, which is bad.  The solution, then, is to detect such a scenario
+// and not actually block inside NS_ProcessNextEvent.
+//
+// But this is a problem, because we want to return the status of
+// NS_ProcessNextEvent to the caller of SpinEventLoopUntil if possible.  In
+// the above scenario, however, we'd stop spinning prematurely and cause
+// all sorts of havoc.  We therefore have this template parameter to
+// control whether errors are ignored or passed out to the caller of
+// SpinEventLoopUntil.  The latter is the default; if you find yourself
+// wanting to use the former, you should think long and hard before doing
+// so, and write a comment like this defending your choice.
+
+enum class ProcessFailureBehavior {
+  IgnoreAndContinue,
+  ReportToCaller,
+};
+
+// SpinEventLoopUntil is a dangerous operation that can result in hangs.
+// In particular during shutdown we want to know if we are hanging
+// inside a nested event loop on the main thread.
+// This is a helper annotation class to keep track of this.
+struct MOZ_STACK_CLASS AutoNestedEventLoopAnnotation {
+  explicit AutoNestedEventLoopAnnotation(const nsACString& aEntry)
+      : mPrev(nullptr) {
+    if (NS_IsMainThread()) {
+      StaticMutexAutoLock lock(sStackMutex);
+      mPrev = sCurrent;
+      sCurrent = this;
+      if (mPrev) {
+        mStack = mPrev->mStack + "|"_ns + aEntry;
+      } else {
+        mStack = aEntry;
+      }
+      AnnotateXPCOMSpinEventLoopStack(mStack);
+    }
+  }
+
+  ~AutoNestedEventLoopAnnotation() {
+    if (NS_IsMainThread()) {
+      StaticMutexAutoLock lock(sStackMutex);
+      MOZ_ASSERT(sCurrent == this);
+      sCurrent = mPrev;
+      if (mPrev) {
+        AnnotateXPCOMSpinEventLoopStack(mPrev->mStack);
+      } else {
+        AnnotateXPCOMSpinEventLoopStack(""_ns);
+      }
+    }
+  }
+
+  static void CopyCurrentStack(nsCString& aNestedSpinStack) {
+    // We need to copy this behind a mutex as the
+    // memory for our instances is stack-bound and
+    // can go away at any time.
+    StaticMutexAutoLock lock(sStackMutex);
+    if (sCurrent) {
+      aNestedSpinStack = sCurrent->mStack;
+    } else {
+      aNestedSpinStack = "(no nested event loop active)"_ns;
+    }
+  }
+
+ private:
+  AutoNestedEventLoopAnnotation(const AutoNestedEventLoopAnnotation&) = delete;
+  AutoNestedEventLoopAnnotation& operator=(
+      const AutoNestedEventLoopAnnotation&) = delete;
+
+  // The declarations of these statics live in nsThreadManager.cpp.
+  static AutoNestedEventLoopAnnotation* sCurrent MOZ_GUARDED_BY(sStackMutex);
+  static StaticMutex sStackMutex;
+
+  // We need this to avoid the inclusion of nsExceptionHandler.h here
+  // which can include windows.h which disturbs some dom/media/gtest.
+  // The implementation lives in nsThreadManager.cpp.
+  static void AnnotateXPCOMSpinEventLoopStack(const nsACString& aStack);
+
+  AutoNestedEventLoopAnnotation* mPrev MOZ_GUARDED_BY(sStackMutex);
+  nsCString mStack MOZ_GUARDED_BY(sStackMutex);
+};
+
+// Please see the above notes for the Behavior template parameter.
+//
+// aVeryGoodReasonToDoThis is usually a literal string unique to each
+//   caller that can be recognized in the XPCOMSpinEventLoopStack
+//   annotation.
+// aPredicate is the condition we wait for.
+// aThread can be used to specify a thread, see the above introduction.
+//   It defaults to the current thread.
+template <
+    ProcessFailureBehavior Behavior = ProcessFailureBehavior::ReportToCaller,
+    typename Pred>
+bool SpinEventLoopUntil(const nsACString& aVeryGoodReasonToDoThis,
+                        Pred&& aPredicate, nsIThread* aThread = nullptr) {
+  // Prepare the annotations
+  AutoNestedEventLoopAnnotation annotation(aVeryGoodReasonToDoThis);
+  AUTO_PROFILER_LABEL_DYNAMIC_NSCSTRING_NONSENSITIVE(
+      "SpinEventLoopUntil", OTHER, aVeryGoodReasonToDoThis);
+  AUTO_PROFILER_MARKER_TEXT("SpinEventLoop", OTHER, MarkerStack::Capture(),
+                            aVeryGoodReasonToDoThis);
+
+  nsIThread* thread = aThread ? aThread : NS_GetCurrentThread();
+
+  // From a latency perspective, spinning the event loop is like leaving script
+  // and returning to the event loop. Tell the watchdog we stopped running
+  // script (until we return).
+  mozilla::Maybe<xpc::AutoScriptActivity> asa;
+  if (NS_IsMainThread()) {
+    asa.emplace(false);
+  }
+
+  while (!aPredicate()) {
+    bool didSomething = NS_ProcessNextEvent(thread, true);
+
+    if (Behavior == ProcessFailureBehavior::IgnoreAndContinue) {
+      // Don't care what happened, continue on.
+      continue;
+    } else if (!didSomething) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+}  // namespace mozilla
+
+#endif  // xpcom_threads_SpinEventLoopUntil_h__
diff --git a/xpcom/threads/StateMirroring.h b/xpcom/threads/StateMirroring.h
new file mode 100644
index 0000000000..9f8ded70f4
--- /dev/null
+++ b/xpcom/threads/StateMirroring.h
@@ -0,0 +1,453 @@
+/* -*- 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/. */
+
+#if !defined(StateMirroring_h_)
+#  define StateMirroring_h_
+
+#  include <cstddef>
+#  include "mozilla/AbstractThread.h"
+#  include "mozilla/AlreadyAddRefed.h"
+#  include "mozilla/Assertions.h"
+#  include "mozilla/Logging.h"
+#  include "mozilla/Maybe.h"
+#  include "mozilla/RefPtr.h"
+#  include "mozilla/StateWatching.h"
+#  include "nsCOMPtr.h"
+#  include "nsIRunnable.h"
+#  include "nsISupports.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+/*
+ * The state-mirroring machinery allows pieces of interesting state to be
+ * observed on multiple thread without locking. The basic strategy is to track
+ * changes in a canonical value and post updates to other threads that hold
+ * mirrors for that value.
+ *
+ * One problem with the naive implementation of such a system is that some
+ * pieces of state need to be updated atomically, and certain other operations
+ * need to wait for these atomic updates to complete before executing. The
+ * state-mirroring machinery solves this problem by requiring that its owner
+ * thread uses tail dispatch, and posting state update events (which should
+ * always be run first by TaskDispatcher implementations) to that tail
+ * dispatcher. This ensures that state changes are always atomic from the
+ * perspective of observing threads.
+ *
+ * Given that state-mirroring is an automatic background process, we try to
+ * avoid burdening the caller with worrying too much about teardown. To that
+ * end, we don't assert dispatch success for any of the notifications, and
+ * assume that any canonical or mirror owned by a thread for whom dispatch fails
+ * will soon be disconnected by its holder anyway.
+ *
+ * Given that semantics may change and comments tend to go out of date, we
+ * deliberately don't provide usage examples here. Grep around to find them.
+ */
+
+namespace mozilla {
+
+// Mirror<T> and Canonical<T> inherit WatchTarget, so we piggy-back on the
+// logging that WatchTarget already does. Given that, it makes sense to share
+// the same log module.
+#  define MIRROR_LOG(x, ...)       \
+    MOZ_ASSERT(gStateWatchingLog); \
+    MOZ_LOG(gStateWatchingLog, LogLevel::Debug, (x, ##__VA_ARGS__))
+
+template <typename T>
+class AbstractMirror;
+
+/*
+ * AbstractCanonical is a superclass from which all Canonical values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Mirror value.
+ */
+template <typename T>
+class AbstractCanonical {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractCanonical)
+  AbstractCanonical(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void AddMirror(AbstractMirror<T>* aMirror) = 0;
+  virtual void RemoveMirror(AbstractMirror<T>* aMirror) = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+
+ protected:
+  virtual ~AbstractCanonical() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * AbstractMirror is a superclass from which all Mirror values must
+ * inherit. It serves as the interface of operations which may be performed (via
+ * asynchronous dispatch) by other threads, in particular by the corresponding
+ * Canonical value.
+ */
+template <typename T>
+class AbstractMirror {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractMirror)
+  AbstractMirror(AbstractThread* aThread) : mOwnerThread(aThread) {}
+  virtual void ConnectedOnCanonicalThread(AbstractCanonical<T>* aCanonical) = 0;
+  virtual void UpdateValue(const T& aNewValue) = 0;
+  virtual void NotifyDisconnected() = 0;
+
+  AbstractThread* OwnerThread() const { return mOwnerThread; }
+
+ protected:
+  virtual ~AbstractMirror() {}
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+/*
+ * Canonical<T> is a wrapper class that allows a given value to be mirrored by
+ * other threads. It maintains a list of active mirrors, and queues updates for
+ * them when the internal value changes. When changing the value, the caller
+ * needs to pass a TaskDispatcher object, which fires the updates at the
+ * appropriate time. Canonical<T> is also a WatchTarget, and may be set up to
+ * trigger other routines (on the same thread) when the canonical value changes.
+ *
+ * Canonical<T> is intended to be used as a member variable, so it doesn't
+ * actually inherit AbstractCanonical<T> (a refcounted type). Rather, it
+ * contains an inner class called |Impl| that implements most of the interesting
+ * logic.
+ */
+template <typename T>
+class Canonical {
+ public:
+  Canonical(AbstractThread* aThread, const T& aInitialValue,
+            const char* aName) {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+  ~Canonical() {}
+
+ private:
+  class Impl : public AbstractCanonical<T>, public WatchTarget {
+   public:
+    using AbstractCanonical<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+        : AbstractCanonical<T>(aThread),
+          WatchTarget(aName),
+          mValue(aInitialValue) {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(),
+                 "Can't get coherency without tail dispatch");
+    }
+
+    void ConnectMirror(AbstractMirror<T>* aMirror) {
+      MIRROR_LOG("%s [%p] canonical-init connecting mirror %p", mName, this,
+                 aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aMirror->OwnerThread()),
+                 "Can't get coherency without tail dispatch");
+      aMirror->ConnectedOnCanonicalThread(this);
+      AddMirror(aMirror);
+    }
+
+    void AddMirror(AbstractMirror<T>* aMirror) override {
+      MIRROR_LOG("%s [%p] adding mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!mMirrors.Contains(aMirror));
+      mMirrors.AppendElement(aMirror);
+      aMirror->OwnerThread()->DispatchStateChange(MakeNotifier(aMirror));
+    }
+
+    void RemoveMirror(AbstractMirror<T>* aMirror) override {
+      MIRROR_LOG("%s [%p] removing mirror %p", mName, this, aMirror);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mMirrors.Contains(aMirror));
+      mMirrors.RemoveElement(aMirror);
+    }
+
+    void DisconnectAll() {
+      MIRROR_LOG("%s [%p] Disconnecting all mirrors", mName, this);
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->Dispatch(
+            NewRunnableMethod("AbstractMirror::NotifyDisconnected", mMirrors[i],
+                              &AbstractMirror<T>::NotifyDisconnected));
+      }
+      mMirrors.Clear();
+    }
+
+    operator const T&() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    void Set(const T& aNewValue) {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+
+      if (aNewValue == mValue) {
+        return;
+      }
+
+      // Notify same-thread watchers. The state watching machinery will make
+      // sure that notifications run at the right time.
+      NotifyWatchers();
+
+      // Check if we've already got a pending update. If so we won't schedule
+      // another one.
+      bool alreadyNotifying = mInitialValue.isSome();
+
+      // Stash the initial value if needed, then update to the new value.
+      if (mInitialValue.isNothing()) {
+        mInitialValue.emplace(mValue);
+      }
+      mValue = aNewValue;
+
+      // We wait until things have stablized before sending state updates so
+      // that we can avoid sending multiple updates, and possibly avoid sending
+      // any updates at all if the value ends up where it started.
+      if (!alreadyNotifying) {
+        AbstractThread::DispatchDirectTask(NewRunnableMethod(
+            "Canonical::Impl::DoNotify", this, &Impl::DoNotify));
+      }
+    }
+
+    Impl& operator=(const T& aNewValue) {
+      Set(aNewValue);
+      return *this;
+    }
+    Impl& operator=(const Impl& aOther) {
+      Set(aOther);
+      return *this;
+    }
+    Impl(const Impl& aOther) = delete;
+
+   protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(mMirrors.IsEmpty()); }
+
+   private:
+    void DoNotify() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(mInitialValue.isSome());
+      bool same = mInitialValue.ref() == mValue;
+      mInitialValue.reset();
+
+      if (same) {
+        MIRROR_LOG("%s [%p] unchanged - not sending update", mName, this);
+        return;
+      }
+
+      for (size_t i = 0; i < mMirrors.Length(); ++i) {
+        mMirrors[i]->OwnerThread()->DispatchStateChange(
+            MakeNotifier(mMirrors[i]));
+      }
+    }
+
+    already_AddRefed<nsIRunnable> MakeNotifier(AbstractMirror<T>* aMirror) {
+      return NewRunnableMethod<T>("AbstractMirror::UpdateValue", aMirror,
+                                  &AbstractMirror<T>::UpdateValue, mValue);
+    }
+
+    T mValue;
+    Maybe<T> mInitialValue;
+    nsTArray<RefPtr<AbstractMirror<T>>> mMirrors;
+  };
+
+ public:
+  /*
+   * Connect this Canonical to aMirror. Note that the canonical value starts
+   * being mirrored to aMirror immediately, and requires one thread hop to
+   * aMirror's owning thread before the connection is established.
+   *
+   * Note that this could race with Mirror::DisconnectIfConnected(). It is up to
+   * the caller to provide the guarantee that disconnection happens after the
+   * connection has been established. There is no race between this and
+   * DisconnectAll().
+   */
+  void ConnectMirror(AbstractMirror<T>* aMirror) {
+    return mImpl->ConnectMirror(aMirror);
+  }
+  void DisconnectAll() { return mImpl->DisconnectAll(); }
+
+  // Access to the Impl.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+  void Set(const T& aNewValue) { mImpl->Set(aNewValue); }
+  Canonical& operator=(const T& aNewValue) {
+    Set(aNewValue);
+    return *this;
+  }
+  Canonical& operator=(const Canonical& aOther) {
+    Set(aOther);
+    return *this;
+  }
+  Canonical(const Canonical& aOther) = delete;
+
+ private:
+  RefPtr<Impl> mImpl;
+};
+
+/*
+ * Mirror<T> is a wrapper class that allows a given value to mirror that of a
+ * Canonical<T> owned by another thread. It registers itself with a
+ * Canonical<T>, and is periodically updated with new values. Mirror<T> is also
+ * a WatchTarget, and may be set up to trigger other routines (on the same
+ * thread) when the mirrored value changes.
+ *
+ * Mirror<T> is intended to be used as a member variable, so it doesn't actually
+ * inherit AbstractMirror<T> (a refcounted type). Rather, it contains an inner
+ * class called |Impl| that implements most of the interesting logic.
+ */
+template <typename T>
+class Mirror {
+ public:
+  Mirror(AbstractThread* aThread, const T& aInitialValue, const char* aName) {
+    mImpl = new Impl(aThread, aInitialValue, aName);
+  }
+
+  ~Mirror() {
+    // As a member of complex objects, a Mirror<T> may be destroyed on a
+    // different thread than its owner, or late in shutdown during CC. Given
+    // that, we require manual disconnection so that callers can put things in
+    // the right place.
+    MOZ_DIAGNOSTIC_ASSERT(!mImpl->IsConnected());
+    mImpl->AssertNoIncomingConnects();
+  }
+
+ private:
+  class Impl : public AbstractMirror<T>, public WatchTarget {
+   public:
+    using AbstractMirror<T>::OwnerThread;
+
+    Impl(AbstractThread* aThread, const T& aInitialValue, const char* aName)
+        : AbstractMirror<T>(aThread),
+          WatchTarget(aName),
+          mValue(aInitialValue) {
+      MIRROR_LOG("%s [%p] initialized", mName, this);
+      MOZ_ASSERT(aThread->SupportsTailDispatch(),
+                 "Can't get coherency without tail dispatch");
+    }
+
+    operator const T&() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      return mValue;
+    }
+
+    void ConnectedOnCanonicalThread(AbstractCanonical<T>* aCanonical) override {
+      MOZ_ASSERT(aCanonical->OwnerThread()->IsCurrentThreadIn());
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      ++mIncomingConnects;
+#  endif
+      OwnerThread()->DispatchStateChange(
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractCanonical<T>>>(
+              "Mirror::Impl::SetCanonical", this, &Impl::SetCanonical,
+              aCanonical));
+    }
+
+    void SetCanonical(AbstractCanonical<T>* aCanonical) {
+      MIRROR_LOG("%s [%p] Canonical-init setting canonical %p", mName, this,
+                 aCanonical);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!IsConnected());
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+      --mIncomingConnects;
+#  endif
+      mCanonical = aCanonical;
+    }
+
+    void UpdateValue(const T& aNewValue) override {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (mValue != aNewValue) {
+        mValue = aNewValue;
+        WatchTarget::NotifyWatchers();
+      }
+    }
+
+    void NotifyDisconnected() override {
+      MIRROR_LOG("%s [%p] Notifed of disconnection from %p", mName, this,
+                 mCanonical.get());
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      mCanonical = nullptr;
+    }
+
+    bool IsConnected() const { return !!mCanonical; }
+
+    void Connect(AbstractCanonical<T>* aCanonical) {
+      MIRROR_LOG("%s [%p] Connecting to %p", mName, this, aCanonical);
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      MOZ_ASSERT(!IsConnected());
+      MOZ_ASSERT(OwnerThread()->RequiresTailDispatch(aCanonical->OwnerThread()),
+                 "Can't get coherency without tail dispatch");
+
+      nsCOMPtr<nsIRunnable> r =
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractMirror<T>>>(
+              "AbstractCanonical::AddMirror", aCanonical,
+              &AbstractCanonical<T>::AddMirror, this);
+      aCanonical->OwnerThread()->Dispatch(r.forget());
+      mCanonical = aCanonical;
+    }
+
+    void DisconnectIfConnected() {
+      MOZ_ASSERT(OwnerThread()->IsCurrentThreadIn());
+      if (!IsConnected()) {
+        return;
+      }
+
+      MIRROR_LOG("%s [%p] Disconnecting from %p", mName, this,
+                 mCanonical.get());
+      nsCOMPtr<nsIRunnable> r =
+          NewRunnableMethod<StoreRefPtrPassByPtr<AbstractMirror<T>>>(
+              "AbstractCanonical::RemoveMirror", mCanonical,
+              &AbstractCanonical<T>::RemoveMirror, this);
+      mCanonical->OwnerThread()->Dispatch(r.forget());
+      mCanonical = nullptr;
+    }
+
+    void AssertNoIncomingConnects() {
+      MOZ_DIAGNOSTIC_ASSERT(mIncomingConnects == 0);
+    }
+
+   protected:
+    ~Impl() { MOZ_DIAGNOSTIC_ASSERT(!IsConnected()); }
+
+   private:
+    T mValue;
+    RefPtr<AbstractCanonical<T>> mCanonical;
+#  ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    std::atomic<size_t> mIncomingConnects = 0;
+#  endif
+  };
+
+ public:
+  // Forward control operations to the Impl<T>.
+  /*
+   * Connect aCanonical to this Mirror. Note that this requires one thread hop
+   * back to aCanonical's owning thread before the canonical value starts
+   * being mirrored, and another to our owning thread before the connection is
+   * established.
+   *
+   * Note that this mirror-initialized connection could race with
+   * Canonical::DisconnectAll(). It is up to the caller to provide the guarantee
+   * that disconnection happens after the connection has been established. There
+   * is no race between this and DisconnectIfConnected().
+   */
+  void Connect(AbstractCanonical<T>* aCanonical) { mImpl->Connect(aCanonical); }
+  void DisconnectIfConnected() { mImpl->DisconnectIfConnected(); }
+
+  // Access to the Impl<T>.
+  operator Impl&() { return *mImpl; }
+  Impl* operator&() { return mImpl; }
+
+  // Access to the T.
+  const T& Ref() const { return *mImpl; }
+  operator const T&() const { return Ref(); }
+
+ private:
+  RefPtr<Impl> mImpl;
+};
+
+#  undef MIRROR_LOG
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/StateWatching.h b/xpcom/threads/StateWatching.h
new file mode 100644
index 0000000000..3da0c63bfe
--- /dev/null
+++ b/xpcom/threads/StateWatching.h
@@ -0,0 +1,302 @@
+/* -*- 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/. */
+
+#if !defined(StateWatching_h_)
+#  define StateWatching_h_
+
+#  include <cstddef>
+#  include <new>
+#  include <utility>
+#  include "mozilla/AbstractThread.h"
+#  include "mozilla/Assertions.h"
+#  include "mozilla/Logging.h"
+#  include "mozilla/RefPtr.h"
+#  include "nsISupports.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+/*
+ * The state-watching machinery automates the process of responding to changes
+ * in various pieces of state.
+ *
+ * A standard programming pattern is as follows:
+ *
+ * mFoo = ...;
+ * NotifyStuffChanged();
+ * ...
+ * mBar = ...;
+ * NotifyStuffChanged();
+ *
+ * This pattern is error-prone and difficult to audit because it requires the
+ * programmer to manually trigger the update routine. This can be especially
+ * problematic when the update routine depends on numerous pieces of state, and
+ * when that state is modified across a variety of helper methods. In these
+ * cases the responsibility for invoking the routine is often unclear, causing
+ * developers to scatter calls to it like pixie dust. This can result in
+ * duplicate invocations (which is wasteful) and missing invocations in corner-
+ * cases (which is a source of bugs).
+ *
+ * This file provides a set of primitives that automatically handle updates and
+ * allow the programmers to explicitly construct a graph of state dependencies.
+ * When used correctly, it eliminates the guess-work and wasted cycles described
+ * above.
+ *
+ * There are two basic pieces:
+ *   (1) Objects that can be watched for updates. These inherit WatchTarget.
+ *   (2) Objects that receive objects and trigger processing. These inherit
+ *       AbstractWatcher. In the current machinery, these exist only internally
+ *       within the WatchManager, though that could change.
+ *
+ * Note that none of this machinery is thread-safe - it must all happen on the
+ * same owning thread. To solve multi-threaded use-cases, use state mirroring
+ * and watch the mirrored value.
+ *
+ * Given that semantics may change and comments tend to go out of date, we
+ * deliberately don't provide usage examples here. Grep around to find them.
+ */
+
+namespace mozilla {
+
+extern LazyLogModule gStateWatchingLog;
+
+#  define WATCH_LOG(x, ...) \
+    MOZ_LOG(gStateWatchingLog, LogLevel::Debug, (x, ##__VA_ARGS__))
+
+/*
+ * AbstractWatcher is a superclass from which all watchers must inherit.
+ */
+class AbstractWatcher {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(AbstractWatcher)
+  AbstractWatcher() : mDestroyed(false) {}
+  bool IsDestroyed() { return mDestroyed; }
+  virtual void Notify() = 0;
+
+ protected:
+  virtual ~AbstractWatcher() { MOZ_ASSERT(mDestroyed); }
+  bool mDestroyed;
+};
+
+/*
+ * WatchTarget is a superclass from which all watchable things must inherit.
+ * Unlike AbstractWatcher, it is a fully-implemented Mix-in, and the subclass
+ * needs only to invoke NotifyWatchers when something changes.
+ *
+ * The functionality that this class provides is not threadsafe, and should only
+ * be used on the thread that owns that WatchTarget.
+ */
+class WatchTarget {
+ public:
+  explicit WatchTarget(const char* aName) : mName(aName) {}
+
+  void AddWatcher(AbstractWatcher* aWatcher) {
+    MOZ_ASSERT(!mWatchers.Contains(aWatcher));
+    mWatchers.AppendElement(aWatcher);
+  }
+
+  void RemoveWatcher(AbstractWatcher* aWatcher) {
+    MOZ_ASSERT(mWatchers.Contains(aWatcher));
+    mWatchers.RemoveElement(aWatcher);
+  }
+
+ protected:
+  void NotifyWatchers() {
+    WATCH_LOG("%s[%p] notifying watchers\n", mName, this);
+    PruneWatchers();
+    for (size_t i = 0; i < mWatchers.Length(); ++i) {
+      mWatchers[i]->Notify();
+    }
+  }
+
+ private:
+  // We don't have Watchers explicitly unregister themselves when they die,
+  // because then they'd need back-references to all the WatchTargets they're
+  // subscribed to, and WatchTargets aren't reference-counted. So instead we
+  // just prune dead ones at appropriate times, which works just fine.
+  void PruneWatchers() {
+    mWatchers.RemoveElementsBy(
+        [](const auto& watcher) { return watcher->IsDestroyed(); });
+  }
+
+  nsTArray<RefPtr<AbstractWatcher>> mWatchers;
+
+ protected:
+  const char* mName;
+};
+
+/*
+ * Watchable is a wrapper class that turns any primitive into a WatchTarget.
+ */
+template <typename T>
+class Watchable : public WatchTarget {
+ public:
+  Watchable(const T& aInitialValue, const char* aName)
+      : WatchTarget(aName), mValue(aInitialValue) {}
+
+  const T& Ref() const { return mValue; }
+  operator const T&() const { return Ref(); }
+  Watchable& operator=(const T& aNewValue) {
+    if (aNewValue != mValue) {
+      mValue = aNewValue;
+      NotifyWatchers();
+    }
+
+    return *this;
+  }
+
+ private:
+  Watchable(const Watchable& aOther) = delete;
+  Watchable& operator=(const Watchable& aOther) = delete;
+
+  T mValue;
+};
+
+// Manager class for state-watching. Declare one of these in any class for which
+// you want to invoke method callbacks.
+//
+// Internally, WatchManager maintains one AbstractWatcher per callback method.
+// Consumers invoke Watch/Unwatch on a particular (WatchTarget, Callback) tuple.
+// This causes an AbstractWatcher for |Callback| to be instantiated if it
+// doesn't already exist, and registers it with |WatchTarget|.
+//
+// Using Direct Tasks on the TailDispatcher, WatchManager ensures that we fire
+// watch callbacks no more than once per task, once all other operations for
+// that task have been completed.
+//
+// WatchManager<OwnerType> is intended to be declared as a member of |OwnerType|
+// objects. Given that, it and its owned objects can't hold permanent strong
+// refs to the owner, since that would keep the owner alive indefinitely.
+// Instead, it _only_ holds strong refs while waiting for Direct Tasks to fire.
+// This ensures that everything is kept alive just long enough.
+template <typename OwnerType>
+class WatchManager {
+ public:
+  typedef void (OwnerType::*CallbackMethod)();
+  explicit WatchManager(OwnerType* aOwner, AbstractThread* aOwnerThread)
+      : mOwner(aOwner), mOwnerThread(aOwnerThread) {}
+
+  ~WatchManager() {
+    if (!IsShutdown()) {
+      Shutdown();
+    }
+  }
+
+  bool IsShutdown() const { return !mOwner; }
+
+  // Shutdown needs to happen on mOwnerThread. If the WatchManager will be
+  // destroyed on a different thread, Shutdown() must be called manually.
+  void Shutdown() {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    for (auto& watcher : mWatchers) {
+      watcher->Destroy();
+    }
+    mWatchers.Clear();
+    mOwner = nullptr;
+  }
+
+  void Watch(WatchTarget& aTarget, CallbackMethod aMethod) {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    aTarget.AddWatcher(&EnsureWatcher(aMethod));
+  }
+
+  void Unwatch(WatchTarget& aTarget, CallbackMethod aMethod) {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    MOZ_ASSERT(watcher);
+    aTarget.RemoveWatcher(watcher);
+  }
+
+  void ManualNotify(CallbackMethod aMethod) {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    MOZ_ASSERT(watcher);
+    watcher->Notify();
+  }
+
+ private:
+  class PerCallbackWatcher : public AbstractWatcher {
+   public:
+    PerCallbackWatcher(OwnerType* aOwner, AbstractThread* aOwnerThread,
+                       CallbackMethod aMethod)
+        : mOwner(aOwner),
+          mOwnerThread(aOwnerThread),
+          mCallbackMethod(aMethod) {}
+
+    void Destroy() {
+      MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+      mDestroyed = true;
+      mOwner = nullptr;
+    }
+
+    void Notify() override {
+      MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+      MOZ_DIAGNOSTIC_ASSERT(mOwner,
+                            "mOwner is only null after destruction, "
+                            "at which point we shouldn't be notified");
+      if (mNotificationPending) {
+        // We've already got a notification job in the pipe.
+        return;
+      }
+      mNotificationPending = true;
+
+      // Queue up our notification jobs to run in a stable state.
+      AbstractThread::DispatchDirectTask(
+          NS_NewRunnableFunction("WatchManager::PerCallbackWatcher::Notify",
+                                 [self = RefPtr<PerCallbackWatcher>(this),
+                                  owner = RefPtr<OwnerType>(mOwner)]() {
+                                   if (!self->mDestroyed) {
+                                     ((*owner).*(self->mCallbackMethod))();
+                                   }
+                                   self->mNotificationPending = false;
+                                 }));
+    }
+
+    bool CallbackMethodIs(CallbackMethod aMethod) const {
+      return mCallbackMethod == aMethod;
+    }
+
+   private:
+    ~PerCallbackWatcher() = default;
+
+    OwnerType* mOwner;  // Never null.
+    bool mNotificationPending = false;
+    RefPtr<AbstractThread> mOwnerThread;
+    CallbackMethod mCallbackMethod;
+  };
+
+  PerCallbackWatcher* GetWatcher(CallbackMethod aMethod) {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    for (auto& watcher : mWatchers) {
+      if (watcher->CallbackMethodIs(aMethod)) {
+        return watcher;
+      }
+    }
+    return nullptr;
+  }
+
+  PerCallbackWatcher& EnsureWatcher(CallbackMethod aMethod) {
+    MOZ_ASSERT(mOwnerThread->IsCurrentThreadIn());
+    PerCallbackWatcher* watcher = GetWatcher(aMethod);
+    if (watcher) {
+      return *watcher;
+    }
+    watcher = mWatchers
+                  .AppendElement(MakeAndAddRef<PerCallbackWatcher>(
+                      mOwner, mOwnerThread, aMethod))
+                  ->get();
+    return *watcher;
+  }
+
+  nsTArray<RefPtr<PerCallbackWatcher>> mWatchers;
+  OwnerType* mOwner;
+  RefPtr<AbstractThread> mOwnerThread;
+};
+
+#  undef WATCH_LOG
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/SyncRunnable.h b/xpcom/threads/SyncRunnable.h
new file mode 100644
index 0000000000..77f82ba313
--- /dev/null
+++ b/xpcom/threads/SyncRunnable.h
@@ -0,0 +1,157 @@
+/* -*- 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_SyncRunnable_h
+#define mozilla_SyncRunnable_h
+
+#include <utility>
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/dom/JSExecutionManager.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+/**
+ * This class will wrap a nsIRunnable and dispatch it to the target thread
+ * synchronously. This is different from
+ * NS_DispatchAndSpinEventLoopUntilComplete: this class does not spin the event
+ * loop waiting for the event to be dispatched. This means that you don't risk
+ * reentrance from pending messages, but you must be sure that the target thread
+ * does not ever block on this thread, or else you will deadlock.
+ *
+ * Typical usage:
+ *   RefPtr<SyncRunnable> sr = new SyncRunnable(new myrunnable...());
+ *   sr->DispatchToThread(t);
+ *
+ * We also provide convenience wrappers:
+ *   SyncRunnable::DispatchToThread(pThread, new myrunnable...());
+ *   SyncRunnable::DispatchToThread(pThread, NS_NewRunnableFunction(...));
+ *
+ */
+class SyncRunnable : public Runnable {
+ public:
+  explicit SyncRunnable(nsIRunnable* aRunnable)
+      : Runnable("SyncRunnable"),
+        mRunnable(aRunnable),
+        mMonitor("SyncRunnable"),
+        mDone(false) {}
+
+  explicit SyncRunnable(already_AddRefed<nsIRunnable> aRunnable)
+      : Runnable("SyncRunnable"),
+        mRunnable(std::move(aRunnable)),
+        mMonitor("SyncRunnable"),
+        mDone(false) {}
+
+  nsresult DispatchToThread(nsIEventTarget* aThread,
+                            bool aForceDispatch = false) {
+    nsresult rv;
+    bool on;
+
+    if (!aForceDispatch) {
+      rv = aThread->IsOnCurrentThread(&on);
+      MOZ_ASSERT(NS_SUCCEEDED(rv));
+      if (NS_SUCCEEDED(rv) && on) {
+        mRunnable->Run();
+        return NS_OK;
+      }
+    }
+
+    rv = aThread->Dispatch(this, NS_DISPATCH_NORMAL);
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      // This could be synchronously dispatching to a thread currently waiting
+      // for JS execution clearance. Yield JS execution.
+      dom::AutoYieldJSThreadExecution yield;
+
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+    return rv;
+  }
+
+  nsresult DispatchToThread(AbstractThread* aThread,
+                            bool aForceDispatch = false) {
+    if (!aForceDispatch && aThread->IsCurrentThreadIn()) {
+      mRunnable->Run();
+      return NS_OK;
+    }
+
+    // Check we don't have tail dispatching here. Otherwise we will deadlock
+    // ourself when spinning the loop below.
+    MOZ_ASSERT(!aThread->RequiresTailDispatchFromCurrentThread());
+
+    nsresult rv = aThread->Dispatch(RefPtr<nsIRunnable>(this).forget());
+    if (NS_SUCCEEDED(rv)) {
+      mozilla::MonitorAutoLock lock(mMonitor);
+      while (!mDone) {
+        lock.Wait();
+      }
+    }
+    return rv;
+  }
+
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   nsIRunnable* aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   nsIRunnable* aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(aRunnable));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   already_AddRefed<nsIRunnable> aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(std::move(aRunnable)));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   already_AddRefed<nsIRunnable> aRunnable,
+                                   bool aForceDispatch = false) {
+    RefPtr<SyncRunnable> s(new SyncRunnable(std::move(aRunnable)));
+    return s->DispatchToThread(aThread, aForceDispatch);
+  }
+
+  // These deleted overloads prevent accidentally (if harmlessly) double-
+  // wrapping SyncRunnable, which was previously a common anti-pattern.
+  static nsresult DispatchToThread(nsIEventTarget* aThread,
+                                   SyncRunnable* aRunnable,
+                                   bool aForceDispatch = false) = delete;
+  static nsresult DispatchToThread(AbstractThread* aThread,
+                                   SyncRunnable* aRunnable,
+                                   bool aForceDispatch = false) = delete;
+
+ protected:
+  NS_IMETHOD Run() override {
+    mRunnable->Run();
+
+    mozilla::MonitorAutoLock lock(mMonitor);
+    MOZ_ASSERT(!mDone);
+
+    mDone = true;
+    mMonitor.Notify();
+
+    return NS_OK;
+  }
+
+ private:
+  nsCOMPtr<nsIRunnable> mRunnable;
+  mozilla::Monitor mMonitor;
+  bool mDone MOZ_GUARDED_BY(mMonitor);
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SyncRunnable_h
diff --git a/xpcom/threads/SynchronizedEventQueue.cpp b/xpcom/threads/SynchronizedEventQueue.cpp
new file mode 100644
index 0000000000..59161b7f9d
--- /dev/null
+++ b/xpcom/threads/SynchronizedEventQueue.cpp
@@ -0,0 +1,26 @@
+/* -*- 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 "SynchronizedEventQueue.h"
+#include "nsIThreadInternal.h"
+
+using namespace mozilla;
+
+void SynchronizedEventQueue::AddObserver(nsIThreadObserver* aObserver) {
+  MOZ_ASSERT(aObserver);
+  MOZ_ASSERT(!mEventObservers.Contains(aObserver));
+  mEventObservers.AppendElement(aObserver);
+}
+
+void SynchronizedEventQueue::RemoveObserver(nsIThreadObserver* aObserver) {
+  MOZ_ASSERT(aObserver);
+  MOZ_ALWAYS_TRUE(mEventObservers.RemoveElement(aObserver));
+}
+
+const nsTObserverArray<nsCOMPtr<nsIThreadObserver>>&
+SynchronizedEventQueue::EventObservers() {
+  return mEventObservers;
+}
diff --git a/xpcom/threads/SynchronizedEventQueue.h b/xpcom/threads/SynchronizedEventQueue.h
new file mode 100644
index 0000000000..e4cf1a62af
--- /dev/null
+++ b/xpcom/threads/SynchronizedEventQueue.h
@@ -0,0 +1,131 @@
+/* -*- 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_SynchronizedEventQueue_h
+#define mozilla_SynchronizedEventQueue_h
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "nsIThreadInternal.h"
+#include "nsCOMPtr.h"
+#include "nsTObserverArray.h"
+
+class nsIEventTarget;
+class nsISerialEventTarget;
+class nsIThreadObserver;
+
+namespace mozilla {
+
+// A SynchronizedEventQueue is an abstract class for event queues that can be
+// used across threads. A SynchronizedEventQueue implementation will typically
+// use locks and condition variables to guarantee consistency. The methods of
+// SynchronizedEventQueue are split between ThreadTargetSink (which contains
+// methods for posting events) and SynchronizedEventQueue (which contains
+// methods for getting events). This split allows event targets (specifically
+// ThreadEventTarget) to use a narrow interface, since they only need to post
+// events.
+//
+// ThreadEventQueue is the canonical implementation of
+// SynchronizedEventQueue. When Quantum DOM is implemented, we will use a
+// different synchronized queue on the main thread, SchedulerEventQueue, which
+// will handle the cooperative threading model.
+
+class ThreadTargetSink {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ThreadTargetSink)
+
+  virtual bool PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                        EventQueuePriority aPriority) = 0;
+
+  // After this method is called, no more events can be posted.
+  virtual void Disconnect(const MutexAutoLock& aProofOfLock) = 0;
+
+  virtual nsresult RegisterShutdownTask(nsITargetShutdownTask* aTask) = 0;
+  virtual nsresult UnregisterShutdownTask(nsITargetShutdownTask* aTask) = 0;
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  // Not const because overrides may need to take a lock
+  virtual size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) = 0;
+
+ protected:
+  virtual ~ThreadTargetSink() = default;
+};
+
+class SynchronizedEventQueue : public ThreadTargetSink {
+ public:
+  virtual already_AddRefed<nsIRunnable> GetEvent(
+      bool aMayWait, mozilla::TimeDuration* aLastEventDelay = nullptr) = 0;
+  virtual bool HasPendingEvent() = 0;
+
+  // This method atomically checks if there are pending events and, if there are
+  // none, forbids future events from being posted. It returns true if there
+  // were no pending events.
+  virtual bool ShutdownIfNoPendingEvents() = 0;
+
+  // These methods provide access to an nsIThreadObserver, whose methods are
+  // called when posting and processing events. SetObserver should only be
+  // called on the thread that processes events. GetObserver can be called from
+  // any thread. GetObserverOnThread must be used from the thread that processes
+  // events; it does not acquire a lock.
+  virtual already_AddRefed<nsIThreadObserver> GetObserver() = 0;
+  virtual already_AddRefed<nsIThreadObserver> GetObserverOnThread() = 0;
+  virtual void SetObserver(nsIThreadObserver* aObserver) = 0;
+
+  void AddObserver(nsIThreadObserver* aObserver);
+  void RemoveObserver(nsIThreadObserver* aObserver);
+  const nsTObserverArray<nsCOMPtr<nsIThreadObserver>>& EventObservers();
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) override {
+    // Normally we'd return
+    // mEventObservers.ShallowSizeOfExcludingThis(aMallocSizeOf); However,
+    // mEventObservers may be being mutated on another thread, and we don't lock
+    // around access, so locking here wouldn't help.  They're small, so
+    return 0;
+  }
+
+  /**
+   * This method causes any events currently enqueued on the thread to be
+   * suppressed until PopEventQueue is called, and any event dispatched to this
+   * thread's nsIEventTarget will queue as well. Calls to PushEventQueue may be
+   * nested and must each be paired with a call to PopEventQueue in order to
+   * restore the original state of the thread. The returned nsIEventTarget may
+   * be used to push events onto the nested queue. Dispatching will be disabled
+   * once the event queue is popped. The thread will only ever process pending
+   * events for the innermost event queue. Must only be called on the target
+   * thread.
+   */
+  virtual already_AddRefed<nsISerialEventTarget> PushEventQueue() = 0;
+
+  /**
+   * Revert a call to PushEventQueue. When an event queue is popped, any events
+   * remaining in the queue are appended to the elder queue. This also causes
+   * the nsIEventTarget returned from PushEventQueue to stop dispatching events.
+   * Must only be called on the target thread, and with the innermost event
+   * queue.
+   */
+  virtual void PopEventQueue(nsIEventTarget* aTarget) = 0;
+
+  /**
+   * Flush the list of shutdown tasks which were previously registered.  After
+   * this is called, new shutdown tasks cannot be registered.
+   */
+  virtual void RunShutdownTasks() = 0;
+
+ protected:
+  virtual ~SynchronizedEventQueue() = default;
+
+ private:
+  nsTObserverArray<nsCOMPtr<nsIThreadObserver>> mEventObservers;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_SynchronizedEventQueue_h
diff --git a/xpcom/threads/TaskController.cpp b/xpcom/threads/TaskController.cpp
new file mode 100644
index 0000000000..8e3aae185a
--- /dev/null
+++ b/xpcom/threads/TaskController.cpp
@@ -0,0 +1,1098 @@
+/* -*- 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 "TaskController.h"
+#include "nsIIdleRunnable.h"
+#include "nsIRunnable.h"
+#include "nsThreadUtils.h"
+#include <algorithm>
+#include "GeckoProfiler.h"
+#include "mozilla/BackgroundHangMonitor.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/Hal.h"
+#include "mozilla/InputTaskManager.h"
+#include "mozilla/VsyncTaskManager.h"
+#include "mozilla/IOInterposer.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/SchedulerGroup.h"
+#include "mozilla/ScopeExit.h"
+#include "nsIThreadInternal.h"
+#include "nsThread.h"
+#include "prenv.h"
+#include "prsystem.h"
+
+namespace mozilla {
+
+StaticAutoPtr<TaskController> TaskController::sSingleton;
+
+thread_local size_t mThreadPoolIndex = -1;
+std::atomic<uint64_t> Task::sCurrentTaskSeqNo = 0;
+
+const int32_t kMinimumPoolThreadCount = 2;
+const int32_t kMaximumPoolThreadCount = 8;
+
+/* static */
+int32_t TaskController::GetPoolThreadCount() {
+  if (PR_GetEnv("MOZ_TASKCONTROLLER_THREADCOUNT")) {
+    return strtol(PR_GetEnv("MOZ_TASKCONTROLLER_THREADCOUNT"), nullptr, 0);
+  }
+
+  int32_t numCores = 0;
+#if defined(XP_MACOSX) && defined(__aarch64__)
+  if (const auto& cpuInfo = hal::GetHeterogeneousCpuInfo()) {
+    // -1 because of the main thread.
+    numCores = cpuInfo->mBigCpus.Count() + cpuInfo->mMediumCpus.Count() - 1;
+  } else
+#endif
+  {
+    numCores = std::max<int32_t>(1, PR_GetNumberOfProcessors());
+  }
+
+  return std::clamp<int32_t>(numCores, kMinimumPoolThreadCount,
+                             kMaximumPoolThreadCount);
+}
+
+#if defined(MOZ_COLLECTING_RUNNABLE_TELEMETRY)
+
+struct TaskMarker : BaseMarkerType<TaskMarker> {
+  static constexpr const char* Name = "Task";
+  static constexpr const char* Description =
+      "Marker representing a task being executed in TaskController.";
+
+  using MS = MarkerSchema;
+  static constexpr MS::PayloadField PayloadFields[] = {
+      {"name", MS::InputType::CString, "Task Name", MS::Format::String,
+       MS::PayloadFlags::Searchable},
+      {"priority", MS::InputType::Uint32, "Priority level",
+       MS::Format::Integer},
+      {"priorityName", MS::InputType::CString, "Priority Name"}};
+
+  static constexpr MS::Location Locations[] = {MS::Location::MarkerChart,
+                                               MS::Location::MarkerTable};
+  static constexpr const char* ChartLabel = "{marker.data.name}";
+  static constexpr const char* TableLabel =
+      "{marker.name} - {marker.data.name} - priority: "
+      "{marker.data.priorityName} ({marker.data.priority})";
+
+  static constexpr MS::ETWMarkerGroup Group = MS::ETWMarkerGroup::Scheduling;
+
+  static void TranslateMarkerInputToSchema(void* aContext,
+                                           const nsCString& aName,
+                                           uint32_t aPriority) {
+    ETW::OutputMarkerSchema(aContext, TaskMarker{}, aName, aPriority,
+                            ProfilerStringView(""));
+  }
+
+  static void StreamJSONMarkerData(baseprofiler::SpliceableJSONWriter& aWriter,
+                                   const nsCString& aName, uint32_t aPriority) {
+    aWriter.StringProperty("name", aName);
+    aWriter.IntProperty("priority", aPriority);
+
+#  define EVENT_PRIORITY(NAME, VALUE)                \
+    if (aPriority == (VALUE)) {                      \
+      aWriter.StringProperty("priorityName", #NAME); \
+    } else
+    EVENT_QUEUE_PRIORITY_LIST(EVENT_PRIORITY)
+#  undef EVENT_PRIORITY
+    {
+      aWriter.StringProperty("priorityName", "Invalid Value");
+    }
+  }
+};
+
+class MOZ_RAII AutoProfileTask {
+ public:
+  explicit AutoProfileTask(nsACString& aName, uint64_t aPriority)
+      : mName(aName), mPriority(aPriority) {
+    if (profiler_is_collecting_markers()) {
+      mStartTime = TimeStamp::Now();
+    }
+  }
+
+  ~AutoProfileTask() {
+    if (!profiler_thread_is_being_profiled_for_markers()) {
+      return;
+    }
+
+    AUTO_PROFILER_LABEL("AutoProfileTask", PROFILER);
+    AUTO_PROFILER_STATS(AUTO_PROFILE_TASK);
+    profiler_add_marker("Runnable", ::mozilla::baseprofiler::category::OTHER,
+                        mStartTime.IsNull()
+                            ? MarkerTiming::IntervalEnd()
+                            : MarkerTiming::IntervalUntilNowFrom(mStartTime),
+                        TaskMarker{}, mName, mPriority);
+  }
+
+ private:
+  TimeStamp mStartTime;
+  nsAutoCString mName;
+  uint32_t mPriority;
+};
+
+#  define AUTO_PROFILE_FOLLOWING_TASK(task)                                  \
+    nsAutoCString name;                                                      \
+    (task)->GetName(name);                                                   \
+    AUTO_PROFILER_LABEL_DYNAMIC_NSCSTRING_NONSENSITIVE("Task", OTHER, name); \
+    mozilla::AutoProfileTask PROFILER_RAII(name, (task)->GetPriority());
+#else
+#  define AUTO_PROFILE_FOLLOWING_TASK(task)
+#endif
+
+bool TaskManager::
+    UpdateCachesForCurrentIterationAndReportPriorityModifierChanged(
+        const MutexAutoLock& aProofOfLock, IterationType aIterationType) {
+  mCurrentSuspended = IsSuspended(aProofOfLock);
+
+  if (aIterationType == IterationType::EVENT_LOOP_TURN && !mCurrentSuspended) {
+    int32_t oldModifier = mCurrentPriorityModifier;
+    mCurrentPriorityModifier =
+        GetPriorityModifierForEventLoopTurn(aProofOfLock);
+
+    if (mCurrentPriorityModifier != oldModifier) {
+      return true;
+    }
+  }
+  return false;
+}
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+class MOZ_RAII AutoSetMainThreadRunnableName {
+ public:
+  explicit AutoSetMainThreadRunnableName(const nsCString& aName) {
+    MOZ_ASSERT(NS_IsMainThread());
+    // We want to record our current runnable's name in a static so
+    // that BHR can record it.
+    mRestoreRunnableName = nsThread::sMainThreadRunnableName;
+
+    // Copy the name into sMainThreadRunnableName's buffer, and append a
+    // terminating null.
+    uint32_t length = std::min((uint32_t)nsThread::kRunnableNameBufSize - 1,
+                               (uint32_t)aName.Length());
+    memcpy(nsThread::sMainThreadRunnableName.begin(), aName.BeginReading(),
+           length);
+    nsThread::sMainThreadRunnableName[length] = '\0';
+  }
+
+  ~AutoSetMainThreadRunnableName() {
+    nsThread::sMainThreadRunnableName = mRestoreRunnableName;
+  }
+
+ private:
+  Array<char, nsThread::kRunnableNameBufSize> mRestoreRunnableName;
+};
+#endif
+
+Task* Task::GetHighestPriorityDependency() {
+  Task* currentTask = this;
+
+  while (!currentTask->mDependencies.empty()) {
+    auto iter = currentTask->mDependencies.begin();
+
+    while (iter != currentTask->mDependencies.end()) {
+      if ((*iter)->mCompleted) {
+        auto oldIter = iter;
+        iter++;
+        // Completed tasks are removed here to prevent needlessly keeping them
+        // alive or iterating over them in the future.
+        currentTask->mDependencies.erase(oldIter);
+        continue;
+      }
+
+      currentTask = iter->get();
+      break;
+    }
+  }
+
+  return currentTask == this ? nullptr : currentTask;
+}
+
+void TaskController::Initialize() {
+  MOZ_ASSERT(!sSingleton);
+  sSingleton = new TaskController();
+}
+
+void ThreadFuncPoolThread(void* aIndex) {
+  mThreadPoolIndex = *reinterpret_cast<int32_t*>(aIndex);
+  delete reinterpret_cast<int32_t*>(aIndex);
+  TaskController::Get()->RunPoolThread();
+}
+
+TaskController::TaskController()
+    : mGraphMutex("TaskController::mGraphMutex"),
+      mThreadPoolCV(mGraphMutex, "TaskController::mThreadPoolCV"),
+      mMainThreadCV(mGraphMutex, "TaskController::mMainThreadCV"),
+      mRunOutOfMTTasksCounter(0) {
+  InputTaskManager::Init();
+  VsyncTaskManager::Init();
+  mMTProcessingRunnable = NS_NewRunnableFunction(
+      "TaskController::ExecutePendingMTTasks()",
+      []() { TaskController::Get()->ProcessPendingMTTask(); });
+  mMTBlockingProcessingRunnable = NS_NewRunnableFunction(
+      "TaskController::ExecutePendingMTTasks()",
+      []() { TaskController::Get()->ProcessPendingMTTask(true); });
+}
+
+// We want our default stack size limit to be approximately 2MB, to be safe for
+// JS helper tasks that can use a lot of stack, but expect most threads to use
+// much less. On Linux, however, requesting a stack of 2MB or larger risks the
+// kernel allocating an entire 2MB huge page for it on first access, which we do
+// not want. To avoid this possibility, we subtract 2 standard VM page sizes
+// from our default.
+constexpr PRUint32 sBaseStackSize = 2048 * 1024 - 2 * 4096;
+
+// TSan enforces a minimum stack size that's just slightly larger than our
+// default helper stack size.  It does this to store blobs of TSan-specific data
+// on each thread's stack.  Unfortunately, that means that even though we'll
+// actually receive a larger stack than we requested, the effective usable space
+// of that stack is significantly less than what we expect.  To offset TSan
+// stealing our stack space from underneath us, double the default.
+//
+// Similarly, ASan requires more stack space due to red-zones.
+#if defined(MOZ_TSAN) || defined(MOZ_ASAN)
+constexpr PRUint32 sStackSize = 2 * sBaseStackSize;
+#else
+constexpr PRUint32 sStackSize = sBaseStackSize;
+#endif
+
+void TaskController::InitializeThreadPool() {
+  mPoolInitializationMutex.AssertCurrentThreadOwns();
+  MOZ_ASSERT(!mThreadPoolInitialized);
+  mThreadPoolInitialized = true;
+
+  int32_t poolSize = GetPoolThreadCount();
+  for (int32_t i = 0; i < poolSize; i++) {
+    int32_t* index = new int32_t(i);
+    mPoolThreads.push_back(
+        {PR_CreateThread(PR_USER_THREAD, ThreadFuncPoolThread, index,
+                         PR_PRIORITY_NORMAL, PR_GLOBAL_THREAD,
+                         PR_JOINABLE_THREAD, sStackSize),
+         nullptr});
+  }
+}
+
+/* static */
+size_t TaskController::GetThreadStackSize() { return sStackSize; }
+
+void TaskController::SetPerformanceCounterState(
+    PerformanceCounterState* aPerformanceCounterState) {
+  mPerformanceCounterState = aPerformanceCounterState;
+}
+
+/* static */
+void TaskController::Shutdown() {
+  InputTaskManager::Cleanup();
+  VsyncTaskManager::Cleanup();
+  if (sSingleton) {
+    sSingleton->ShutdownThreadPoolInternal();
+    sSingleton = nullptr;
+  }
+  MOZ_ASSERT(!sSingleton);
+}
+
+void TaskController::ShutdownThreadPoolInternal() {
+  {
+    // Prevent race condition on mShuttingDown and wait.
+    MutexAutoLock lock(mGraphMutex);
+    mShuttingDown = true;
+    mThreadPoolCV.NotifyAll();
+  }
+  for (PoolThread& thread : mPoolThreads) {
+    PR_JoinThread(thread.mThread);
+  }
+}
+
+void TaskController::RunPoolThread() {
+  IOInterposer::RegisterCurrentThread();
+
+  // This is used to hold on to a task to make sure it is released outside the
+  // lock. This is required since it's perfectly feasible for task destructors
+  // to post events themselves.
+  RefPtr<Task> lastTask;
+
+  nsAutoCString threadName;
+  threadName.AppendLiteral("TaskController #");
+  threadName.AppendInt(static_cast<int64_t>(mThreadPoolIndex));
+  AUTO_PROFILER_REGISTER_THREAD(threadName.BeginReading());
+
+  MutexAutoLock lock(mGraphMutex);
+  while (true) {
+    bool ranTask = false;
+
+    if (!mThreadableTasks.empty()) {
+      for (auto iter = mThreadableTasks.begin(); iter != mThreadableTasks.end();
+           ++iter) {
+        // Search for the highest priority dependency of the highest priority
+        // task.
+
+        // We work with rawptrs to avoid needless refcounting. All our tasks
+        // are always kept alive by the graph. If one is removed from the graph
+        // it is kept alive by mPoolThreads[mThreadPoolIndex].mCurrentTask.
+        Task* task = iter->get();
+
+        MOZ_ASSERT(!task->mTaskManager);
+
+        mPoolThreads[mThreadPoolIndex].mEffectiveTaskPriority =
+            task->GetPriority();
+
+        Task* nextTask;
+        while ((nextTask = task->GetHighestPriorityDependency())) {
+          task = nextTask;
+        }
+
+        if (task->GetKind() == Task::Kind::MainThreadOnly ||
+            task->mInProgress) {
+          continue;
+        }
+
+        mPoolThreads[mThreadPoolIndex].mCurrentTask = task;
+        mThreadableTasks.erase(task->mIterator);
+        task->mIterator = mThreadableTasks.end();
+        task->mInProgress = true;
+
+        if (!mThreadableTasks.empty()) {
+          // Ensure at least one additional thread is woken up if there are
+          // more threadable tasks to process. Notifying all threads at once
+          // isn't actually better for performance since they all need the
+          // GraphMutex to proceed anyway.
+          mThreadPoolCV.Notify();
+        }
+
+        bool taskCompleted = false;
+        {
+          MutexAutoUnlock unlock(mGraphMutex);
+          lastTask = nullptr;
+          AUTO_PROFILE_FOLLOWING_TASK(task);
+          taskCompleted = task->Run() == Task::TaskResult::Complete;
+          ranTask = true;
+        }
+
+        task->mInProgress = false;
+
+        if (!taskCompleted) {
+          // Presumably this task was interrupted, leave its dependencies
+          // unresolved and reinsert into the queue.
+          auto insertion = mThreadableTasks.insert(
+              mPoolThreads[mThreadPoolIndex].mCurrentTask);
+          MOZ_ASSERT(insertion.second);
+          task->mIterator = insertion.first;
+        } else {
+          task->mCompleted = true;
+#ifdef DEBUG
+          task->mIsInGraph = false;
+#endif
+          task->mDependencies.clear();
+          // This may have unblocked a main thread task. We could do this only
+          // if there was a main thread task before this one in the dependency
+          // chain.
+          mMayHaveMainThreadTask = true;
+          // Since this could have multiple dependencies thare are restricted
+          // to the main thread. Let's make sure that's awake.
+          EnsureMainThreadTasksScheduled();
+
+          MaybeInterruptTask(GetHighestPriorityMTTask());
+        }
+
+        // Store last task for release next time we release the lock or enter
+        // wait state.
+        lastTask = mPoolThreads[mThreadPoolIndex].mCurrentTask.forget();
+        break;
+      }
+    }
+
+    // Ensure the last task is released before we enter the wait state.
+    if (lastTask) {
+      MutexAutoUnlock unlock(mGraphMutex);
+      lastTask = nullptr;
+
+      // Run another loop iteration, while we were unlocked there was an
+      // opportunity for another task to be posted or shutdown to be initiated.
+      continue;
+    }
+
+    if (!ranTask) {
+      if (mShuttingDown) {
+        IOInterposer::UnregisterCurrentThread();
+        MOZ_ASSERT(mThreadableTasks.empty());
+        return;
+      }
+
+      AUTO_PROFILER_LABEL("TaskController::RunPoolThread", IDLE);
+      mThreadPoolCV.Wait();
+    }
+  }
+}
+
+void TaskController::AddTask(already_AddRefed<Task>&& aTask) {
+  RefPtr<Task> task(aTask);
+
+  if (task->GetKind() == Task::Kind::OffMainThreadOnly) {
+    MutexAutoLock lock(mPoolInitializationMutex);
+    if (!mThreadPoolInitialized) {
+      InitializeThreadPool();
+    }
+  }
+
+  MutexAutoLock lock(mGraphMutex);
+
+  if (TaskManager* manager = task->GetManager()) {
+    if (manager->mTaskCount == 0) {
+      mTaskManagers.insert(manager);
+    }
+    manager->DidQueueTask();
+
+    // Set this here since if this manager's priority modifier doesn't change
+    // we will not reprioritize when iterating over the queue.
+    task->mPriorityModifier = manager->mCurrentPriorityModifier;
+  }
+
+  if (profiler_is_active_and_unpaused()) {
+    task->mInsertionTime = TimeStamp::Now();
+  }
+
+#ifdef DEBUG
+  task->mIsInGraph = true;
+
+  for (const RefPtr<Task>& otherTask : task->mDependencies) {
+    MOZ_ASSERT(!otherTask->mTaskManager ||
+               otherTask->mTaskManager == task->mTaskManager);
+  }
+#endif
+
+  LogTask::LogDispatch(task);
+
+  std::pair<std::set<RefPtr<Task>, Task::PriorityCompare>::iterator, bool>
+      insertion;
+  switch (task->GetKind()) {
+    case Task::Kind::MainThreadOnly:
+      if (task->GetPriority() >=
+              static_cast<uint32_t>(EventQueuePriority::Normal) &&
+          !mMainThreadTasks.empty()) {
+        insertion = std::pair(
+            mMainThreadTasks.insert(--mMainThreadTasks.end(), std::move(task)),
+            true);
+      } else {
+        insertion = mMainThreadTasks.insert(std::move(task));
+      }
+      break;
+    case Task::Kind::OffMainThreadOnly:
+      insertion = mThreadableTasks.insert(std::move(task));
+      break;
+  }
+  (*insertion.first)->mIterator = insertion.first;
+  MOZ_ASSERT(insertion.second);
+
+  MaybeInterruptTask(*insertion.first);
+}
+
+void TaskController::WaitForTaskOrMessage() {
+  MutexAutoLock lock(mGraphMutex);
+  while (!mMayHaveMainThreadTask) {
+    AUTO_PROFILER_LABEL("TaskController::WaitForTaskOrMessage", IDLE);
+    mMainThreadCV.Wait();
+  }
+}
+
+void TaskController::ExecuteNextTaskOnlyMainThread() {
+  MOZ_ASSERT(NS_IsMainThread());
+  MutexAutoLock lock(mGraphMutex);
+  ExecuteNextTaskOnlyMainThreadInternal(lock);
+}
+
+void TaskController::ProcessPendingMTTask(bool aMayWait) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MutexAutoLock lock(mGraphMutex);
+
+  for (;;) {
+    // We only ever process one event here. However we may sometimes
+    // not actually process a real event because of suspended tasks.
+    // This loop allows us to wait until we've processed something
+    // in that scenario.
+
+    mMTTaskRunnableProcessedTask = ExecuteNextTaskOnlyMainThreadInternal(lock);
+
+    if (mMTTaskRunnableProcessedTask || !aMayWait) {
+      break;
+    }
+
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+    // Unlock before calling into the BackgroundHangMonitor API as it uses
+    // the timer API.
+    {
+      MutexAutoUnlock unlock(mGraphMutex);
+      BackgroundHangMonitor().NotifyWait();
+    }
+#endif
+
+    {
+      // ProcessNextEvent will also have attempted to wait, however we may have
+      // given it a Runnable when all the tasks in our task graph were suspended
+      // but we weren't able to cheaply determine that.
+      AUTO_PROFILER_LABEL("TaskController::ProcessPendingMTTask", IDLE);
+      mMainThreadCV.Wait();
+    }
+
+#ifdef MOZ_ENABLE_BACKGROUND_HANG_MONITOR
+    {
+      MutexAutoUnlock unlock(mGraphMutex);
+      BackgroundHangMonitor().NotifyActivity();
+    }
+#endif
+  }
+
+  if (mMayHaveMainThreadTask) {
+    EnsureMainThreadTasksScheduled();
+  }
+}
+
+void TaskController::ReprioritizeTask(Task* aTask, uint32_t aPriority) {
+  MutexAutoLock lock(mGraphMutex);
+  std::set<RefPtr<Task>, Task::PriorityCompare>* queue = &mMainThreadTasks;
+  if (aTask->GetKind() == Task::Kind::OffMainThreadOnly) {
+    queue = &mThreadableTasks;
+  }
+
+  MOZ_ASSERT(aTask->mIterator != queue->end());
+  queue->erase(aTask->mIterator);
+
+  aTask->mPriority = aPriority;
+
+  auto insertion = queue->insert(aTask);
+  MOZ_ASSERT(insertion.second);
+  aTask->mIterator = insertion.first;
+
+  MaybeInterruptTask(aTask);
+}
+
+// Code supporting runnable compatibility.
+// Task that wraps a runnable.
+class RunnableTask : public Task {
+ public:
+  RunnableTask(already_AddRefed<nsIRunnable>&& aRunnable, int32_t aPriority,
+               Kind aKind)
+      : Task(aKind, aPriority), mRunnable(aRunnable) {}
+
+  virtual TaskResult Run() override {
+    mRunnable->Run();
+    mRunnable = nullptr;
+    return TaskResult::Complete;
+  }
+
+  void SetIdleDeadline(TimeStamp aDeadline) override {
+    nsCOMPtr<nsIIdleRunnable> idleRunnable = do_QueryInterface(mRunnable);
+    if (idleRunnable) {
+      idleRunnable->SetDeadline(aDeadline);
+    }
+  }
+
+  virtual bool GetName(nsACString& aName) override {
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+    if (nsCOMPtr<nsINamed> named = do_QueryInterface(mRunnable)) {
+      MOZ_ALWAYS_TRUE(NS_SUCCEEDED(named->GetName(aName)));
+    } else {
+      aName.AssignLiteral("non-nsINamed runnable");
+    }
+    if (aName.IsEmpty()) {
+      aName.AssignLiteral("anonymous runnable");
+    }
+    return true;
+#else
+    return false;
+#endif
+  }
+
+ private:
+  RefPtr<nsIRunnable> mRunnable;
+};
+
+void TaskController::DispatchRunnable(already_AddRefed<nsIRunnable>&& aRunnable,
+                                      uint32_t aPriority,
+                                      TaskManager* aManager) {
+  RefPtr<RunnableTask> task = new RunnableTask(std::move(aRunnable), aPriority,
+                                               Task::Kind::MainThreadOnly);
+
+  task->SetManager(aManager);
+  TaskController::Get()->AddTask(task.forget());
+}
+
+nsIRunnable* TaskController::GetRunnableForMTTask(bool aReallyWait) {
+  MutexAutoLock lock(mGraphMutex);
+
+  while (mMainThreadTasks.empty()) {
+    if (!aReallyWait) {
+      return nullptr;
+    }
+
+    AUTO_PROFILER_LABEL("TaskController::GetRunnableForMTTask::Wait", IDLE);
+    mMainThreadCV.Wait();
+  }
+
+  return aReallyWait ? mMTBlockingProcessingRunnable : mMTProcessingRunnable;
+}
+
+bool TaskController::HasMainThreadPendingTasks() {
+  MOZ_ASSERT(NS_IsMainThread());
+  auto resetIdleState = MakeScopeExit([&idleManager = mIdleTaskManager] {
+    if (idleManager) {
+      idleManager->State().ClearCachedIdleDeadline();
+    }
+  });
+
+  for (bool considerIdle : {false, true}) {
+    if (considerIdle && !mIdleTaskManager) {
+      continue;
+    }
+
+    MutexAutoLock lock(mGraphMutex);
+
+    if (considerIdle) {
+      mIdleTaskManager->State().ForgetPendingTaskGuarantee();
+      // Temporarily unlock so we can peek our idle deadline.
+      // XXX We could do this _before_ we take the lock if the API would let us.
+      // We do want to do this before looking at mMainThreadTasks, in case
+      // someone adds one while we're unlocked.
+      {
+        MutexAutoUnlock unlock(mGraphMutex);
+        mIdleTaskManager->State().CachePeekedIdleDeadline(unlock);
+      }
+    }
+
+    // Return early if there's no tasks at all.
+    if (mMainThreadTasks.empty()) {
+      return false;
+    }
+
+    // We can cheaply count how many tasks are suspended.
+    uint64_t totalSuspended = 0;
+    for (TaskManager* manager : mTaskManagers) {
+      DebugOnly<bool> modifierChanged =
+          manager
+              ->UpdateCachesForCurrentIterationAndReportPriorityModifierChanged(
+                  lock, TaskManager::IterationType::NOT_EVENT_LOOP_TURN);
+      MOZ_ASSERT(!modifierChanged);
+
+      // The idle manager should be suspended unless we're doing the idle pass.
+      MOZ_ASSERT(manager != mIdleTaskManager || manager->mCurrentSuspended ||
+                     considerIdle,
+                 "Why are idle tasks not suspended here?");
+
+      if (manager->mCurrentSuspended) {
+        // XXX - If managers manage off-main-thread tasks this breaks! This
+        // scenario is explicitly not supported.
+        //
+        // This is only incremented inside the lock -or- decremented on the main
+        // thread so this is safe.
+        totalSuspended += manager->mTaskCount;
+      }
+    }
+
+    // This would break down if we have a non-suspended task depending on a
+    // suspended task. This is why for the moment we do not allow tasks
+    // to be dependent on tasks managed by another taskmanager.
+    if (mMainThreadTasks.size() > totalSuspended) {
+      // If mIdleTaskManager->mTaskCount is 0, we never updated the suspended
+      // state of mIdleTaskManager above, hence shouldn't even check it here.
+      // But in that case idle tasks are not contributing to our suspended task
+      // count anyway.
+      if (mIdleTaskManager && mIdleTaskManager->mTaskCount &&
+          !mIdleTaskManager->mCurrentSuspended) {
+        MOZ_ASSERT(considerIdle, "Why is mIdleTaskManager not suspended?");
+        // Check whether the idle tasks were really needed to make our "we have
+        // an unsuspended task" decision.  If they were, we need to force-enable
+        // idle tasks until we run our next task.
+        if (mMainThreadTasks.size() - mIdleTaskManager->mTaskCount <=
+            totalSuspended) {
+          mIdleTaskManager->State().EnforcePendingTaskGuarantee();
+        }
+      }
+      return true;
+    }
+  }
+  return false;
+}
+
+uint64_t TaskController::PendingMainthreadTaskCountIncludingSuspended() {
+  MutexAutoLock lock(mGraphMutex);
+  return mMainThreadTasks.size();
+}
+
+bool TaskController::ExecuteNextTaskOnlyMainThreadInternal(
+    const MutexAutoLock& aProofOfLock) {
+  MOZ_ASSERT(NS_IsMainThread());
+  mGraphMutex.AssertCurrentThreadOwns();
+  // Block to make it easier to jump to our cleanup.
+  bool taskRan = false;
+  do {
+    taskRan = DoExecuteNextTaskOnlyMainThreadInternal(aProofOfLock);
+    if (taskRan) {
+      if (mIdleTaskManager && mIdleTaskManager->mTaskCount &&
+          mIdleTaskManager->IsSuspended(aProofOfLock)) {
+        uint32_t activeTasks = mMainThreadTasks.size();
+        for (TaskManager* manager : mTaskManagers) {
+          if (manager->IsSuspended(aProofOfLock)) {
+            activeTasks -= manager->mTaskCount;
+          } else {
+            break;
+          }
+        }
+
+        if (!activeTasks) {
+          // We have only idle (and maybe other suspended) tasks left, so need
+          // to update the idle state. We need to temporarily release the lock
+          // while we do that.
+          MutexAutoUnlock unlock(mGraphMutex);
+          mIdleTaskManager->State().RequestIdleDeadlineIfNeeded(unlock);
+        }
+      }
+      break;
+    }
+
+    if (!mIdleTaskManager) {
+      break;
+    }
+
+    if (mIdleTaskManager->mTaskCount) {
+      // We have idle tasks that we may not have gotten above because
+      // our idle state is not up to date.  We need to update the idle state
+      // and try again.  We need to temporarily release the lock while we do
+      // that.
+      MutexAutoUnlock unlock(mGraphMutex);
+      mIdleTaskManager->State().UpdateCachedIdleDeadline(unlock);
+    } else {
+      MutexAutoUnlock unlock(mGraphMutex);
+      mIdleTaskManager->State().RanOutOfTasks(unlock);
+    }
+
+    // When we unlocked, someone may have queued a new task on us.  So try to
+    // see whether we can run things again.
+    taskRan = DoExecuteNextTaskOnlyMainThreadInternal(aProofOfLock);
+  } while (false);
+
+  if (mIdleTaskManager) {
+    // The pending task guarantee is not needed anymore, since we just tried
+    // running a task
+    mIdleTaskManager->State().ForgetPendingTaskGuarantee();
+
+    if (mMainThreadTasks.empty()) {
+      ++mRunOutOfMTTasksCounter;
+
+      // XXX the IdlePeriodState API demands we have a MutexAutoUnlock for it.
+      // Otherwise we could perhaps just do this after we exit the locked block,
+      // by pushing the lock down into this method.  Though it's not clear that
+      // we could check mMainThreadTasks.size() once we unlock, and whether we
+      // could maybe substitute mMayHaveMainThreadTask for that check.
+      MutexAutoUnlock unlock(mGraphMutex);
+      mIdleTaskManager->State().RanOutOfTasks(unlock);
+    }
+  }
+
+  return taskRan;
+}
+
+bool TaskController::DoExecuteNextTaskOnlyMainThreadInternal(
+    const MutexAutoLock& aProofOfLock) {
+  mGraphMutex.AssertCurrentThreadOwns();
+
+  nsCOMPtr<nsIThread> mainIThread;
+  NS_GetMainThread(getter_AddRefs(mainIThread));
+
+  nsThread* mainThread = static_cast<nsThread*>(mainIThread.get());
+  if (mainThread) {
+    mainThread->SetRunningEventDelay(TimeDuration(), TimeStamp());
+  }
+
+  uint32_t totalSuspended = 0;
+  for (TaskManager* manager : mTaskManagers) {
+    bool modifierChanged =
+        manager
+            ->UpdateCachesForCurrentIterationAndReportPriorityModifierChanged(
+                aProofOfLock, TaskManager::IterationType::EVENT_LOOP_TURN);
+    if (modifierChanged) {
+      ProcessUpdatedPriorityModifier(manager);
+    }
+    if (manager->mCurrentSuspended) {
+      totalSuspended += manager->mTaskCount;
+    }
+  }
+
+  MOZ_ASSERT(mMainThreadTasks.size() >= totalSuspended);
+
+  // This would break down if we have a non-suspended task depending on a
+  // suspended task. This is why for the moment we do not allow tasks
+  // to be dependent on tasks managed by another taskmanager.
+  if (mMainThreadTasks.size() > totalSuspended) {
+    for (auto iter = mMainThreadTasks.begin(); iter != mMainThreadTasks.end();
+         iter++) {
+      Task* task = iter->get();
+
+      if (task->mTaskManager && task->mTaskManager->mCurrentSuspended) {
+        // Even though we may want to run some dependencies of this task, we
+        // will run them at their own priority level and not the priority
+        // level of their dependents.
+        continue;
+      }
+
+      task = GetFinalDependency(task);
+
+      if (task->GetKind() == Task::Kind::OffMainThreadOnly ||
+          task->mInProgress ||
+          (task->mTaskManager && task->mTaskManager->mCurrentSuspended)) {
+        continue;
+      }
+
+      mCurrentTasksMT.push(task);
+      mMainThreadTasks.erase(task->mIterator);
+      task->mIterator = mMainThreadTasks.end();
+      task->mInProgress = true;
+      TaskManager* manager = task->GetManager();
+      bool result = false;
+
+      {
+        MutexAutoUnlock unlock(mGraphMutex);
+        if (manager) {
+          manager->WillRunTask();
+          if (manager != mIdleTaskManager) {
+            // Notify the idle period state that we're running a non-idle task.
+            // This needs to happen while our mutex is not locked!
+            mIdleTaskManager->State().FlagNotIdle();
+          } else {
+            TimeStamp idleDeadline =
+                mIdleTaskManager->State().GetCachedIdleDeadline();
+            MOZ_ASSERT(
+                idleDeadline,
+                "How can we not have a deadline if our manager is enabled?");
+            task->SetIdleDeadline(idleDeadline);
+          }
+        }
+        if (mIdleTaskManager) {
+          // We found a task to run; we can clear the idle deadline on our idle
+          // task manager.  This _must_ be done before we actually run the task,
+          // because running the task could reenter via spinning the event loop
+          // and we want to make sure there's no cached idle deadline at that
+          // point.  But we have to make sure we do it after out SetIdleDeadline
+          // call above, in the case when the task is actually an idle task.
+          mIdleTaskManager->State().ClearCachedIdleDeadline();
+        }
+
+        TimeStamp now = TimeStamp::Now();
+
+        if (mainThread) {
+          if (task->GetPriority() < uint32_t(EventQueuePriority::InputHigh) ||
+              task->mInsertionTime.IsNull()) {
+            mainThread->SetRunningEventDelay(TimeDuration(), now);
+          } else {
+            mainThread->SetRunningEventDelay(now - task->mInsertionTime, now);
+          }
+        }
+
+        nsAutoCString name;
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+        task->GetName(name);
+#endif
+
+        PerformanceCounterState::Snapshot snapshot =
+            mPerformanceCounterState->RunnableWillRun(
+                now, manager == mIdleTaskManager);
+
+        {
+          LogTask::Run log(task);
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+          AutoSetMainThreadRunnableName nameGuard(name);
+#endif
+          AUTO_PROFILE_FOLLOWING_TASK(task);
+          result = task->Run() == Task::TaskResult::Complete;
+        }
+
+        // Task itself should keep manager alive.
+        if (manager) {
+          manager->DidRunTask();
+        }
+
+        mPerformanceCounterState->RunnableDidRun(name, std::move(snapshot));
+      }
+
+      // Task itself should keep manager alive.
+      if (manager && result && manager->mTaskCount == 0) {
+        mTaskManagers.erase(manager);
+      }
+
+      task->mInProgress = false;
+
+      if (!result) {
+        // Presumably this task was interrupted, leave its dependencies
+        // unresolved and reinsert into the queue.
+        auto insertion =
+            mMainThreadTasks.insert(std::move(mCurrentTasksMT.top()));
+        MOZ_ASSERT(insertion.second);
+        task->mIterator = insertion.first;
+        manager->WillRunTask();
+      } else {
+        task->mCompleted = true;
+#ifdef DEBUG
+        task->mIsInGraph = false;
+#endif
+        // Clear dependencies to release references.
+        task->mDependencies.clear();
+
+        if (!mThreadableTasks.empty()) {
+          // We're going to wake up a single thread in our pool. This thread
+          // is responsible for waking up additional threads in the situation
+          // where more than one task became available.
+          mThreadPoolCV.Notify();
+        }
+      }
+
+      mCurrentTasksMT.pop();
+      return true;
+    }
+  }
+
+  mMayHaveMainThreadTask = false;
+  if (mIdleTaskManager) {
+    // We did not find a task to run.  We still need to clear the cached idle
+    // deadline on our idle state, because that deadline was only relevant to
+    // the execution of this function.  Had we found a task, we would have
+    // cleared the deadline before running that task.
+    mIdleTaskManager->State().ClearCachedIdleDeadline();
+  }
+  return false;
+}
+
+Task* TaskController::GetFinalDependency(Task* aTask) {
+  Task* nextTask;
+
+  while ((nextTask = aTask->GetHighestPriorityDependency())) {
+    aTask = nextTask;
+  }
+
+  return aTask;
+}
+
+void TaskController::MaybeInterruptTask(Task* aTask) {
+  mGraphMutex.AssertCurrentThreadOwns();
+
+  if (!aTask) {
+    return;
+  }
+
+  // This optimization prevents many slow lookups in long chains of similar
+  // priority.
+  if (!aTask->mDependencies.empty()) {
+    Task* firstDependency = aTask->mDependencies.begin()->get();
+    if (aTask->GetPriority() <= firstDependency->GetPriority() &&
+        !firstDependency->mCompleted &&
+        aTask->GetKind() == firstDependency->GetKind()) {
+      // This task has the same or a higher priority as one of its dependencies,
+      // never any need to interrupt.
+      return;
+    }
+  }
+
+  Task* finalDependency = GetFinalDependency(aTask);
+
+  if (finalDependency->mInProgress) {
+    // No need to wake anything, we can't schedule this task right now anyway.
+    return;
+  }
+
+  if (aTask->GetKind() == Task::Kind::MainThreadOnly) {
+    mMayHaveMainThreadTask = true;
+
+    EnsureMainThreadTasksScheduled();
+
+    if (mCurrentTasksMT.empty()) {
+      return;
+    }
+
+    // We could go through the steps above here and interrupt an off main
+    // thread task in case it has a lower priority.
+    if (finalDependency->GetKind() == Task::Kind::OffMainThreadOnly) {
+      return;
+    }
+
+    if (mCurrentTasksMT.top()->GetPriority() < aTask->GetPriority()) {
+      mCurrentTasksMT.top()->RequestInterrupt(aTask->GetPriority());
+    }
+  } else {
+    Task* lowestPriorityTask = nullptr;
+    for (PoolThread& thread : mPoolThreads) {
+      if (!thread.mCurrentTask) {
+        mThreadPoolCV.Notify();
+        // There's a free thread, no need to interrupt anything.
+        return;
+      }
+
+      if (!lowestPriorityTask) {
+        lowestPriorityTask = thread.mCurrentTask.get();
+        continue;
+      }
+
+      // This should possibly select the lowest priority task which was started
+      // the latest. But for now we ignore that optimization.
+      // This also doesn't guarantee a task is interruptable, so that's an
+      // avenue for improvements as well.
+      if (lowestPriorityTask->GetPriority() > thread.mEffectiveTaskPriority) {
+        lowestPriorityTask = thread.mCurrentTask.get();
+      }
+    }
+
+    if (lowestPriorityTask->GetPriority() < aTask->GetPriority()) {
+      lowestPriorityTask->RequestInterrupt(aTask->GetPriority());
+    }
+
+    // We choose not to interrupt main thread tasks for tasks which may be
+    // executed off the main thread.
+  }
+}
+
+Task* TaskController::GetHighestPriorityMTTask() {
+  mGraphMutex.AssertCurrentThreadOwns();
+
+  if (!mMainThreadTasks.empty()) {
+    return mMainThreadTasks.begin()->get();
+  }
+  return nullptr;
+}
+
+void TaskController::EnsureMainThreadTasksScheduled() {
+  if (mObserver) {
+    mObserver->OnDispatchedEvent();
+  }
+  if (mExternalCondVar) {
+    mExternalCondVar->Notify();
+  }
+  mMainThreadCV.Notify();
+}
+
+void TaskController::ProcessUpdatedPriorityModifier(TaskManager* aManager) {
+  mGraphMutex.AssertCurrentThreadOwns();
+
+  MOZ_ASSERT(NS_IsMainThread());
+
+  int32_t modifier = aManager->mCurrentPriorityModifier;
+
+  std::vector<RefPtr<Task>> storedTasks;
+  // Find all relevant tasks.
+  for (auto iter = mMainThreadTasks.begin(); iter != mMainThreadTasks.end();) {
+    if ((*iter)->mTaskManager == aManager) {
+      storedTasks.push_back(*iter);
+      iter = mMainThreadTasks.erase(iter);
+    } else {
+      iter++;
+    }
+  }
+
+  // Reinsert found tasks with their new priorities.
+  for (RefPtr<Task>& ref : storedTasks) {
+    // Kept alive at first by the vector and then by mMainThreadTasks.
+    Task* task = ref;
+    task->mPriorityModifier = modifier;
+    auto insertion = mMainThreadTasks.insert(std::move(ref));
+    MOZ_ASSERT(insertion.second);
+    task->mIterator = insertion.first;
+  }
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/TaskController.h b/xpcom/threads/TaskController.h
new file mode 100644
index 0000000000..f51e61f4cd
--- /dev/null
+++ b/xpcom/threads/TaskController.h
@@ -0,0 +1,459 @@
+/* -*- 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_TaskController_h
+#define mozilla_TaskController_h
+
+#include "MainThreadUtils.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/IdlePeriodState.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/EventQueue.h"
+#include "nsISupportsImpl.h"
+
+#include <atomic>
+#include <vector>
+#include <set>
+#include <stack>
+
+class nsIRunnable;
+class nsIThreadObserver;
+
+namespace mozilla {
+
+class Task;
+class TaskController;
+class PerformanceCounter;
+class PerformanceCounterState;
+
+const EventQueuePriority kDefaultPriorityValue = EventQueuePriority::Normal;
+
+// This file contains the core classes to access the Gecko scheduler. The
+// scheduler forms a graph of prioritize tasks, and is responsible for ensuring
+// the execution of tasks or their dependencies in order of inherited priority.
+//
+// The core class is the 'Task' class. The task class describes a single unit of
+// work. Users scheduling work implement this class and are required to
+// reimplement the 'Run' function in order to do work.
+//
+// The TaskManager class is reimplemented by users that require
+// the ability to reprioritize or suspend tasks.
+//
+// The TaskController is responsible for scheduling the work itself. The AddTask
+// function is used to schedule work. The ReprioritizeTask function may be used
+// to change the priority of a task already in the task graph, without
+// unscheduling it.
+
+// The TaskManager is the baseclass used to atomically manage a large set of
+// tasks. API users reimplementing TaskManager may reimplement a number of
+// functions that they may use to indicate to the scheduler changes in the state
+// for any tasks they manage. They may be used to reprioritize or suspend tasks
+// under their control, and will also be notified before and after tasks under
+// their control are executed. Their methods will only be called once per event
+// loop turn, however they may still incur some performance overhead. In
+// addition to this frequent reprioritizations may incur a significant
+// performance overhead and are discouraged. A TaskManager may currently only be
+// used to manage tasks that are bound to the Gecko Main Thread.
+class TaskManager {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(TaskManager)
+
+  TaskManager() : mTaskCount(0) {}
+
+  // Subclasses implementing task manager will have this function called to
+  // determine whether their associated tasks are currently suspended. This
+  // will only be called once per iteration of the task queue, this means that
+  // suspension of tasks managed by a single TaskManager may be assumed to
+  // occur atomically.
+  virtual bool IsSuspended(const MutexAutoLock& aProofOfLock) { return false; }
+
+  // Subclasses may implement this in order to supply a priority adjustment
+  // to their managed tasks. This is called once per iteration of the task
+  // queue, and may be assumed to occur atomically for all managed tasks.
+  virtual int32_t GetPriorityModifierForEventLoopTurn(
+      const MutexAutoLock& aProofOfLock) {
+    return 0;
+  }
+
+  void DidQueueTask() { ++mTaskCount; }
+  // This is called when a managed task is about to be executed by the
+  // scheduler. Anyone reimplementing this should ensure to call the parent or
+  // decrement mTaskCount.
+  virtual void WillRunTask() { --mTaskCount; }
+  // This is called when a managed task has finished being executed by the
+  // scheduler.
+  virtual void DidRunTask() {}
+  uint32_t PendingTaskCount() { return mTaskCount; }
+
+ protected:
+  virtual ~TaskManager() {}
+
+ private:
+  friend class TaskController;
+
+  enum class IterationType { NOT_EVENT_LOOP_TURN, EVENT_LOOP_TURN };
+  bool UpdateCachesForCurrentIterationAndReportPriorityModifierChanged(
+      const MutexAutoLock& aProofOfLock, IterationType aIterationType);
+
+  bool mCurrentSuspended = false;
+  int32_t mCurrentPriorityModifier = 0;
+
+  std::atomic<uint32_t> mTaskCount;
+};
+
+// A Task is the the base class for any unit of work that may be scheduled.
+//
+// Subclasses may specify their priority and whether they should be bound to
+// either the Gecko Main thread or off main thread. When not bound to the main
+// thread tasks may be executed on any available thread excluding the main
+// thread, but they may also be executed in parallel to any other task they do
+// not have a dependency relationship with.
+//
+// Tasks will be run in order of object creation.
+class Task {
+ public:
+  enum class Kind : uint8_t {
+    // This task should be executed on any available thread excluding the Gecko
+    // Main thread.
+    OffMainThreadOnly,
+
+    // This task should be executed on the Gecko Main thread.
+    MainThreadOnly
+
+    // NOTE: "any available thread including the main thread" option is not
+    //       supported (See bug 1839102).
+  };
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(Task)
+
+  Kind GetKind() { return mKind; }
+
+  // This returns the current task priority with its modifier applied.
+  uint32_t GetPriority() { return mPriority + mPriorityModifier; }
+  uint64_t GetSeqNo() { return mSeqNo; }
+
+  // Callee needs to assume this may be called on any thread.
+  // aInterruptPriority passes the priority of the higher priority task that
+  // is ready to be executed. The task may safely ignore this function, or
+  // interrupt any work being done. It may return 'false' from its run function
+  // in order to be run automatically in the future, or true if it will
+  // reschedule incomplete work manually.
+  virtual void RequestInterrupt(uint32_t aInterruptPriority) {}
+
+  // At the moment this -must- be called before the task is added to the
+  // controller. Calling this after tasks have been added to the controller
+  // results in undefined behavior!
+  // At submission, tasks must depend only on tasks managed by the same, or
+  // no idle manager.
+  void AddDependency(Task* aTask) {
+    MOZ_ASSERT(aTask);
+    MOZ_ASSERT(!mIsInGraph);
+    mDependencies.insert(aTask);
+  }
+
+  // This sets the TaskManager for the current task. Calling this after the
+  // task has been added to the TaskController results in undefined behavior.
+  void SetManager(TaskManager* aManager) {
+    MOZ_ASSERT(mKind == Kind::MainThreadOnly);
+    MOZ_ASSERT(!mIsInGraph);
+    mTaskManager = aManager;
+  }
+  TaskManager* GetManager() { return mTaskManager; }
+
+  struct PriorityCompare {
+    bool operator()(const RefPtr<Task>& aTaskA,
+                    const RefPtr<Task>& aTaskB) const {
+      uint32_t prioA = aTaskA->GetPriority();
+      uint32_t prioB = aTaskB->GetPriority();
+      return (prioA > prioB) ||
+             (prioA == prioB && (aTaskA->GetSeqNo() < aTaskB->GetSeqNo()));
+    }
+  };
+
+  // Tell the task about its idle deadline.  Will only be called for
+  // tasks managed by an IdleTaskManager, right before the task runs.
+  virtual void SetIdleDeadline(TimeStamp aDeadline) {}
+
+  virtual PerformanceCounter* GetPerformanceCounter() const { return nullptr; }
+
+  // Get a name for this task. This returns false if the task has no name.
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  virtual bool GetName(nsACString& aName) = 0;
+#else
+  virtual bool GetName(nsACString& aName) { return false; }
+#endif
+
+ protected:
+  Task(Kind aKind,
+       uint32_t aPriority = static_cast<uint32_t>(kDefaultPriorityValue))
+      : mKind(aKind), mSeqNo(sCurrentTaskSeqNo++), mPriority(aPriority) {}
+
+  Task(Kind aKind, EventQueuePriority aPriority = kDefaultPriorityValue)
+      : mKind(aKind),
+        mSeqNo(sCurrentTaskSeqNo++),
+        mPriority(static_cast<uint32_t>(aPriority)) {}
+
+  virtual ~Task() {}
+
+  friend class TaskController;
+
+  enum class TaskResult {
+    Complete,
+    Incomplete,
+  };
+
+  // When this returns TaskResult::Incomplete, it will be rescheduled at the
+  // current 'mPriority' level.
+  virtual TaskResult Run() = 0;
+
+ private:
+  Task* GetHighestPriorityDependency();
+
+  // Iterator pointing to this task's position in
+  // mThreadableTasks/mMainThreadTasks if, and only if this task is currently
+  // scheduled to be executed. This allows fast access to the task's position
+  // in the set, allowing for fast removal.
+  // This is safe, and remains valid unless the task is removed from the set.
+  // See also iterator invalidation in:
+  // https://en.cppreference.com/w/cpp/container
+  //
+  // Or the spec:
+  // "All Associative Containers: The insert and emplace members shall not
+  // affect the validity of iterators and references to the container
+  // [26.2.6/9]" "All Associative Containers: The erase members shall invalidate
+  // only iterators and references to the erased elements [26.2.6/9]"
+  std::set<RefPtr<Task>, PriorityCompare>::iterator mIterator;
+  std::set<RefPtr<Task>, PriorityCompare> mDependencies;
+
+  RefPtr<TaskManager> mTaskManager;
+
+  // Access to these variables is protected by the GraphMutex.
+  Kind mKind;
+  bool mCompleted = false;
+  bool mInProgress = false;
+#ifdef DEBUG
+  bool mIsInGraph = false;
+#endif
+
+  static std::atomic<uint64_t> sCurrentTaskSeqNo;
+  int64_t mSeqNo;
+  uint32_t mPriority;
+  // Modifier currently being applied to this task by its taskmanager.
+  int32_t mPriorityModifier = 0;
+  // Time this task was inserted into the task graph, this is used by the
+  // profiler.
+  mozilla::TimeStamp mInsertionTime;
+};
+
+struct PoolThread {
+  PRThread* mThread;
+  RefPtr<Task> mCurrentTask;
+  // This may be higher than mCurrentTask's priority due to priority
+  // propagation. This is -only- valid when mCurrentTask != nullptr.
+  uint32_t mEffectiveTaskPriority;
+};
+
+// A task manager implementation for priority levels that should only
+// run during idle periods.
+class IdleTaskManager : public TaskManager {
+ public:
+  explicit IdleTaskManager(already_AddRefed<nsIIdlePeriod>&& aIdlePeriod)
+      : mIdlePeriodState(std::move(aIdlePeriod)), mProcessedTaskCount(0) {}
+
+  IdlePeriodState& State() { return mIdlePeriodState; }
+
+  bool IsSuspended(const MutexAutoLock& aProofOfLock) override {
+    TimeStamp idleDeadline = State().GetCachedIdleDeadline();
+    return !idleDeadline;
+  }
+
+  void DidRunTask() override {
+    TaskManager::DidRunTask();
+    ++mProcessedTaskCount;
+  }
+
+  uint64_t ProcessedTaskCount() { return mProcessedTaskCount; }
+
+ private:
+  // Tracking of our idle state of various sorts.
+  IdlePeriodState mIdlePeriodState;
+
+  std::atomic<uint64_t> mProcessedTaskCount;
+};
+
+// The TaskController is the core class of the scheduler. It is used to
+// schedule tasks to be executed, as well as to reprioritize tasks that have
+// already been scheduled. The core functions to do this are AddTask and
+// ReprioritizeTask.
+class TaskController {
+ public:
+  TaskController();
+
+  static TaskController* Get() {
+    MOZ_ASSERT(sSingleton.get());
+    return sSingleton.get();
+  }
+
+  static void Initialize();
+
+  void SetThreadObserver(nsIThreadObserver* aObserver) {
+    MutexAutoLock lock(mGraphMutex);
+    mObserver = aObserver;
+  }
+  void SetConditionVariable(CondVar* aExternalCondVar) {
+    MutexAutoLock lock(mGraphMutex);
+    mExternalCondVar = aExternalCondVar;
+  }
+
+  void SetIdleTaskManager(IdleTaskManager* aIdleTaskManager) {
+    mIdleTaskManager = aIdleTaskManager;
+  }
+  IdleTaskManager* GetIdleTaskManager() { return mIdleTaskManager.get(); }
+
+  uint64_t RunOutOfMTTasksCount() { return mRunOutOfMTTasksCounter; }
+
+  // Initialization and shutdown code.
+  void SetPerformanceCounterState(
+      PerformanceCounterState* aPerformanceCounterState);
+
+  static void Shutdown();
+
+  // This adds a task to the TaskController graph.
+  // This may be called on any thread.
+  void AddTask(already_AddRefed<Task>&& aTask);
+
+  // This wait function is the theoretical function you would need if our main
+  // thread needs to also process OS messages or something along those lines.
+  void WaitForTaskOrMessage();
+
+  // This gets the next (highest priority) task that is only allowed to execute
+  // on the main thread.
+  void ExecuteNextTaskOnlyMainThread();
+
+  // Process all pending main thread tasks.
+  void ProcessPendingMTTask(bool aMayWait = false);
+
+  // This allows reprioritization of a task already in the task graph.
+  // This may be called on any thread.
+  void ReprioritizeTask(Task* aTask, uint32_t aPriority);
+
+  void DispatchRunnable(already_AddRefed<nsIRunnable>&& aRunnable,
+                        uint32_t aPriority, TaskManager* aManager = nullptr);
+
+  nsIRunnable* GetRunnableForMTTask(bool aReallyWait);
+
+  bool HasMainThreadPendingTasks();
+
+  uint64_t PendingMainthreadTaskCountIncludingSuspended();
+
+  // Let users know whether the last main thread task runnable did work.
+  bool MTTaskRunnableProcessedTask() {
+    MOZ_ASSERT(NS_IsMainThread());
+    return mMTTaskRunnableProcessedTask;
+  }
+
+  static int32_t GetPoolThreadCount();
+  static size_t GetThreadStackSize();
+
+ private:
+  friend void ThreadFuncPoolThread(void* aIndex);
+  static StaticAutoPtr<TaskController> sSingleton;
+
+  void InitializeThreadPool();
+
+  // This gets the next (highest priority) task that is only allowed to execute
+  // on the main thread, if any, and executes it.
+  // Returns true if it succeeded.
+  bool ExecuteNextTaskOnlyMainThreadInternal(const MutexAutoLock& aProofOfLock);
+
+  // The guts of ExecuteNextTaskOnlyMainThreadInternal, which get idle handling
+  // wrapped around them.  Returns whether a task actually ran.
+  bool DoExecuteNextTaskOnlyMainThreadInternal(
+      const MutexAutoLock& aProofOfLock);
+
+  Task* GetFinalDependency(Task* aTask);
+  void MaybeInterruptTask(Task* aTask);
+  Task* GetHighestPriorityMTTask();
+
+  void EnsureMainThreadTasksScheduled();
+
+  void ProcessUpdatedPriorityModifier(TaskManager* aManager);
+
+  void ShutdownThreadPoolInternal();
+
+  void RunPoolThread();
+
+  // This protects access to the task graph.
+  Mutex mGraphMutex MOZ_UNANNOTATED;
+
+  // This protects thread pool initialization. We cannot do this from within
+  // the GraphMutex, since thread creation on Windows can generate events on
+  // the main thread that need to be handled.
+  Mutex mPoolInitializationMutex =
+      Mutex("TaskController::mPoolInitializationMutex");
+  // Created under the PoolInitialization mutex, then never extended, and
+  // only freed when the object is freed.  mThread is set at creation time;
+  // mCurrentTask and mEffectiveTaskPriority are only accessed from the
+  // thread, so no locking is needed to access this.
+  std::vector<PoolThread> mPoolThreads;
+
+  CondVar mThreadPoolCV;
+  CondVar mMainThreadCV;
+
+  // Variables below are protected by mGraphMutex.
+
+  std::stack<RefPtr<Task>> mCurrentTasksMT;
+
+  // A list of all tasks ordered by priority.
+  std::set<RefPtr<Task>, Task::PriorityCompare> mThreadableTasks;
+  std::set<RefPtr<Task>, Task::PriorityCompare> mMainThreadTasks;
+
+  // TaskManagers currently active.
+  // We can use a raw pointer since tasks always hold on to their TaskManager.
+  std::set<TaskManager*> mTaskManagers;
+
+  // This ensures we keep running the main thread if we processed a task there.
+  bool mMayHaveMainThreadTask = true;
+  bool mShuttingDown = false;
+
+  // This stores whether the last main thread task runnable did work.
+  // Accessed only on MainThread
+  bool mMTTaskRunnableProcessedTask = false;
+
+  // Whether our thread pool is initialized. We use this currently to avoid
+  // starting the threads in processes where it's never used. This is protected
+  // by mPoolInitializationMutex.
+  bool mThreadPoolInitialized = false;
+
+  // Whether we have scheduled a runnable on the main thread event loop.
+  // This is used for nsIRunnable compatibility.
+  RefPtr<nsIRunnable> mMTProcessingRunnable;
+  RefPtr<nsIRunnable> mMTBlockingProcessingRunnable;
+
+  // XXX - Thread observer to notify when a new event has been dispatched
+  // Set immediately, then simply accessed from any thread
+  nsIThreadObserver* mObserver = nullptr;
+  // XXX - External condvar to notify when we have received an event
+  CondVar* mExternalCondVar = nullptr;
+  // Idle task manager so we can properly do idle state stuff.
+  RefPtr<IdleTaskManager> mIdleTaskManager;
+
+  // How many times the main thread was empty.
+  std::atomic<uint64_t> mRunOutOfMTTasksCounter;
+
+  // Our tracking of our performance counter and long task state,
+  // shared with nsThread.
+  // Set once when MainThread is created, never changed, only accessed from
+  // DoExecuteNextTaskOnlyMainThreadInternal()
+  PerformanceCounterState* mPerformanceCounterState = nullptr;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_TaskController_h
diff --git a/xpcom/threads/TaskDispatcher.h b/xpcom/threads/TaskDispatcher.h
new file mode 100644
index 0000000000..1f27c32c7d
--- /dev/null
+++ b/xpcom/threads/TaskDispatcher.h
@@ -0,0 +1,304 @@
+/* -*- 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/. */
+
+#if !defined(TaskDispatcher_h_)
+#  define TaskDispatcher_h_
+
+#  include <queue>
+
+#  include "mozilla/AbstractThread.h"
+#  include "mozilla/Maybe.h"
+#  include "mozilla/ProfilerRunnable.h"
+#  include "mozilla/UniquePtr.h"
+#  include "nsIDirectTaskDispatcher.h"
+#  include "nsISupportsImpl.h"
+#  include "nsTArray.h"
+#  include "nsThreadUtils.h"
+
+namespace mozilla {
+
+class SimpleTaskQueue {
+ public:
+  SimpleTaskQueue() = default;
+  virtual ~SimpleTaskQueue() = default;
+
+  void AddTask(already_AddRefed<nsIRunnable> aRunnable) {
+    if (!mTasks) {
+      mTasks.emplace();
+    }
+    mTasks->push(std::move(aRunnable));
+  }
+
+  void DrainTasks() {
+    if (!mTasks) {
+      return;
+    }
+    auto& queue = mTasks.ref();
+    while (!queue.empty()) {
+      nsCOMPtr<nsIRunnable> r = std::move(queue.front());
+      queue.pop();
+      AUTO_PROFILE_FOLLOWING_RUNNABLE(r);
+      r->Run();
+    }
+  }
+
+  bool HaveTasks() const { return mTasks && !mTasks->empty(); }
+
+ private:
+  // We use a Maybe<> because (a) when used for DirectTasks it often doesn't get
+  // anything put into it, and (b) the std::queue implementation in GNU
+  // libstdc++ does two largish heap allocations when creating a new std::queue.
+  Maybe<std::queue<nsCOMPtr<nsIRunnable>>> mTasks;
+};
+
+/*
+ * A classic approach to cross-thread communication is to dispatch asynchronous
+ * runnables to perform updates on other threads. This generally works well, but
+ * there are sometimes reasons why we might want to delay the actual dispatch of
+ * these tasks until a specified moment. At present, this is primarily useful to
+ * ensure that mirrored state gets updated atomically - but there may be other
+ * applications as well.
+ *
+ * TaskDispatcher is a general abstract class that accepts tasks and dispatches
+ * them at some later point. These groups of tasks are per-target-thread, and
+ * contain separate queues for several kinds of tasks (see comments  below). -
+ * "state change tasks" (which run first, and are intended to be used to update
+ * the value held by mirrors), and regular tasks, which are other arbitrary
+ * operations that the are gated to run after all the state changes have
+ * completed.
+ */
+class TaskDispatcher {
+ public:
+  TaskDispatcher() = default;
+  virtual ~TaskDispatcher() = default;
+
+  // Direct tasks are run directly (rather than dispatched asynchronously) when
+  // the tail dispatcher fires. A direct task may cause other tasks to be added
+  // to the tail dispatcher.
+  virtual void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) = 0;
+
+  // State change tasks are dispatched asynchronously always run before regular
+  // tasks. They are intended to be used to update the value held by mirrors
+  // before any other dispatched tasks are run on the target thread.
+  virtual void AddStateChangeTask(AbstractThread* aThread,
+                                  already_AddRefed<nsIRunnable> aRunnable) = 0;
+
+  // Regular tasks are dispatched asynchronously, and run after state change
+  // tasks.
+  virtual nsresult AddTask(AbstractThread* aThread,
+                           already_AddRefed<nsIRunnable> aRunnable) = 0;
+
+  virtual nsresult DispatchTasksFor(AbstractThread* aThread) = 0;
+  virtual bool HasTasksFor(AbstractThread* aThread) = 0;
+  virtual void DrainDirectTasks() = 0;
+};
+
+/*
+ * AutoTaskDispatcher is a stack-scoped TaskDispatcher implementation that fires
+ * its queued tasks when it is popped off the stack.
+ */
+class AutoTaskDispatcher : public TaskDispatcher {
+ public:
+  explicit AutoTaskDispatcher(nsIDirectTaskDispatcher* aDirectTaskDispatcher,
+                              bool aIsTailDispatcher = false)
+      : mDirectTaskDispatcher(aDirectTaskDispatcher),
+        mIsTailDispatcher(aIsTailDispatcher) {}
+
+  ~AutoTaskDispatcher() {
+    // Given that direct tasks may trigger other code that uses the tail
+    // dispatcher, it's better to avoid processing them in the tail dispatcher's
+    // destructor. So we require TailDispatchers to manually invoke
+    // DrainDirectTasks before the AutoTaskDispatcher gets destroyed. In truth,
+    // this is only necessary in the case where this AutoTaskDispatcher can be
+    // accessed by the direct tasks it dispatches (true for TailDispatchers, but
+    // potentially not true for other hypothetical AutoTaskDispatchers). Feel
+    // free to loosen this restriction to apply only to mIsTailDispatcher if a
+    // use-case requires it.
+    MOZ_ASSERT(!HaveDirectTasks());
+
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      DispatchTaskGroup(std::move(mTaskGroups[i]));
+    }
+  }
+
+  bool HaveDirectTasks() {
+    return mDirectTaskDispatcher && mDirectTaskDispatcher->HaveDirectTasks();
+  }
+
+  void DrainDirectTasks() override {
+    if (mDirectTaskDispatcher) {
+      mDirectTaskDispatcher->DrainDirectTasks();
+    }
+  }
+
+  void AddDirectTask(already_AddRefed<nsIRunnable> aRunnable) override {
+    MOZ_ASSERT(mDirectTaskDispatcher);
+    mDirectTaskDispatcher->DispatchDirectTask(std::move(aRunnable));
+  }
+
+  void AddStateChangeTask(AbstractThread* aThread,
+                          already_AddRefed<nsIRunnable> aRunnable) override {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    MOZ_RELEASE_ASSERT(r);
+    EnsureTaskGroup(aThread).mStateChangeTasks.AppendElement(r.forget());
+  }
+
+  nsresult AddTask(AbstractThread* aThread,
+                   already_AddRefed<nsIRunnable> aRunnable) override {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    MOZ_RELEASE_ASSERT(r);
+    // To preserve the event order, we need to append a new group if the last
+    // group is not targeted for |aThread|.
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=1318226&mark=0-3#c0
+    // for the details of the issue.
+    if (mTaskGroups.Length() == 0 ||
+        mTaskGroups.LastElement()->mThread != aThread) {
+      mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
+    }
+
+    PerThreadTaskGroup& group = *mTaskGroups.LastElement();
+    group.mRegularTasks.AppendElement(r.forget());
+
+    return NS_OK;
+  }
+
+  bool HasTasksFor(AbstractThread* aThread) override {
+    return !!GetTaskGroup(aThread) ||
+           (aThread == AbstractThread::GetCurrent() && HaveDirectTasks());
+  }
+
+  nsresult DispatchTasksFor(AbstractThread* aThread) override {
+    nsresult rv = NS_OK;
+
+    // Dispatch all groups that match |aThread|.
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      if (mTaskGroups[i]->mThread == aThread) {
+        nsresult rv2 = DispatchTaskGroup(std::move(mTaskGroups[i]));
+
+        if (NS_WARN_IF(NS_FAILED(rv2)) && NS_SUCCEEDED(rv)) {
+          // We should try our best to call DispatchTaskGroup() as much as
+          // possible and return an error if any of DispatchTaskGroup() calls
+          // failed.
+          rv = rv2;
+        }
+
+        mTaskGroups.RemoveElementAt(i--);
+      }
+    }
+
+    return rv;
+  }
+
+ private:
+  struct PerThreadTaskGroup {
+   public:
+    explicit PerThreadTaskGroup(AbstractThread* aThread) : mThread(aThread) {
+      MOZ_COUNT_CTOR(PerThreadTaskGroup);
+    }
+
+    MOZ_COUNTED_DTOR(PerThreadTaskGroup)
+
+    RefPtr<AbstractThread> mThread;
+    nsTArray<nsCOMPtr<nsIRunnable>> mStateChangeTasks;
+    nsTArray<nsCOMPtr<nsIRunnable>> mRegularTasks;
+  };
+
+  class TaskGroupRunnable : public Runnable {
+   public:
+    explicit TaskGroupRunnable(UniquePtr<PerThreadTaskGroup>&& aTasks)
+        : Runnable("AutoTaskDispatcher::TaskGroupRunnable"),
+          mTasks(std::move(aTasks)) {}
+
+    NS_IMETHOD Run() override {
+      // State change tasks get run all together before any code is run, so
+      // that all state changes are made in an atomic unit.
+      for (size_t i = 0; i < mTasks->mStateChangeTasks.Length(); ++i) {
+        mTasks->mStateChangeTasks[i]->Run();
+      }
+
+      // Once the state changes have completed, drain any direct tasks
+      // generated by those state changes (i.e. watcher notification tasks).
+      // This needs to be outside the loop because we don't want to run code
+      // that might observe intermediate states.
+      MaybeDrainDirectTasks();
+
+      for (size_t i = 0; i < mTasks->mRegularTasks.Length(); ++i) {
+        AUTO_PROFILE_FOLLOWING_RUNNABLE(mTasks->mRegularTasks[i]);
+        mTasks->mRegularTasks[i]->Run();
+
+        // Scope direct tasks tightly to the task that generated them.
+        MaybeDrainDirectTasks();
+      }
+
+      return NS_OK;
+    }
+
+   private:
+    void MaybeDrainDirectTasks() {
+      AbstractThread* currentThread = AbstractThread::GetCurrent();
+      if (currentThread && currentThread->MightHaveTailTasks()) {
+        currentThread->TailDispatcher().DrainDirectTasks();
+      }
+    }
+
+    UniquePtr<PerThreadTaskGroup> mTasks;
+  };
+
+  PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread) {
+    PerThreadTaskGroup* existing = GetTaskGroup(aThread);
+    if (existing) {
+      return *existing;
+    }
+
+    mTaskGroups.AppendElement(new PerThreadTaskGroup(aThread));
+    return *mTaskGroups.LastElement();
+  }
+
+  PerThreadTaskGroup* GetTaskGroup(AbstractThread* aThread) {
+    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
+      if (mTaskGroups[i]->mThread == aThread) {
+        return mTaskGroups[i].get();
+      }
+    }
+
+    // Not found.
+    return nullptr;
+  }
+
+  nsresult DispatchTaskGroup(UniquePtr<PerThreadTaskGroup> aGroup) {
+    RefPtr<AbstractThread> thread = aGroup->mThread;
+
+    AbstractThread::DispatchReason reason =
+        mIsTailDispatcher ? AbstractThread::TailDispatch
+                          : AbstractThread::NormalDispatch;
+    nsCOMPtr<nsIRunnable> r = new TaskGroupRunnable(std::move(aGroup));
+    return thread->Dispatch(r.forget(), reason);
+  }
+
+  // Task groups, organized by thread.
+  nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;
+
+  nsCOMPtr<nsIDirectTaskDispatcher> mDirectTaskDispatcher;
+  // True if this TaskDispatcher represents the tail dispatcher for the thread
+  // upon which it runs.
+  const bool mIsTailDispatcher;
+};
+
+// Little utility class to allow declaring AutoTaskDispatcher as a default
+// parameter for methods that take a TaskDispatcher&.
+template <typename T>
+class PassByRef {
+ public:
+  PassByRef() = default;
+  operator T&() { return mVal; }
+
+ private:
+  T mVal;
+};
+
+}  // namespace mozilla
+
+#endif
diff --git a/xpcom/threads/TaskQueue.cpp b/xpcom/threads/TaskQueue.cpp
new file mode 100644
index 0000000000..febb609784
--- /dev/null
+++ b/xpcom/threads/TaskQueue.cpp
@@ -0,0 +1,345 @@
+/* -*- 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/TaskQueue.h"
+
+#include "mozilla/ProfilerRunnable.h"
+#include "nsIEventTarget.h"
+#include "nsITargetShutdownTask.h"
+#include "nsThreadUtils.h"
+#include "nsQueryObject.h"
+
+namespace mozilla {
+
+// Handle for a TaskQueue being tracked by a TaskQueueTracker. When created,
+// it is registered with the TaskQueueTracker, and when destroyed it is
+// unregistered. Holds a threadsafe weak reference to the TaskQueue.
+class TaskQueueTrackerEntry final
+    : private LinkedListElement<TaskQueueTrackerEntry> {
+ public:
+  TaskQueueTrackerEntry(TaskQueueTracker* aTracker,
+                        const RefPtr<TaskQueue>& aQueue)
+      : mTracker(aTracker), mQueue(aQueue) {
+    MutexAutoLock lock(mTracker->mMutex);
+    mTracker->mEntries.insertFront(this);
+  }
+  ~TaskQueueTrackerEntry() {
+    MutexAutoLock lock(mTracker->mMutex);
+    removeFrom(mTracker->mEntries);
+  }
+
+  TaskQueueTrackerEntry(const TaskQueueTrackerEntry&) = delete;
+  TaskQueueTrackerEntry(TaskQueueTrackerEntry&&) = delete;
+  TaskQueueTrackerEntry& operator=(const TaskQueueTrackerEntry&) = delete;
+  TaskQueueTrackerEntry& operator=(TaskQueueTrackerEntry&&) = delete;
+
+  RefPtr<TaskQueue> GetQueue() const { return RefPtr<TaskQueue>(mQueue); }
+
+ private:
+  friend class LinkedList<TaskQueueTrackerEntry>;
+  friend class LinkedListElement<TaskQueueTrackerEntry>;
+
+  const RefPtr<TaskQueueTracker> mTracker;
+  const ThreadSafeWeakPtr<TaskQueue> mQueue;
+};
+
+RefPtr<TaskQueue> TaskQueue::Create(already_AddRefed<nsIEventTarget> aTarget,
+                                    const char* aName,
+                                    bool aSupportsTailDispatch) {
+  nsCOMPtr<nsIEventTarget> target(std::move(aTarget));
+  RefPtr<TaskQueue> queue =
+      new TaskQueue(do_AddRef(target), aName, aSupportsTailDispatch);
+
+  // If |target| is a TaskQueueTracker, register this TaskQueue with it. It will
+  // be unregistered when the TaskQueue is destroyed or shut down.
+  if (RefPtr<TaskQueueTracker> tracker = do_QueryObject(target)) {
+    MonitorAutoLock lock(queue->mQueueMonitor);
+    queue->mTrackerEntry = MakeUnique<TaskQueueTrackerEntry>(tracker, queue);
+  }
+
+  return queue;
+}
+
+TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
+                     const char* aName, bool aSupportsTailDispatch)
+    : AbstractThread(aSupportsTailDispatch),
+      mTarget(aTarget),
+      mQueueMonitor("TaskQueue::Queue"),
+      mTailDispatcher(nullptr),
+      mIsRunning(false),
+      mIsShutdown(false),
+      mName(aName) {}
+
+TaskQueue::~TaskQueue() {
+  // We should never free the TaskQueue if it was destroyed abnormally, meaning
+  // that all cleanup tasks should be complete if we do.
+  MOZ_ASSERT(mShutdownTasks.IsEmpty());
+}
+
+NS_IMPL_ADDREF_INHERITED(TaskQueue, SupportsThreadSafeWeakPtr<TaskQueue>)
+NS_IMPL_RELEASE_INHERITED(TaskQueue, SupportsThreadSafeWeakPtr<TaskQueue>)
+NS_IMPL_QUERY_INTERFACE(TaskQueue, nsIDirectTaskDispatcher,
+                        nsISerialEventTarget, nsIEventTarget)
+
+TaskDispatcher& TaskQueue::TailDispatcher() {
+  MOZ_ASSERT(IsCurrentThreadIn());
+  MOZ_ASSERT(mTailDispatcher);
+  return *mTailDispatcher;
+}
+
+// Note aRunnable is passed by ref to support conditional ownership transfer.
+// See Dispatch() in TaskQueue.h for more details.
+nsresult TaskQueue::DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable,
+                                   uint32_t aFlags, DispatchReason aReason) {
+  mQueueMonitor.AssertCurrentThreadOwns();
+
+  // Continue to allow dispatches after shutdown until the last message has been
+  // processed, at which point no more messages will be accepted.
+  if (mIsShutdown && !mIsRunning) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  AbstractThread* currentThread;
+  if (aReason != TailDispatch && (currentThread = GetCurrent()) &&
+      RequiresTailDispatch(currentThread) &&
+      currentThread->IsTailDispatcherAvailable()) {
+    MOZ_ASSERT(aFlags == NS_DISPATCH_NORMAL,
+               "Tail dispatch doesn't support flags");
+    return currentThread->TailDispatcher().AddTask(this, aRunnable.forget());
+  }
+
+  LogRunnable::LogDispatch(aRunnable);
+  mTasks.Push({std::move(aRunnable), aFlags});
+
+  if (mIsRunning) {
+    return NS_OK;
+  }
+  RefPtr<nsIRunnable> runner(new Runner(this));
+  nsresult rv = mTarget->Dispatch(runner.forget(), aFlags);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("Failed to dispatch runnable to run TaskQueue");
+    return rv;
+  }
+  mIsRunning = true;
+
+  return NS_OK;
+}
+
+nsresult TaskQueue::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  NS_ENSURE_ARG(aTask);
+
+  MonitorAutoLock mon(mQueueMonitor);
+  if (mIsShutdown) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  MOZ_ASSERT(!mShutdownTasks.Contains(aTask));
+  mShutdownTasks.AppendElement(aTask);
+  return NS_OK;
+}
+
+nsresult TaskQueue::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  NS_ENSURE_ARG(aTask);
+
+  MonitorAutoLock mon(mQueueMonitor);
+  if (mIsShutdown) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  return mShutdownTasks.RemoveElement(aTask) ? NS_OK : NS_ERROR_UNEXPECTED;
+}
+
+void TaskQueue::AwaitIdle() {
+  MonitorAutoLock mon(mQueueMonitor);
+  AwaitIdleLocked();
+}
+
+void TaskQueue::AwaitIdleLocked() {
+  // Make sure there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
+
+  mQueueMonitor.AssertCurrentThreadOwns();
+  MOZ_ASSERT(mIsRunning || mTasks.IsEmpty());
+  while (mIsRunning) {
+    mQueueMonitor.Wait();
+  }
+}
+
+void TaskQueue::AwaitShutdownAndIdle() {
+  MOZ_ASSERT(!IsCurrentThreadIn());
+  // Make sure there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->HasTailTasksFor(this));
+
+  MonitorAutoLock mon(mQueueMonitor);
+  while (!mIsShutdown) {
+    mQueueMonitor.Wait();
+  }
+  AwaitIdleLocked();
+}
+RefPtr<ShutdownPromise> TaskQueue::BeginShutdown() {
+  // Dispatch any tasks for this queue waiting in the caller's tail dispatcher,
+  // since this is the last opportunity to do so.
+  if (AbstractThread* currentThread = AbstractThread::GetCurrent()) {
+    currentThread->TailDispatchTasksFor(this);
+  }
+
+  MonitorAutoLock mon(mQueueMonitor);
+  // Dispatch any cleanup tasks to the queue before we put it into full
+  // shutdown.
+  for (auto& task : mShutdownTasks) {
+    nsCOMPtr runnable{task->AsRunnable()};
+    MOZ_ALWAYS_SUCCEEDS(
+        DispatchLocked(runnable, NS_DISPATCH_NORMAL, TailDispatch));
+  }
+  mShutdownTasks.Clear();
+  mIsShutdown = true;
+
+  RefPtr<ShutdownPromise> p = mShutdownPromise.Ensure(__func__);
+  MaybeResolveShutdown();
+  mon.NotifyAll();
+  return p;
+}
+
+void TaskQueue::MaybeResolveShutdown() {
+  mQueueMonitor.AssertCurrentThreadOwns();
+  if (mIsShutdown && !mIsRunning) {
+    mShutdownPromise.ResolveIfExists(true, __func__);
+    // Disconnect from our target as we won't try to dispatch any more events.
+    mTrackerEntry = nullptr;
+    mTarget = nullptr;
+  }
+}
+
+bool TaskQueue::IsEmpty() {
+  MonitorAutoLock mon(mQueueMonitor);
+  return mTasks.IsEmpty();
+}
+
+bool TaskQueue::IsCurrentThreadIn() const {
+  bool in = mRunningThread == PR_GetCurrentThread();
+  return in;
+}
+
+nsresult TaskQueue::Runner::Run() {
+  TaskStruct event;
+  {
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    MOZ_ASSERT(mQueue->mIsRunning);
+    if (mQueue->mTasks.IsEmpty()) {
+      mQueue->mIsRunning = false;
+      mQueue->MaybeResolveShutdown();
+      mon.NotifyAll();
+      return NS_OK;
+    }
+    event = mQueue->mTasks.Pop();
+  }
+  MOZ_ASSERT(event.event);
+
+  // Note that dropping the queue monitor before running the task, and
+  // taking the monitor again after the task has run ensures we have memory
+  // fences enforced. This means that if the object we're calling wasn't
+  // designed to be threadsafe, it will be, provided we're only calling it
+  // in this task queue.
+  {
+    AutoTaskGuard g(mQueue);
+    SerialEventTargetGuard tg(mQueue);
+    {
+      LogRunnable::Run log(event.event);
+
+      AUTO_PROFILE_FOLLOWING_RUNNABLE(event.event);
+      event.event->Run();
+
+      // Drop the reference to event. The event will hold a reference to the
+      // object it's calling, and we don't want to keep it alive, it may be
+      // making assumptions what holds references to it. This is especially
+      // the case if the object is waiting for us to shutdown, so that it
+      // can shutdown (like in the MediaDecoderStateMachine's SHUTDOWN case).
+      event.event = nullptr;
+    }
+  }
+
+  {
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    if (mQueue->mTasks.IsEmpty()) {
+      // No more events to run. Exit the task runner.
+      mQueue->mIsRunning = false;
+      mQueue->MaybeResolveShutdown();
+      mon.NotifyAll();
+      return NS_OK;
+    }
+  }
+
+  // There's at least one more event that we can run. Dispatch this Runner
+  // to the target again to ensure it runs again. Note that we don't just
+  // run in a loop here so that we don't hog the target. This means we may
+  // run on another thread next time, but we rely on the memory fences from
+  // mQueueMonitor for thread safety of non-threadsafe tasks.
+  nsresult rv;
+  {
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    rv = mQueue->mTarget->Dispatch(
+        this, mQueue->mTasks.FirstElement().flags | NS_DISPATCH_AT_END);
+  }
+  if (NS_FAILED(rv)) {
+    // Failed to dispatch, shutdown!
+    MonitorAutoLock mon(mQueue->mQueueMonitor);
+    mQueue->mIsRunning = false;
+    mQueue->mIsShutdown = true;
+    mQueue->MaybeResolveShutdown();
+    mon.NotifyAll();
+  }
+
+  return NS_OK;
+}
+
+//-----------------------------------------------------------------------------
+// nsIDirectTaskDispatcher
+//-----------------------------------------------------------------------------
+
+NS_IMETHODIMP
+TaskQueue::DispatchDirectTask(already_AddRefed<nsIRunnable> aEvent) {
+  if (!IsCurrentThreadIn()) {
+    return NS_ERROR_FAILURE;
+  }
+  mDirectTasks.AddTask(std::move(aEvent));
+  return NS_OK;
+}
+
+NS_IMETHODIMP TaskQueue::DrainDirectTasks() {
+  if (!IsCurrentThreadIn()) {
+    return NS_ERROR_FAILURE;
+  }
+  mDirectTasks.DrainTasks();
+  return NS_OK;
+}
+
+NS_IMETHODIMP TaskQueue::HaveDirectTasks(bool* aValue) {
+  if (!IsCurrentThreadIn()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aValue = mDirectTasks.HaveTasks();
+  return NS_OK;
+}
+
+nsTArray<RefPtr<TaskQueue>> TaskQueueTracker::GetAllTrackedTaskQueues() {
+  MutexAutoLock lock(mMutex);
+  nsTArray<RefPtr<TaskQueue>> queues;
+  for (auto* entry : mEntries) {
+    if (auto queue = entry->GetQueue()) {
+      queues.AppendElement(queue);
+    }
+  }
+  return queues;
+}
+
+TaskQueueTracker::~TaskQueueTracker() = default;
+
+}  // namespace mozilla
diff --git a/xpcom/threads/TaskQueue.h b/xpcom/threads/TaskQueue.h
new file mode 100644
index 0000000000..0d99d385d3
--- /dev/null
+++ b/xpcom/threads/TaskQueue.h
@@ -0,0 +1,276 @@
+/* -*- 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 TaskQueue_h_
+#define TaskQueue_h_
+
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/MozPromise.h"
+#include "mozilla/Queue.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/ThreadSafeWeakPtr.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+typedef MozPromise<bool, bool, false> ShutdownPromise;
+
+class TaskQueueTrackerEntry;
+
+// Abstracts executing runnables in order on an arbitrary event target. The
+// runnables dispatched to the TaskQueue will be executed in the order in which
+// they're received, and are guaranteed to not be executed concurrently.
+// They may be executed on different threads, and a memory barrier is used
+// to make this threadsafe for objects that aren't already threadsafe.
+//
+// Note, since a TaskQueue can also be converted to an nsIEventTarget using
+// WrapAsEventTarget() its possible to construct a hierarchy of TaskQueues.
+// Consider these three TaskQueues:
+//
+//  TQ1 dispatches to the main thread
+//  TQ2 dispatches to TQ1
+//  TQ3 dispatches to TQ1
+//
+// This ensures there is only ever a single runnable from the entire chain on
+// the main thread.  It also ensures that TQ2 and TQ3 only have a single
+// runnable in TQ1 at any time.
+//
+// This arrangement lets you prioritize work by dispatching runnables directly
+// to TQ1.  You can issue many runnables for important work.  Meanwhile the TQ2
+// and TQ3 work will always execute at most one runnable and then yield.
+//
+// A TaskQueue does not require explicit shutdown, however it provides a
+// BeginShutdown() method that places TaskQueue in a shut down state and returns
+// a promise that gets resolved once all pending tasks have completed
+class TaskQueue final : public AbstractThread,
+                        public nsIDirectTaskDispatcher,
+                        public SupportsThreadSafeWeakPtr<TaskQueue> {
+  class EventTargetWrapper;
+
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIDIRECTTASKDISPATCHER
+  MOZ_DECLARE_REFCOUNTED_TYPENAME(TaskQueue)
+
+  static RefPtr<TaskQueue> Create(already_AddRefed<nsIEventTarget> aTarget,
+                                  const char* aName,
+                                  bool aSupportsTailDispatch = false);
+
+  TaskDispatcher& TailDispatcher() override;
+
+  NS_IMETHOD Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                      uint32_t aFlags) override {
+    nsCOMPtr<nsIRunnable> runnable = aEvent;
+    {
+      MonitorAutoLock mon(mQueueMonitor);
+      return DispatchLocked(/* passed by ref */ runnable, aFlags,
+                            NormalDispatch);
+    }
+    // If the ownership of |r| is not transferred in DispatchLocked() due to
+    // dispatch failure, it will be deleted here outside the lock. We do so
+    // since the destructor of the runnable might access TaskQueue and result
+    // in deadlocks.
+  }
+
+  [[nodiscard]] nsresult Dispatch(
+      already_AddRefed<nsIRunnable> aRunnable,
+      DispatchReason aReason = NormalDispatch) override {
+    nsCOMPtr<nsIRunnable> r = aRunnable;
+    {
+      MonitorAutoLock mon(mQueueMonitor);
+      return DispatchLocked(/* passed by ref */ r, NS_DISPATCH_NORMAL, aReason);
+    }
+    // If the ownership of |r| is not transferred in DispatchLocked() due to
+    // dispatch failure, it will be deleted here outside the lock. We do so
+    // since the destructor of the runnable might access TaskQueue and result
+    // in deadlocks.
+  }
+
+  // So we can access nsIEventTarget::Dispatch(nsIRunnable*, uint32_t aFlags)
+  using nsIEventTarget::Dispatch;
+
+  NS_IMETHOD RegisterShutdownTask(nsITargetShutdownTask* aTask) override;
+  NS_IMETHOD UnregisterShutdownTask(nsITargetShutdownTask* aTask) override;
+
+  using CancelPromise = MozPromise<bool, bool, false>;
+
+  // Puts the queue in a shutdown state and returns immediately. The queue will
+  // remain alive at least until all the events are drained, because the Runners
+  // hold a strong reference to the task queue, and one of them is always held
+  // by the target event queue when the task queue is non-empty.
+  //
+  // The returned promise is resolved when the queue goes empty.
+  RefPtr<ShutdownPromise> BeginShutdown();
+
+  // Blocks until all task finish executing.
+  void AwaitIdle();
+
+  // Blocks until the queue is flagged for shutdown and all tasks have finished
+  // executing.
+  void AwaitShutdownAndIdle();
+
+  bool IsEmpty();
+
+  // Returns true if the current thread is currently running a Runnable in
+  // the task queue.
+  bool IsCurrentThreadIn() const override;
+  using nsISerialEventTarget::IsOnCurrentThread;
+
+ private:
+  friend class SupportsThreadSafeWeakPtr<TaskQueue>;
+
+  TaskQueue(already_AddRefed<nsIEventTarget> aTarget, const char* aName,
+            bool aSupportsTailDispatch);
+
+  virtual ~TaskQueue();
+
+  // Blocks until all task finish executing. Called internally by methods
+  // that need to wait until the task queue is idle.
+  // mQueueMonitor must be held.
+  void AwaitIdleLocked();
+
+  nsresult DispatchLocked(nsCOMPtr<nsIRunnable>& aRunnable, uint32_t aFlags,
+                          DispatchReason aReason = NormalDispatch);
+
+  void MaybeResolveShutdown();
+
+  nsCOMPtr<nsIEventTarget> mTarget MOZ_GUARDED_BY(mQueueMonitor);
+
+  // Handle for this TaskQueue being registered with our target if it implements
+  // TaskQueueTracker.
+  UniquePtr<TaskQueueTrackerEntry> mTrackerEntry MOZ_GUARDED_BY(mQueueMonitor);
+
+  // Monitor that protects the queue, mIsRunning, mIsShutdown and
+  // mShutdownTasks;
+  Monitor mQueueMonitor;
+
+  typedef struct TaskStruct {
+    nsCOMPtr<nsIRunnable> event;
+    uint32_t flags;
+  } TaskStruct;
+
+  // Queue of tasks to run.
+  Queue<TaskStruct> mTasks MOZ_GUARDED_BY(mQueueMonitor);
+
+  // List of tasks to run during shutdown.
+  nsTArray<nsCOMPtr<nsITargetShutdownTask>> mShutdownTasks
+      MOZ_GUARDED_BY(mQueueMonitor);
+
+  // The thread currently running the task queue. We store a reference
+  // to this so that IsCurrentThreadIn() can tell if the current thread
+  // is the thread currently running in the task queue.
+  //
+  // This may be read on any thread, but may only be written on mRunningThread.
+  // The thread can't die while we're running in it, and we only use it for
+  // pointer-comparison with the current thread anyway - so we make it atomic
+  // and don't refcount it.
+  Atomic<PRThread*> mRunningThread;
+
+  // RAII class that gets instantiated for each dispatched task.
+  class AutoTaskGuard {
+   public:
+    explicit AutoTaskGuard(TaskQueue* aQueue)
+        : mQueue(aQueue), mLastCurrentThread(nullptr) {
+      // NB: We don't hold the lock to aQueue here. Don't do anything that
+      // might require it.
+      MOZ_ASSERT(!mQueue->mTailDispatcher);
+      mTaskDispatcher.emplace(aQueue,
+                              /* aIsTailDispatcher = */ true);
+      mQueue->mTailDispatcher = mTaskDispatcher.ptr();
+
+      mLastCurrentThread = sCurrentThreadTLS.get();
+      sCurrentThreadTLS.set(aQueue);
+
+      MOZ_ASSERT(mQueue->mRunningThread == nullptr);
+      mQueue->mRunningThread = PR_GetCurrentThread();
+    }
+
+    ~AutoTaskGuard() {
+      mTaskDispatcher->DrainDirectTasks();
+      mTaskDispatcher.reset();
+
+      MOZ_ASSERT(mQueue->mRunningThread == PR_GetCurrentThread());
+      mQueue->mRunningThread = nullptr;
+
+      sCurrentThreadTLS.set(mLastCurrentThread);
+      mQueue->mTailDispatcher = nullptr;
+    }
+
+   private:
+    Maybe<AutoTaskDispatcher> mTaskDispatcher;
+    TaskQueue* mQueue;
+    AbstractThread* mLastCurrentThread;
+  };
+
+  TaskDispatcher* mTailDispatcher;
+
+  // True if we've dispatched an event to the target to execute events from
+  // the queue.
+  bool mIsRunning MOZ_GUARDED_BY(mQueueMonitor);
+
+  // True if we've started our shutdown process.
+  bool mIsShutdown MOZ_GUARDED_BY(mQueueMonitor);
+  MozPromiseHolder<ShutdownPromise> mShutdownPromise
+      MOZ_GUARDED_BY(mQueueMonitor);
+
+  // The name of this TaskQueue. Useful when debugging dispatch failures.
+  const char* const mName;
+
+  SimpleTaskQueue mDirectTasks;
+
+  class Runner : public Runnable {
+   public:
+    explicit Runner(TaskQueue* aQueue)
+        : Runnable("TaskQueue::Runner"), mQueue(aQueue) {}
+    NS_IMETHOD Run() override;
+
+   private:
+    RefPtr<TaskQueue> mQueue;
+  };
+};
+
+#define MOZILLA_TASKQUEUETRACKER_IID                 \
+  {                                                  \
+    0x765c4b56, 0xd5f6, 0x4a9f, {                    \
+      0x91, 0xcf, 0x51, 0x47, 0xb3, 0xc1, 0x7e, 0xa6 \
+    }                                                \
+  }
+
+// XPCOM "interface" which may be implemented by nsIEventTarget implementations
+// which want to keep track of what TaskQueue instances are currently targeting
+// them. This may be used to asynchronously shutdown TaskQueues targeting a
+// threadpool or other event target before the threadpool goes away.
+//
+// This explicitly TaskQueue-aware tracker is used instead of
+// `nsITargetShutdownTask` as the operations required to shut down a TaskQueue
+// are asynchronous, which is not a requirement of that interface.
+class TaskQueueTracker : public nsISupports {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(MOZILLA_TASKQUEUETRACKER_IID)
+
+  // Get a strong reference to every TaskQueue currently tracked by this
+  // TaskQueueTracker. May be called from any thraed.
+  nsTArray<RefPtr<TaskQueue>> GetAllTrackedTaskQueues();
+
+ protected:
+  virtual ~TaskQueueTracker();
+
+ private:
+  friend class TaskQueueTrackerEntry;
+
+  Mutex mMutex{"TaskQueueTracker"};
+  LinkedList<TaskQueueTrackerEntry> mEntries MOZ_GUARDED_BY(mMutex);
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(TaskQueueTracker, MOZILLA_TASKQUEUETRACKER_IID)
+
+}  // namespace mozilla
+
+#endif  // TaskQueue_h_
diff --git a/xpcom/threads/ThreadBound.h b/xpcom/threads/ThreadBound.h
new file mode 100644
index 0000000000..4d5e0088b5
--- /dev/null
+++ b/xpcom/threads/ThreadBound.h
@@ -0,0 +1,143 @@
+/* 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 class for values only accessible from a single designated thread.
+
+#ifndef mozilla_ThreadBound_h
+#define mozilla_ThreadBound_h
+
+#include "mozilla/Atomics.h"
+#include "prthread.h"
+
+#include <type_traits>
+
+namespace mozilla {
+
+template <typename T>
+class ThreadBound;
+
+namespace detail {
+
+template <bool Condition, typename T>
+struct AddConstIf {
+  using type = T;
+};
+
+template <typename T>
+struct AddConstIf<true, T> {
+  using type = typename std::add_const<T>::type;
+};
+
+}  // namespace detail
+
+// A ThreadBound<T> is a T that can only be accessed by a specific
+// thread. To enforce this rule, the inner T is only accessible
+// through a non-copyable, immovable accessor object.
+// Given a ThreadBound<T> threadBoundData, it can be accessed like so:
+//
+//   auto innerData = threadBoundData.Access();
+//   innerData->DoStuff();
+//
+// Trying to access a ThreadBound<T> from a different thread will
+// trigger a MOZ_DIAGNOSTIC_ASSERT.
+//   The encapsulated T is constructed during the construction of the
+// enclosing ThreadBound<T> by forwarding all of the latter's
+// constructor parameters to the former. A newly constructed
+// ThreadBound<T> is bound to the thread it's constructed in. It's
+// possible to rebind the data to some otherThread by calling
+//
+//   threadBoundData.Transfer(otherThread);
+//
+// on the thread that threadBoundData is currently bound to, as long
+// as it's not currently being accessed. (Trying to rebind from
+// another thread or while an accessor exists will trigger an
+// assertion.)
+//
+// Note: A ThreadBound<T> may be destructed from any thread, not just
+// its designated thread at the time the destructor is invoked.
+template <typename T>
+class ThreadBound final {
+ public:
+  template <typename... Args>
+  explicit ThreadBound(Args&&... aArgs)
+      : mData(std::forward<Args>(aArgs)...),
+        mThread(PR_GetCurrentThread()),
+        mAccessCount(0) {}
+
+  ~ThreadBound() { AssertIsNotCurrentlyAccessed(); }
+
+  void Transfer(const PRThread* const aDest) {
+    AssertIsCorrectThread();
+    AssertIsNotCurrentlyAccessed();
+    mThread = aDest;
+  }
+
+ private:
+  T mData;
+
+  // This member is (potentially) accessed by multiple threads and is
+  // thus the first point of synchronization between them.
+  Atomic<const PRThread*, ReleaseAcquire> mThread;
+
+  // In order to support nested accesses (e.g. from different stack
+  // frames) it's necessary to maintain a counter of the existing
+  // accessor. Since it's possible to access a const ThreadBound, the
+  // counter is mutable. It's atomic because accessing it synchronizes
+  // access to mData (see comment in Accessor's constructor).
+  using AccessCountType = Atomic<int, ReleaseAcquire>;
+  mutable AccessCountType mAccessCount;
+
+ public:
+  template <bool IsConst>
+  class MOZ_STACK_CLASS Accessor final {
+    using DataType = typename detail::AddConstIf<IsConst, T>::type;
+
+   public:
+    explicit Accessor(
+        typename detail::AddConstIf<IsConst, ThreadBound>::type& aThreadBound)
+        : mData(aThreadBound.mData), mAccessCount(aThreadBound.mAccessCount) {
+      aThreadBound.AssertIsCorrectThread();
+
+      // This load/store serves as a memory fence that guards mData
+      // against accesses that would trip the thread assertion.
+      // (Otherwise one of the loads in the caller's instruction
+      // stream might be scheduled before the assertion.)
+      ++mAccessCount;
+    }
+
+    Accessor(const Accessor&) = delete;
+    Accessor(Accessor&&) = delete;
+    Accessor& operator=(const Accessor&) = delete;
+    Accessor& operator=(Accessor&&) = delete;
+
+    ~Accessor() { --mAccessCount; }
+
+    DataType* operator->() { return &mData; }
+
+   private:
+    DataType& mData;
+    AccessCountType& mAccessCount;
+  };
+
+  auto Access() { return Accessor<false>{*this}; }
+
+  auto Access() const { return Accessor<true>{*this}; }
+
+ private:
+  bool IsCorrectThread() const { return mThread == PR_GetCurrentThread(); }
+
+  bool IsNotCurrentlyAccessed() const { return mAccessCount == 0; }
+
+#define MOZ_DEFINE_THREAD_BOUND_ASSERT(predicate) \
+  void Assert##predicate() const { MOZ_DIAGNOSTIC_ASSERT(predicate()); }
+
+  MOZ_DEFINE_THREAD_BOUND_ASSERT(IsCorrectThread)
+  MOZ_DEFINE_THREAD_BOUND_ASSERT(IsNotCurrentlyAccessed)
+
+#undef MOZ_DEFINE_THREAD_BOUND_ASSERT
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ThreadBound_h
diff --git a/xpcom/threads/ThreadDelay.cpp b/xpcom/threads/ThreadDelay.cpp
new file mode 100644
index 0000000000..1c38e25510
--- /dev/null
+++ b/xpcom/threads/ThreadDelay.cpp
@@ -0,0 +1,38 @@
+/* -*- 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 "ThreadDelay.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/ChaosMode.h"
+
+#if defined(XP_WIN)
+#  include <windows.h>
+#else
+#  include <unistd.h>
+#endif
+
+namespace mozilla {
+
+void DelayForChaosMode(ChaosFeature aFeature,
+                       const uint32_t aMicrosecondLimit) {
+  if (!ChaosMode::isActive(aFeature)) {
+    return;
+  }
+
+  MOZ_ASSERT(aMicrosecondLimit <= 1000);
+#if defined(XP_WIN)
+  // Windows doesn't support sleeping at less than millisecond resolution.
+  // We could spin here, or we could just sleep for one millisecond.
+  // Sleeping for a full millisecond causes heavy delays, so we don't do
+  // anything here for now until we have found a good way to sleep more
+  // precisely here.
+#else
+  const uint32_t duration = ChaosMode::randomUint32LessThan(aMicrosecondLimit);
+  ::usleep(duration);
+#endif
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/ThreadDelay.h b/xpcom/threads/ThreadDelay.h
new file mode 100644
index 0000000000..5cfc116e4d
--- /dev/null
+++ b/xpcom/threads/ThreadDelay.h
@@ -0,0 +1,16 @@
+/* -*- 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/ChaosMode.h"
+
+namespace mozilla {
+
+// Sleep for a random number of microseconds less than aMicrosecondLimit
+// if aFeature is enabled.  On Windows, the sleep will always be 1 millisecond
+// due to platform limitations.
+void DelayForChaosMode(ChaosFeature aFeature, const uint32_t aMicrosecondLimit);
+
+}  // namespace mozilla
diff --git a/xpcom/threads/ThreadEventQueue.cpp b/xpcom/threads/ThreadEventQueue.cpp
new file mode 100644
index 0000000000..dec317beef
--- /dev/null
+++ b/xpcom/threads/ThreadEventQueue.cpp
@@ -0,0 +1,324 @@
+/* -*- 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/ThreadEventQueue.h"
+#include "mozilla/EventQueue.h"
+
+#include "LeakRefPtr.h"
+#include "nsComponentManagerUtils.h"
+#include "nsITargetShutdownTask.h"
+#include "nsIThreadInternal.h"
+#include "nsThreadUtils.h"
+#include "nsThread.h"
+#include "ThreadEventTarget.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/TaskController.h"
+#include "mozilla/StaticPrefs_threads.h"
+
+using namespace mozilla;
+
+class ThreadEventQueue::NestedSink : public ThreadTargetSink {
+ public:
+  NestedSink(EventQueue* aQueue, ThreadEventQueue* aOwner)
+      : mQueue(aQueue), mOwner(aOwner) {}
+
+  bool PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                EventQueuePriority aPriority) final {
+    return mOwner->PutEventInternal(std::move(aEvent), aPriority, this);
+  }
+
+  void Disconnect(const MutexAutoLock& aProofOfLock) final { mQueue = nullptr; }
+
+  nsresult RegisterShutdownTask(nsITargetShutdownTask* aTask) final {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  nsresult UnregisterShutdownTask(nsITargetShutdownTask* aTask) final {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) {
+    if (mQueue) {
+      return mQueue->SizeOfIncludingThis(aMallocSizeOf);
+    }
+    return 0;
+  }
+
+ private:
+  friend class ThreadEventQueue;
+
+  // This is a non-owning reference. It must live at least until Disconnect is
+  // called to clear it out.
+  EventQueue* mQueue;
+  RefPtr<ThreadEventQueue> mOwner;
+};
+
+ThreadEventQueue::ThreadEventQueue(UniquePtr<EventQueue> aQueue,
+                                   bool aIsMainThread)
+    : mBaseQueue(std::move(aQueue)),
+      mLock("ThreadEventQueue"),
+      mEventsAvailable(mLock, "EventsAvail"),
+      mIsMainThread(aIsMainThread) {
+  if (aIsMainThread) {
+    TaskController::Get()->SetConditionVariable(&mEventsAvailable);
+  }
+}
+
+ThreadEventQueue::~ThreadEventQueue() { MOZ_ASSERT(mNestedQueues.IsEmpty()); }
+
+bool ThreadEventQueue::PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                                EventQueuePriority aPriority) {
+  return PutEventInternal(std::move(aEvent), aPriority, nullptr);
+}
+
+bool ThreadEventQueue::PutEventInternal(already_AddRefed<nsIRunnable>&& aEvent,
+                                        EventQueuePriority aPriority,
+                                        NestedSink* aSink) {
+  // We want to leak the reference when we fail to dispatch it, so that
+  // we won't release the event in a wrong thread.
+  LeakRefPtr<nsIRunnable> event(std::move(aEvent));
+  nsCOMPtr<nsIThreadObserver> obs;
+
+  {
+    // Check if the runnable wants to override the passed-in priority.
+    // Do this outside the lock, so runnables implemented in JS can QI
+    // (and possibly GC) outside of the lock.
+    if (mIsMainThread) {
+      auto* e = event.get();  // can't do_QueryInterface on LeakRefPtr.
+      if (nsCOMPtr<nsIRunnablePriority> runnablePrio = do_QueryInterface(e)) {
+        uint32_t prio = nsIRunnablePriority::PRIORITY_NORMAL;
+        runnablePrio->GetPriority(&prio);
+        if (prio == nsIRunnablePriority::PRIORITY_CONTROL) {
+          aPriority = EventQueuePriority::Control;
+        } else if (prio == nsIRunnablePriority::PRIORITY_RENDER_BLOCKING) {
+          aPriority = EventQueuePriority::RenderBlocking;
+        } else if (prio == nsIRunnablePriority::PRIORITY_VSYNC) {
+          aPriority = EventQueuePriority::Vsync;
+        } else if (prio == nsIRunnablePriority::PRIORITY_INPUT_HIGH) {
+          aPriority = EventQueuePriority::InputHigh;
+        } else if (prio == nsIRunnablePriority::PRIORITY_MEDIUMHIGH) {
+          aPriority = EventQueuePriority::MediumHigh;
+        } else if (prio == nsIRunnablePriority::PRIORITY_DEFERRED_TIMERS) {
+          aPriority = EventQueuePriority::DeferredTimers;
+        } else if (prio == nsIRunnablePriority::PRIORITY_IDLE) {
+          aPriority = EventQueuePriority::Idle;
+        } else if (prio == nsIRunnablePriority::PRIORITY_LOW) {
+          aPriority = EventQueuePriority::Low;
+        }
+      }
+
+      if (aPriority == EventQueuePriority::Control &&
+          !StaticPrefs::threads_control_event_queue_enabled()) {
+        aPriority = EventQueuePriority::MediumHigh;
+      }
+    }
+
+    MutexAutoLock lock(mLock);
+
+    if (mEventsAreDoomed) {
+      return false;
+    }
+
+    if (aSink) {
+      if (!aSink->mQueue) {
+        return false;
+      }
+
+      aSink->mQueue->PutEvent(event.take(), aPriority, lock);
+    } else {
+      mBaseQueue->PutEvent(event.take(), aPriority, lock);
+    }
+
+    mEventsAvailable.Notify();
+
+    // Make sure to grab the observer before dropping the lock, otherwise the
+    // event that we just placed into the queue could run and eventually delete
+    // this nsThread before the calling thread is scheduled again. We would then
+    // crash while trying to access a dead nsThread.
+    obs = mObserver;
+  }
+
+  if (obs) {
+    obs->OnDispatchedEvent();
+  }
+
+  return true;
+}
+
+already_AddRefed<nsIRunnable> ThreadEventQueue::GetEvent(
+    bool aMayWait, mozilla::TimeDuration* aLastEventDelay) {
+  nsCOMPtr<nsIRunnable> event;
+  {
+    // Scope for lock.  When we are about to return, we will exit this
+    // scope so we can do some work after releasing the lock but
+    // before returning.
+    MutexAutoLock lock(mLock);
+
+    for (;;) {
+      const bool noNestedQueue = mNestedQueues.IsEmpty();
+      if (noNestedQueue) {
+        event = mBaseQueue->GetEvent(lock, aLastEventDelay);
+      } else {
+        // We always get events from the topmost queue when there are nested
+        // queues.
+        event =
+            mNestedQueues.LastElement().mQueue->GetEvent(lock, aLastEventDelay);
+      }
+
+      if (event) {
+        break;
+      }
+
+      // No runnable available.  Sleep waiting for one if if we're supposed to.
+      // Otherwise just go ahead and return null.
+      if (!aMayWait) {
+        break;
+      }
+
+      AUTO_PROFILER_LABEL("ThreadEventQueue::GetEvent::Wait", IDLE);
+      mEventsAvailable.Wait();
+    }
+  }
+
+  return event.forget();
+}
+
+bool ThreadEventQueue::HasPendingEvent() {
+  MutexAutoLock lock(mLock);
+
+  // We always get events from the topmost queue when there are nested queues.
+  if (mNestedQueues.IsEmpty()) {
+    return mBaseQueue->HasReadyEvent(lock);
+  } else {
+    return mNestedQueues.LastElement().mQueue->HasReadyEvent(lock);
+  }
+}
+
+bool ThreadEventQueue::ShutdownIfNoPendingEvents() {
+  MutexAutoLock lock(mLock);
+  if (mNestedQueues.IsEmpty() && mBaseQueue->IsEmpty(lock)) {
+    mEventsAreDoomed = true;
+    return true;
+  }
+  return false;
+}
+
+already_AddRefed<nsISerialEventTarget> ThreadEventQueue::PushEventQueue() {
+  auto queue = MakeUnique<EventQueue>();
+  RefPtr<NestedSink> sink = new NestedSink(queue.get(), this);
+  RefPtr<ThreadEventTarget> eventTarget =
+      new ThreadEventTarget(sink, NS_IsMainThread(), false);
+
+  MutexAutoLock lock(mLock);
+
+  mNestedQueues.AppendElement(NestedQueueItem(std::move(queue), eventTarget));
+  return eventTarget.forget();
+}
+
+void ThreadEventQueue::PopEventQueue(nsIEventTarget* aTarget) {
+  MutexAutoLock lock(mLock);
+
+  MOZ_ASSERT(!mNestedQueues.IsEmpty());
+
+  NestedQueueItem& item = mNestedQueues.LastElement();
+
+  MOZ_ASSERT(aTarget == item.mEventTarget);
+
+  // Disconnect the event target that will be popped.
+  item.mEventTarget->Disconnect(lock);
+
+  EventQueue* prevQueue =
+      mNestedQueues.Length() == 1
+          ? mBaseQueue.get()
+          : mNestedQueues[mNestedQueues.Length() - 2].mQueue.get();
+
+  // Move events from the old queue to the new one.
+  nsCOMPtr<nsIRunnable> event;
+  TimeDuration delay;
+  while ((event = item.mQueue->GetEvent(lock, &delay))) {
+    // preserve the event delay so far
+    prevQueue->PutEvent(event.forget(), EventQueuePriority::Normal, lock,
+                        &delay);
+  }
+
+  mNestedQueues.RemoveLastElement();
+}
+
+size_t ThreadEventQueue::SizeOfExcludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) {
+  size_t n = 0;
+
+  {
+    MutexAutoLock lock(mLock);
+    n += mBaseQueue->SizeOfIncludingThis(aMallocSizeOf);
+    n += mNestedQueues.ShallowSizeOfExcludingThis(aMallocSizeOf);
+    for (auto& queue : mNestedQueues) {
+      n += queue.mEventTarget->SizeOfIncludingThis(aMallocSizeOf);
+    }
+  }
+
+  return SynchronizedEventQueue::SizeOfExcludingThis(aMallocSizeOf) + n;
+}
+
+already_AddRefed<nsIThreadObserver> ThreadEventQueue::GetObserver() {
+  MutexAutoLock lock(mLock);
+  return do_AddRef(mObserver);
+}
+
+already_AddRefed<nsIThreadObserver> ThreadEventQueue::GetObserverOnThread()
+    MOZ_NO_THREAD_SAFETY_ANALYSIS {
+  // only written on this thread
+  return do_AddRef(mObserver);
+}
+
+void ThreadEventQueue::SetObserver(nsIThreadObserver* aObserver) {
+  // Always called from the thread - single writer.
+  nsCOMPtr<nsIThreadObserver> observer = aObserver;
+  {
+    MutexAutoLock lock(mLock);
+    mObserver.swap(observer);
+  }
+  if (NS_IsMainThread()) {
+    TaskController::Get()->SetThreadObserver(aObserver);
+  }
+}
+
+nsresult ThreadEventQueue::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  NS_ENSURE_ARG(aTask);
+  MutexAutoLock lock(mLock);
+  if (mEventsAreDoomed || mShutdownTasksRun) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  MOZ_ASSERT(!mShutdownTasks.Contains(aTask));
+  mShutdownTasks.AppendElement(aTask);
+  return NS_OK;
+}
+
+nsresult ThreadEventQueue::UnregisterShutdownTask(
+    nsITargetShutdownTask* aTask) {
+  NS_ENSURE_ARG(aTask);
+  MutexAutoLock lock(mLock);
+  if (mEventsAreDoomed || mShutdownTasksRun) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  return mShutdownTasks.RemoveElement(aTask) ? NS_OK : NS_ERROR_UNEXPECTED;
+}
+
+void ThreadEventQueue::RunShutdownTasks() {
+  nsTArray<nsCOMPtr<nsITargetShutdownTask>> shutdownTasks;
+  {
+    MutexAutoLock lock(mLock);
+    shutdownTasks = std::move(mShutdownTasks);
+    mShutdownTasks.Clear();
+    mShutdownTasksRun = true;
+  }
+  for (auto& task : shutdownTasks) {
+    task->TargetShutdown();
+  }
+}
+
+ThreadEventQueue::NestedQueueItem::NestedQueueItem(
+    UniquePtr<EventQueue> aQueue, ThreadEventTarget* aEventTarget)
+    : mQueue(std::move(aQueue)), mEventTarget(aEventTarget) {}
diff --git a/xpcom/threads/ThreadEventQueue.h b/xpcom/threads/ThreadEventQueue.h
new file mode 100644
index 0000000000..5244acb3dc
--- /dev/null
+++ b/xpcom/threads/ThreadEventQueue.h
@@ -0,0 +1,95 @@
+/* -*- 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_ThreadEventQueue_h
+#define mozilla_ThreadEventQueue_h
+
+#include "mozilla/EventQueue.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/SynchronizedEventQueue.h"
+#include "nsCOMPtr.h"
+#include "nsTArray.h"
+
+class nsIEventTarget;
+class nsISerialEventTarget;
+class nsIThreadObserver;
+
+namespace mozilla {
+
+class EventQueue;
+class ThreadEventTarget;
+
+// A ThreadEventQueue implements normal monitor-style synchronization over the
+// EventQueue. It also implements PushEventQueue and PopEventQueue for workers
+// (see the documentation below for an explanation of those). All threads use a
+// ThreadEventQueue as their event queue. Although for the main thread this
+// simply forwards events to the TaskController.
+class ThreadEventQueue final : public SynchronizedEventQueue {
+ public:
+  explicit ThreadEventQueue(UniquePtr<EventQueue> aQueue,
+                            bool aIsMainThread = false);
+
+  bool PutEvent(already_AddRefed<nsIRunnable>&& aEvent,
+                EventQueuePriority aPriority) final;
+
+  already_AddRefed<nsIRunnable> GetEvent(
+      bool aMayWait, mozilla::TimeDuration* aLastEventDelay = nullptr) final;
+  bool HasPendingEvent() final;
+
+  bool ShutdownIfNoPendingEvents() final;
+
+  void Disconnect(const MutexAutoLock& aProofOfLock) final {}
+
+  nsresult RegisterShutdownTask(nsITargetShutdownTask* aTask) final;
+  nsresult UnregisterShutdownTask(nsITargetShutdownTask* aTask) final;
+  void RunShutdownTasks() final;
+
+  already_AddRefed<nsISerialEventTarget> PushEventQueue() final;
+  void PopEventQueue(nsIEventTarget* aTarget) final;
+
+  already_AddRefed<nsIThreadObserver> GetObserver() final;
+  already_AddRefed<nsIThreadObserver> GetObserverOnThread() final;
+  void SetObserver(nsIThreadObserver* aObserver) final;
+
+  Mutex& MutexRef() { return mLock; }
+
+  size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) override;
+
+ private:
+  class NestedSink;
+
+  virtual ~ThreadEventQueue();
+
+  bool PutEventInternal(already_AddRefed<nsIRunnable>&& aEvent,
+                        EventQueuePriority aPriority, NestedSink* aQueue);
+
+  const UniquePtr<EventQueue> mBaseQueue MOZ_GUARDED_BY(mLock);
+
+  struct NestedQueueItem {
+    UniquePtr<EventQueue> mQueue;
+    RefPtr<ThreadEventTarget> mEventTarget;
+
+    NestedQueueItem(UniquePtr<EventQueue> aQueue,
+                    ThreadEventTarget* aEventTarget);
+  };
+
+  nsTArray<NestedQueueItem> mNestedQueues MOZ_GUARDED_BY(mLock);
+
+  Mutex mLock;
+  CondVar mEventsAvailable MOZ_GUARDED_BY(mLock);
+
+  bool mEventsAreDoomed MOZ_GUARDED_BY(mLock) = false;
+  nsCOMPtr<nsIThreadObserver> mObserver MOZ_GUARDED_BY(mLock);
+  nsTArray<nsCOMPtr<nsITargetShutdownTask>> mShutdownTasks
+      MOZ_GUARDED_BY(mLock);
+  bool mShutdownTasksRun MOZ_GUARDED_BY(mLock) = false;
+
+  const bool mIsMainThread;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ThreadEventQueue_h
diff --git a/xpcom/threads/ThreadEventTarget.cpp b/xpcom/threads/ThreadEventTarget.cpp
new file mode 100644
index 0000000000..17844d4886
--- /dev/null
+++ b/xpcom/threads/ThreadEventTarget.cpp
@@ -0,0 +1,137 @@
+/* -*- 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 "ThreadEventTarget.h"
+#include "mozilla/ThreadEventQueue.h"
+
+#include "LeakRefPtr.h"
+#include "mozilla/DelayedRunnable.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/TimeStamp.h"
+#include "nsComponentManagerUtils.h"
+#include "nsITimer.h"
+#include "nsThreadManager.h"
+#include "nsThreadSyncDispatch.h"
+#include "nsThreadUtils.h"
+#include "ThreadDelay.h"
+
+using namespace mozilla;
+
+#ifdef DEBUG
+// This flag will be set right after XPCOMShutdownThreads finished but before
+// we continue with other processing. It is exclusively meant to prime the
+// assertion of ThreadEventTarget::Dispatch as early as possible.
+// Please use AppShutdown::IsInOrBeyond(ShutdownPhase::???)
+// elsewhere to check for shutdown phases.
+static mozilla::Atomic<bool, mozilla::SequentiallyConsistent>
+    gXPCOMThreadsShutDownNotified(false);
+#endif
+
+ThreadEventTarget::ThreadEventTarget(ThreadTargetSink* aSink,
+                                     bool aIsMainThread, bool aBlockDispatch)
+    : mSink(aSink),
+#ifdef DEBUG
+      mIsMainThread(aIsMainThread),
+#endif
+      mBlockDispatch(aBlockDispatch) {
+  mThread = PR_GetCurrentThread();
+}
+
+ThreadEventTarget::~ThreadEventTarget() = default;
+
+void ThreadEventTarget::SetCurrentThread(PRThread* aThread) {
+  mThread = aThread;
+}
+
+void ThreadEventTarget::ClearCurrentThread() { mThread = nullptr; }
+
+NS_IMPL_ISUPPORTS(ThreadEventTarget, nsIEventTarget, nsISerialEventTarget)
+
+NS_IMETHODIMP
+ThreadEventTarget::DispatchFromScript(nsIRunnable* aRunnable, uint32_t aFlags) {
+  return Dispatch(do_AddRef(aRunnable), aFlags);
+}
+
+#ifdef DEBUG
+// static
+void ThreadEventTarget::XPCOMShutdownThreadsNotificationFinished() {
+  gXPCOMThreadsShutDownNotified = true;
+}
+#endif
+
+NS_IMETHODIMP
+ThreadEventTarget::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                            uint32_t aFlags) {
+  // We want to leak the reference when we fail to dispatch it, so that
+  // we won't release the event in a wrong thread.
+  LeakRefPtr<nsIRunnable> event(std::move(aEvent));
+  if (NS_WARN_IF(!event)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  NS_ASSERTION(!gXPCOMThreadsShutDownNotified || mIsMainThread ||
+                   PR_GetCurrentThread() == mThread ||
+                   (aFlags & NS_DISPATCH_IGNORE_BLOCK_DISPATCH),
+               "Dispatch to non-main thread after xpcom-shutdown-threads");
+
+  if (mBlockDispatch && !(aFlags & NS_DISPATCH_IGNORE_BLOCK_DISPATCH)) {
+    MOZ_DIAGNOSTIC_ASSERT(
+        false,
+        "Attempt to dispatch to thread which does not usually process "
+        "dispatched runnables until shutdown");
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  LogRunnable::LogDispatch(event.get());
+
+  NS_ASSERTION((aFlags & (NS_DISPATCH_AT_END |
+                          NS_DISPATCH_IGNORE_BLOCK_DISPATCH)) == aFlags,
+               "unexpected dispatch flags");
+  if (!mSink->PutEvent(event.take(), EventQueuePriority::Normal)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  // Delay to encourage the receiving task to run before we do work.
+  DelayForChaosMode(ChaosFeature::TaskDispatching, 1000);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+ThreadEventTarget::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                                   uint32_t aDelayMs) {
+  nsCOMPtr<nsIRunnable> event = aEvent;
+  NS_ENSURE_TRUE(!!aDelayMs, NS_ERROR_UNEXPECTED);
+
+  RefPtr<DelayedRunnable> r =
+      new DelayedRunnable(do_AddRef(this), event.forget(), aDelayMs);
+  nsresult rv = r->Init();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return Dispatch(r.forget(), NS_DISPATCH_NORMAL);
+}
+
+NS_IMETHODIMP
+ThreadEventTarget::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return mSink->RegisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+ThreadEventTarget::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return mSink->UnregisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+ThreadEventTarget::IsOnCurrentThread(bool* aIsOnCurrentThread) {
+  *aIsOnCurrentThread = IsOnCurrentThread();
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+ThreadEventTarget::IsOnCurrentThreadInfallible() {
+  // This method is only going to be called if `mThread` is null, which
+  // only happens when the thread has exited the event loop.  Therefore, when
+  // we are called, we can never be on this thread.
+  return false;
+}
diff --git a/xpcom/threads/ThreadEventTarget.h b/xpcom/threads/ThreadEventTarget.h
new file mode 100644
index 0000000000..b78411aa80
--- /dev/null
+++ b/xpcom/threads/ThreadEventTarget.h
@@ -0,0 +1,63 @@
+/* -*- 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_ThreadEventTarget_h
+#define mozilla_ThreadEventTarget_h
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/SynchronizedEventQueue.h"  // for ThreadTargetSink
+#include "nsISerialEventTarget.h"
+
+namespace mozilla {
+class DelayedRunnable;
+
+// ThreadEventTarget handles the details of posting an event to a thread. It can
+// be used with any ThreadTargetSink implementation.
+class ThreadEventTarget final : public nsISerialEventTarget {
+ public:
+  ThreadEventTarget(ThreadTargetSink* aSink, bool aIsMainThread,
+                    bool aBlockDispatch);
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+
+  // Disconnects the target so that it can no longer post events.
+  void Disconnect(const MutexAutoLock& aProofOfLock) {
+    mSink->Disconnect(aProofOfLock);
+  }
+
+  // Sets the thread for which IsOnCurrentThread returns true to the current
+  // thread.
+  void SetCurrentThread(PRThread* aThread);
+  // Call ClearCurrentThread() before the PRThread is deleted on thread join.
+  void ClearCurrentThread();
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
+    size_t n = 0;
+    if (mSink) {
+      n += mSink->SizeOfIncludingThis(aMallocSizeOf);
+    }
+    return aMallocSizeOf(this) + n;
+  }
+
+#ifdef DEBUG
+  static void XPCOMShutdownThreadsNotificationFinished();
+#endif
+
+ private:
+  ~ThreadEventTarget();
+
+  RefPtr<ThreadTargetSink> mSink;
+#ifdef DEBUG
+  const bool mIsMainThread;
+#endif
+  const bool mBlockDispatch;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ThreadEventTarget_h
diff --git a/xpcom/threads/ThreadLocalVariables.cpp b/xpcom/threads/ThreadLocalVariables.cpp
new file mode 100644
index 0000000000..606c0c6384
--- /dev/null
+++ b/xpcom/threads/ThreadLocalVariables.cpp
@@ -0,0 +1,16 @@
+/* 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/ThreadLocal.h"
+
+// This variable is used to ensure creating new URI doesn't put us in an
+// infinite loop
+MOZ_THREAD_LOCAL(uint32_t) gTlsURLRecursionCount;
+
+void InitThreadLocalVariables() {
+  if (!gTlsURLRecursionCount.init()) {
+    MOZ_CRASH("Could not init gTlsURLRecursionCount");
+  }
+  gTlsURLRecursionCount.set(0);
+}
diff --git a/xpcom/threads/ThrottledEventQueue.cpp b/xpcom/threads/ThrottledEventQueue.cpp
new file mode 100644
index 0000000000..9e4219b305
--- /dev/null
+++ b/xpcom/threads/ThrottledEventQueue.cpp
@@ -0,0 +1,459 @@
+/* -*- 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 "ThrottledEventQueue.h"
+
+#include "mozilla/Atomics.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Unused.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+
+namespace {}  // anonymous namespace
+
+// The ThrottledEventQueue is designed with inner and outer objects:
+//
+//       XPCOM code     base event target
+//            |               |
+//            v               v
+//        +-------+       +--------+
+//        | Outer |   +-->|executor|
+//        +-------+   |   +--------+
+//            |       |       |
+//            |   +-------+   |
+//            +-->| Inner |<--+
+//                +-------+
+//
+// Client code references the outer nsIEventTarget which in turn references
+// an inner object, which actually holds the queue of runnables.
+//
+// Whenever the queue is non-empty (and not paused), it keeps an "executor"
+// runnable dispatched to the base event target. Each time the executor is run,
+// it draws the next event from Inner's queue and runs it. If that queue has
+// more events, the executor is dispatched to the base again.
+//
+// The executor holds a strong reference to the Inner object. This means that if
+// the outer object is dereferenced and destroyed, the Inner object will remain
+// live for as long as the executor exists - that is, until the Inner's queue is
+// empty.
+//
+// A Paused ThrottledEventQueue does not enqueue an executor when new events are
+// added. Any executor previously queued on the base event target draws no
+// events from a Paused ThrottledEventQueue, and returns without re-enqueueing
+// itself. Since there is no executor keeping the Inner object alive until its
+// queue is empty, dropping a Paused ThrottledEventQueue may drop the Inner
+// while it still owns events. This is the correct behavior: if there are no
+// references to it, it will never be Resumed, and thus it will never dispatch
+// events again.
+//
+// Resuming a ThrottledEventQueue must dispatch an executor, so calls to Resume
+// are fallible for the same reasons as calls to Dispatch.
+//
+// The xpcom shutdown process drains the main thread's event queue several
+// times, so if a ThrottledEventQueue is being driven by the main thread, it
+// should get emptied out by the time we reach the "eventq shutdown" phase.
+class ThrottledEventQueue::Inner final : public nsISupports {
+  // The runnable which is dispatched to the underlying base target.  Since
+  // we only execute one event at a time we just re-use a single instance
+  // of this class while there are events left in the queue.
+  class Executor final : public Runnable, public nsIRunnablePriority {
+    // The Inner whose runnables we execute. mInner->mExecutor points
+    // to this executor, forming a reference loop.
+    RefPtr<Inner> mInner;
+
+    ~Executor() = default;
+
+   public:
+    explicit Executor(Inner* aInner)
+        : Runnable("ThrottledEventQueue::Inner::Executor"), mInner(aInner) {}
+
+    NS_DECL_ISUPPORTS_INHERITED
+
+    NS_IMETHODIMP
+    Run() override {
+      mInner->ExecuteRunnable();
+      return NS_OK;
+    }
+
+    NS_IMETHODIMP
+    GetPriority(uint32_t* aPriority) override {
+      *aPriority = mInner->mPriority;
+      return NS_OK;
+    }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+    NS_IMETHODIMP
+    GetName(nsACString& aName) override { return mInner->CurrentName(aName); }
+#endif
+  };
+
+  mutable Mutex mMutex;
+  mutable CondVar mIdleCondVar MOZ_GUARDED_BY(mMutex);
+
+  // As-of-yet unexecuted runnables queued on this ThrottledEventQueue.
+  //
+  // Used from any thread; protected by mMutex. Signals mIdleCondVar when
+  // emptied.
+  EventQueueSized<64> mEventQueue MOZ_GUARDED_BY(mMutex);
+
+  // The event target we dispatch our events (actually, just our Executor) to.
+  //
+  // Written only during construction. Readable by any thread without locking.
+  const nsCOMPtr<nsISerialEventTarget> mBaseTarget;
+
+  // The Executor that we dispatch to mBaseTarget to draw runnables from our
+  // queue. mExecutor->mInner points to this Inner, forming a reference loop.
+  //
+  // Used from any thread; protected by mMutex.
+  nsCOMPtr<nsIRunnable> mExecutor MOZ_GUARDED_BY(mMutex);
+
+  const char* const mName;
+
+  const uint32_t mPriority;
+
+  // True if this queue is currently paused.
+  // Used from any thread; protected by mMutex.
+  bool mIsPaused MOZ_GUARDED_BY(mMutex);
+
+  explicit Inner(nsISerialEventTarget* aBaseTarget, const char* aName,
+                 uint32_t aPriority)
+      : mMutex("ThrottledEventQueue"),
+        mIdleCondVar(mMutex, "ThrottledEventQueue:Idle"),
+        mBaseTarget(aBaseTarget),
+        mName(aName),
+        mPriority(aPriority),
+        mIsPaused(false) {
+    MOZ_ASSERT(mName, "Must pass a valid name!");
+  }
+
+  ~Inner() {
+#ifdef DEBUG
+    MutexAutoLock lock(mMutex);
+
+    // As long as an executor exists, it had better keep us alive, since it's
+    // going to call ExecuteRunnable on us.
+    MOZ_ASSERT(!mExecutor);
+
+    // If we have any events in our queue, there should be an executor queued
+    // for them, and that should have kept us alive. The exception is that, if
+    // we're paused, we don't enqueue an executor.
+    MOZ_ASSERT(mEventQueue.IsEmpty(lock) || IsPaused(lock));
+
+    // Some runnables are only safe to drop on the main thread, so if our queue
+    // isn't empty, we'd better be on the main thread.
+    MOZ_ASSERT_IF(!mEventQueue.IsEmpty(lock), NS_IsMainThread());
+#endif
+  }
+
+  // Make sure an executor has been queued on our base target. If we already
+  // have one, do nothing; otherwise, create and dispatch it.
+  nsresult EnsureExecutor(MutexAutoLock& lock) MOZ_REQUIRES(mMutex) {
+    if (mExecutor) return NS_OK;
+
+    // Note, this creates a ref cycle keeping the inner alive
+    // until the queue is drained.
+    mExecutor = new Executor(this);
+    nsresult rv = mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      mExecutor = nullptr;
+      return rv;
+    }
+
+    return NS_OK;
+  }
+
+  nsresult CurrentName(nsACString& aName) {
+    nsCOMPtr<nsIRunnable> event;
+
+#ifdef DEBUG
+    bool currentThread = false;
+    mBaseTarget->IsOnCurrentThread(&currentThread);
+    MOZ_ASSERT(currentThread);
+#endif
+
+    {
+      MutexAutoLock lock(mMutex);
+      event = mEventQueue.PeekEvent(lock);
+      // It is possible that mEventQueue wasn't empty when the executor
+      // was added to the queue, but someone processed events from mEventQueue
+      // before the executor, this is why mEventQueue is empty here
+      if (!event) {
+        aName.AssignLiteral("no runnables left in the ThrottledEventQueue");
+        return NS_OK;
+      }
+    }
+
+    if (nsCOMPtr<nsINamed> named = do_QueryInterface(event)) {
+      nsresult rv = named->GetName(aName);
+      return rv;
+    }
+
+    aName.AssignASCII(mName);
+    return NS_OK;
+  }
+
+  void ExecuteRunnable() {
+    // Any thread
+    nsCOMPtr<nsIRunnable> event;
+
+#ifdef DEBUG
+    bool currentThread = false;
+    mBaseTarget->IsOnCurrentThread(&currentThread);
+    MOZ_ASSERT(currentThread);
+#endif
+
+    {
+      MutexAutoLock lock(mMutex);
+
+      // Normally, a paused queue doesn't dispatch any executor, but we might
+      // have been paused after the executor was already in flight. There's no
+      // way to yank the executor out of the base event target, so we just check
+      // for a paused queue here and return without running anything. We'll
+      // create a new executor when we're resumed.
+      if (IsPaused(lock)) {
+        // Note, this breaks a ref cycle.
+        mExecutor = nullptr;
+        return;
+      }
+
+      // We only dispatch an executor runnable when we know there is something
+      // in the queue, so this should never fail.
+      event = mEventQueue.GetEvent(lock);
+      MOZ_ASSERT(event);
+
+      // If there are more events in the queue, then dispatch the next
+      // executor.  We do this now, before running the event, because
+      // the event might spin the event loop and we don't want to stall
+      // the queue.
+      if (mEventQueue.HasReadyEvent(lock)) {
+        // Dispatch the next base target runnable to attempt to execute
+        // the next throttled event.  We must do this before executing
+        // the event in case the event spins the event loop.
+        MOZ_ALWAYS_SUCCEEDS(
+            mBaseTarget->Dispatch(mExecutor, NS_DISPATCH_NORMAL));
+      }
+
+      // Otherwise the queue is empty and we can stop dispatching the
+      // executor.
+      else {
+        // Break the Executor::mInner / Inner::mExecutor reference loop.
+        mExecutor = nullptr;
+        mIdleCondVar.NotifyAll();
+      }
+    }
+
+    // Execute the event now that we have unlocked.
+    LogRunnable::Run log(event);
+    Unused << event->Run();
+
+    // To cover the event's destructor code in the LogRunnable log
+    event = nullptr;
+  }
+
+ public:
+  static already_AddRefed<Inner> Create(nsISerialEventTarget* aBaseTarget,
+                                        const char* aName, uint32_t aPriority) {
+    MOZ_ASSERT(NS_IsMainThread());
+    // FIXME: This assertion only worked when `sCurrentShutdownPhase` was not
+    // being updated.
+    // MOZ_ASSERT(ClearOnShutdown_Internal::sCurrentShutdownPhase ==
+    //            ShutdownPhase::NotInShutdown);
+
+    RefPtr<Inner> ref = new Inner(aBaseTarget, aName, aPriority);
+    return ref.forget();
+  }
+
+  bool IsEmpty() const {
+    // Any thread
+    return Length() == 0;
+  }
+
+  uint32_t Length() const {
+    // Any thread
+    MutexAutoLock lock(mMutex);
+    return mEventQueue.Count(lock);
+  }
+
+  already_AddRefed<nsIRunnable> GetEvent() {
+    MutexAutoLock lock(mMutex);
+    return mEventQueue.GetEvent(lock);
+  }
+
+  void AwaitIdle() const {
+    // Any thread, except the main thread or our base target.  Blocking the
+    // main thread is forbidden.  Blocking the base target is guaranteed to
+    // produce a deadlock.
+    MOZ_ASSERT(!NS_IsMainThread());
+#ifdef DEBUG
+    bool onBaseTarget = false;
+    Unused << mBaseTarget->IsOnCurrentThread(&onBaseTarget);
+    MOZ_ASSERT(!onBaseTarget);
+#endif
+
+    MutexAutoLock lock(mMutex);
+    while (mExecutor || IsPaused(lock)) {
+      mIdleCondVar.Wait();
+    }
+  }
+
+  bool IsPaused() const {
+    MutexAutoLock lock(mMutex);
+    return IsPaused(lock);
+  }
+
+  bool IsPaused(const MutexAutoLock& aProofOfLock) const MOZ_REQUIRES(mMutex) {
+    return mIsPaused;
+  }
+
+  nsresult SetIsPaused(bool aIsPaused) {
+    MutexAutoLock lock(mMutex);
+
+    // If we will be unpaused, and we have events in our queue, make sure we
+    // have an executor queued on the base event target to run them. Do this
+    // before we actually change mIsPaused, since this is fallible.
+    if (!aIsPaused && !mEventQueue.IsEmpty(lock)) {
+      nsresult rv = EnsureExecutor(lock);
+      if (NS_FAILED(rv)) {
+        return rv;
+      }
+    }
+
+    mIsPaused = aIsPaused;
+    return NS_OK;
+  }
+
+  nsresult DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+    // Any thread
+    nsCOMPtr<nsIRunnable> r = aEvent;
+    return Dispatch(r.forget(), aFlags);
+  }
+
+  nsresult Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) {
+    MOZ_ASSERT(aFlags == NS_DISPATCH_NORMAL || aFlags == NS_DISPATCH_AT_END);
+
+    // Any thread
+    MutexAutoLock lock(mMutex);
+
+    if (!IsPaused(lock)) {
+      // Make sure we have an executor in flight to process events. This is
+      // fallible, so do it first. Our lock will prevent the executor from
+      // accessing the event queue before we add the event below.
+      nsresult rv = EnsureExecutor(lock);
+      if (NS_FAILED(rv)) return rv;
+    }
+
+    // Only add the event to the underlying queue if are able to
+    // dispatch to our base target.
+    nsCOMPtr<nsIRunnable> event(aEvent);
+    LogRunnable::LogDispatch(event);
+    mEventQueue.PutEvent(event.forget(), EventQueuePriority::Normal, lock);
+    return NS_OK;
+  }
+
+  nsresult DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                           uint32_t aDelay) {
+    // The base target may implement this, but we don't.  Always fail
+    // to provide consistent behavior.
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  nsresult RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+    return mBaseTarget->RegisterShutdownTask(aTask);
+  }
+
+  nsresult UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+    return mBaseTarget->UnregisterShutdownTask(aTask);
+  }
+
+  bool IsOnCurrentThread() { return mBaseTarget->IsOnCurrentThread(); }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(ThrottledEventQueue::Inner, nsISupports);
+
+NS_IMPL_ISUPPORTS_INHERITED(ThrottledEventQueue::Inner::Executor, Runnable,
+                            nsIRunnablePriority)
+
+NS_IMPL_ISUPPORTS(ThrottledEventQueue, ThrottledEventQueue, nsIEventTarget,
+                  nsISerialEventTarget);
+
+ThrottledEventQueue::ThrottledEventQueue(already_AddRefed<Inner> aInner)
+    : mInner(aInner) {
+  MOZ_ASSERT(mInner);
+}
+
+already_AddRefed<ThrottledEventQueue> ThrottledEventQueue::Create(
+    nsISerialEventTarget* aBaseTarget, const char* aName, uint32_t aPriority) {
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aBaseTarget);
+
+  RefPtr<Inner> inner = Inner::Create(aBaseTarget, aName, aPriority);
+
+  RefPtr<ThrottledEventQueue> ref = new ThrottledEventQueue(inner.forget());
+  return ref.forget();
+}
+
+bool ThrottledEventQueue::IsEmpty() const { return mInner->IsEmpty(); }
+
+uint32_t ThrottledEventQueue::Length() const { return mInner->Length(); }
+
+// Get the next runnable from the queue
+already_AddRefed<nsIRunnable> ThrottledEventQueue::GetEvent() {
+  return mInner->GetEvent();
+}
+
+void ThrottledEventQueue::AwaitIdle() const { return mInner->AwaitIdle(); }
+
+nsresult ThrottledEventQueue::SetIsPaused(bool aIsPaused) {
+  return mInner->SetIsPaused(aIsPaused);
+}
+
+bool ThrottledEventQueue::IsPaused() const { return mInner->IsPaused(); }
+
+NS_IMETHODIMP
+ThrottledEventQueue::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+  return mInner->DispatchFromScript(aEvent, aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::Dispatch(already_AddRefed<nsIRunnable> aEvent,
+                              uint32_t aFlags) {
+  return mInner->Dispatch(std::move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                                     uint32_t aFlags) {
+  return mInner->DelayedDispatch(std::move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return mInner->RegisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return mInner->UnregisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+ThrottledEventQueue::IsOnCurrentThread(bool* aResult) {
+  *aResult = mInner->IsOnCurrentThread();
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+ThrottledEventQueue::IsOnCurrentThreadInfallible() {
+  return mInner->IsOnCurrentThread();
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/ThrottledEventQueue.h b/xpcom/threads/ThrottledEventQueue.h
new file mode 100644
index 0000000000..cf37a10a6d
--- /dev/null
+++ b/xpcom/threads/ThrottledEventQueue.h
@@ -0,0 +1,118 @@
+/* -*- 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/. */
+
+// nsIEventTarget wrapper for throttling event dispatch.
+
+#ifndef mozilla_ThrottledEventQueue_h
+#define mozilla_ThrottledEventQueue_h
+
+#include "nsISerialEventTarget.h"
+
+#define NS_THROTTLEDEVENTQUEUE_IID                   \
+  {                                                  \
+    0x8f3cf7dc, 0xfc14, 0x4ad5, {                    \
+      0x9f, 0xd5, 0xdb, 0x79, 0xbc, 0xe6, 0xd5, 0x08 \
+    }                                                \
+  }
+
+namespace mozilla {
+
+// A ThrottledEventQueue is an event target that can be used to throttle
+// events being dispatched to another base target.  It maintains its
+// own queue of events and only dispatches one at a time to the wrapped
+// target.  This can be used to avoid flooding the base target.
+//
+// Flooding is avoided via a very simple principle.  Runnables dispatched
+// to the ThrottledEventQueue are only dispatched to the base target
+// one at a time.  Only once that runnable has executed will we dispatch
+// the next runnable to the base target.  This in effect makes all
+// runnables passing through the ThrottledEventQueue yield to other work
+// on the base target.
+//
+// ThrottledEventQueue keeps runnables waiting to be dispatched to the
+// base in its own internal queue.  Code can query the length of this
+// queue using IsEmpty() and Length().  Further, code implement back
+// pressure by checking the depth of the queue and deciding to stop
+// issuing runnables if they see the ThrottledEventQueue is backed up.
+// Code running on other threads could even use AwaitIdle() to block
+// all operation until the ThrottledEventQueue drains.
+//
+// Note, this class is similar to TaskQueue, but also differs in a few
+// ways.  First, it is a very simple nsIEventTarget implementation.  It
+// does not use the AbstractThread API.
+//
+// In addition, ThrottledEventQueue currently dispatches its next
+// runnable to the base target *before* running the current event.  This
+// allows the event code to spin the event loop without stalling the
+// ThrottledEventQueue.  In contrast, TaskQueue only dispatches its next
+// runnable after running the current event.  That approach is necessary
+// for TaskQueue in order to work with thread pool targets.
+//
+// So, if you are targeting a thread pool you probably want a TaskQueue.
+// If you are targeting a single thread or other non-concurrent event
+// target, you probably want a ThrottledEventQueue.
+//
+// If you drop a ThrottledEventQueue while its queue still has events to be run,
+// they will continue to be dispatched as usual to the base. Only once the last
+// event has run will all the ThrottledEventQueue's memory be freed.
+class ThrottledEventQueue final : public nsISerialEventTarget {
+  class Inner;
+  RefPtr<Inner> mInner;
+
+  explicit ThrottledEventQueue(already_AddRefed<Inner> aInner);
+  ~ThrottledEventQueue() = default;
+
+ public:
+  // Create a ThrottledEventQueue for the given target.
+  static already_AddRefed<ThrottledEventQueue> Create(
+      nsISerialEventTarget* aBaseTarget, const char* aName,
+      uint32_t aPriority = nsIRunnablePriority::PRIORITY_NORMAL);
+
+  // Determine if there are any events pending in the queue.
+  bool IsEmpty() const;
+
+  // Determine how many events are pending in the queue.
+  uint32_t Length() const;
+
+  already_AddRefed<nsIRunnable> GetEvent();
+
+  // Block the current thread until the queue is empty. This may not be called
+  // on the main thread or the base target. The ThrottledEventQueue must not be
+  // paused.
+  void AwaitIdle() const;
+
+  // If |aIsPaused| is true, pause execution of events from this queue. No
+  // events from this queue will be run until this is called with |aIsPaused|
+  // false.
+  //
+  // To un-pause a ThrottledEventQueue, we need to dispatch a runnable to the
+  // underlying event target. That operation may fail, so this method is
+  // fallible as well.
+  //
+  // Note that, although ThrottledEventQueue's behavior is descibed as queueing
+  // events on the base target, an event queued on a TEQ is never actually moved
+  // to any other queue. What is actually dispatched to the base is an
+  // "executor" event which, when run, removes an event from the TEQ and runs it
+  // immediately. This means that you can pause a TEQ even after the executor
+  // has been queued on the base target, and even so, no events from the TEQ
+  // will run. When the base target gets around to running the executor, the
+  // executor will see that the TEQ is paused, and do nothing.
+  [[nodiscard]] nsresult SetIsPaused(bool aIsPaused);
+
+  // Return true if this ThrottledEventQueue is paused.
+  bool IsPaused() const;
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_THROTTLEDEVENTQUEUE_IID);
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(ThrottledEventQueue, NS_THROTTLEDEVENTQUEUE_IID);
+
+}  // namespace mozilla
+
+#endif  // mozilla_ThrottledEventQueue_h
diff --git a/xpcom/threads/TimerThread.cpp b/xpcom/threads/TimerThread.cpp
new file mode 100644
index 0000000000..d0ff778f20
--- /dev/null
+++ b/xpcom/threads/TimerThread.cpp
@@ -0,0 +1,1565 @@
+/* -*- 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 "nsTimerImpl.h"
+#include "TimerThread.h"
+
+#include "GeckoProfiler.h"
+#include "nsThreadUtils.h"
+
+#include "nsIObserverService.h"
+#include "nsIPropertyBag2.h"
+#include "mozilla/Services.h"
+#include "mozilla/ChaosMode.h"
+#include "mozilla/ArenaAllocator.h"
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/OperatorNewExtensions.h"
+#include "mozilla/StaticPrefs_timer.h"
+
+#include "mozilla/glean/GleanMetrics.h"
+
+#include <math.h>
+
+using namespace mozilla;
+
+#ifdef XP_WIN
+// Include Windows header required for enabling high-precision timers.
+#  include <windows.h>
+#  include <mmsystem.h>
+
+static constexpr UINT kTimerPeriodHiRes = 1;
+static constexpr UINT kTimerPeriodLowRes = 16;
+
+// Helper functions to determine what Windows timer resolution to target.
+static constexpr UINT GetDesiredTimerPeriod(const bool aOnBatteryPower,
+                                            const bool aLowProcessPriority) {
+  const bool useLowResTimer = aOnBatteryPower || aLowProcessPriority;
+  return useLowResTimer ? kTimerPeriodLowRes : kTimerPeriodHiRes;
+}
+
+static_assert(GetDesiredTimerPeriod(true, false) == kTimerPeriodLowRes);
+static_assert(GetDesiredTimerPeriod(false, true) == kTimerPeriodLowRes);
+static_assert(GetDesiredTimerPeriod(true, true) == kTimerPeriodLowRes);
+static_assert(GetDesiredTimerPeriod(false, false) == kTimerPeriodHiRes);
+
+UINT TimerThread::ComputeDesiredTimerPeriod() const {
+  const bool lowPriorityProcess =
+      mCachedPriority.load(std::memory_order_relaxed) <
+      hal::PROCESS_PRIORITY_FOREGROUND;
+
+  // NOTE: Using short-circuiting here to avoid call to GetSystemPowerStatus()
+  // when we know that that result will not affect the final result. (As
+  // confirmed by the static_assert's above, onBatteryPower does not affect the
+  // result when the lowPriorityProcess is true.)
+  SYSTEM_POWER_STATUS status;
+  const bool onBatteryPower = !lowPriorityProcess &&
+                              GetSystemPowerStatus(&status) &&
+                              (status.ACLineStatus == 0);
+
+  return GetDesiredTimerPeriod(onBatteryPower, lowPriorityProcess);
+}
+#endif
+
+// Uncomment the following line to enable runtime stats during development.
+// #define TIMERS_RUNTIME_STATS
+
+#ifdef TIMERS_RUNTIME_STATS
+// This class gathers durations and displays some basic stats when destroyed.
+// It is intended to be used as a static variable (see `AUTO_TIMERS_STATS`
+// below), to display stats at the end of the program.
+class StaticTimersStats {
+ public:
+  explicit StaticTimersStats(const char* aName) : mName(aName) {}
+
+  ~StaticTimersStats() {
+    // Using unsigned long long for computations and printfs.
+    using ULL = unsigned long long;
+    ULL n = static_cast<ULL>(mCount);
+    if (n == 0) {
+      printf("[%d] Timers stats `%s`: (nothing)\n",
+             int(profiler_current_process_id().ToNumber()), mName);
+    } else if (ULL sumNs = static_cast<ULL>(mSumDurationsNs); sumNs == 0) {
+      printf("[%d] Timers stats `%s`: %llu\n",
+             int(profiler_current_process_id().ToNumber()), mName, n);
+    } else {
+      printf("[%d] Timers stats `%s`: %llu ns / %llu = %llu ns, max %llu ns\n",
+             int(profiler_current_process_id().ToNumber()), mName, sumNs, n,
+             sumNs / n, static_cast<ULL>(mLongestDurationNs));
+    }
+  }
+
+  void AddDurationFrom(TimeStamp aStart) {
+    // Duration between aStart and now, rounded to the nearest nanosecond.
+    DurationNs duration = static_cast<DurationNs>(
+        (TimeStamp::Now() - aStart).ToMicroseconds() * 1000 + 0.5);
+    mSumDurationsNs += duration;
+    ++mCount;
+    // Update mLongestDurationNs if this one is longer.
+    for (;;) {
+      DurationNs longest = mLongestDurationNs;
+      if (MOZ_LIKELY(longest >= duration)) {
+        // This duration is not the longest, nothing to do.
+        break;
+      }
+      if (MOZ_LIKELY(mLongestDurationNs.compareExchange(longest, duration))) {
+        // Successfully updated `mLongestDurationNs` with the new value.
+        break;
+      }
+      // Otherwise someone else just updated `mLongestDurationNs`, we need to
+      // try again by looping.
+    }
+  }
+
+  void AddCount() {
+    MOZ_ASSERT(mSumDurationsNs == 0, "Don't mix counts and durations");
+    ++mCount;
+  }
+
+ private:
+  using DurationNs = uint64_t;
+  using Count = uint32_t;
+
+  Atomic<DurationNs> mSumDurationsNs{0};
+  Atomic<DurationNs> mLongestDurationNs{0};
+  Atomic<Count> mCount{0};
+  const char* mName;
+};
+
+// RAII object that measures its scoped lifetime duration and reports it to a
+// `StaticTimersStats`.
+class MOZ_RAII AutoTimersStats {
+ public:
+  explicit AutoTimersStats(StaticTimersStats& aStats)
+      : mStats(aStats), mStart(TimeStamp::Now()) {}
+
+  ~AutoTimersStats() { mStats.AddDurationFrom(mStart); }
+
+ private:
+  StaticTimersStats& mStats;
+  TimeStamp mStart;
+};
+
+// Macro that should be used to collect basic statistics from measurements of
+// block durations, from where this macro is, until the end of its enclosing
+// scope. The name is used in the static variable name and when displaying stats
+// at the end of the program; Another location could use the same name but their
+// stats will not be combined, so use different name if these locations should
+// be distinguished.
+#  define AUTO_TIMERS_STATS(name)                  \
+    static ::StaticTimersStats sStat##name(#name); \
+    ::AutoTimersStats autoStat##name(sStat##name);
+
+// This macro only counts the number of times it's used, not durations.
+// Don't mix with AUTO_TIMERS_STATS!
+#  define COUNT_TIMERS_STATS(name)                 \
+    static ::StaticTimersStats sStat##name(#name); \
+    sStat##name.AddCount();
+
+#else  // TIMERS_RUNTIME_STATS
+
+#  define AUTO_TIMERS_STATS(name)
+#  define COUNT_TIMERS_STATS(name)
+
+#endif  // TIMERS_RUNTIME_STATS else
+
+NS_IMPL_ISUPPORTS_INHERITED(TimerThread, Runnable, nsIObserver)
+
+TimerThread::TimerThread()
+    : Runnable("TimerThread"),
+      mInitialized(false),
+      mMonitor("TimerThread.mMonitor"),
+      mShutdown(false),
+      mWaiting(false),
+      mNotified(false),
+      mSleeping(false),
+      mAllowedEarlyFiringMicroseconds(0) {}
+
+TimerThread::~TimerThread() {
+  mThread = nullptr;
+
+  NS_ASSERTION(mTimers.IsEmpty(), "Timers remain in TimerThread::~TimerThread");
+
+#if TIMER_THREAD_STATISTICS
+  {
+    MonitorAutoLock lock(mMonitor);
+    PrintStatistics();
+  }
+#endif
+}
+
+namespace {
+
+class TimerObserverRunnable : public Runnable {
+ public:
+  explicit TimerObserverRunnable(nsIObserver* aObserver)
+      : mozilla::Runnable("TimerObserverRunnable"), mObserver(aObserver) {}
+
+  NS_DECL_NSIRUNNABLE
+
+ private:
+  nsCOMPtr<nsIObserver> mObserver;
+};
+
+NS_IMETHODIMP
+TimerObserverRunnable::Run() {
+  nsCOMPtr<nsIObserverService> observerService =
+      mozilla::services::GetObserverService();
+  if (observerService) {
+    observerService->AddObserver(mObserver, "sleep_notification", false);
+    observerService->AddObserver(mObserver, "wake_notification", false);
+    observerService->AddObserver(mObserver, "suspend_process_notification",
+                                 false);
+    observerService->AddObserver(mObserver, "resume_process_notification",
+                                 false);
+    observerService->AddObserver(mObserver, "ipc:process-priority-changed",
+                                 false);
+  }
+  return NS_OK;
+}
+
+}  // namespace
+
+namespace {
+
+// TimerEventAllocator is a thread-safe allocator used only for nsTimerEvents.
+// It's needed to avoid contention over the default allocator lock when
+// firing timer events (see bug 733277).  The thread-safety is required because
+// nsTimerEvent objects are allocated on the timer thread, and freed on another
+// thread.  Because TimerEventAllocator has its own lock, contention over that
+// lock is limited to the allocation and deallocation of nsTimerEvent objects.
+//
+// Because this is layered over ArenaAllocator, it never shrinks -- even
+// "freed" nsTimerEvents aren't truly freed, they're just put onto a free-list
+// for later recycling.  So the amount of memory consumed will always be equal
+// to the high-water mark consumption.  But nsTimerEvents are small and it's
+// unusual to have more than a few hundred of them, so this shouldn't be a
+// problem in practice.
+
+class TimerEventAllocator {
+ private:
+  struct FreeEntry {
+    FreeEntry* mNext;
+  };
+
+  ArenaAllocator<4096> mPool MOZ_GUARDED_BY(mMonitor);
+  FreeEntry* mFirstFree MOZ_GUARDED_BY(mMonitor);
+  mozilla::Monitor mMonitor;
+
+ public:
+  TimerEventAllocator()
+      : mFirstFree(nullptr), mMonitor("TimerEventAllocator") {}
+
+  ~TimerEventAllocator() = default;
+
+  void* Alloc(size_t aSize);
+  void Free(void* aPtr);
+};
+
+}  // namespace
+
+// This is a nsICancelableRunnable because we can dispatch it to Workers and
+// those can be shut down at any time, and in these cases, Cancel() is called
+// instead of Run().
+class nsTimerEvent final : public CancelableRunnable {
+ public:
+  NS_IMETHOD Run() override;
+
+  nsresult Cancel() override {
+    mTimer->Cancel();
+    return NS_OK;
+  }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_IMETHOD GetName(nsACString& aName) override;
+#endif
+
+  explicit nsTimerEvent(already_AddRefed<nsTimerImpl> aTimer,
+                        ProfilerThreadId aTimerThreadId)
+      : mozilla::CancelableRunnable("nsTimerEvent"),
+        mTimer(aTimer),
+        mGeneration(mTimer->GetGeneration()),
+        mTimerThreadId(aTimerThreadId) {
+    // Note: We override operator new for this class, and the override is
+    // fallible!
+    sAllocatorUsers++;
+
+    if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug) ||
+        profiler_thread_is_being_profiled_for_markers(mTimerThreadId)) {
+      mInitTime = TimeStamp::Now();
+    }
+  }
+
+  static void Init();
+  static void Shutdown();
+  static void DeleteAllocatorIfNeeded();
+
+  static void* operator new(size_t aSize) noexcept(true) {
+    return sAllocator->Alloc(aSize);
+  }
+  void operator delete(void* aPtr) {
+    sAllocator->Free(aPtr);
+    sAllocatorUsers--;
+    DeleteAllocatorIfNeeded();
+  }
+
+  already_AddRefed<nsTimerImpl> ForgetTimer() { return mTimer.forget(); }
+
+ private:
+  nsTimerEvent(const nsTimerEvent&) = delete;
+  nsTimerEvent& operator=(const nsTimerEvent&) = delete;
+  nsTimerEvent& operator=(const nsTimerEvent&&) = delete;
+
+  ~nsTimerEvent() {
+    MOZ_ASSERT(!sCanDeleteAllocator || sAllocatorUsers > 0,
+               "This will result in us attempting to deallocate the "
+               "nsTimerEvent allocator twice");
+  }
+
+  TimeStamp mInitTime;
+  RefPtr<nsTimerImpl> mTimer;
+  const int32_t mGeneration;
+  ProfilerThreadId mTimerThreadId;
+
+  static TimerEventAllocator* sAllocator;
+
+  static Atomic<int32_t, SequentiallyConsistent> sAllocatorUsers;
+  static Atomic<bool, SequentiallyConsistent> sCanDeleteAllocator;
+};
+
+TimerEventAllocator* nsTimerEvent::sAllocator = nullptr;
+Atomic<int32_t, SequentiallyConsistent> nsTimerEvent::sAllocatorUsers;
+Atomic<bool, SequentiallyConsistent> nsTimerEvent::sCanDeleteAllocator;
+
+namespace {
+
+void* TimerEventAllocator::Alloc(size_t aSize) {
+  MOZ_ASSERT(aSize == sizeof(nsTimerEvent));
+
+  mozilla::MonitorAutoLock lock(mMonitor);
+
+  void* p;
+  if (mFirstFree) {
+    p = mFirstFree;
+    mFirstFree = mFirstFree->mNext;
+  } else {
+    p = mPool.Allocate(aSize, fallible);
+  }
+
+  return p;
+}
+
+void TimerEventAllocator::Free(void* aPtr) {
+  mozilla::MonitorAutoLock lock(mMonitor);
+
+  FreeEntry* entry = reinterpret_cast<FreeEntry*>(aPtr);
+
+  entry->mNext = mFirstFree;
+  mFirstFree = entry;
+}
+
+}  // namespace
+
+struct TimerMarker {
+  static constexpr Span<const char> MarkerTypeName() {
+    return MakeStringSpan("Timer");
+  }
+  static void StreamJSONMarkerData(baseprofiler::SpliceableJSONWriter& aWriter,
+                                   uint32_t aDelay, uint8_t aType,
+                                   MarkerThreadId aThreadId, bool aCanceled) {
+    aWriter.IntProperty("delay", aDelay);
+    if (!aThreadId.IsUnspecified()) {
+      // Tech note: If `ToNumber()` returns a uint64_t, the conversion to
+      // int64_t is "implementation-defined" before C++20. This is
+      // acceptable here, because this is a one-way conversion to a unique
+      // identifier that's used to visually separate data by thread on the
+      // front-end.
+      aWriter.IntProperty(
+          "threadId", static_cast<int64_t>(aThreadId.ThreadId().ToNumber()));
+    }
+    if (aCanceled) {
+      aWriter.BoolProperty("canceled", true);
+      // Show a red 'X' as a prefix on the marker chart for canceled timers.
+      aWriter.StringProperty("prefix", "❌");
+    }
+
+    // The string property for the timer type is not written when the type is
+    // one shot, as that's the type used almost all the time, and that would
+    // consume space in the profiler buffer and then in the profile JSON,
+    // getting in the way of capturing long power profiles.
+    // Bug 1815677 might make this cheap to capture.
+    if (aType != nsITimer::TYPE_ONE_SHOT) {
+      if (aType == nsITimer::TYPE_REPEATING_SLACK) {
+        aWriter.StringProperty("ttype", "repeating slack");
+      } else if (aType == nsITimer::TYPE_REPEATING_PRECISE) {
+        aWriter.StringProperty("ttype", "repeating precise");
+      } else if (aType == nsITimer::TYPE_REPEATING_PRECISE_CAN_SKIP) {
+        aWriter.StringProperty("ttype", "repeating precise can skip");
+      } else if (aType == nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY) {
+        aWriter.StringProperty("ttype", "repeating slack low priority");
+      } else if (aType == nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY) {
+        aWriter.StringProperty("ttype", "low priority");
+      }
+    }
+  }
+  static MarkerSchema MarkerTypeDisplay() {
+    using MS = MarkerSchema;
+    MS schema{MS::Location::MarkerChart, MS::Location::MarkerTable};
+    schema.AddKeyLabelFormat("delay", "Delay", MS::Format::Milliseconds);
+    schema.AddKeyLabelFormat("ttype", "Timer Type", MS::Format::String);
+    schema.AddKeyLabelFormat("canceled", "Canceled", MS::Format::String);
+    schema.SetChartLabel("{marker.data.prefix} {marker.data.delay}");
+    schema.SetTableLabel(
+        "{marker.name} - {marker.data.prefix} {marker.data.delay}");
+    return schema;
+  }
+};
+
+struct AddRemoveTimerMarker {
+  static constexpr Span<const char> MarkerTypeName() {
+    return MakeStringSpan("AddRemoveTimer");
+  }
+  static void StreamJSONMarkerData(baseprofiler::SpliceableJSONWriter& aWriter,
+                                   const ProfilerString8View& aTimerName,
+                                   uint32_t aDelay, MarkerThreadId aThreadId) {
+    aWriter.StringProperty("name", aTimerName);
+    aWriter.IntProperty("delay", aDelay);
+    if (!aThreadId.IsUnspecified()) {
+      // Tech note: If `ToNumber()` returns a uint64_t, the conversion to
+      // int64_t is "implementation-defined" before C++20. This is
+      // acceptable here, because this is a one-way conversion to a unique
+      // identifier that's used to visually separate data by thread on the
+      // front-end.
+      aWriter.IntProperty(
+          "threadId", static_cast<int64_t>(aThreadId.ThreadId().ToNumber()));
+    }
+  }
+  static MarkerSchema MarkerTypeDisplay() {
+    using MS = MarkerSchema;
+    MS schema{MS::Location::MarkerChart, MS::Location::MarkerTable};
+    schema.AddKeyLabelFormatSearchable("name", "Name", MS::Format::String,
+                                       MS::Searchable::Searchable);
+    schema.AddKeyLabelFormat("delay", "Delay", MS::Format::Milliseconds);
+    schema.SetTableLabel(
+        "{marker.name} - {marker.data.name} - {marker.data.delay}");
+    return schema;
+  }
+};
+
+void nsTimerEvent::Init() { sAllocator = new TimerEventAllocator(); }
+
+void nsTimerEvent::Shutdown() {
+  sCanDeleteAllocator = true;
+  DeleteAllocatorIfNeeded();
+}
+
+void nsTimerEvent::DeleteAllocatorIfNeeded() {
+  if (sCanDeleteAllocator && sAllocatorUsers == 0) {
+    delete sAllocator;
+    sAllocator = nullptr;
+  }
+}
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+NS_IMETHODIMP
+nsTimerEvent::GetName(nsACString& aName) {
+  bool current;
+  MOZ_RELEASE_ASSERT(
+      NS_SUCCEEDED(mTimer->mEventTarget->IsOnCurrentThread(&current)) &&
+      current);
+
+  mTimer->GetName(aName);
+  return NS_OK;
+}
+#endif
+
+NS_IMETHODIMP
+nsTimerEvent::Run() {
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    TimeStamp now = TimeStamp::Now();
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("[this=%p] time between PostTimerEvent() and Fire(): %fms\n", this,
+             (now - mInitTime).ToMilliseconds()));
+  }
+
+  if (profiler_thread_is_being_profiled_for_markers(mTimerThreadId)) {
+    MutexAutoLock lock(mTimer->mMutex);
+    nsAutoCString name;
+    mTimer->GetName(name, lock);
+    // This adds a marker with the timer name as the marker name, to make it
+    // obvious which timers are being used. This marker will be useful to
+    // understand which timers might be added and firing excessively often.
+    profiler_add_marker(
+        name, geckoprofiler::category::TIMER,
+        MarkerOptions(MOZ_LIKELY(mInitTime)
+                          ? MarkerTiming::Interval(
+                                mTimer->mTimeout - mTimer->mDelay, mInitTime)
+                          : MarkerTiming::IntervalUntilNowFrom(
+                                mTimer->mTimeout - mTimer->mDelay),
+                      MarkerThreadId(mTimerThreadId)),
+        TimerMarker{}, mTimer->mDelay.ToMilliseconds(), mTimer->mType,
+        MarkerThreadId::CurrentThread(), false);
+    // This marker is meant to help understand the behavior of the timer thread.
+    profiler_add_marker(
+        "PostTimerEvent", geckoprofiler::category::OTHER,
+        MarkerOptions(MOZ_LIKELY(mInitTime)
+                          ? MarkerTiming::IntervalUntilNowFrom(mInitTime)
+                          : MarkerTiming::InstantNow(),
+                      MarkerThreadId(mTimerThreadId)),
+        AddRemoveTimerMarker{}, name, mTimer->mDelay.ToMilliseconds(),
+        MarkerThreadId::CurrentThread());
+  }
+
+  mTimer->Fire(mGeneration);
+
+  return NS_OK;
+}
+
+nsresult TimerThread::Init() {
+  mMonitor.AssertCurrentThreadOwns();
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+          ("TimerThread::Init [%d]\n", mInitialized));
+
+  if (!mInitialized) {
+    nsTimerEvent::Init();
+
+    // We hold on to mThread to keep the thread alive.
+    nsresult rv =
+        NS_NewNamedThread("Timer", getter_AddRefs(mThread), this,
+                          {.stackSize = nsIThreadManager::DEFAULT_STACK_SIZE,
+                           .blockDispatch = true});
+    if (NS_FAILED(rv)) {
+      mThread = nullptr;
+    } else {
+      RefPtr<TimerObserverRunnable> r = new TimerObserverRunnable(this);
+      if (NS_IsMainThread()) {
+        r->Run();
+      } else {
+        NS_DispatchToMainThread(r);
+      }
+    }
+
+    mInitialized = true;
+  }
+
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+
+  return NS_OK;
+}
+
+nsresult TimerThread::Shutdown() {
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug, ("TimerThread::Shutdown begin\n"));
+
+  if (!mThread) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  nsTArray<RefPtr<nsTimerImpl>> timers;
+  {
+    // lock scope
+    MonitorAutoLock lock(mMonitor);
+
+    mShutdown = true;
+
+    // notify the cond var so that Run() can return
+    if (mWaiting) {
+      mNotified = true;
+      mMonitor.Notify();
+    }
+
+    // Need to copy content of mTimers array to a local array
+    // because call to timers' Cancel() (and release its self)
+    // must not be done under the lock. Destructor of a callback
+    // might potentially call some code reentering the same lock
+    // that leads to unexpected behavior or deadlock.
+    // See bug 422472.
+    timers.SetCapacity(mTimers.Length());
+    for (Entry& entry : mTimers) {
+      if (entry.Value()) {
+        timers.AppendElement(entry.Take());
+      }
+    }
+
+    mTimers.Clear();
+  }
+
+  for (const RefPtr<nsTimerImpl>& timer : timers) {
+    MOZ_ASSERT(timer);
+    timer->Cancel();
+  }
+
+  mThread->Shutdown();  // wait for the thread to die
+
+  nsTimerEvent::Shutdown();
+
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug, ("TimerThread::Shutdown end\n"));
+  return NS_OK;
+}
+
+namespace {
+
+struct MicrosecondsToInterval {
+  PRIntervalTime operator[](size_t aMs) const {
+    return PR_MicrosecondsToInterval(aMs);
+  }
+};
+
+struct IntervalComparator {
+  int operator()(PRIntervalTime aInterval) const {
+    return (0 < aInterval) ? -1 : 1;
+  }
+};
+
+}  // namespace
+
+#ifdef DEBUG
+void TimerThread::VerifyTimerListConsistency() const {
+  mMonitor.AssertCurrentThreadOwns();
+
+  // Find the first non-canceled timer (and check its cached timeout if we find
+  // it).
+  const size_t timerCount = mTimers.Length();
+  size_t lastNonCanceledTimerIndex = 0;
+  while (lastNonCanceledTimerIndex < timerCount &&
+         !mTimers[lastNonCanceledTimerIndex].Value()) {
+    ++lastNonCanceledTimerIndex;
+  }
+  MOZ_ASSERT(lastNonCanceledTimerIndex == timerCount ||
+             mTimers[lastNonCanceledTimerIndex].Value());
+  MOZ_ASSERT(lastNonCanceledTimerIndex == timerCount ||
+             mTimers[lastNonCanceledTimerIndex].Value()->mTimeout ==
+                 mTimers[lastNonCanceledTimerIndex].Timeout());
+
+  // Verify that mTimers is sorted and the cached timeouts are consistent.
+  for (size_t timerIndex = lastNonCanceledTimerIndex + 1;
+       timerIndex < timerCount; ++timerIndex) {
+    if (mTimers[timerIndex].Value()) {
+      MOZ_ASSERT(mTimers[timerIndex].Timeout() ==
+                 mTimers[timerIndex].Value()->mTimeout);
+      MOZ_ASSERT(mTimers[timerIndex].Timeout() >=
+                 mTimers[lastNonCanceledTimerIndex].Timeout());
+      lastNonCanceledTimerIndex = timerIndex;
+    }
+  }
+}
+#endif
+
+size_t TimerThread::ComputeTimerInsertionIndex(const TimeStamp& timeout) const {
+  mMonitor.AssertCurrentThreadOwns();
+
+  const size_t timerCount = mTimers.Length();
+
+  size_t firstGtIndex = 0;
+  while (firstGtIndex < timerCount &&
+         (!mTimers[firstGtIndex].Value() ||
+          mTimers[firstGtIndex].Timeout() <= timeout)) {
+    ++firstGtIndex;
+  }
+
+  return firstGtIndex;
+}
+
+TimeStamp TimerThread::ComputeWakeupTimeFromTimers() const {
+  mMonitor.AssertCurrentThreadOwns();
+
+  // Timer list should be non-empty and first timer should always be
+  // non-canceled at this point and we rely on that here.
+  MOZ_ASSERT(!mTimers.IsEmpty());
+  MOZ_ASSERT(mTimers[0].Value());
+
+  // Overview: Find the last timer in the list that can be "bundled" together in
+  // the same wake-up with mTimers[0] and use its timeout as our target wake-up
+  // time.
+
+  // bundleWakeup is when we should wake up in order to be able to fire all of
+  // the timers in our selected bundle. It will always be the timeout of the
+  // last timer in the bundle.
+  TimeStamp bundleWakeup = mTimers[0].Timeout();
+
+  // cutoffTime is the latest that we can wake up for the timers currently
+  // accepted into the bundle. These needs to be updated as we go through the
+  // list because later timers may have more strict delay tolerances.
+  const TimeDuration minTimerDelay = TimeDuration::FromMilliseconds(
+      StaticPrefs::timer_minimum_firing_delay_tolerance_ms());
+  const TimeDuration maxTimerDelay = TimeDuration::FromMilliseconds(
+      StaticPrefs::timer_maximum_firing_delay_tolerance_ms());
+  TimeStamp cutoffTime =
+      bundleWakeup + ComputeAcceptableFiringDelay(mTimers[0].Delay(),
+                                                  minTimerDelay, maxTimerDelay);
+
+  const size_t timerCount = mTimers.Length();
+  for (size_t entryIndex = 1; entryIndex < timerCount; ++entryIndex) {
+    const Entry& curEntry = mTimers[entryIndex];
+    const nsTimerImpl* curTimer = curEntry.Value();
+    if (!curTimer) {
+      // Canceled timer - skip it
+      continue;
+    }
+
+    const TimeStamp curTimerDue = curEntry.Timeout();
+    if (curTimerDue > cutoffTime) {
+      // Can't include this timer in the bundle - it fires too late.
+      break;
+    }
+
+    // This timer can be included in the bundle. Update bundleWakeup and
+    // cutoffTime.
+    bundleWakeup = curTimerDue;
+    cutoffTime = std::min(
+        curTimerDue + ComputeAcceptableFiringDelay(
+                          curEntry.Delay(), minTimerDelay, maxTimerDelay),
+        cutoffTime);
+    MOZ_ASSERT(bundleWakeup <= cutoffTime);
+  }
+
+#if !defined(XP_WIN)
+  // Due to the fact that, on Windows, each TimeStamp object holds two distinct
+  // "values", this assert is not valid there. See bug 1829983 for the details.
+  MOZ_ASSERT(bundleWakeup - mTimers[0].Timeout() <=
+             ComputeAcceptableFiringDelay(mTimers[0].Delay(), minTimerDelay,
+                                          maxTimerDelay));
+#endif
+
+  return bundleWakeup;
+}
+
+TimeDuration TimerThread::ComputeAcceptableFiringDelay(
+    TimeDuration timerDuration, TimeDuration minDelay,
+    TimeDuration maxDelay) const {
+  // Use the timer's duration divided by this value as a base for how much
+  // firing delay a timer can accept. 8 was chosen specifically because it is a
+  // power of two which means that this division turns nicely into a shift.
+  constexpr int64_t timerDurationDivider = 8;
+  static_assert(IsPowerOfTwo(static_cast<uint64_t>(timerDurationDivider)));
+  const TimeDuration tmp = timerDuration / timerDurationDivider;
+  return std::min(std::max(minDelay, tmp), maxDelay);
+}
+
+NS_IMETHODIMP
+TimerThread::Run() {
+  MonitorAutoLock lock(mMonitor);
+
+  mProfilerThreadId = profiler_current_thread_id();
+
+  // TODO: Make mAllowedEarlyFiringMicroseconds const and initialize it in the
+  // constructor.
+  mAllowedEarlyFiringMicroseconds = 250;
+  const TimeDuration allowedEarlyFiring =
+      TimeDuration::FromMicroseconds(mAllowedEarlyFiringMicroseconds);
+
+  bool forceRunNextTimer = false;
+
+  // Queue for tracking of how many timers are fired on each wake-up. We need to
+  // buffer these locally and only send off to glean occasionally to avoid
+  // performance hit.
+  static constexpr size_t kMaxQueuedTimerFired = 128;
+  size_t queuedTimerFiredCount = 0;
+  AutoTArray<uint64_t, kMaxQueuedTimerFired> queuedTimersFiredPerWakeup;
+  queuedTimersFiredPerWakeup.SetLengthAndRetainStorage(kMaxQueuedTimerFired);
+
+#ifdef XP_WIN
+  // kTimerPeriodEvalIntervalSec is the minimum amount of time that must pass
+  // before we will consider changing the timer period again.
+  static constexpr float kTimerPeriodEvalIntervalSec = 2.0f;
+  const TimeDuration timerPeriodEvalInterval =
+      TimeDuration::FromSeconds(kTimerPeriodEvalIntervalSec);
+  TimeStamp nextTimerPeriodEval = TimeStamp::Now() + timerPeriodEvalInterval;
+
+  // If this is false, we will perform all of the logic but will stop short of
+  // actually changing the timer period.
+  const bool adjustTimerPeriod =
+      StaticPrefs::timer_auto_increase_timer_resolution();
+  UINT lastTimePeriodSet = ComputeDesiredTimerPeriod();
+
+  if (adjustTimerPeriod) {
+    timeBeginPeriod(lastTimePeriodSet);
+  }
+#endif
+
+  uint64_t timersFiredThisWakeup = 0;
+  while (!mShutdown) {
+    // Have to use PRIntervalTime here, since PR_WaitCondVar takes it
+    TimeDuration waitFor;
+    bool forceRunThisTimer = forceRunNextTimer;
+    forceRunNextTimer = false;
+
+#ifdef DEBUG
+    VerifyTimerListConsistency();
+#endif
+
+    if (mSleeping) {
+      // Sleep for 0.1 seconds while not firing timers.
+      uint32_t milliseconds = 100;
+      if (ChaosMode::isActive(ChaosFeature::TimerScheduling)) {
+        milliseconds = ChaosMode::randomUint32LessThan(200);
+      }
+      waitFor = TimeDuration::FromMilliseconds(milliseconds);
+    } else {
+      waitFor = TimeDuration::Forever();
+      TimeStamp now = TimeStamp::Now();
+
+#ifdef XP_WIN
+      if (now >= nextTimerPeriodEval) {
+        const UINT newTimePeriod = ComputeDesiredTimerPeriod();
+        if (newTimePeriod != lastTimePeriodSet) {
+          if (adjustTimerPeriod) {
+            timeEndPeriod(lastTimePeriodSet);
+            timeBeginPeriod(newTimePeriod);
+          }
+          lastTimePeriodSet = newTimePeriod;
+        }
+        nextTimerPeriodEval = now + timerPeriodEvalInterval;
+      }
+#endif
+
+#if TIMER_THREAD_STATISTICS
+      if (!mNotified && !mIntendedWakeupTime.IsNull() &&
+          now < mIntendedWakeupTime) {
+        ++mEarlyWakeups;
+        const double earlinessms = (mIntendedWakeupTime - now).ToMilliseconds();
+        mTotalEarlyWakeupTime += earlinessms;
+      }
+#endif
+
+      RemoveLeadingCanceledTimersInternal();
+
+      if (!mTimers.IsEmpty()) {
+        if (now + allowedEarlyFiring >= mTimers[0].Value()->mTimeout ||
+            forceRunThisTimer) {
+        next:
+          // NB: AddRef before the Release under RemoveTimerInternal to avoid
+          // mRefCnt passing through zero, in case all other refs than the one
+          // from mTimers have gone away (the last non-mTimers[i]-ref's Release
+          // must be racing with us, blocked in gThread->RemoveTimer waiting
+          // for TimerThread::mMonitor, under nsTimerImpl::Release.
+
+          RefPtr<nsTimerImpl> timerRef(mTimers[0].Take());
+          RemoveFirstTimerInternal();
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+                  ("Timer thread woke up %fms from when it was supposed to\n",
+                   fabs((now - timerRef->mTimeout).ToMilliseconds())));
+
+          // We are going to let the call to PostTimerEvent here handle the
+          // release of the timer so that we don't end up releasing the timer
+          // on the TimerThread instead of on the thread it targets.
+          {
+            ++timersFiredThisWakeup;
+            LogTimerEvent::Run run(timerRef.get());
+            PostTimerEvent(timerRef.forget());
+          }
+
+          if (mShutdown) {
+            break;
+          }
+
+          // Update now, as PostTimerEvent plus the locking may have taken a
+          // tick or two, and we may goto next below.
+          now = TimeStamp::Now();
+        }
+      }
+
+      RemoveLeadingCanceledTimersInternal();
+
+      if (!mTimers.IsEmpty()) {
+        TimeStamp timeout = mTimers[0].Value()->mTimeout;
+
+        // Don't wait at all (even for PR_INTERVAL_NO_WAIT) if the next timer
+        // is due now or overdue.
+        //
+        // Note that we can only sleep for integer values of a certain
+        // resolution. We use mAllowedEarlyFiringMicroseconds, calculated
+        // before, to do the optimal rounding (i.e., of how to decide what
+        // interval is so small we should not wait at all).
+        double microseconds = (timeout - now).ToMicroseconds();
+
+        // The mean value of sFractions must be 1 to ensure that the average of
+        // a long sequence of timeouts converges to the actual sum of their
+        // times.
+        static constexpr double sChaosFractions[] = {0.0, 0.25, 0.5, 0.75,
+                                                     1.0, 1.75, 2.75};
+        if (ChaosMode::isActive(ChaosFeature::TimerScheduling)) {
+          microseconds *= sChaosFractions[ChaosMode::randomUint32LessThan(
+              ArrayLength(sChaosFractions))];
+          forceRunNextTimer = true;
+        }
+
+        if (microseconds < mAllowedEarlyFiringMicroseconds) {
+          forceRunNextTimer = false;
+          goto next;  // round down; execute event now
+        }
+
+        // TECHNICAL NOTE: Determining waitFor (by subtracting |now| from our
+        // desired wake-up time) at this point is not ideal. For one thing, the
+        // |now| that we have at this point is somewhat old. Secondly, there is
+        // quite a bit of code between here and where we actually use waitFor to
+        // request sleep. If I am thinking about this correctly, both of these
+        // will contribute to us requesting more sleep than is actually needed
+        // to wake up at our desired time. We could avoid this problem by only
+        // determining our desired wake-up time here and then calculating the
+        // wait time when we're actually about to sleep.
+        const TimeStamp wakeupTime = ComputeWakeupTimeFromTimers();
+        waitFor = wakeupTime - now;
+
+        // If this were to fail that would mean that we had more timers that we
+        // should have fired.
+        MOZ_ASSERT(!waitFor.IsZero());
+
+        if (ChaosMode::isActive(ChaosFeature::TimerScheduling)) {
+          // If chaos mode is active then mess with the amount of time that we
+          // request to sleep (without changing what we record as our expected
+          // wake-up time). This will simulate unintended early/late wake-ups.
+          const double waitInMs = waitFor.ToMilliseconds();
+          const double chaosWaitInMs =
+              waitInMs * sChaosFractions[ChaosMode::randomUint32LessThan(
+                             ArrayLength(sChaosFractions))];
+          waitFor = TimeDuration::FromMilliseconds(chaosWaitInMs);
+        }
+
+        mIntendedWakeupTime = wakeupTime;
+      } else {
+        mIntendedWakeupTime = TimeStamp{};
+      }
+
+      if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+        if (waitFor == TimeDuration::Forever())
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug, ("waiting forever\n"));
+        else
+          MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+                  ("waiting for %f\n", waitFor.ToMilliseconds()));
+      }
+    }
+
+    {
+      // About to sleep - let's make note of how many timers we processed and
+      // see if we should send out a new batch of telemetry.
+      queuedTimersFiredPerWakeup[queuedTimerFiredCount] = timersFiredThisWakeup;
+      ++queuedTimerFiredCount;
+      if (queuedTimerFiredCount == kMaxQueuedTimerFired) {
+        glean::timer_thread::timers_fired_per_wakeup.AccumulateSamples(
+            queuedTimersFiredPerWakeup);
+        queuedTimerFiredCount = 0;
+      }
+    }
+
+#if TIMER_THREAD_STATISTICS
+    {
+      size_t bucketIndex = 0;
+      while (bucketIndex < sTimersFiredPerWakeupBucketCount - 1 &&
+             timersFiredThisWakeup >
+                 sTimersFiredPerWakeupThresholds[bucketIndex]) {
+        ++bucketIndex;
+      }
+      MOZ_ASSERT(bucketIndex < sTimersFiredPerWakeupBucketCount);
+      ++mTimersFiredPerWakeup[bucketIndex];
+
+      ++mTotalWakeupCount;
+      if (mNotified) {
+        ++mTimersFiredPerNotifiedWakeup[bucketIndex];
+        ++mTotalNotifiedWakeupCount;
+      } else {
+        ++mTimersFiredPerUnnotifiedWakeup[bucketIndex];
+        ++mTotalUnnotifiedWakeupCount;
+      }
+    }
+#endif
+
+    timersFiredThisWakeup = 0;
+
+    mWaiting = true;
+    mNotified = false;
+
+    {
+      AUTO_PROFILER_TRACING_MARKER("TimerThread", "Wait", OTHER);
+      mMonitor.Wait(waitFor);
+    }
+    if (mNotified) {
+      forceRunNextTimer = false;
+    }
+    mWaiting = false;
+  }
+
+  // About to shut down - let's send out the final batch of timers fired counts.
+  if (queuedTimerFiredCount != 0) {
+    queuedTimersFiredPerWakeup.SetLengthAndRetainStorage(queuedTimerFiredCount);
+    glean::timer_thread::timers_fired_per_wakeup.AccumulateSamples(
+        queuedTimersFiredPerWakeup);
+  }
+
+#ifdef XP_WIN
+  // About to shut down - let's finish off the last time period that we set.
+  if (adjustTimerPeriod) {
+    timeEndPeriod(lastTimePeriodSet);
+  }
+#endif
+
+  return NS_OK;
+}
+
+nsresult TimerThread::AddTimer(nsTimerImpl* aTimer,
+                               const MutexAutoLock& aProofOfLock) {
+  MonitorAutoLock lock(mMonitor);
+  AUTO_TIMERS_STATS(TimerThread_AddTimer);
+
+  if (!aTimer->mEventTarget) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  nsresult rv = Init();
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  // Awaken the timer thread if:
+  // - This timer needs to fire *before* the Timer Thread is scheduled to wake
+  //   up.
+  // AND/OR
+  // - The delay is 0, which is usually meant to be run as soon as possible.
+  //   Note: Even if the thread is scheduled to wake up now/soon, on some
+  //   systems there could be a significant delay compared to notifying, which
+  //   is almost immediate; and some users of 0-delay depend on it being this
+  //   fast!
+  const TimeDuration minTimerDelay = TimeDuration::FromMilliseconds(
+      StaticPrefs::timer_minimum_firing_delay_tolerance_ms());
+  const TimeDuration maxTimerDelay = TimeDuration::FromMilliseconds(
+      StaticPrefs::timer_maximum_firing_delay_tolerance_ms());
+  const TimeDuration firingDelay = ComputeAcceptableFiringDelay(
+      aTimer->mDelay, minTimerDelay, maxTimerDelay);
+  const bool firingBeforeNextWakeup =
+      mIntendedWakeupTime.IsNull() ||
+      (aTimer->mTimeout + firingDelay < mIntendedWakeupTime);
+  const bool wakeUpTimerThread =
+      mWaiting && (firingBeforeNextWakeup || aTimer->mDelay.IsZero());
+
+#if TIMER_THREAD_STATISTICS
+  if (mTotalTimersAdded == 0) {
+    mFirstTimerAdded = TimeStamp::Now();
+  }
+  ++mTotalTimersAdded;
+#endif
+
+  // Add the timer to our list.
+  if (!AddTimerInternal(*aTimer)) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (wakeUpTimerThread) {
+    mNotified = true;
+    mMonitor.Notify();
+  }
+
+  if (profiler_thread_is_being_profiled_for_markers(mProfilerThreadId)) {
+    nsAutoCString name;
+    aTimer->GetName(name, aProofOfLock);
+
+    nsLiteralCString prefix("Anonymous_");
+    profiler_add_marker(
+        "AddTimer", geckoprofiler::category::OTHER,
+        MarkerOptions(MarkerThreadId(mProfilerThreadId),
+                      MarkerStack::MaybeCapture(
+                          name.Equals("nonfunction:JS") ||
+                          StringHead(name, prefix.Length()) == prefix)),
+        AddRemoveTimerMarker{}, name, aTimer->mDelay.ToMilliseconds(),
+        MarkerThreadId::CurrentThread());
+  }
+
+  return NS_OK;
+}
+
+nsresult TimerThread::RemoveTimer(nsTimerImpl* aTimer,
+                                  const MutexAutoLock& aProofOfLock) {
+  MonitorAutoLock lock(mMonitor);
+  AUTO_TIMERS_STATS(TimerThread_RemoveTimer);
+
+  // Remove the timer from our array.  Tell callers that aTimer was not found
+  // by returning NS_ERROR_NOT_AVAILABLE.
+
+  if (!RemoveTimerInternal(*aTimer)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+#if TIMER_THREAD_STATISTICS
+  ++mTotalTimersRemoved;
+#endif
+
+  // Note: The timer thread is *not* awoken.
+  // The removed-timer entry is just left null, and will be reused (by a new or
+  // re-set timer) or discarded (when the timer thread logic handles non-null
+  // timers around it).
+  // If this was the front timer, and in the unlikely case that its entry is not
+  // soon reused by a re-set timer, the timer thread will wake up at the
+  // previously-scheduled time, but will quickly notice that there is no actual
+  // pending timer, and will restart its wait until the following real timeout.
+
+  if (profiler_thread_is_being_profiled_for_markers(mProfilerThreadId)) {
+    nsAutoCString name;
+    aTimer->GetName(name, aProofOfLock);
+
+    nsLiteralCString prefix("Anonymous_");
+    // This marker is meant to help understand the behavior of the timer thread.
+    profiler_add_marker(
+        "RemoveTimer", geckoprofiler::category::OTHER,
+        MarkerOptions(MarkerThreadId(mProfilerThreadId),
+                      MarkerStack::MaybeCapture(
+                          name.Equals("nonfunction:JS") ||
+                          StringHead(name, prefix.Length()) == prefix)),
+        AddRemoveTimerMarker{}, name, aTimer->mDelay.ToMilliseconds(),
+        MarkerThreadId::CurrentThread());
+    // This adds a marker with the timer name as the marker name, to make it
+    // obvious which timers are being used. This marker will be useful to
+    // understand which timers might be added and removed excessively often.
+    profiler_add_marker(name, geckoprofiler::category::TIMER,
+                        MarkerOptions(MarkerTiming::IntervalUntilNowFrom(
+                                          aTimer->mTimeout - aTimer->mDelay),
+                                      MarkerThreadId(mProfilerThreadId)),
+                        TimerMarker{}, aTimer->mDelay.ToMilliseconds(),
+                        aTimer->mType, MarkerThreadId::CurrentThread(), true);
+  }
+
+  return NS_OK;
+}
+
+TimeStamp TimerThread::FindNextFireTimeForCurrentThread(TimeStamp aDefault,
+                                                        uint32_t aSearchBound) {
+  MonitorAutoLock lock(mMonitor);
+  AUTO_TIMERS_STATS(TimerThread_FindNextFireTimeForCurrentThread);
+
+  for (const Entry& entry : mTimers) {
+    const nsTimerImpl* timer = entry.Value();
+    if (timer) {
+      if (entry.Timeout() > aDefault) {
+        return aDefault;
+      }
+
+      // Don't yield to timers created with the *_LOW_PRIORITY type.
+      if (!timer->IsLowPriority()) {
+        bool isOnCurrentThread = false;
+        nsresult rv =
+            timer->mEventTarget->IsOnCurrentThread(&isOnCurrentThread);
+        if (NS_SUCCEEDED(rv) && isOnCurrentThread) {
+          return entry.Timeout();
+        }
+      }
+
+      if (aSearchBound == 0) {
+        // Couldn't find any non-low priority timers for the current thread.
+        // Return a compromise between a very short and a long idle time.
+        TimeStamp fallbackDeadline =
+            TimeStamp::Now() + TimeDuration::FromMilliseconds(16);
+        return fallbackDeadline < aDefault ? fallbackDeadline : aDefault;
+      }
+
+      --aSearchBound;
+    }
+  }
+
+  // No timers for this thread, return the default.
+  return aDefault;
+}
+
+// This function must be called from within a lock
+// Also: we hold the mutex for the nsTimerImpl.
+bool TimerThread::AddTimerInternal(nsTimerImpl& aTimer) {
+  mMonitor.AssertCurrentThreadOwns();
+  aTimer.mMutex.AssertCurrentThreadOwns();
+  AUTO_TIMERS_STATS(TimerThread_AddTimerInternal);
+  if (mShutdown) {
+    return false;
+  }
+
+  LogTimerEvent::LogDispatch(&aTimer);
+
+  const TimeStamp& timeout = aTimer.mTimeout;
+  const size_t insertionIndex = ComputeTimerInsertionIndex(timeout);
+
+  if (insertionIndex != 0 && !mTimers[insertionIndex - 1].Value()) {
+    // Very common scenario in practice: The timer just before the insertion
+    // point is canceled, overwrite it.
+    AUTO_TIMERS_STATS(TimerThread_AddTimerInternal_overwrite_before);
+    mTimers[insertionIndex - 1] = Entry{aTimer};
+    return true;
+  }
+
+  const size_t length = mTimers.Length();
+  if (insertionIndex == length) {
+    // We're at the end (including it's the very first insertion), add new timer
+    // at the end.
+    AUTO_TIMERS_STATS(TimerThread_AddTimerInternal_append);
+    return mTimers.AppendElement(Entry{aTimer}, mozilla::fallible);
+  }
+
+  if (!mTimers[insertionIndex].Value()) {
+    // The timer at the insertion point is canceled, overwrite it.
+    AUTO_TIMERS_STATS(TimerThread_AddTimerInternal_overwrite);
+    mTimers[insertionIndex] = Entry{aTimer};
+    return true;
+  }
+
+  // The new timer has to be inserted.
+  AUTO_TIMERS_STATS(TimerThread_AddTimerInternal_insert);
+  // The capacity should be checked first, because if it needs to be increased
+  // and the memory allocation fails, only the new timer should be lost.
+  if (length == mTimers.Capacity() && mTimers[length - 1].Value()) {
+    // We have reached capacity, and the last entry is not canceled, so we
+    // really want to increase the capacity in case the extra slot is required.
+    // To force-expand the array, append a canceled-timer entry with a timestamp
+    // far in the future.
+    // This empty Entry may be used below to receive the moved-from previous
+    // entry. If not, it may be used in a later call if we need to append a new
+    // timer at the end.
+    AUTO_TIMERS_STATS(TimerThread_AddTimerInternal_insert_expand);
+    if (!mTimers.AppendElement(
+            Entry{mTimers[length - 1].Timeout() +
+                  TimeDuration::FromSeconds(365.0 * 24.0 * 60.0 * 60.0)},
+            mozilla::fallible)) {
+      return false;
+    }
+  }
+
+  // Extract the timer at the insertion point, and put the new timer in its
+  // place.
+  Entry extractedEntry = std::exchange(mTimers[insertionIndex], Entry{aTimer});
+  // Following entries can be pushed until we hit a canceled timer or the end.
+  for (size_t i = insertionIndex + 1; i < length; ++i) {
+    Entry& entryRef = mTimers[i];
+    if (!entryRef.Value()) {
+      // Canceled entry, overwrite it with the extracted entry from before.
+      COUNT_TIMERS_STATS(TimerThread_AddTimerInternal_insert_overwrite);
+      entryRef = std::move(extractedEntry);
+      return true;
+    }
+    // Write extracted entry from before, and extract current entry.
+    COUNT_TIMERS_STATS(TimerThread_AddTimerInternal_insert_shifts);
+    std::swap(entryRef, extractedEntry);
+  }
+  // We've reached the end of the list, with still one extracted entry to
+  // re-insert. We've checked the capacity above, this cannot fail.
+  COUNT_TIMERS_STATS(TimerThread_AddTimerInternal_insert_append);
+  mTimers.AppendElement(std::move(extractedEntry));
+  return true;
+}
+
+// This function must be called from within a lock
+// Also: we hold the mutex for the nsTimerImpl.
+bool TimerThread::RemoveTimerInternal(nsTimerImpl& aTimer) {
+  mMonitor.AssertCurrentThreadOwns();
+  aTimer.mMutex.AssertCurrentThreadOwns();
+  AUTO_TIMERS_STATS(TimerThread_RemoveTimerInternal);
+  if (!aTimer.IsInTimerThread()) {
+    COUNT_TIMERS_STATS(TimerThread_RemoveTimerInternal_not_in_list);
+    return false;
+  }
+  AUTO_TIMERS_STATS(TimerThread_RemoveTimerInternal_in_list);
+  for (auto& entry : mTimers) {
+    if (entry.Value() == &aTimer) {
+      entry.Forget();
+      return true;
+    }
+  }
+  MOZ_ASSERT(!aTimer.IsInTimerThread(),
+             "Not found in the list but it should be!?");
+  return false;
+}
+
+void TimerThread::RemoveLeadingCanceledTimersInternal() {
+  mMonitor.AssertCurrentThreadOwns();
+  AUTO_TIMERS_STATS(TimerThread_RemoveLeadingCanceledTimersInternal);
+
+  size_t toRemove = 0;
+  while (toRemove < mTimers.Length() && !mTimers[toRemove].Value()) {
+    ++toRemove;
+  }
+  mTimers.RemoveElementsAt(0, toRemove);
+}
+
+void TimerThread::RemoveFirstTimerInternal() {
+  mMonitor.AssertCurrentThreadOwns();
+  AUTO_TIMERS_STATS(TimerThread_RemoveFirstTimerInternal);
+  MOZ_ASSERT(!mTimers.IsEmpty());
+  mTimers.RemoveElementAt(0);
+}
+
+void TimerThread::PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef) {
+  mMonitor.AssertCurrentThreadOwns();
+  AUTO_TIMERS_STATS(TimerThread_PostTimerEvent);
+
+  RefPtr<nsTimerImpl> timer(aTimerRef);
+
+#if TIMER_THREAD_STATISTICS
+  const double actualFiringDelay =
+      std::max((TimeStamp::Now() - timer->mTimeout).ToMilliseconds(), 0.0);
+  if (mNotified) {
+    ++mTotalTimersFiredNotified;
+    mTotalActualTimerFiringDelayNotified += actualFiringDelay;
+  } else {
+    ++mTotalTimersFiredUnnotified;
+    mTotalActualTimerFiringDelayUnnotified += actualFiringDelay;
+  }
+#endif
+
+  if (!timer->mEventTarget) {
+    NS_ERROR("Attempt to post timer event to NULL event target");
+    return;
+  }
+
+  // XXX we may want to reuse this nsTimerEvent in the case of repeating timers.
+
+  // Since we already addref'd 'timer', we don't need to addref here.
+  // We will release either in ~nsTimerEvent(), or pass the reference back to
+  // the caller. We need to copy the generation number from this timer into the
+  // event, so we can avoid firing a timer that was re-initialized after being
+  // canceled.
+
+  nsCOMPtr<nsIEventTarget> target = timer->mEventTarget;
+
+  void* p = nsTimerEvent::operator new(sizeof(nsTimerEvent));
+  if (!p) {
+    return;
+  }
+  RefPtr<nsTimerEvent> event =
+      ::new (KnownNotNull, p) nsTimerEvent(timer.forget(), mProfilerThreadId);
+
+  nsresult rv;
+  {
+    // We release mMonitor around the Dispatch because if the Dispatch interacts
+    // with the timer API we'll deadlock.
+    MonitorAutoUnlock unlock(mMonitor);
+    rv = target->Dispatch(event, NS_DISPATCH_NORMAL);
+    if (NS_FAILED(rv)) {
+      timer = event->ForgetTimer();
+      // We do this to avoid possible deadlock by taking the two locks in a
+      // different order than is used in RemoveTimer().  RemoveTimer() has
+      // aTimer->mMutex first.   We use timer.get() to keep static analysis
+      // happy
+      // NOTE: I'm not sure that any of the below is actually necessary. It
+      // seems to me that the timer that we're trying to fire will have already
+      // been removed prior to this.
+      MutexAutoLock lock1(timer.get()->mMutex);
+      MonitorAutoLock lock2(mMonitor);
+      RemoveTimerInternal(*timer);
+    }
+  }
+}
+
+void TimerThread::DoBeforeSleep() {
+  // Mainthread
+  MonitorAutoLock lock(mMonitor);
+  mSleeping = true;
+}
+
+// Note: wake may be notified without preceding sleep notification
+void TimerThread::DoAfterSleep() {
+  // Mainthread
+  MonitorAutoLock lock(mMonitor);
+  mSleeping = false;
+
+  // Wake up the timer thread to re-process the array to ensure the sleep delay
+  // is correct, and fire any expired timers (perhaps quite a few)
+  mNotified = true;
+  PROFILER_MARKER_UNTYPED("AfterSleep", OTHER,
+                          MarkerThreadId(mProfilerThreadId));
+  mMonitor.Notify();
+}
+
+NS_IMETHODIMP
+TimerThread::Observe(nsISupports* aSubject, const char* aTopic,
+                     const char16_t* aData) {
+  if (strcmp(aTopic, "ipc:process-priority-changed") == 0) {
+    nsCOMPtr<nsIPropertyBag2> props = do_QueryInterface(aSubject);
+    MOZ_ASSERT(props != nullptr);
+
+    int32_t priority = static_cast<int32_t>(hal::PROCESS_PRIORITY_UNKNOWN);
+    props->GetPropertyAsInt32(u"priority"_ns, &priority);
+    mCachedPriority.store(static_cast<hal::ProcessPriority>(priority),
+                          std::memory_order_relaxed);
+  }
+
+  if (StaticPrefs::timer_ignore_sleep_wake_notifications()) {
+    return NS_OK;
+  }
+
+  if (strcmp(aTopic, "sleep_notification") == 0 ||
+      strcmp(aTopic, "suspend_process_notification") == 0) {
+    DoBeforeSleep();
+  } else if (strcmp(aTopic, "wake_notification") == 0 ||
+             strcmp(aTopic, "resume_process_notification") == 0) {
+    DoAfterSleep();
+  }
+
+  return NS_OK;
+}
+
+uint32_t TimerThread::AllowedEarlyFiringMicroseconds() {
+  MonitorAutoLock lock(mMonitor);
+  return mAllowedEarlyFiringMicroseconds;
+}
+
+#if TIMER_THREAD_STATISTICS
+void TimerThread::PrintStatistics() const {
+  mMonitor.AssertCurrentThreadOwns();
+
+  const TimeStamp freshNow = TimeStamp::Now();
+  const double timeElapsed = mFirstTimerAdded.IsNull()
+                                 ? 0.0
+                                 : (freshNow - mFirstTimerAdded).ToSeconds();
+  printf_stderr("TimerThread Stats (Total time %8.2fs)\n", timeElapsed);
+
+  printf_stderr("Added: %6llu Removed: %6llu Fired: %6llu\n", mTotalTimersAdded,
+                mTotalTimersRemoved,
+                mTotalTimersFiredNotified + mTotalTimersFiredUnnotified);
+
+  auto PrintTimersFiredBucket =
+      [](const AutoTArray<size_t, sTimersFiredPerWakeupBucketCount>& buckets,
+         const size_t wakeupCount, const size_t timersFiredCount,
+         const double totalTimerDelay, const char* label) {
+        printf_stderr("%s : [", label);
+        for (size_t bucketVal : buckets) {
+          printf_stderr(" %5llu", bucketVal);
+        }
+        printf_stderr(
+            " ] Wake-ups/timer %6llu / %6llu (%7.4f) Avg Timer Delay %7.4f\n",
+            wakeupCount, timersFiredCount,
+            static_cast<double>(wakeupCount) / timersFiredCount,
+            totalTimerDelay / timersFiredCount);
+      };
+
+  printf_stderr("Wake-ups:\n");
+  PrintTimersFiredBucket(
+      mTimersFiredPerWakeup, mTotalWakeupCount,
+      mTotalTimersFiredNotified + mTotalTimersFiredUnnotified,
+      mTotalActualTimerFiringDelayNotified +
+          mTotalActualTimerFiringDelayUnnotified,
+      "Total      ");
+  PrintTimersFiredBucket(mTimersFiredPerNotifiedWakeup,
+                         mTotalNotifiedWakeupCount, mTotalTimersFiredNotified,
+                         mTotalActualTimerFiringDelayNotified, "Notified   ");
+  PrintTimersFiredBucket(mTimersFiredPerUnnotifiedWakeup,
+                         mTotalUnnotifiedWakeupCount,
+                         mTotalTimersFiredUnnotified,
+                         mTotalActualTimerFiringDelayUnnotified, "Unnotified ");
+
+  printf_stderr("Early Wake-ups: %6llu Avg: %7.4fms\n", mEarlyWakeups,
+                mTotalEarlyWakeupTime / mEarlyWakeups);
+}
+#endif
+
+/* This nsReadOnlyTimer class is used for the values returned by the
+ * TimerThread::GetTimers method.
+ * It is not possible to return a strong reference to the nsTimerImpl
+ * instance (that could extend the lifetime of the timer and cause it to fire
+ * a callback pointing to already freed memory) or a weak reference
+ * (nsSupportsWeakReference doesn't support freeing the referee on a thread
+ * that isn't the thread that owns the weak reference), so instead the timer
+ * name, delay and type are copied to a new object. */
+class nsReadOnlyTimer final : public nsITimer {
+ public:
+  explicit nsReadOnlyTimer(const nsACString& aName, uint32_t aDelay,
+                           uint32_t aType)
+      : mName(aName), mDelay(aDelay), mType(aType) {}
+  NS_DECL_ISUPPORTS
+
+  NS_IMETHOD Init(nsIObserver* aObserver, uint32_t aDelayInMs,
+                  uint32_t aType) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD InitWithCallback(nsITimerCallback* aCallback, uint32_t aDelayInMs,
+                              uint32_t aType) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD InitHighResolutionWithCallback(nsITimerCallback* aCallback,
+                                            const mozilla::TimeDuration& aDelay,
+                                            uint32_t aType) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD Cancel(void) override { return NS_ERROR_NOT_IMPLEMENTED; }
+  NS_IMETHOD InitWithNamedFuncCallback(nsTimerCallbackFunc aCallback,
+                                       void* aClosure, uint32_t aDelay,
+                                       uint32_t aType,
+                                       const char* aName) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD InitHighResolutionWithNamedFuncCallback(
+      nsTimerCallbackFunc aCallback, void* aClosure,
+      const mozilla::TimeDuration& aDelay, uint32_t aType,
+      const char* aName) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD GetName(nsACString& aName) override {
+    aName = mName;
+    return NS_OK;
+  }
+  NS_IMETHOD GetDelay(uint32_t* aDelay) override {
+    *aDelay = mDelay;
+    return NS_OK;
+  }
+  NS_IMETHOD SetDelay(uint32_t aDelay) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD GetType(uint32_t* aType) override {
+    *aType = mType;
+    return NS_OK;
+  }
+  NS_IMETHOD SetType(uint32_t aType) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD GetClosure(void** aClosure) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD GetCallback(nsITimerCallback** aCallback) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD GetTarget(nsIEventTarget** aTarget) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD SetTarget(nsIEventTarget* aTarget) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  NS_IMETHOD GetAllowedEarlyFiringMicroseconds(
+      uint32_t* aAllowedEarlyFiringMicroseconds) override {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) override {
+    return sizeof(*this);
+  }
+
+ private:
+  nsCString mName;
+  uint32_t mDelay;
+  uint32_t mType;
+  ~nsReadOnlyTimer() = default;
+};
+
+NS_IMPL_ISUPPORTS(nsReadOnlyTimer, nsITimer)
+
+nsresult TimerThread::GetTimers(nsTArray<RefPtr<nsITimer>>& aRetVal) {
+  nsTArray<RefPtr<nsTimerImpl>> timers;
+  {
+    MonitorAutoLock lock(mMonitor);
+    for (const auto& entry : mTimers) {
+      nsTimerImpl* timer = entry.Value();
+      if (!timer) {
+        continue;
+      }
+      timers.AppendElement(timer);
+    }
+  }
+
+  for (nsTimerImpl* timer : timers) {
+    nsAutoCString name;
+    timer->GetName(name);
+
+    uint32_t delay;
+    timer->GetDelay(&delay);
+
+    uint32_t type;
+    timer->GetType(&type);
+
+    aRetVal.AppendElement(new nsReadOnlyTimer(name, delay, type));
+  }
+
+  return NS_OK;
+}
diff --git a/xpcom/threads/TimerThread.h b/xpcom/threads/TimerThread.h
new file mode 100644
index 0000000000..f6a5827b94
--- /dev/null
+++ b/xpcom/threads/TimerThread.h
@@ -0,0 +1,247 @@
+/* -*- 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 TimerThread_h___
+#define TimerThread_h___
+
+#include "nsIObserver.h"
+#include "nsIRunnable.h"
+#include "nsIThread.h"
+
+#include "nsTimerImpl.h"
+#include "nsThreadUtils.h"
+
+#include "nsTArray.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/HalTypes.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/ProfilerUtils.h"
+
+// Enable this to compute lots of interesting statistics and print them out when
+// PrintStatistics() is called.
+#define TIMER_THREAD_STATISTICS 0
+
+class TimerThread final : public mozilla::Runnable, public nsIObserver {
+ public:
+  typedef mozilla::Monitor Monitor;
+  typedef mozilla::MutexAutoLock MutexAutoLock;
+  typedef mozilla::TimeStamp TimeStamp;
+  typedef mozilla::TimeDuration TimeDuration;
+
+  TimerThread();
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSIOBSERVER
+
+  nsresult Shutdown();
+
+  nsresult AddTimer(nsTimerImpl* aTimer, const MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(aTimer->mMutex);
+  nsresult RemoveTimer(nsTimerImpl* aTimer, const MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(aTimer->mMutex);
+  // Considering only the first 'aSearchBound' timers (in firing order), returns
+  // the timeout of the first non-low-priority timer, on the current thread,
+  // that will fire before 'aDefault'. If no such timer exists, 'aDefault' is
+  // returned.
+  TimeStamp FindNextFireTimeForCurrentThread(TimeStamp aDefault,
+                                             uint32_t aSearchBound);
+
+  void DoBeforeSleep();
+  void DoAfterSleep();
+
+  bool IsOnTimerThread() const { return mThread->IsOnCurrentThread(); }
+
+  uint32_t AllowedEarlyFiringMicroseconds();
+  nsresult GetTimers(nsTArray<RefPtr<nsITimer>>& aRetVal);
+
+ private:
+  ~TimerThread();
+
+  bool mInitialized;
+
+  // These internal helper methods must be called while mMonitor is held.
+  // AddTimerInternal returns false if the insertion failed.
+  bool AddTimerInternal(nsTimerImpl& aTimer) MOZ_REQUIRES(mMonitor);
+  bool RemoveTimerInternal(nsTimerImpl& aTimer)
+      MOZ_REQUIRES(mMonitor, aTimer.mMutex);
+  void RemoveLeadingCanceledTimersInternal() MOZ_REQUIRES(mMonitor);
+  void RemoveFirstTimerInternal() MOZ_REQUIRES(mMonitor);
+  nsresult Init() MOZ_REQUIRES(mMonitor);
+
+  void PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef)
+      MOZ_REQUIRES(mMonitor);
+
+  // Using atomic because this value is written to in one place, and read from
+  // in another, and those two locations are likely to be executed from separate
+  // threads. Reads/writes to an aligned value this size should be atomic even
+  // without using std::atomic, but doing this explicitly provides a good
+  // reminder that this is accessed from multiple threads.
+  std::atomic<mozilla::hal::ProcessPriority> mCachedPriority =
+      mozilla::hal::PROCESS_PRIORITY_UNKNOWN;
+
+  nsCOMPtr<nsIThread> mThread;
+  // Lock ordering requirements:
+  // (optional) ThreadWrapper::sMutex ->
+  // (optional) nsTimerImpl::mMutex   ->
+  // TimerThread::mMonitor
+  Monitor mMonitor;
+
+  bool mShutdown MOZ_GUARDED_BY(mMonitor);
+  bool mWaiting MOZ_GUARDED_BY(mMonitor);
+  bool mNotified MOZ_GUARDED_BY(mMonitor);
+  bool mSleeping MOZ_GUARDED_BY(mMonitor);
+
+  class Entry final {
+   public:
+    explicit Entry(nsTimerImpl& aTimerImpl)
+        : mTimeout(aTimerImpl.mTimeout),
+          mDelay(aTimerImpl.mDelay),
+          mTimerImpl(&aTimerImpl) {
+      aTimerImpl.SetIsInTimerThread(true);
+    }
+
+    // Create an already-canceled entry with the given timeout.
+    explicit Entry(TimeStamp aTimeout)
+        : mTimeout(std::move(aTimeout)), mTimerImpl(nullptr) {}
+
+    // Don't allow copies, otherwise which one would manage `IsInTimerThread`?
+    Entry(const Entry&) = delete;
+    Entry& operator=(const Entry&) = delete;
+
+    // Move-only.
+    Entry(Entry&&) = default;
+    Entry& operator=(Entry&&) = default;
+
+    ~Entry() {
+      if (mTimerImpl) {
+        mTimerImpl->mMutex.AssertCurrentThreadOwns();
+        mTimerImpl->SetIsInTimerThread(false);
+      }
+    }
+
+    nsTimerImpl* Value() const { return mTimerImpl; }
+
+    void Forget() {
+      if (MOZ_UNLIKELY(!mTimerImpl)) {
+        return;
+      }
+      mTimerImpl->mMutex.AssertCurrentThreadOwns();
+      mTimerImpl->SetIsInTimerThread(false);
+      mTimerImpl = nullptr;
+    }
+
+    // Called with the Monitor held, but not the TimerImpl's mutex
+    already_AddRefed<nsTimerImpl> Take() {
+      if (MOZ_LIKELY(mTimerImpl)) {
+        MOZ_ASSERT(mTimerImpl->IsInTimerThread());
+        mTimerImpl->SetIsInTimerThread(false);
+      }
+      return mTimerImpl.forget();
+    }
+
+    const TimeStamp& Timeout() const { return mTimeout; }
+    const TimeDuration& Delay() const { return mDelay; }
+
+   private:
+    // These values are simply cached from the timer. Keeping them here is good
+    // for cache usage and allows us to avoid worrying about locking conflicts
+    // with the timer.
+    TimeStamp mTimeout;
+    TimeDuration mDelay;
+
+    RefPtr<nsTimerImpl> mTimerImpl;
+  };
+
+  // Computes and returns the index in mTimers at which a new timer with the
+  // specified timeout should be inserted in order to maintain "sorted" order.
+  size_t ComputeTimerInsertionIndex(const TimeStamp& timeout) const
+      MOZ_REQUIRES(mMonitor);
+
+  // Computes and returns when we should next try to wake up in order to handle
+  // the triggering of the timers in mTimers. Currently this is very simple and
+  // we always just plan to wake up for the next timer in the list. In the
+  // future this will be more sophisticated.
+  TimeStamp ComputeWakeupTimeFromTimers() const MOZ_REQUIRES(mMonitor);
+
+  // Computes how late a timer can acceptably fire.
+  // timerDuration is the duration of the timer whose delay we are calculating.
+  // Longer timers can tolerate longer firing delays.
+  // minDelay is an amount by which any timer can be delayed.
+  // This function will never return a value smaller than minDelay (unless this
+  // conflicts with maxDelay). maxDelay is the upper limit on the amount by
+  // which we will ever delay any timer. Takes precedence over minDelay if there
+  // is a conflict. (Zero will effectively disable timer coalescing.)
+  TimeDuration ComputeAcceptableFiringDelay(TimeDuration timerDuration,
+                                            TimeDuration minDelay,
+                                            TimeDuration maxDelay) const;
+
+#ifdef XP_WIN
+  UINT ComputeDesiredTimerPeriod() const;
+#endif
+
+#ifdef DEBUG
+  // Checks mTimers to see if any entries are out of order or any cached
+  // timeouts are incorrect and will assert if any inconsistency is found. Has
+  // no side effects other than asserting so has no use in non-DEBUG builds.
+  void VerifyTimerListConsistency() const MOZ_REQUIRES(mMonitor);
+#endif
+
+  // mTimers is maintained in a "pseudo-sorted" order wrt the timeouts.
+  // Specifcally, mTimers is sorted according to the timeouts *if you ignore the
+  // canceled entries* (those whose mTimerImpl is nullptr). Notably this means
+  // that you cannot use a binary search on this list.
+  nsTArray<Entry> mTimers MOZ_GUARDED_BY(mMonitor);
+
+  // Set only at the start of the thread's Run():
+  uint32_t mAllowedEarlyFiringMicroseconds MOZ_GUARDED_BY(mMonitor);
+
+  ProfilerThreadId mProfilerThreadId MOZ_GUARDED_BY(mMonitor);
+
+  // Time at which we were intending to wake up the last time that we slept.
+  // Is "null" if we have never slept or if our last sleep was "forever".
+  TimeStamp mIntendedWakeupTime;
+
+#if TIMER_THREAD_STATISTICS
+  static constexpr size_t sTimersFiredPerWakeupBucketCount = 16;
+  static inline constexpr std::array<size_t, sTimersFiredPerWakeupBucketCount>
+      sTimersFiredPerWakeupThresholds = {
+          0, 1, 2, 3, 4, 5, 6, 7, 8, 12, 20, 30, 40, 50, 70, (size_t)(-1)};
+
+  mutable AutoTArray<size_t, sTimersFiredPerWakeupBucketCount>
+      mTimersFiredPerWakeup MOZ_GUARDED_BY(mMonitor) = {0, 0, 0, 0, 0, 0, 0, 0,
+                                                        0, 0, 0, 0, 0, 0, 0, 0};
+  mutable AutoTArray<size_t, sTimersFiredPerWakeupBucketCount>
+      mTimersFiredPerUnnotifiedWakeup MOZ_GUARDED_BY(mMonitor) = {
+          0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+  mutable AutoTArray<size_t, sTimersFiredPerWakeupBucketCount>
+      mTimersFiredPerNotifiedWakeup MOZ_GUARDED_BY(mMonitor) = {
+          0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+
+  mutable size_t mTotalTimersAdded MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable size_t mTotalTimersRemoved MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable size_t mTotalTimersFiredNotified MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable size_t mTotalTimersFiredUnnotified MOZ_GUARDED_BY(mMonitor) = 0;
+
+  mutable size_t mTotalWakeupCount MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable size_t mTotalUnnotifiedWakeupCount MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable size_t mTotalNotifiedWakeupCount MOZ_GUARDED_BY(mMonitor) = 0;
+
+  mutable double mTotalActualTimerFiringDelayNotified MOZ_GUARDED_BY(mMonitor) =
+      0.0;
+  mutable double mTotalActualTimerFiringDelayUnnotified
+      MOZ_GUARDED_BY(mMonitor) = 0.0;
+
+  mutable TimeStamp mFirstTimerAdded MOZ_GUARDED_BY(mMonitor);
+
+  mutable size_t mEarlyWakeups MOZ_GUARDED_BY(mMonitor) = 0;
+  mutable double mTotalEarlyWakeupTime MOZ_GUARDED_BY(mMonitor) = 0.0;
+
+  void PrintStatistics() const;
+#endif
+};
+#endif /* TimerThread_h___ */
diff --git a/xpcom/threads/VsyncTaskManager.cpp b/xpcom/threads/VsyncTaskManager.cpp
new file mode 100644
index 0000000000..ba4201af45
--- /dev/null
+++ b/xpcom/threads/VsyncTaskManager.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 "VsyncTaskManager.h"
+#include "InputTaskManager.h"
+
+namespace mozilla {
+
+StaticRefPtr<VsyncTaskManager> VsyncTaskManager::gHighPriorityTaskManager;
+
+void VsyncTaskManager::Init() {
+  gHighPriorityTaskManager = new VsyncTaskManager();
+}
+
+void VsyncTaskManager::WillRunTask() {
+  TaskManager::WillRunTask();
+  InputTaskManager::Get()->NotifyVsync();
+};
+}  // namespace mozilla
diff --git a/xpcom/threads/VsyncTaskManager.h b/xpcom/threads/VsyncTaskManager.h
new file mode 100644
index 0000000000..e284ebf47b
--- /dev/null
+++ b/xpcom/threads/VsyncTaskManager.h
@@ -0,0 +1,26 @@
+/* -*- 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_VsyncTaskManager_h
+#define mozilla_VsyncTaskManager_h
+
+#include "TaskController.h"
+#include "mozilla/StaticPtr.h"
+
+namespace mozilla {
+class VsyncTaskManager : public TaskManager {
+ public:
+  static VsyncTaskManager* Get() { return gHighPriorityTaskManager.get(); }
+  static void Cleanup() { gHighPriorityTaskManager = nullptr; }
+  static void Init();
+
+  void WillRunTask() override;
+
+ private:
+  static StaticRefPtr<VsyncTaskManager> gHighPriorityTaskManager;
+};
+}  // namespace mozilla
+#endif
diff --git a/xpcom/threads/WinHandleWatcher.cpp b/xpcom/threads/WinHandleWatcher.cpp
new file mode 100644
index 0000000000..d475993925
--- /dev/null
+++ b/xpcom/threads/WinHandleWatcher.cpp
@@ -0,0 +1,306 @@
+/* -*- 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 <threadpoolapiset.h>
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/ThreadSafety.h"
+#include "mozilla/WinHandleWatcher.h"
+
+#include "nsCOMPtr.h"
+#include "nsIRunnable.h"
+#include "nsISerialEventTarget.h"
+#include "nsISupportsImpl.h"
+#include "nsITargetShutdownTask.h"
+#include "nsIWeakReferenceUtils.h"
+#include "nsThreadUtils.h"
+
+mozilla::LazyLogModule sHWLog("HandleWatcher");
+
+namespace mozilla {
+namespace details {
+struct WaitHandleDeleter {
+  void operator()(PTP_WAIT waitHandle) {
+    MOZ_LOG(sHWLog, LogLevel::Debug, ("Closing PTP_WAIT %p", waitHandle));
+    ::CloseThreadpoolWait(waitHandle);
+  }
+};
+}  // namespace details
+using WaitHandlePtr = UniquePtr<TP_WAIT, details::WaitHandleDeleter>;
+
+// HandleWatcher::Impl
+//
+// The backing implementation of HandleWatcher is a PTP_WAIT, an OS-threadpool
+// wait-object. Windows doesn't actually create a new thread per wait-object;
+// OS-threadpool threads are assigned to wait-objects only when their associated
+// handle become signaled -- although explicit documentation of this fact is
+// somewhat obscurely placed. [1]
+//
+// Throughout this class, we use manual locking and unlocking guarded by Clang's
+// thread-safety warnings, rather than scope-based lock-guards. See `Replace()`
+// for an explanation and justification.
+//
+// [1]https://learn.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitformultipleobjects#remarks
+class HandleWatcher::Impl final : public nsITargetShutdownTask {
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+ public:
+  Impl() = default;
+
+ private:
+  ~Impl() { MOZ_ASSERT(IsStopped()); }
+
+  struct Data {
+    // The watched handle and its callback.
+    HANDLE handle;
+    RefPtr<nsIEventTarget> target;
+    nsCOMPtr<nsIRunnable> runnable;
+
+    // Handle to the threadpool wait-object.
+    WaitHandlePtr waitHandle;
+    // A pointer to ourselves, notionally owned by the wait-object.
+    RefPtr<Impl> self;
+
+    // (We can't actually do this because a) it has annoying consequences in
+    // C++20 thanks to P1008R1, and b) Clang just ignores it anyway.)
+    //
+    // ~Data() MOZ_EXCLUDES(mMutex) = default;
+  };
+
+  mozilla::Mutex mMutex{"HandleWatcher::Impl"};
+  Data mData MOZ_GUARDED_BY(mMutex) = {};
+
+  // Callback from OS threadpool wait-object.
+  static void CALLBACK WaitCallback(PTP_CALLBACK_INSTANCE, void* ctx,
+                                    PTP_WAIT aWaitHandle,
+                                    TP_WAIT_RESULT aResult) {
+    static_cast<Impl*>(ctx)->OnWaitCompleted(aWaitHandle, aResult);
+  }
+
+  void OnWaitCompleted(PTP_WAIT aWaitHandle, TP_WAIT_RESULT aResult)
+      MOZ_EXCLUDES(mMutex) {
+    MOZ_ASSERT(aResult == WAIT_OBJECT_0);
+
+    mMutex.Lock();
+    // If this callback is no longer the active callback, skip out.
+    // All cleanup is someone else's problem.
+    if (aWaitHandle != mData.waitHandle.get()) {
+      MOZ_LOG(sHWLog, LogLevel::Debug,
+              ("Recv'd already-stopped callback: HW %p | PTP_WAIT %p", this,
+               aWaitHandle));
+      mMutex.Unlock();
+      return;
+    }
+
+    // Take our self-pointer so that we release it on exit.
+    RefPtr<Impl> self = std::move(mData.self);
+
+    MOZ_LOG(sHWLog, LogLevel::Info,
+            ("Recv'd callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+             this, mData.handle, mData.target.get(), aWaitHandle));
+
+    // This may fail if (for example) `mData.target` is being shut down, but we
+    // have not yet received the shutdown callback.
+    mData.target->Dispatch(mData.runnable.forget());
+    Replace(Data{});
+  }
+
+ public:
+  static RefPtr<Impl> Create(HANDLE aHandle, nsIEventTarget* aTarget,
+                             already_AddRefed<nsIRunnable> aRunnable) {
+    auto impl = MakeRefPtr<Impl>();
+    bool const ok [[maybe_unused]] =
+        impl->Watch(aHandle, aTarget, std::move(aRunnable));
+    MOZ_ASSERT(ok);
+    return impl;
+  }
+
+ private:
+  bool Watch(HANDLE aHandle, nsIEventTarget* aTarget,
+             already_AddRefed<nsIRunnable> aRunnable) MOZ_EXCLUDES(mMutex) {
+    MOZ_ASSERT(aHandle);
+    MOZ_ASSERT(aTarget);
+
+    RefPtr<nsIEventTarget> target(aTarget);
+
+    WaitHandlePtr waitHandle{
+        ::CreateThreadpoolWait(&WaitCallback, this, nullptr)};
+    if (!waitHandle) {
+      return false;
+    }
+
+    {
+      mMutex.Lock();
+
+      nsresult const ret = aTarget->RegisterShutdownTask(this);
+      if (NS_FAILED(ret)) {
+        mMutex.Unlock();
+        return false;
+      }
+
+      MOZ_LOG(sHWLog, LogLevel::Info,
+              ("Setting callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+               this, aHandle, aTarget, waitHandle.get()));
+
+      // returns `void`; presumably always succeeds given a successful
+      // `::CreateThreadpoolWait()`
+      ::SetThreadpoolWait(waitHandle.get(), aHandle, nullptr);
+      // After this point, you must call `FlushWaitHandle(waitHandle.get())`
+      // before destroying the wait handle. (Note that this must be done while
+      // *not* holding `mMutex`!)
+
+      Replace(Data{.handle = aHandle,
+                   .target = std::move(target),
+                   .runnable = aRunnable,
+                   .waitHandle = std::move(waitHandle),
+                   .self = this});
+    }
+
+    return true;
+  }
+
+  void TargetShutdown() MOZ_EXCLUDES(mMutex) override final {
+    mMutex.Lock();
+
+    MOZ_LOG(sHWLog, LogLevel::Debug,
+            ("Target shutdown: HW %p | handle %p | target %p | PTP_WAIT %p",
+             this, mData.handle, mData.target.get(), mData.waitHandle.get()));
+
+    // Clear mData.target, since there's no need to unregister the shutdown task
+    // anymore. Hold onto it until we release the mutex, though, to avoid any
+    // reentrancy issues.
+    //
+    // This is more for internal consistency than safety: someone has to be
+    // shutting `target` down, and that someone isn't us, so there's necessarily
+    // another reference out there. (Although decrementing the refcount might
+    // still have arbitrary effects if someone's been excessively clever with
+    // nsISupports::Release...)
+    auto const oldTarget = std::move(mData.target);
+    Replace(Data{});
+    // (Static-assert that the mutex has indeed been released.)
+    ([&]() MOZ_EXCLUDES(mMutex) {})();
+  }
+
+ public:
+  void Stop() MOZ_EXCLUDES(mMutex) {
+    mMutex.Lock();
+    Replace(Data{});
+  }
+
+  bool IsStopped() MOZ_EXCLUDES(mMutex) {
+    mozilla::MutexAutoLock lock(mMutex);
+    return !mData.handle;
+  }
+
+ private:
+  // Throughout this class, we use manual locking and unlocking guarded by
+  // Clang's thread-safety warnings, rather than scope-based lock-guards. This
+  // is largely driven by `Replace()`, below, which performs both operations
+  // which require the mutex to be held and operations which require it to not
+  // be held, and therefore must explicitly sequence the mutex release.
+  //
+  // These explicit locks, unlocks, and annotations are both alien to C++ and
+  // offensively tedious; but they _are_ still checked for state consistency at
+  // scope boundaries. (The concerned reader is invited to test this by
+  // deliberately removing an `mMutex.Unlock()` call from anywhere in the class
+  // and viewing the resultant compiler diagnostics.)
+  //
+  // A more principled, or at least differently-principled, implementation might
+  // create a scope-based lock-guard and pass it to `Replace()` to dispose of at
+  // the proper time. Alas, it cannot be communicated to Clang's thread-safety
+  // checker that such a guard is associated with `mMutex`.
+  //
+  void Replace(Data&& aData) MOZ_CAPABILITY_RELEASE(mMutex) {
+    // either both handles are NULL, or neither is
+    MOZ_ASSERT(!!aData.handle == !!aData.waitHandle);
+
+    if (mData.handle) {
+      MOZ_LOG(sHWLog, LogLevel::Info,
+              ("Stop callback: HW %p | handle %p | target %p | PTP_WAIT %p",
+               this, mData.handle, mData.target.get(), mData.waitHandle.get()));
+    }
+
+    if (mData.target) {
+      mData.target->UnregisterShutdownTask(this);
+    }
+
+    // Extract the old data and insert the new -- but hold onto the old data for
+    // now. (See [1] and [2], below.)
+    Data oldData = std::exchange(mData, std::move(aData));
+
+    ////////////////////////////////////////////////////////////////////////////
+    // Release the mutex.
+    mMutex.Unlock();
+    ////////////////////////////////////////////////////////////////////////////
+
+    // [1] `oldData.self` will be unset if the old callback already ran (or if
+    // there was no old callback in the first place). If it's set, though, we
+    // need to explicitly clear out the wait-object first.
+    if (oldData.self) {
+      MOZ_ASSERT(oldData.waitHandle);
+      FlushWaitHandle(oldData.waitHandle.get());
+    }
+
+    // [2] oldData also includes several other reference-counted pointers. It's
+    // possible that these may be the last pointer to something, so releasing
+    // them may have arbitrary side-effects -- like calling this->Stop(), which
+    // will try to reacquire the mutex.
+    //
+    // Now that we've released the mutex, we can (implicitly) release them all
+    // here.
+  }
+
+  // Either confirm as complete or cancel any callbacks on aWaitHandle. Block
+  // until this is done. (See documentation for ::CloseThreadpoolWait().)
+  void FlushWaitHandle(PTP_WAIT aWaitHandle) MOZ_EXCLUDES(mMutex) {
+    ::SetThreadpoolWait(aWaitHandle, nullptr, nullptr);
+    // This might block on `OnWaitCompleted()`, so we can't hold `mMutex` here.
+    ::WaitForThreadpoolWaitCallbacks(aWaitHandle, TRUE);
+    // ::CloseThreadpoolWait() itself is the caller's responsibility.
+  }
+};
+
+NS_IMPL_ISUPPORTS(HandleWatcher::Impl, nsITargetShutdownTask)
+
+//////
+// HandleWatcher member function implementations
+
+HandleWatcher::HandleWatcher() : mImpl{} {}
+HandleWatcher::~HandleWatcher() {
+  if (mImpl) {
+    MOZ_ASSERT(mImpl->IsStopped());
+    mImpl->Stop();  // just in case, in release
+  }
+}
+
+HandleWatcher::HandleWatcher(HandleWatcher&&) noexcept = default;
+HandleWatcher& HandleWatcher::operator=(HandleWatcher&&) noexcept = default;
+
+void HandleWatcher::Watch(HANDLE aHandle, nsIEventTarget* aTarget,
+                          already_AddRefed<nsIRunnable> aRunnable) {
+  auto impl = Impl::Create(aHandle, aTarget, std::move(aRunnable));
+  MOZ_ASSERT(impl);
+
+  if (mImpl) {
+    mImpl->Stop();
+  }
+  mImpl = std::move(impl);
+}
+
+void HandleWatcher::Stop() {
+  if (mImpl) {
+    mImpl->Stop();
+  }
+}
+
+bool HandleWatcher::IsStopped() { return !mImpl || mImpl->IsStopped(); }
+
+}  // namespace mozilla
diff --git a/xpcom/threads/WinHandleWatcher.h b/xpcom/threads/WinHandleWatcher.h
new file mode 100644
index 0000000000..eb1e1c0245
--- /dev/null
+++ b/xpcom/threads/WinHandleWatcher.h
@@ -0,0 +1,117 @@
+/* -*- 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 WinHandleWatcher_h__
+#define WinHandleWatcher_h__
+
+#include <minwindef.h>
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
+
+#include "nsIEventTarget.h"
+#include "nsIRunnable.h"
+#include "nsIThread.h"
+#include "nsThreadUtils.h"
+
+namespace mozilla {
+///////////////////////////////////////////////////////////////////////
+// HandleWatcher
+//
+// Enqueues a task onto an event target when a watched Win32 synchronization
+// object [1] enters the signaled state.
+//
+// The HandleWatcher must be stopped before either it or the synchronization
+// object is destroyed.
+//
+//////
+//
+// Example of use:
+//
+// ```
+// class MyClass {
+//   /* ... */
+//
+//   HANDLE CreateThing();
+//   void OnComplete();
+//  public:
+//   void Fire() {
+//     mHandle.set(CreateThing());
+//     mWatcher.Watch(
+//         mHandle.get(), NS_GetCurrentThread(),  // (or any other thread)
+//         NS_NewRunnableFunction("OnComplete", [this] { OnComplete(); }));
+//   }
+//
+//   ~MyClass() { mWatcher.Stop(); }
+//
+//   HandleWatcher mWatcher;
+//   HandlePtr mHandle;  // calls ::CloseHandle() on destruction
+// };
+// ```
+//
+// Note: this example demonstrates why an explicit `Stop()` is necessary in
+// MyClass's destructor. Without it, the `HandlePtr` would destroy the HANDLE --
+// and possibly whatever other data `OnComplete()` depends on -- before the
+// watch was stopped!
+//
+// Rather than make code correctness silently dependent on member object order,
+// we require that HandleWatcher already be stopped at its destruction time.
+// (This does not guarantee correctness, as the task may still reference a
+// partially-destroyed transitive owner; but, short of RIIR, a guarantee of
+// correctness is probably not possible here.)
+//
+//////
+//
+// [1]https://learn.microsoft.com/en-us/windows/win32/sync/synchronization-objects
+class HandleWatcher {
+ public:
+  class Impl;
+
+  HandleWatcher();
+  ~HandleWatcher();
+
+  HandleWatcher(HandleWatcher const&) = delete;
+  HandleWatcher& operator=(HandleWatcher const&) = delete;
+
+  HandleWatcher(HandleWatcher&&) noexcept;
+  HandleWatcher& operator=(HandleWatcher&&) noexcept;
+
+  // Watches the given Win32 HANDLE, which must be a synchronization object. As
+  // soon as the HANDLE is signaled, posts `aRunnable` to `aTarget`.
+  //
+  // `aHandle` is merely borrowed for the duration of the watch: the
+  // HandleWatcher does not attempt to close it, and its lifetime must exceed
+  // that of the watch.
+  //
+  // If the watch is stopped for any reason other than completion, `aRunnable`
+  // is released immediately, on the same thread from which the Watch was
+  // stopped.
+  //
+  // The watch is stopped when any of the following occurs:
+  //   * `Stop()` is called.
+  //   * `Watch()` is called again, even without an intervening `Stop()`.
+  //   * This object is destroyed.
+  //   * `aTarget` shuts down.
+  //   * `aHandle` becomes signaled.
+  //
+  void Watch(HANDLE aHandle, nsIEventTarget* aTarget,
+             already_AddRefed<nsIRunnable> aRunnable);
+
+  // Cancels the current watch, if any.
+  //
+  // Idempotent. Thread-safe with respect to other calls of `Stop()`.
+  void Stop();
+
+  // Potentially racy. Only intended for tests.
+  bool IsStopped();
+
+ private:
+  RefPtr<Impl> mImpl;
+};
+}  // namespace mozilla
+
+#endif  // WinHandleWatcher_h__
diff --git a/xpcom/threads/components.conf b/xpcom/threads/components.conf
new file mode 100644
index 0000000000..53f76d3b89
--- /dev/null
+++ b/xpcom/threads/components.conf
@@ -0,0 +1,29 @@
+# -*- 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': '{03d68f92-9513-4e25-9be9-7cb239874172}',
+        'contract_ids': ['@mozilla.org/process/environment;1'],
+        'legacy_constructor': 'nsEnvironment::Create',
+        'headers': ['/xpcom/threads/nsEnvironment.h'],
+        'js_name': 'env',
+        'interfaces': ['nsIEnvironment'],
+    },
+    {
+        'cid': '{5ff24248-1dd2-11b2-8427-fbab44f29bc8}',
+        'contract_ids': ['@mozilla.org/timer;1'],
+        'legacy_constructor': 'nsTimer::XPCOMConstructor',
+        'headers': ['/xpcom/threads/nsTimerImpl.h'],
+        'processes': ProcessSelector.ALLOW_IN_GPU_RDD_SOCKET_AND_UTILITY_PROCESS,
+    },
+    {
+        'cid': '{d39a8904-2e09-4a3a-a273-c3bec7db2bfe}',
+        'contract_ids': ['@mozilla.org/timer-manager;1'],
+        'headers': ['/xpcom/threads/nsTimerImpl.h'],
+        'type': 'nsTimerManager',
+    },
+]
diff --git a/xpcom/threads/moz.build b/xpcom/threads/moz.build
new file mode 100644
index 0000000000..06d10ad331
--- /dev/null
+++ b/xpcom/threads/moz.build
@@ -0,0 +1,144 @@
+# -*- 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 += [
+    "nsIDirectTaskDispatcher.idl",
+    "nsIEnvironment.idl",
+    "nsIEventTarget.idl",
+    "nsIIdlePeriod.idl",
+    "nsINamed.idl",
+    "nsIProcess.idl",
+    "nsIRunnable.idl",
+    "nsISerialEventTarget.idl",
+    "nsISupportsPriority.idl",
+    "nsIThread.idl",
+    "nsIThreadInternal.idl",
+    "nsIThreadManager.idl",
+    "nsIThreadPool.idl",
+    "nsIThreadShutdown.idl",
+    "nsITimer.idl",
+]
+
+XPIDL_MODULE = "xpcom_threads"
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+EXPORTS += [
+    "MainThreadUtils.h",
+    "nsICancelableRunnable.h",
+    "nsIDiscardableRunnable.h",
+    "nsIIdleRunnable.h",
+    "nsITargetShutdownTask.h",
+    "nsMemoryPressure.h",
+    "nsProcess.h",
+    "nsProxyRelease.h",
+    "nsThread.h",
+    "nsThreadManager.h",
+    "nsThreadPool.h",
+    "nsThreadUtils.h",
+]
+
+EXPORTS.mozilla += [
+    "AbstractThread.h",
+    "BlockingResourceBase.h",
+    "CondVar.h",
+    "CPUUsageWatcher.h",
+    "DataMutex.h",
+    "DeadlockDetector.h",
+    "DelayedRunnable.h",
+    "EventQueue.h",
+    "EventTargetCapability.h",
+    "IdlePeriodState.h",
+    "IdleTaskRunner.h",
+    "InputTaskManager.h",
+    "LazyIdleThread.h",
+    "MainThreadIdlePeriod.h",
+    "Monitor.h",
+    "MozPromise.h",
+    "Mutex.h",
+    "Queue.h",
+    "RecursiveMutex.h",
+    "ReentrantMonitor.h",
+    "RWLock.h",
+    "SchedulerGroup.h",
+    "SharedThreadPool.h",
+    "SpinEventLoopUntil.h",
+    "StateMirroring.h",
+    "StateWatching.h",
+    "SynchronizedEventQueue.h",
+    "SyncRunnable.h",
+    "TaskController.h",
+    "TaskDispatcher.h",
+    "TaskQueue.h",
+    "ThreadBound.h",
+    "ThreadEventQueue.h",
+    "ThrottledEventQueue.h",
+    "VsyncTaskManager.h",
+]
+
+SOURCES += [
+    "IdlePeriodState.cpp",
+    "IdleTaskRunner.cpp",
+    "ThreadDelay.cpp",
+]
+
+UNIFIED_SOURCES += [
+    "AbstractThread.cpp",
+    "BlockingResourceBase.cpp",
+    "CPUUsageWatcher.cpp",
+    "DelayedRunnable.cpp",
+    "EventQueue.cpp",
+    "InputTaskManager.cpp",
+    "LazyIdleThread.cpp",
+    "MainThreadIdlePeriod.cpp",
+    "nsEnvironment.cpp",
+    "nsMemoryPressure.cpp",
+    "nsProcessCommon.cpp",
+    "nsProxyRelease.cpp",
+    "nsThread.cpp",
+    "nsThreadManager.cpp",
+    "nsThreadPool.cpp",
+    "nsThreadUtils.cpp",
+    "nsTimerImpl.cpp",
+    "RecursiveMutex.cpp",
+    "RWLock.cpp",
+    "SchedulerGroup.cpp",
+    "SharedThreadPool.cpp",
+    "SynchronizedEventQueue.cpp",
+    "TaskController.cpp",
+    "TaskQueue.cpp",
+    "ThreadEventQueue.cpp",
+    "ThreadEventTarget.cpp",
+    "ThreadLocalVariables.cpp",
+    "ThrottledEventQueue.cpp",
+    "TimerThread.cpp",
+    "VsyncTaskManager.cpp",
+]
+
+if CONFIG["OS_ARCH"] == "WINNT":
+    EXPORTS.mozilla += ["WinHandleWatcher.h"]
+    UNIFIED_SOURCES += ["WinHandleWatcher.cpp"]
+
+# Should match the conditions in toolkit/components/backgroundhangmonitor/moz.build
+if (
+    CONFIG["NIGHTLY_BUILD"]
+    and not CONFIG["MOZ_DEBUG"]
+    and not CONFIG["MOZ_TSAN"]
+    and not CONFIG["MOZ_ASAN"]
+):
+    DEFINES["MOZ_ENABLE_BACKGROUND_HANG_MONITOR"] = 1
+
+LOCAL_INCLUDES += [
+    "../build",
+    "/caps",
+    "/tools/profiler",
+]
+
+FINAL_LIBRARY = "xul"
+
+include("/ipc/chromium/chromium-config.mozbuild")
diff --git a/xpcom/threads/nsEnvironment.cpp b/xpcom/threads/nsEnvironment.cpp
new file mode 100644
index 0000000000..54efd9194a
--- /dev/null
+++ b/xpcom/threads/nsEnvironment.cpp
@@ -0,0 +1,136 @@
+/* -*- 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 "nsEnvironment.h"
+#include "prenv.h"
+#include "nsBaseHashtable.h"
+#include "nsHashKeys.h"
+#include "nsPromiseFlatString.h"
+#include "nsDependentString.h"
+#include "nsNativeCharsetUtils.h"
+#include "mozilla/Printf.h"
+#include "mozilla/StaticMutex.h"
+
+using namespace mozilla;
+
+NS_IMPL_ISUPPORTS(nsEnvironment, nsIEnvironment)
+
+nsresult nsEnvironment::Create(REFNSIID aIID, void** aResult) {
+  nsresult rv;
+  *aResult = nullptr;
+
+  nsEnvironment* obj = new nsEnvironment();
+
+  rv = obj->QueryInterface(aIID, aResult);
+  if (NS_FAILED(rv)) {
+    delete obj;
+  }
+  return rv;
+}
+
+NS_IMETHODIMP
+nsEnvironment::Exists(const nsAString& aName, bool* aOutValue) {
+  nsAutoCString nativeName;
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsAutoCString nativeVal;
+#if defined(XP_UNIX)
+  /* For Unix/Linux platforms we follow the Unix definition:
+   * An environment variable exists when |getenv()| returns a non-nullptr
+   * value. An environment variable does not exist when |getenv()| returns
+   * nullptr.
+   */
+  const char* value = PR_GetEnv(nativeName.get());
+  *aOutValue = value && *value;
+#else
+  /* For non-Unix/Linux platforms we have to fall back to a
+   * "portable" definition (which is incorrect for Unix/Linux!!!!)
+   * which simply checks whether the string returned by |Get()| is empty
+   * or not.
+   */
+  nsAutoString value;
+  Get(aName, value);
+  *aOutValue = !value.IsEmpty();
+#endif /* XP_UNIX */
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsEnvironment::Get(const nsAString& aName, nsAString& aOutValue) {
+  nsAutoCString nativeName;
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  nsAutoCString nativeVal;
+  const char* value = PR_GetEnv(nativeName.get());
+  if (value && *value) {
+    rv = NS_CopyNativeToUnicode(nsDependentCString(value), aOutValue);
+  } else {
+    aOutValue.Truncate();
+    rv = NS_OK;
+  }
+
+  return rv;
+}
+
+/* Environment strings must have static duration; We're gonna leak all of this
+ * at shutdown: this is by design, caused how Unix/Linux implement environment
+ * vars.
+ */
+
+typedef nsBaseHashtableET<nsCharPtrHashKey, char*> EnvEntryType;
+typedef nsTHashtable<EnvEntryType> EnvHashType;
+
+static StaticMutex gEnvHashMutex;
+static EnvHashType* gEnvHash MOZ_GUARDED_BY(gEnvHashMutex) = nullptr;
+
+static EnvHashType* EnsureEnvHash() MOZ_REQUIRES(gEnvHashMutex) {
+  if (!gEnvHash) {
+    gEnvHash = new EnvHashType;
+  }
+  return gEnvHash;
+}
+
+NS_IMETHODIMP
+nsEnvironment::Set(const nsAString& aName, const nsAString& aValue) {
+  nsAutoCString nativeName;
+  nsAutoCString nativeVal;
+
+  nsresult rv = NS_CopyUnicodeToNative(aName, nativeName);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  rv = NS_CopyUnicodeToNative(aValue, nativeVal);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  StaticMutexAutoLock lock(gEnvHashMutex);
+  EnvEntryType* entry = EnsureEnvHash()->PutEntry(nativeName.get());
+  if (!entry) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  SmprintfPointer newData =
+      mozilla::Smprintf("%s=%s", nativeName.get(), nativeVal.get());
+  if (!newData) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  PR_SetEnv(newData.get());
+  if (entry->GetData()) {
+    free(entry->GetData());
+  }
+  entry->SetData(newData.release());
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsEnvironment.h b/xpcom/threads/nsEnvironment.h
new file mode 100644
index 0000000000..d371050ec5
--- /dev/null
+++ b/xpcom/threads/nsEnvironment.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 nsEnvironment_h__
+#define nsEnvironment_h__
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Mutex.h"
+#include "nsIEnvironment.h"
+
+#define NS_ENVIRONMENT_CID                           \
+  {                                                  \
+    0X3D68F92UL, 0X9513, 0X4E25, {                   \
+      0X9B, 0XE9, 0X7C, 0XB2, 0X39, 0X87, 0X41, 0X72 \
+    }                                                \
+  }
+#define NS_ENVIRONMENT_CONTRACTID "@mozilla.org/process/environment;1"
+
+class nsEnvironment final : public nsIEnvironment {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIENVIRONMENT
+
+  static nsresult Create(REFNSIID aIID, void** aResult);
+
+ private:
+  nsEnvironment() {}
+  ~nsEnvironment() = default;
+};
+
+#endif /* !nsEnvironment_h__ */
diff --git a/xpcom/threads/nsICancelableRunnable.h b/xpcom/threads/nsICancelableRunnable.h
new file mode 100644
index 0000000000..7aa98c86b6
--- /dev/null
+++ b/xpcom/threads/nsICancelableRunnable.h
@@ -0,0 +1,40 @@
+/* -*- 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 nsICancelableRunnable_h__
+#define nsICancelableRunnable_h__
+
+#include "nsISupports.h"
+
+#define NS_ICANCELABLERUNNABLE_IID                   \
+  {                                                  \
+    0xde93dc4c, 0x5eea, 0x4eb7, {                    \
+      0xb6, 0xd1, 0xdb, 0xf1, 0xe0, 0xce, 0xf6, 0x5c \
+    }                                                \
+  }
+
+class nsICancelableRunnable : public nsISupports {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_ICANCELABLERUNNABLE_IID)
+
+  /*
+   * Cancels a pending task, so that calling run() on the task is a no-op.
+   * Calling cancel after the task execution has begun will be a no-op.
+   * Calling this method twice is considered an error.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the runnable has already been canceled.
+   */
+  virtual nsresult Cancel() = 0;
+
+ protected:
+  nsICancelableRunnable() = default;
+  virtual ~nsICancelableRunnable() = default;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsICancelableRunnable, NS_ICANCELABLERUNNABLE_IID)
+
+#endif  // nsICancelableRunnable_h__
diff --git a/xpcom/threads/nsIDirectTaskDispatcher.idl b/xpcom/threads/nsIDirectTaskDispatcher.idl
new file mode 100644
index 0000000000..7d44608708
--- /dev/null
+++ b/xpcom/threads/nsIDirectTaskDispatcher.idl
@@ -0,0 +1,57 @@
+/* -*- 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 "nsIRunnable.idl"
+
+%{C++
+#include "mozilla/AlreadyAddRefed.h"
+%}
+
+native alreadyAddRefed_nsIRunnable(already_AddRefed<nsIRunnable>);
+
+/*
+ * The primary use of this interface is to allow any nsISerialEventTarget to
+ * provide Direct Task dispatching which is similar (but not identical to) the
+ * microtask semantics of JS promises.
+ * New direct task may be dispatched when a current direct task is running. In
+ * which case they will be run in FIFO order.
+ */
+[uuid(e05bf0fe-94b7-4e28-8462-a8368da9c136)]
+interface nsIDirectTaskDispatcher : nsISupports
+{
+  /**
+   * Dispatch an event for the nsISerialEventTarget, using the direct task
+   * queue.
+   *
+   * This function must be called from the same nsISerialEventTarget
+   * implementing direct task dispatching.
+   *
+   * @param event
+   *   The alreadyAddRefed<> event to dispatch.
+   *
+   */
+  [noscript] void dispatchDirectTask(in alreadyAddRefed_nsIRunnable event);
+
+  /**
+   * Synchronously run any pending direct tasks queued.
+   */
+  [noscript] void drainDirectTasks();
+
+  /**
+   * Returns true if any direct tasks are pending.
+   */
+  [noscript] bool haveDirectTasks();
+
+  %{C++
+  // Infallible version of the above. Will assert that it is successful.
+  bool HaveDirectTasks() {
+    bool value = false;
+    MOZ_ALWAYS_SUCCEEDS(HaveDirectTasks(&value));
+    return value;
+  }
+  %}
+
+};
diff --git a/xpcom/threads/nsIDiscardableRunnable.h b/xpcom/threads/nsIDiscardableRunnable.h
new file mode 100644
index 0000000000..873b1f5d93
--- /dev/null
+++ b/xpcom/threads/nsIDiscardableRunnable.h
@@ -0,0 +1,41 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=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_THREADS_NSIDISCARDABLERUNNABLE_H_
+#define XPCOM_THREADS_NSIDISCARDABLERUNNABLE_H_
+
+#include "nsISupports.h"
+
+/**
+ * An interface implemented by nsIRunnable tasks for which nsIRunnable::Run()
+ * might not be called.
+ */
+#define NS_IDISCARDABLERUNNABLE_IID                  \
+  {                                                  \
+    0xde93dc4c, 0x755c, 0x4cdc, {                    \
+      0x96, 0x76, 0x35, 0xc6, 0x48, 0x81, 0x59, 0x78 \
+    }                                                \
+  }
+
+class NS_NO_VTABLE nsIDiscardableRunnable : public nsISupports {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_IDISCARDABLERUNNABLE_IID)
+
+  /**
+   * Called exactly once on a queued task only if nsIRunnable::Run() is not
+   * called.
+   */
+  virtual void OnDiscard() = 0;
+
+ protected:
+  nsIDiscardableRunnable() = default;
+  virtual ~nsIDiscardableRunnable() = default;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsIDiscardableRunnable,
+                              NS_IDISCARDABLERUNNABLE_IID)
+
+#endif  // XPCOM_THREADS_NSIDISCARDABLERUNNABLE_H_
diff --git a/xpcom/threads/nsIEnvironment.idl b/xpcom/threads/nsIEnvironment.idl
new file mode 100644
index 0000000000..60da8ba76a
--- /dev/null
+++ b/xpcom/threads/nsIEnvironment.idl
@@ -0,0 +1,54 @@
+/* -*- Mode: C++; tab-width: 8; 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"
+
+/**
+ * Scriptable access to the current process environment.
+ *
+ */
+[scriptable, uuid(101d5941-d820-4e85-a266-9a3469940807)]
+interface nsIEnvironment : nsISupports
+{
+    /**
+     * Set the value of an environment variable.
+     *
+     * @param aName   the variable name to set.
+     * @param aValue  the value to set.
+     */
+    void set(in AString aName, in AString aValue);
+
+    /**
+     * Get the value of an environment variable.
+     *
+     * @param aName   the variable name to retrieve.
+     * @return        returns the value of the env variable. An empty string
+     *                will be returned when the env variable does not exist or
+     *                when the value itself is an empty string - please use
+     *                |exists()| to probe whether the env variable exists
+     *                or not.
+     */
+    AString get(in AString aName);
+
+    /**
+     * Check the existence of an environment variable.
+     * This method checks whether an environment variable is present in
+     * the environment or not.
+     *
+     * - For Unix/Linux platforms we follow the Unix definition:
+     * An environment variable exists when |getenv()| returns a non-NULL value.
+     * An environment variable does not exist when |getenv()| returns NULL.
+     * - For non-Unix/Linux platforms we have to fall back to a
+     * "portable" definition (which is incorrect for Unix/Linux!!!!)
+     * which simply checks whether the string returned by |Get()| is empty
+     * or not.
+     *
+     * @param aName   the variable name to probe.
+     * @return        if the variable has been set, the value returned is
+     *                PR_TRUE. If the variable was not defined in the
+     *                environment PR_FALSE will be returned.
+     */
+    boolean exists(in AString aName);
+};
diff --git a/xpcom/threads/nsIEventTarget.idl b/xpcom/threads/nsIEventTarget.idl
new file mode 100644
index 0000000000..edd49f306d
--- /dev/null
+++ b/xpcom/threads/nsIEventTarget.idl
@@ -0,0 +1,227 @@
+/* -*- 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"
+#include "nsIRunnable.idl"
+%{C++
+#include "nsCOMPtr.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Atomics.h"
+
+class nsITargetShutdownTask;
+%}
+
+native alreadyAddRefed_nsIRunnable(already_AddRefed<nsIRunnable>);
+[ptr] native nsITargetShutdownTask(nsITargetShutdownTask);
+
+[builtinclass, scriptable, rust_sync, uuid(a03b8b63-af8b-4164-b0e5-c41e8b2b7cfa)]
+interface nsIEventTarget : nsISupports
+{
+  /* until we can get rid of all uses, keep the non-alreadyAddRefed<> version */
+%{C++
+    nsresult Dispatch(nsIRunnable* aEvent, uint32_t aFlags) {
+      return Dispatch(nsCOMPtr<nsIRunnable>(aEvent).forget(), aFlags);
+    }
+%}
+
+  /**
+   * This flag specifies the default mode of event dispatch, whereby the event
+   * is simply queued for later processing.  When this flag is specified,
+   * dispatch returns immediately after the event is queued.
+   */
+  const unsigned long DISPATCH_NORMAL = 0;
+
+  // NOTE: 1 used to be DISPATCH_SYNC
+
+  /**
+   * This flag specifies that the dispatch is occurring from a running event
+   * that was dispatched to the same event target, and that event is about to
+   * finish.
+   *
+   * A thread pool can use this as an optimization hint to not spin up
+   * another thread, since the current thread is about to become idle.
+   *
+   * These events are always async.
+   */
+  const unsigned long DISPATCH_AT_END = 2;
+
+  /**
+   * This flag specifies that the dispatched event may block the thread on
+   * which it executes, usually by doing some sort of I/O.  This information
+   * may be used by the event target to execute the job on a thread
+   * specifically dedicated to doing I/O, leaving other threads available for
+   * CPU-intensive work.
+   */
+  const unsigned long DISPATCH_EVENT_MAY_BLOCK = 4;
+
+  /**
+   * This flag specifies that the dispatched event should be delivered to the
+   * target thread even if the thread has been configured to block dispatching
+   * of runnables. This is generally done for threads which have their own
+   * internal event loop, such as thread pools or the timer thread, and will not
+   * service runnables dispatched to them until shutdown.
+   */
+  const unsigned long DISPATCH_IGNORE_BLOCK_DISPATCH = 8;
+
+  /**
+   * IsOnCurrentThread() should return true if events dispatched to this target
+   * can possibly run on the current thread, and false otherwise. In the case
+   * of an nsIEventTarget for a thread pool, it should return true on all
+   * threads in the pool. In the case of a non-thread nsIEventTarget such as
+   * ThrottledEventQueue, it should return true on the thread where events are
+   * expected to be processed, even if no events from the queue are actually
+   * being processed right now.
+   *
+   * When called on an nsISerialEventTarget, IsOnCurrentThread can be used to
+   * ensure that no other thread has "ownership" of the event target. As such,
+   * it's useful for asserting that an object is only used on a particular
+   * thread. IsOnCurrentThread can't guarantee that the current event has been
+   * dispatched through a particular event target.
+   *
+   * The infallible version of IsOnCurrentThread() is optimized to avoid a
+   * virtual call for non-thread event targets. Thread targets should set
+   * mThread to their virtual PRThread. Non-thread targets should leave
+   * mThread null and implement IsOnCurrentThreadInfallible() to
+   * return the correct answer.
+   *
+   * The fallible version of IsOnCurrentThread may return errors, such as during
+   * shutdown. If it does not return an error, it should return the same result
+   * as the infallible version. The infallible method should return the correct
+   * result regardless of whether the fallible method returns an error.
+   */
+  %{C++
+public:
+  // Infallible. Defined in nsThreadUtils.cpp. Delegates to
+  // IsOnCurrentThreadInfallible when mThread is null.
+  bool IsOnCurrentThread();
+
+protected:
+  mozilla::Atomic<PRThread*> mThread;
+
+  nsIEventTarget() : mThread(nullptr) {}
+  %}
+  // Note that this method is protected.  We define it through IDL, rather than
+  // in a %{C++ block, to ensure that the correct method indices are recorded
+  // for XPConnect purposes.
+  [noscript,notxpcom] boolean isOnCurrentThreadInfallible();
+  %{C++
+public:
+  %}
+
+  // Fallible version of IsOnCurrentThread.
+  boolean isOnCurrentThread();
+
+  /**
+   * Dispatch an event to this event target.  This function may be called from
+   * any thread, and it may be called re-entrantly.
+   *
+   * @param event
+   *   The alreadyAddRefed<> event to dispatch.
+   *   NOTE that the event will be leaked if it fails to dispatch.
+   * @param flags
+   *   The flags modifying event dispatch.  The flags are described in detail
+   *   below.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched.
+   */
+  [noscript, binaryname(Dispatch)] void dispatchFromC(in alreadyAddRefed_nsIRunnable event,
+                                                      [default(DISPATCH_NORMAL)] in unsigned long flags);
+  /**
+   * Version of Dispatch to expose to JS, which doesn't require an alreadyAddRefed<>
+   * (it will be converted to that internally)
+   *
+   * @param event
+   *   The (raw) event to dispatch.
+   * @param flags
+   *   The flags modifying event dispatch.  The flags are described in detail
+   *   below.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched.
+   */
+  [binaryname(DispatchFromScript)] void dispatch(in nsIRunnable event, in unsigned long flags);
+  /**
+   * Dispatch an event to this event target, but do not run it before delay
+   * milliseconds have passed.  This function may be called from any thread.
+   *
+   * @param event
+   *   The alreadyAddrefed<> event to dispatch.
+   * @param delay
+   *   The delay (in ms) before running the event.  If event does not rise to
+   *   the top of the event queue before the delay has passed, it will be set
+   *   aside to execute once the delay has passed.  Otherwise, it will be
+   *   executed immediately.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched, or
+   * that delay is zero.
+   */
+  [noscript] void delayedDispatch(in alreadyAddRefed_nsIRunnable event, in unsigned long delay);
+
+  /**
+   * Register an task to be run on this event target when it begins shutting
+   * down.  Shutdown tasks may be run in any order, and this function may be
+   * called from any thread.
+   *
+   * The event target may or may not continue accepting events during or after
+   * the shutdown task. The precise behaviour here depends on the event target.
+   *
+   * @param task
+   *   The task to be registered to the target thread.
+   *   NOTE that unlike `dispatch`, this will not leak the task if it fails.
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that task is null.
+   * @throws NS_ERROR_NOT_IMPLEMENTED
+   *   Indicates that this event target doesn't support shutdown tasks.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is already shutting down, and no longer
+   *   accepting events.
+   */
+  [noscript] void registerShutdownTask(in nsITargetShutdownTask task);
+
+  /**
+   * Unregisters an task previously registered with registerShutdownTask.  This
+   * function may be called from any thread.
+   *
+   * @param task
+   *   The task previously registered with registerShutdownTask
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that task is null.
+   * @throws NS_ERROR_NOT_IMPLEMENTED
+   *   Indicates that this event target doesn't support shutdown tasks.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is already shutting down, and no longer
+   *   accepting events, or that the shutdown task cannot be found.
+   */
+  [noscript] void unregisterShutdownTask(in nsITargetShutdownTask task);
+};
+
+%{C++
+// convenient aliases:
+#define NS_DISPATCH_NORMAL nsIEventTarget::DISPATCH_NORMAL
+#define NS_DISPATCH_AT_END nsIEventTarget::DISPATCH_AT_END
+#define NS_DISPATCH_EVENT_MAY_BLOCK nsIEventTarget::DISPATCH_EVENT_MAY_BLOCK
+#define NS_DISPATCH_IGNORE_BLOCK_DISPATCH nsIEventTarget::DISPATCH_IGNORE_BLOCK_DISPATCH
+
+// Convenient NS_DECL variant that includes some C++-only methods.
+#define NS_DECL_NSIEVENTTARGET_FULL                                   \
+    NS_DECL_NSIEVENTTARGET                                            \
+    /* Avoid hiding these methods */                                  \
+    using nsIEventTarget::Dispatch;                                   \
+    using nsIEventTarget::IsOnCurrentThread;
+%}
diff --git a/xpcom/threads/nsIIdlePeriod.idl b/xpcom/threads/nsIIdlePeriod.idl
new file mode 100644
index 0000000000..03ab45d80d
--- /dev/null
+++ b/xpcom/threads/nsIIdlePeriod.idl
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 2; 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"
+
+%{C++
+namespace mozilla {
+class TimeStamp;
+}
+%}
+
+native TimeStamp(mozilla::TimeStamp);
+
+/**
+ * An instance implementing nsIIdlePeriod is used by an associated
+ * nsIThread to estimate when it is likely that it will receive an
+ * event.
+ */
+[uuid(21dd35a2-eae9-4bd8-b470-0dfa35a0e3b9)]
+interface nsIIdlePeriod : nsISupports
+{
+    /**
+     * Return an estimate of a point in time in the future when we
+     * think that the associated thread will become busy. Should
+     * return TimeStamp() (i.e. the null time) or a time less than
+     * TimeStamp::Now() if the thread is currently busy or will become
+     * busy very soon.
+     */
+    TimeStamp getIdlePeriodHint();
+};
diff --git a/xpcom/threads/nsIIdleRunnable.h b/xpcom/threads/nsIIdleRunnable.h
new file mode 100644
index 0000000000..7fe6149154
--- /dev/null
+++ b/xpcom/threads/nsIIdleRunnable.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/. */
+
+#ifndef nsIIdleRunnable_h__
+#define nsIIdleRunnable_h__
+
+#include "nsISupports.h"
+#include "mozilla/TimeStamp.h"
+
+#define NS_IIDLERUNNABLE_IID                         \
+  {                                                  \
+    0x688be92e, 0x7ade, 0x4fdc, {                    \
+      0x9d, 0x83, 0x74, 0xcb, 0xef, 0xf4, 0xa5, 0x2c \
+    }                                                \
+  }
+
+class nsIEventTarget;
+
+/**
+ * A task interface for tasks that can schedule their work to happen
+ * in increments bounded by a deadline.
+ */
+class nsIIdleRunnable : public nsISupports {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_IIDLERUNNABLE_IID)
+
+  /**
+   * Notify the task of a point in time in the future when the task
+   * should stop executing.
+   */
+  virtual void SetDeadline(mozilla::TimeStamp aDeadline){};
+  virtual void SetTimer(uint32_t aDelay, nsIEventTarget* aTarget) {
+    MOZ_ASSERT_UNREACHABLE(
+        "The nsIIdleRunnable instance does not support "
+        "idle dispatch with timeout!");
+  };
+
+ protected:
+  nsIIdleRunnable() = default;
+  virtual ~nsIIdleRunnable() = default;
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsIIdleRunnable, NS_IIDLERUNNABLE_IID)
+
+#endif  // nsIIdleRunnable_h__
diff --git a/xpcom/threads/nsINamed.idl b/xpcom/threads/nsINamed.idl
new file mode 100644
index 0000000000..cdb7d88f30
--- /dev/null
+++ b/xpcom/threads/nsINamed.idl
@@ -0,0 +1,24 @@
+/* -*- Mode: C++; tab-width: 2; 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"
+
+/**
+ * Represents an object with a name, such as a runnable or a timer.
+ */
+
+[scriptable, uuid(0c5fe7de-7e83-4d0d-a8a6-4a6518b9a7b3)]
+interface nsINamed : nsISupports
+{
+    /*
+     * A string describing the purpose of the runnable/timer/whatever. Useful
+     * for debugging. This attribute is read-only, but you can change it to a
+     * compile-time string literal with setName.
+     *
+     * WARNING: This attribute will be included in telemetry, so it should
+     * never contain privacy sensitive information.
+     */
+    readonly attribute AUTF8String name;
+};
diff --git a/xpcom/threads/nsIProcess.idl b/xpcom/threads/nsIProcess.idl
new file mode 100644
index 0000000000..c15ded7a2f
--- /dev/null
+++ b/xpcom/threads/nsIProcess.idl
@@ -0,0 +1,112 @@
+/* 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 nsIFile;
+interface nsIObserver;
+
+[scriptable, uuid(609610de-9954-4a63-8a7c-346350a86403)]
+interface nsIProcess : nsISupports
+{
+  /**
+   * Initialises the process with an executable to be run. Call the run method
+   * to run the executable.
+   * @param executable The executable to run.
+   */
+  void init(in nsIFile executable);
+
+  /**
+   * Kills the running process. After exiting the process will either have
+   * been killed or a failure will have been returned.
+   */
+  void kill();
+
+  /**
+   * Executes the file this object was initialized with
+   * @param blocking   Whether to wait until the process terminates before
+                       returning or not.
+   * @param args       An array of arguments to pass to the process in the
+   *                   native character set.
+   * @param count      The length of the args array.
+   */
+  void run(in boolean blocking, [array, size_is(count)] in string args,
+           in unsigned long count);
+
+  /**
+   * Executes the file this object was initialized with optionally calling
+   * an observer after the process has finished running.
+   * @param args       An array of arguments to pass to the process in the
+   *                   native character set.
+   * @param count      The length of the args array.
+   * @param observer   An observer to notify when the process has completed. It
+   *                   will receive this process instance as the subject and
+   *                   "process-finished" or "process-failed" as the topic. The
+   *                   observer will be notified on the main thread.
+   * @param holdWeak   Whether to use a weak reference to hold the observer.
+   */
+  void runAsync([array, size_is(count)] in string args, in unsigned long count,
+                [optional] in nsIObserver observer, [optional] in boolean holdWeak);
+
+  /**
+   * Executes the file this object was initialized with
+   * @param blocking   Whether to wait until the process terminates before
+                       returning or not.
+   * @param args       An array of arguments to pass to the process in UTF-16
+   * @param count      The length of the args array.
+   */
+  void runw(in boolean blocking, [array, size_is(count)] in wstring args,
+            in unsigned long count);
+
+  /**
+   * Executes the file this object was initialized with optionally calling
+   * an observer after the process has finished running.
+   * @param args       An array of arguments to pass to the process in UTF-16
+   * @param count      The length of the args array.
+   * @param observer   An observer to notify when the process has completed. It
+   *                   will receive this process instance as the subject and
+   *                   "process-finished" or "process-failed" as the topic. The
+   *                   observer will be notified on the main thread.
+   * @param holdWeak   Whether to use a weak reference to hold the observer.
+   */
+  void runwAsync([array, size_is(count)] in wstring args,
+                 in unsigned long count,
+                 [optional] in nsIObserver observer, [optional] in boolean holdWeak);
+
+  /**
+   * When set to true the process will not open a new window when started and
+   * will run hidden from the user. This currently affects only the Windows
+   * platform.
+   */
+  attribute boolean startHidden;
+
+  /**
+   * When set to true the process will be launched directly without using the
+   * shell. This currently affects only the Windows platform.
+   */
+  attribute boolean noShell;
+
+  /**
+   * The process identifier of the currently running process. This will only
+   * be available after the process has started and may not be available on
+   * some platforms.
+   */
+  readonly attribute unsigned long pid;
+
+  /**
+   * The exit value of the process. This is only valid after the process has
+   * exited.
+   */
+  readonly attribute long exitValue;
+
+  /**
+   * Returns whether the process is currently running or not.
+   */
+  readonly attribute boolean isRunning;
+};
+
+%{C++
+
+#define NS_PROCESS_CONTRACTID "@mozilla.org/process/util;1"
+%}
diff --git a/xpcom/threads/nsIRunnable.idl b/xpcom/threads/nsIRunnable.idl
new file mode 100644
index 0000000000..bfe9669a9f
--- /dev/null
+++ b/xpcom/threads/nsIRunnable.idl
@@ -0,0 +1,45 @@
+/* -*- Mode: C++; tab-width: 2; 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"
+
+/**
+ * Represents a task which can be dispatched to a thread for execution.
+ */
+
+[scriptable, function, uuid(4a2abaf0-6886-11d3-9382-00104ba0fd40)]
+interface nsIRunnable : nsISupports
+{
+    /**
+     * The function implementing the task to be run.
+     */
+    void run();
+};
+
+[scriptable, uuid(e75aa42a-80a9-11e6-afb5-e89d87348e2c)]
+interface nsIRunnablePriority : nsISupports
+{
+    const unsigned long PRIORITY_IDLE = 0;
+    const unsigned long PRIORITY_DEFERRED_TIMERS = 1;
+    const unsigned long PRIORITY_LOW = 2;
+    // INPUT_LOW isn't supposed to be used directly.
+    // const unsigned long PRIORITY_INPUT_LOW = 3;
+    const unsigned long PRIORITY_NORMAL = 4;
+    const unsigned long PRIORITY_MEDIUMHIGH = 5;
+    const unsigned long PRIORITY_INPUT_HIGH = 6;
+    const unsigned long PRIORITY_VSYNC = 7;
+    // INPUT_HIGHEST is InputTaskManager's internal priority
+    //const unsigned long PRIORITY_INPUT_HIGHEST = 8;
+    const unsigned long PRIORITY_RENDER_BLOCKING = 9;
+    const unsigned long PRIORITY_CONTROL = 10;
+
+    readonly attribute unsigned long priority;
+};
+
+[uuid(3114c36c-a482-4c6e-9523-1dcfc6f605b9)]
+interface nsIRunnableIPCMessageType : nsISupports
+{
+    readonly attribute unsigned long type;
+};
diff --git a/xpcom/threads/nsISerialEventTarget.idl b/xpcom/threads/nsISerialEventTarget.idl
new file mode 100644
index 0000000000..9cc8ff9066
--- /dev/null
+++ b/xpcom/threads/nsISerialEventTarget.idl
@@ -0,0 +1,27 @@
+/* -*- 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 "nsIEventTarget.idl"
+
+/**
+ * A serial event target is an event dispatching interface like
+ * nsIEventTarget. Runnables dispatched to an nsISerialEventTarget are required
+ * to execute serially. That is, two different runnables dispatched to the
+ * target should never be allowed to execute simultaneously. One exception to
+ * this rule is nested event loops. If a runnable spins a nested event loop,
+ * causing another runnable dispatched to the target to run, the target may
+ * still be considered "serial".
+ *
+ * Examples:
+ * - nsIThread is a serial event target.
+ * - Thread pools are not serial event targets.
+ * - However, one can "convert" a thread pool into an nsISerialEventTarget
+ *   by putting a TaskQueue in front of it.
+ */
+[builtinclass, scriptable, rust_sync, uuid(9f982380-24b4-49f3-88f6-45e2952036c7)]
+interface nsISerialEventTarget : nsIEventTarget
+{
+};
diff --git a/xpcom/threads/nsISupportsPriority.idl b/xpcom/threads/nsISupportsPriority.idl
new file mode 100644
index 0000000000..d0b8b9a3dd
--- /dev/null
+++ b/xpcom/threads/nsISupportsPriority.idl
@@ -0,0 +1,45 @@
+/* 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 exposes the general notion of a scheduled object with a
+ * integral priority value.  Following UNIX conventions, smaller (and possibly
+ * negative) values have higher priority.
+ *
+ * This interface does not strictly define what happens when the priority of an
+ * object is changed.  An implementation of this interface is free to define
+ * the side-effects of changing the priority of an object.  In some cases,
+ * changing the priority of an object may be disallowed (resulting in an
+ * exception being thrown) or may simply be ignored.
+ */
+[scriptable, uuid(aa578b44-abd5-4c19-8b14-36d4de6fdc36)]
+interface nsISupportsPriority : nsISupports
+{
+  /**
+   * Typical priority values.
+   */
+  const long PRIORITY_HIGHEST = -20;
+  const long PRIORITY_HIGH    = -10;
+  const long PRIORITY_NORMAL  =   0;
+  const long PRIORITY_LOW     =  10;
+  const long PRIORITY_LOWEST  =  20;
+
+  /**
+   * This attribute may be modified to change the priority of this object.  The
+   * implementation of this interface is free to truncate a given priority
+   * value to whatever limits are appropriate.  Typically, this attribute is
+   * initialized to PRIORITY_NORMAL, but implementations may choose to assign a
+   * different initial value.
+   */
+  attribute long priority;
+
+  /**
+   * This method adjusts the priority attribute by a given delta.  It helps
+   * reduce the amount of coding required to increment or decrement the value
+   * of the priority attribute.
+   */
+  void adjustPriority(in long delta);
+};
diff --git a/xpcom/threads/nsITargetShutdownTask.h b/xpcom/threads/nsITargetShutdownTask.h
new file mode 100644
index 0000000000..09ac3c5e5f
--- /dev/null
+++ b/xpcom/threads/nsITargetShutdownTask.h
@@ -0,0 +1,37 @@
+/* -*- 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_THREADS_NSITARGETSHUTDOWNTASK_H_
+#define XPCOM_THREADS_NSITARGETSHUTDOWNTASK_H_
+
+#include "nsISupports.h"
+#include "nsIEventTarget.h"
+#include "nsThreadUtils.h"
+
+#define NS_ITARGETSHUTDOWNTASK_IID                   \
+  {                                                  \
+    0xb08647aa, 0xcfb5, 0x4630, {                    \
+      0x8e, 0x26, 0x9a, 0xbe, 0xb3, 0x3f, 0x08, 0x40 \
+    }                                                \
+  }
+
+class NS_NO_VTABLE nsITargetShutdownTask : public nsISupports {
+ public:
+  NS_DECLARE_STATIC_IID_ACCESSOR(NS_ISHUTDOWNTASK_IID)
+
+  virtual void TargetShutdown() = 0;
+
+  already_AddRefed<nsIRunnable> AsRunnable() {
+    // FIXME: Try QI to nsINamed if available?
+    return mozilla::NewRunnableMethod("nsITargetShutdownTask::TargetShutdown",
+                                      this,
+                                      &nsITargetShutdownTask::TargetShutdown);
+  }
+};
+
+NS_DEFINE_STATIC_IID_ACCESSOR(nsITargetShutdownTask, NS_ITARGETSHUTDOWNTASK_IID)
+
+#endif
diff --git a/xpcom/threads/nsIThread.idl b/xpcom/threads/nsIThread.idl
new file mode 100644
index 0000000000..3ba764ac12
--- /dev/null
+++ b/xpcom/threads/nsIThread.idl
@@ -0,0 +1,222 @@
+/* -*- 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 "nsISerialEventTarget.idl"
+#include "nsIThreadShutdown.idl"
+
+%{C++
+#include "mozilla/AlreadyAddRefed.h"
+
+namespace mozilla {
+class TimeStamp;
+class TimeDurationValueCalculator;
+template <typename T> class BaseTimeDuration;
+typedef BaseTimeDuration<TimeDurationValueCalculator> TimeDuration;
+enum class EventQueuePriority;
+}
+%}
+
+[ptr] native PRThread(PRThread);
+native EventQueuePriority(mozilla::EventQueuePriority);
+
+native nsIEventTargetPtr(nsIEventTarget*);
+native nsISerialEventTargetPtr(nsISerialEventTarget*);
+native TimeStamp(mozilla::TimeStamp);
+native TimeDuration(mozilla::TimeDuration);
+
+/**
+ * This interface provides a high-level abstraction for an operating system
+ * thread.
+ *
+ * Threads have a built-in event queue, and a thread is an event target that
+ * can receive nsIRunnable objects (events) to be processed on the thread.
+ *
+ * See nsIThreadManager for the API used to create and locate threads.
+ */
+[builtinclass, scriptable, rust_sync, uuid(5801d193-29d1-4964-a6b7-70eb697ddf2b)]
+interface nsIThread : nsISerialEventTarget
+{
+  /**
+   * @returns
+   *   The NSPR thread object corresponding to this nsIThread.
+   */
+  [noscript] readonly attribute PRThread PRThread;
+
+  /**
+   * @returns
+   *  Whether or not this thread may call into JS. Used in the profiler
+   *  to avoid some unnecessary locking.
+   */
+  [noscript] attribute boolean CanInvokeJS;
+
+  /**
+   * Thread QoS priorities. Currently only supported on MacOS.
+   */
+
+  cenum QoSPriority : 32 {
+    QOS_PRIORITY_NORMAL,
+    QOS_PRIORITY_LOW
+  };
+
+  /**
+   * Shutdown the thread.  This method prevents further dispatch of events to
+   * the thread, and it causes any pending events to run to completion before
+   * the thread joins (see PR_JoinThread) with the current thread.  During this
+   * method call, events for the current thread may be processed.
+   *
+   * This method MAY NOT be executed from the thread itself.  Instead, it is
+   * meant to be executed from another thread (usually the thread that created
+   * this thread or the main application thread).  When this function returns,
+   * the thread will be shutdown, and it will no longer be possible to dispatch
+   * events to the thread.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   the current thread, that this thread was not created with a call to
+   *   nsIThreadManager::NewThread, or if this method was called more than once
+   *   on the thread object.
+   */
+  void shutdown();
+
+  /**
+   * This method may be called to determine if there are any events ready to be
+   * processed.  It may only be called when this thread is the current thread.
+   *
+   * Because events may be added to this thread by another thread, a "false"
+   * result does not mean that this thread has no pending events.  It only
+   * means that there were no pending events when this method was called.
+   *
+   * @returns
+   *   A boolean value that if "true" indicates that this thread has one or
+   *   more pending events.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   not the current thread.
+   */
+  boolean hasPendingEvents();
+
+  /**
+   * Similar to above, but checks only possible high priority queue.
+   */
+  boolean hasPendingHighPriorityEvents();
+
+  /**
+   * Process the next event.  If there are no pending events, then this method
+   * may wait -- depending on the value of the mayWait parameter -- until an
+   * event is dispatched to this thread.  This method is re-entrant but may
+   * only be called if this thread is the current thread.
+   *
+   * @param mayWait
+   *   A boolean parameter that if "true" indicates that the method may block
+   *   the calling thread to wait for a pending event.
+   *
+   * @returns
+   *   A boolean value that if "true" indicates that an event was processed.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   not the current thread.
+   */
+  boolean processNextEvent(in boolean mayWait);
+
+  /**
+   * Shutdown the thread asynchronously.  This method immediately prevents
+   * further dispatch of events to the thread, and it causes any pending events
+   * to run to completion before this thread joins with the current thread.
+   *
+   * UNLIKE shutdown() this does not process events on the current thread.
+   * Instead it merely ensures that the current thread continues running until
+   * this thread has shut down.
+   *
+   * This method MAY NOT be executed from the thread itself.  Instead, it is
+   * meant to be executed from another thread (usually the thread that created
+   * this thread or the main application thread).  When this function returns,
+   * the thread will continue running until it exhausts its event queue.
+   *
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that this method was erroneously called when this thread was
+   *   the current thread, that this thread was not created with a call to
+   *   nsIThreadManager::NewNamedThread, or that this method was called more
+   *   than once on the thread object.
+   */
+  void asyncShutdown();
+
+  /**
+   * Like `asyncShutdown`, but also returns a nsIThreadShutdown instance to
+   * allow observing and controlling the thread's async shutdown progress.
+   */
+  nsIThreadShutdown beginShutdown();
+
+  /**
+   * Dispatch an event to a specified queue for the thread.  This function
+   * may be called from any thread, and it may be called re-entrantly.
+   * Most users should use the NS_Dispatch*() functions in nsThreadUtils instead
+   * of calling this directly.
+   *
+   * @param event
+   *   The alreadyAddRefed<> event to dispatch.
+   *   NOTE that the event will be leaked if it fails to dispatch.
+   * @param queue
+   *   Which event priority queue this should be added to
+   *
+   * @throws NS_ERROR_INVALID_ARG
+   *   Indicates that event is null.
+   * @throws NS_ERROR_UNEXPECTED
+   *   Indicates that the thread is shutting down and has finished processing
+   * events, so this event would never run and has not been dispatched.
+   */
+  [noscript] void dispatchToQueue(in alreadyAddRefed_nsIRunnable event,
+                                  in EventQueuePriority queue);
+
+  /**
+   * This is set to the end of the last 50+ms event that was executed on
+   * this thread (for MainThread only).  Otherwise returns a null TimeStamp.
+   */
+  [noscript] readonly attribute TimeStamp lastLongTaskEnd;
+  [noscript] readonly attribute TimeStamp lastLongNonIdleTaskEnd;
+
+  /**
+   * Get information on the timing of the currently-running event.
+   *
+   * @param delay
+   *   The amount of time the current running event in the specified queue waited
+   *   to run. Will return TimeDuration() if the queue is empty or has not run any
+   *   new events since event delay monitoring started.  NOTE: delay will be
+   *   TimeDuration() if this thread uses a PrioritizedEventQueue (i.e. MainThread)
+   *   and the event priority is below Input.
+   * @param start
+   *   The time the currently running event began to run, or TimeStamp() if no
+   *   event is running.
+   */
+  [noscript] void getRunningEventDelay(out TimeDuration delay, out TimeStamp start);
+
+  /**
+   * Set information on the timing of the currently-running event.
+   * Overrides the values returned by getRunningEventDelay
+   *
+   * @param delay
+   *   Delay the running event spent in queues, or TimeDuration() if
+   *   there's no running event.
+   * @param start
+   *   The time the currently running event began to run, or TimeStamp() if no
+   *   event is running.
+   */
+  [noscript] void setRunningEventDelay(in TimeDuration delay, in TimeStamp start);
+
+  [noscript] void setNameForWakeupTelemetry(in ACString name);
+
+  /**
+   * Set the QoS priority of threads where this may be available. Currently
+   * restricted to MacOS. Must be on the thread to call this method.
+   *
+   * @param aPriority
+   *   The specified priority we will adjust to. Can be low (background) or
+   *   normal (default / user-interactive)
+   */
+  [noscript] void setThreadQoS(in nsIThread_QoSPriority aPriority);
+
+};
diff --git a/xpcom/threads/nsIThreadInternal.idl b/xpcom/threads/nsIThreadInternal.idl
new file mode 100644
index 0000000000..ecc0f540f1
--- /dev/null
+++ b/xpcom/threads/nsIThreadInternal.idl
@@ -0,0 +1,110 @@
+/* -*- 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 "nsIThread.idl"
+
+interface nsIRunnable;
+interface nsIThreadObserver;
+
+/**
+ * The XPCOM thread object implements this interface, which allows a consumer
+ * to observe dispatch activity on the thread.
+ */
+[builtinclass, scriptable, rust_sync, uuid(a3a72e5f-71d9-4add-8f30-59a78fb6d5eb)]
+interface nsIThreadInternal : nsIThread
+{
+  /**
+   * Get/set the current thread observer (may be null).  This attribute may be
+   * read from any thread, but must only be set on the thread corresponding to
+   * this thread object.  The observer will be released on the thread
+   * corresponding to this thread object after all other events have been
+   * processed during a call to Shutdown.
+   */
+  attribute nsIThreadObserver observer;
+
+  /**
+   * Add an observer that will *only* receive onProcessNextEvent,
+   * beforeProcessNextEvent. and afterProcessNextEvent callbacks. Always called
+   * on the target thread, and the implementation does not have to be
+   * threadsafe. Order of callbacks is not guaranteed (i.e.
+   * afterProcessNextEvent may be called first depending on whether or not the
+   * observer is added in a nested loop). Holds a strong ref.
+   */
+  void addObserver(in nsIThreadObserver observer);
+
+  /**
+   * Remove an observer added via the addObserver call. Once removed the
+   * observer will never be called again by the thread.
+   */
+  void removeObserver(in nsIThreadObserver observer);
+};
+
+/**
+ * This interface provides the observer with hooks to implement a layered
+ * event queue.  For example, it is possible to overlay processing events
+ * for a GUI toolkit on top of the events for a thread:
+ *
+ *   var NativeQueue;
+ *   Observer = {
+ *     onDispatchedEvent() {
+ *       NativeQueue.signal();
+ *     }
+ *     onProcessNextEvent(thread, mayWait) {
+ *       if (NativeQueue.hasNextEvent())
+ *         NativeQueue.processNextEvent();
+ *       while (mayWait && !thread.hasPendingEvent()) {
+ *         NativeQueue.wait();
+ *         NativeQueue.processNextEvent();
+ *       }
+ *     }
+ *   };
+ *
+ * NOTE: The implementation of this interface must be threadsafe.
+ *
+ * NOTE: It is valid to change the thread's observer during a call to an
+ *       observer method.
+ *
+ * NOTE: Will be split into two interfaces soon: one for onProcessNextEvent and
+ *       afterProcessNextEvent, then another that inherits the first and adds
+ *       onDispatchedEvent.
+ */
+[uuid(cc8da053-1776-44c2-9199-b5a629d0a19d)]
+interface nsIThreadObserver : nsISupports
+{
+  /**
+   * This method is called after an event has been dispatched to the thread.
+   * This method may be called from any thread.
+   */
+  void onDispatchedEvent();
+
+  /**
+   * This method is called when nsIThread::ProcessNextEvent is called.  It does
+   * not guarantee that an event is actually going to be processed.  This method
+   * is only called on the target thread.
+   *
+   * @param thread
+   *   The thread being asked to process another event.
+   * @param mayWait
+   *   Indicates whether or not the method is allowed to block the calling
+   *   thread.  For example, this parameter is false during thread shutdown.
+   */
+  void onProcessNextEvent(in nsIThreadInternal thread, in boolean mayWait);
+
+  /**
+   * This method is called (from nsIThread::ProcessNextEvent) after an event
+   * is processed.  It does not guarantee that an event was actually processed
+   * (depends on the value of |eventWasProcessed|.  This method is only called
+   * on the target thread.  DO NOT EVER RUN SCRIPT FROM THIS CALLBACK!!!
+   *
+   * @param thread
+   *   The thread that processed another event.
+   * @param eventWasProcessed
+   *   Indicates whether an event was actually processed. May be false if the
+   *   |mayWait| flag was false when calling nsIThread::ProcessNextEvent().
+   */
+  void afterProcessNextEvent(in nsIThreadInternal thread,
+                             in bool eventWasProcessed);
+};
diff --git a/xpcom/threads/nsIThreadManager.idl b/xpcom/threads/nsIThreadManager.idl
new file mode 100644
index 0000000000..879ec05e3a
--- /dev/null
+++ b/xpcom/threads/nsIThreadManager.idl
@@ -0,0 +1,182 @@
+/* -*- 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"
+
+[ptr] native PRThread(PRThread);
+      native ThreadCreationOptions(nsIThreadManager::ThreadCreationOptions);
+
+interface nsIEventTarget;
+interface nsIRunnable;
+interface nsIThread;
+
+%{ C++
+#include "mozilla/Maybe.h"
+%}
+
+[scriptable, function, uuid(039a227d-0cb7-44a5-a8f9-dbb7071979f2)]
+interface nsINestedEventLoopCondition : nsISupports
+{
+  /**
+   * Returns true if the current nested event loop should stop spinning.
+   */
+  bool isDone();
+};
+
+/**
+ * An interface for creating and locating nsIThread instances.
+ */
+[scriptable, uuid(1be89eca-e2f7-453b-8d38-c11ba247f6f3)]
+interface nsIThreadManager : nsISupports
+{
+  /**
+   * Default number of bytes reserved for a thread's stack, if no stack size
+   * is specified in newThread().
+   *
+   * Defaults can be a little overzealous for many platforms.
+   *
+   * On Linux and OS X, for instance, the default thread stack size is whatever
+   * getrlimit(RLIMIT_STACK) returns, which is often set at 8MB. Or, on Linux,
+   * if the stack size is unlimited, we fall back to 2MB. This causes particular
+   * problems on Linux, which allocates 2MB huge VM pages, and will often
+   * immediately allocate them for any stacks which are 2MB or larger.
+   *
+   * The default on Windows is 1MB, which is a little more reasonable. But the
+   * vast majority of our threads don't need anywhere near that much space.
+   *
+   * ASan, TSan and non-opt builds, however, often need a bit more, so give
+   * them the platform default.
+   */
+%{C++
+#if defined(MOZ_ASAN) || defined(MOZ_TSAN) || !defined(__OPTIMIZE__)
+  static constexpr uint32_t DEFAULT_STACK_SIZE = 0;
+#else
+  static constexpr uint32_t DEFAULT_STACK_SIZE = 256 * 1024;
+#endif
+
+  static const uint32_t kThreadPoolStackSize = DEFAULT_STACK_SIZE;
+
+  struct ThreadCreationOptions {
+    // The size in bytes to reserve for the thread's stack. A value of `0` means
+    // to use the platform default.
+    uint32_t stackSize = nsIThreadManager::DEFAULT_STACK_SIZE;
+
+    // If set to `true`, any attempts to dispatch runnables to this thread
+    // without `DISPATCH_IGNORE_BLOCK_DISPATCH` will fail.
+    //
+    // This is intended to be used for threads which are expected to generally
+    // only service a single runnable (other than thread lifecycle runnables),
+    // and perform their own event dispatching internaly, such as thread pool
+    // threads or the timer thread.
+    bool blockDispatch = false;
+
+    // (Windows-only) Whether the thread should have a MessageLoop capable of
+    // processing native UI events. Defaults to false.
+    bool isUiThread = false;
+
+    // If set, long task markers will be collected for tasks
+    // longer than longTaskLength ms when profiling is enabled.
+    // See https://www.w3.org/TR/longtasks
+    mozilla::Maybe<uint32_t> longTaskLength;
+  };
+%}
+
+  /**
+   * Create a new thread (a global, user PRThread) with the specified name.
+   *
+   * @param name
+   *   The name of the thread. If it is empty the thread will not be named.
+   * @param options
+   *   Configuration options for the newly created thread.
+   *
+   * @returns
+   *   The newly created nsIThread object.
+   */
+  [noscript] nsIThread newNamedThread(in ACString name, in ThreadCreationOptions options);
+
+  /**
+   * Get the main thread.
+   */
+  readonly attribute nsIThread mainThread;
+
+  /**
+   * Get the current thread.  If the calling thread does not already have a
+   * nsIThread associated with it, then a new nsIThread will be created and
+   * associated with the current PRThread.
+   */
+  readonly attribute nsIThread currentThread;
+
+  /**
+   * This queues a runnable to the main thread. It's a shortcut for JS callers
+   * to be used instead of
+   *   .mainThread.dispatch(runnable, Ci.nsIEventTarget.DISPATCH_NORMAL);
+   * or
+   *   .currentThread.dispatch(runnable, Ci.nsIEventTarget.DISPATCH_NORMAL);
+   * C++ callers should instead use NS_DispatchToMainThread.
+   */
+  [optional_argc]
+  void dispatchToMainThread(in nsIRunnable event, [optional] in uint32_t priority);
+
+  /**
+   * Similar to dispatchToMainThread, but wraps the event with extra
+   * runnable that allocates nsAutoMicroTask.
+   */
+  [optional_argc]
+  void dispatchToMainThreadWithMicroTask(in nsIRunnable event, [optional] in uint32_t priority);
+
+  /**
+   * This queues a runnable to the main thread's idle queue.
+   *
+   * @param event
+   *   The event to dispatch.
+   * @param timeout
+   *   The time in milliseconds until this event should be moved from the idle
+   *   queue to the regular queue if it hasn't been executed by then.  If not
+   *   passed or a zero value is specified, the event will never be moved to
+   *   the regular queue.
+   */
+  void idleDispatchToMainThread(in nsIRunnable event,
+                                [optional] in uint32_t timeout);
+
+  /*
+   * A helper method to dispatch a task through nsIDirectTaskDispatcher to the
+   * current thread.
+   */
+  void dispatchDirectTaskToCurrentThread(in nsIRunnable event);
+
+  /**
+   * Enter a nested event loop on the current thread, waiting on, and
+   * processing events until condition.isDone() returns true.
+   *
+   * If condition.isDone() throws, this function will throw as well.
+   *
+   * C++ code should not use this function, instead preferring
+   * mozilla::SpinEventLoopUntil.
+   */
+  void spinEventLoopUntil(in ACString aVeryGoodReasonToDoThis, in nsINestedEventLoopCondition condition);
+
+  /**
+   * Similar to the previous method, but the spinning of the event loop
+   * terminates when the quit application shutting down starts.
+   *
+   * C++ code should not use this function, instead preferring
+   * mozilla::SpinEventLoopUntil.
+   */
+  void spinEventLoopUntilOrQuit(in ACString aVeryGoodReasonToDoThis, in nsINestedEventLoopCondition condition);
+
+  /**
+   * Spin the current thread's event loop until there are no more pending
+   * events.  This could be done with spinEventLoopUntil, but that would
+   * require access to the current thread from JavaScript, which we are
+   * moving away from.
+   */
+  void spinEventLoopUntilEmpty();
+
+  /**
+   * Return the EventTarget for the main thread.
+   */
+  readonly attribute nsIEventTarget mainThreadEventTarget;
+};
diff --git a/xpcom/threads/nsIThreadPool.idl b/xpcom/threads/nsIThreadPool.idl
new file mode 100644
index 0000000000..48b4a465d8
--- /dev/null
+++ b/xpcom/threads/nsIThreadPool.idl
@@ -0,0 +1,115 @@
+/* -*- 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 "nsIEventTarget.idl"
+#include "nsIThread.idl"
+
+[uuid(ef194cab-3f86-4b61-b132-e5e96a79e5d1)]
+interface nsIThreadPoolListener : nsISupports
+{
+  /**
+   * Called when a new thread is created by the thread pool. The notification
+   * happens on the newly-created thread.
+   */
+  void onThreadCreated();
+
+  /**
+   * Called when a thread is about to be destroyed by the thread pool. The
+   * notification happens on the thread that is about to be destroyed.
+   */
+  void onThreadShuttingDown();
+};
+
+/**
+ * An interface to a thread pool.  A thread pool creates a limited number of
+ * anonymous (unnamed) worker threads.  An event dispatched to the thread pool
+ * will be run on the next available worker thread.
+ */
+[rust_sync, uuid(76ce99c9-8e43-489a-9789-f27cc4424965)]
+interface nsIThreadPool : nsIEventTarget
+{
+  /**
+   * Set the entire pool's QoS priority. If the priority has not changed, do nothing.
+   * Existing threads will update their QoS priority the next time they become
+   * active, and newly created threads will set this QoS priority upon creation.
+   */
+   [noscript] void setQoSForThreads(in nsIThread_QoSPriority aPriority);
+
+  /**
+   * Shutdown the thread pool.  This method may not be executed from any thread
+   * in the thread pool.  Instead, it is meant to be executed from another
+   * thread (usually the thread that created this thread pool).  When this
+   * function returns, the thread pool and all of its threads will be shutdown,
+   * and it will no longer be possible to dispatch tasks to the thread pool.
+   *
+   * As a side effect, events on the current thread will be processed.
+   */
+  void shutdown();
+
+  /**
+   * Shutdown the thread pool, but only wait for aTimeoutMs. After the timeout
+   * expires, any threads that have not shutdown yet are leaked and will not
+   * block shutdown.
+   *
+   * This method should only be used at during shutdown to cleanup threads that
+   * made blocking calls to code outside our control, and can't be safely
+   * terminated. We choose to leak them intentionally to avoid a shutdown hang.
+   */
+  [noscript] void shutdownWithTimeout(in long aTimeoutMs);
+
+  /**
+   * Get/set the maximum number of threads allowed at one time in this pool.
+   */
+  attribute unsigned long threadLimit;
+
+  /**
+   * Get/set the maximum number of idle threads kept alive.
+   */
+  attribute unsigned long idleThreadLimit;
+
+  /**
+   * Get/set the amount of time in milliseconds before an idle thread is
+   * destroyed.
+   */
+  attribute unsigned long idleThreadTimeout;
+
+  /**
+   * If set to true the idle timeout will be calculated as idleThreadTimeout
+   * divideded by the number of idle threads at the moment.  This may help
+   * save memory allocations but still keep reasonable amount of idle threads.
+   * Default is false, use |idleThreadTimeout| for all threads.
+   */
+  attribute boolean idleThreadTimeoutRegressive;
+
+  /**
+   * Get/set the number of bytes reserved for the stack of all threads in
+   * the pool. By default this is nsIThreadManager::DEFAULT_STACK_SIZE.
+   */
+  attribute unsigned long threadStackSize;
+
+  /**
+   * An optional listener that will be notified when a thread is created or
+   * destroyed in the course of the thread pool's operation.
+   *
+   * A listener will only receive notifications about threads created after the
+   * listener is set so it is recommended that the consumer set the listener
+   * before dispatching the first event. A listener that receives an
+   * onThreadCreated() notification is guaranteed to always receive the
+   * corresponding onThreadShuttingDown() notification.
+   *
+   * The thread pool takes ownership of the listener and releases it when the
+   * shutdown() method is called. Threads created after the listener is set will
+   * also take ownership of the listener so that the listener will be kept alive
+   * long enough to receive the guaranteed onThreadShuttingDown() notification.
+   */
+  attribute nsIThreadPoolListener listener;
+
+  /**
+   * Set the label for threads in the pool. All threads will be named
+   * "<aName> #<n>", where <n> is a serial number.
+   */
+  void setName(in ACString aName);
+};
diff --git a/xpcom/threads/nsIThreadShutdown.idl b/xpcom/threads/nsIThreadShutdown.idl
new file mode 100644
index 0000000000..a08d64165b
--- /dev/null
+++ b/xpcom/threads/nsIThreadShutdown.idl
@@ -0,0 +1,57 @@
+/* -*- 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"
+
+interface nsIRunnable;
+
+/**
+ * Handle for the ongoing shutdown progress of a given thread which can be used
+ * to observe and interrupt async shutdown progress. Methods on this interface
+ * may generally only be used on the thread which called
+ * `nsIThread::beginShutdown`.
+ */
+[scriptable, builtinclass, uuid(70a43748-6130-4ea6-a440-7c74e1b7dd7c)]
+interface nsIThreadShutdown : nsISupports
+{
+  /**
+   * Register a runnable to be executed when the thread has completed shutdown,
+   * or shutdown has been cancelled due to `stopWaitingAndLeakThread()`.
+   *
+   * If the thread has already completed or cancelled shutdown, the runnable
+   * may be executed synchronously.
+   *
+   * May only be called on the thread which invoked `nsIThread::beginShutdown`.
+   */
+  void onCompletion(in nsIRunnable aEvent);
+
+  /**
+   * Check if the target thread has completed shutdown.
+   *
+   * May only be accessed on the thread which called `nsIThread::beginShutdown`.
+   */
+  [infallible] readonly attribute boolean completed;
+
+  /**
+   * Give up on waiting for the shutting down thread to exit. Calling this
+   * method will allow the thread to continue running, no longer block shutdown,
+   * and the thread will never be joined or have its resources reclaimed.
+   *
+   * Completion callbacks attached to this `nsIThreadShutdown` may be executed
+   * during this call.
+   *
+   * This method should NOT be called except in exceptional circumstances during
+   * shutdown, as it will cause resources for the shutting down thread to be
+   * leaked.
+   *
+   * May only be called on the thread which called `nsIThread::beginShutdown`
+   *
+   * @throws NS_ERROR_NOT_AVAILABLE
+   *   Indicates that the target thread has already stopped running and a
+   *   request to be joined is already being dispatched to the waiting thread.
+   */
+  void stopWaitingAndLeakThread();
+};
diff --git a/xpcom/threads/nsITimer.idl b/xpcom/threads/nsITimer.idl
new file mode 100644
index 0000000000..5d20c315b4
--- /dev/null
+++ b/xpcom/threads/nsITimer.idl
@@ -0,0 +1,376 @@
+/* -*- 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 "nsISupports.idl"
+#include  "nsINamed.idl"
+
+interface nsIObserver;
+interface nsIEventTarget;
+
+%{C++
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/TimeStamp.h"
+#include <functional>
+
+/**
+ * The signature of the timer callback function passed to initWithFuncCallback.
+ * This is the function that will get called when the timer expires if the
+ * timer is initialized via initWithFuncCallback.
+ *
+ * @param aTimer the timer which has expired
+ * @param aClosure opaque parameter passed to initWithFuncCallback
+ */
+class nsITimer;
+typedef void (*nsTimerCallbackFunc) (nsITimer *aTimer, void *aClosure);
+%}
+
+native MallocSizeOf(mozilla::MallocSizeOf);
+native nsTimerCallbackFunc(nsTimerCallbackFunc);
+[ref] native TimeDuration(mozilla::TimeDuration);
+
+/**
+ * The callback interface for timers.
+ */
+interface nsITimer;
+
+[function, scriptable, uuid(a796816d-7d47-4348-9ab8-c7aeb3216a7d)]
+interface nsITimerCallback : nsISupports
+{
+  /**
+   * @param aTimer the timer which has expired
+   */
+  void notify(in nsITimer timer);
+};
+
+%{C++
+// Two timer deadlines must differ by less than half the PRIntervalTime domain.
+#define DELAY_INTERVAL_LIMIT    PR_BIT(8 * sizeof(PRIntervalTime) - 1)
+%}
+
+/**
+ * nsITimer instances must be initialized by calling one of the "init" methods
+ * documented below.  You may also re-initialize (using one of the init()
+ * methods) an existing instance to avoid the overhead of destroying and
+ * creating a timer.  It is not necessary to cancel the timer in that case.
+ *
+ * By default a timer will fire on the thread that created it.  Set the .target
+ * attribute to fire on a different thread.  Once you have set a timer's .target
+ * and called one of its init functions, any further interactions with the timer
+ * (calling cancel(), changing member fields, etc) should only be done by the
+ * target thread, or races may occur with bad results like timers firing after
+ * they've been canceled, and/or not firing after re-initiatization.
+ */
+[scriptable, builtinclass, uuid(3de4b105-363c-482c-a409-baac83a01bfc)]
+interface nsITimer : nsISupports
+{
+  /* Timer types */
+
+  /**
+   * Type of a timer that fires once only.
+   */
+  const short TYPE_ONE_SHOT = 0;
+
+  /**
+   * After firing, a TYPE_REPEATING_SLACK timer is stopped and not restarted
+   * until its callback completes.  Specified timer period will be at least
+   * the time between when processing for last firing the callback completes
+   * and when the next firing occurs.
+   *
+   * This is the preferable repeating type for most situations.
+   */
+  const short TYPE_REPEATING_SLACK = 1;
+
+  /**
+   * TYPE_REPEATING_PRECISE is just a synonym for
+   * TYPE_REPEATING_PRECISE_CAN_SKIP. They used to be distinct, but the old
+   * TYPE_REPEATING_PRECISE kind was similar to TYPE_REPEATING_PRECISE_CAN_SKIP
+   * while also being less useful. So the distinction was removed.
+   */
+  const short TYPE_REPEATING_PRECISE = 2;
+
+  /**
+   * A TYPE_REPEATING_PRECISE_CAN_SKIP repeating timer aims to have constant
+   * period between firings.  The processing time for each timer callback will
+   * not influence the timer period.  If the callback finishes after the next
+   * firing(s) should have happened (either because the callback took a long
+   * time, or the callback was called extremely late), that firing(s) is
+   * skipped, but the following sequence of firing times will not be altered.
+   * This timer type guarantees that it will not queue up new events to fire
+   * the callback until the previous callback event finishes firing.  This is
+   * the only non-slack timer available.
+   */
+  const short TYPE_REPEATING_PRECISE_CAN_SKIP = 3;
+
+  /**
+   * Same as TYPE_REPEATING_SLACK with the exception that idle events
+   * won't yield to timers with this type.  Use this when you want an
+   * idle callback to be scheduled to run even though this timer is
+   * about to fire.
+   */
+  const short TYPE_REPEATING_SLACK_LOW_PRIORITY = 4;
+
+  /**
+   * Same as TYPE_ONE_SHOT with the exception that idle events won't
+   * yield to timers with this type.  Use this when you want an idle
+   * callback to be scheduled to run even though this timer is about
+   * to fire.
+   */
+  const short TYPE_ONE_SHOT_LOW_PRIORITY = 5;
+
+  /**
+   * Initialize a timer that will fire after the said delay.
+   * A user must keep a reference to this timer till it is
+   * is no longer needed or has been cancelled.
+   *
+   * @param aObserver   the callback object that observes the
+   *                    ``timer-callback'' topic with the subject being
+   *                    the timer itself when the timer fires:
+   *
+   *                    observe(nsISupports aSubject, => nsITimer
+   *                            string aTopic,        => ``timer-callback''
+   *                            wstring data          =>  null
+   *
+   * @param aDelayInMs  delay in milliseconds for timer to fire
+   * @param aType       timer type per TYPE* consts defined above
+   */
+  void init(in nsIObserver aObserver, in unsigned long aDelayInMs,
+            in unsigned long aType);
+
+
+  /**
+   * Initialize a timer to fire after the given millisecond interval.
+   * This version takes a callback object.
+   *
+   * @param aFunc       nsITimerCallback interface to call when timer expires
+   * @param aDelayInMs  The millisecond interval
+   * @param aType       Timer type per TYPE* consts defined above
+   */
+  void initWithCallback(in nsITimerCallback aCallback,
+                        in unsigned long aDelayInMs,
+                        in unsigned long aType);
+
+  /**
+   * Initialize a timer to fire after the high resolution TimeDuration.
+   * This version takes a callback object.
+   *
+   * @param aFunc      nsITimerCallback interface to call when timer expires
+   * @param aDelay     The high resolution interval
+   * @param aType      Timer type per TYPE* consts defined above
+   */
+  [noscript] void initHighResolutionWithCallback(in nsITimerCallback aCallback,
+                                                 [const] in TimeDuration aDelay,
+                                                 in unsigned long aType);
+
+  /**
+   * Cancel the timer.  This method works on all types, not just on repeating
+   * timers -- you might want to cancel a TYPE_ONE_SHOT timer, and even reuse
+   * it by re-initializing it (to avoid object destruction and creation costs
+   * by conserving one timer instance).
+   */
+  void cancel();
+
+  /**
+   * Like initWithFuncCallback, but also takes a name for the timer; the name
+   * will be used when timer profiling is enabled via the "TimerFirings" log
+   * module.
+   *
+   * @param aFunc      The function to invoke
+   * @param aClosure   An opaque pointer to pass to that function
+   * @param aDelay     The millisecond interval
+   * @param aType      Timer type per TYPE* consts defined above
+   * @param aName      The timer's name
+   */
+  [noscript] void initWithNamedFuncCallback(in nsTimerCallbackFunc aCallback,
+                                            in voidPtr aClosure,
+                                            in unsigned long aDelay,
+                                            in unsigned long aType,
+                                            in string aName);
+
+  /**
+   * Initialize a timer to fire after the high resolution TimeDuration.
+   * This version takes a named function callback.
+   *
+   * @param aFunc      The function to invoke
+   * @param aClosure   An opaque pointer to pass to that function
+   * @param aDelay     The high resolution interval
+   * @param aType      Timer type per TYPE* consts defined above
+   * @param aName      The timer's name
+   */
+  [noscript] void initHighResolutionWithNamedFuncCallback(
+                      in nsTimerCallbackFunc aCallback,
+                      in voidPtr aClosure,
+                      [const] in TimeDuration aDelay,
+                      in unsigned long aType,
+                      in string aName);
+
+  /**
+   * The millisecond delay of the timeout.
+   *
+   * NOTE: Re-setting the delay on a one-shot timer that has already fired
+   * doesn't restart the timer. Call one of the init() methods to restart
+   * a one-shot timer.
+   */
+  attribute unsigned long delay;
+
+  /**
+   * The timer type - one of the above TYPE_* constants.
+   */
+  attribute unsigned long type;
+
+  /**
+   * The opaque pointer pass to initWithFuncCallback.
+   */
+  [noscript] readonly attribute voidPtr closure;
+
+  /**
+   * The nsITimerCallback object passed to initWithCallback.
+   */
+  readonly attribute nsITimerCallback callback;
+
+  /**
+   * The nsIEventTarget where the callback will be dispatched. Note that this
+   * target may only be set before the call to one of the init methods above.
+   *
+   * By default the target is the thread that created the timer.
+   */
+  attribute nsIEventTarget target;
+
+  readonly attribute ACString name;
+
+  /**
+   * The number of microseconds this nsITimer implementation can possibly
+   * fire early.
+   */
+  [noscript] readonly attribute unsigned long allowedEarlyFiringMicroseconds;
+
+  [notxpcom, nostdcall] size_t sizeOfIncludingThis(in MallocSizeOf aMallocSizeOf);
+};
+
+%{C++
+#include "nsCOMPtr.h"
+
+already_AddRefed<nsITimer> NS_NewTimer();
+
+already_AddRefed<nsITimer> NS_NewTimer(nsIEventTarget* aTarget);
+
+nsresult
+NS_NewTimerWithObserver(nsITimer** aTimer,
+                        nsIObserver* aObserver,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithObserver(nsIObserver* aObserver,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithCallback(nsITimer** aTimer,
+                        nsITimerCallback* aCallback,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithCallback(nsITimerCallback* aCallback,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithCallback(nsITimer** aTimer,
+                        nsITimerCallback* aCallback,
+                        const mozilla::TimeDuration& aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithCallback(nsITimerCallback* aCallback,
+                        const mozilla::TimeDuration& aDelay,
+                        uint32_t aType,
+                        nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithCallback(nsITimer** aTimer,
+                        std::function<void(nsITimer*)>&& aCallback,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        const char* aNameString,
+                        nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithCallback(std::function<void(nsITimer*)>&& aCallback,
+                        uint32_t aDelay,
+                        uint32_t aType,
+                        const char* aNameString,
+                        nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithCallback(nsITimer** aTimer,
+                        std::function<void(nsITimer*)>&& aCallback,
+                        const mozilla::TimeDuration& aDelay,
+                        uint32_t aType,
+                        const char* aNameString,
+                        nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithCallback(std::function<void(nsITimer*)>&& aCallback,
+                        const mozilla::TimeDuration& aDelay,
+                        uint32_t aType,
+                        const char* aNameString,
+                        nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithFuncCallback(nsITimer** aTimer,
+                            nsTimerCallbackFunc aCallback,
+                            void* aClosure,
+                            uint32_t aDelay,
+                            uint32_t aType,
+                            const char* aNameString,
+                            nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithFuncCallback(nsTimerCallbackFunc aCallback,
+                            void* aClosure,
+                            uint32_t aDelay,
+                            uint32_t aType,
+                            const char* aNameString,
+                            nsIEventTarget* aTarget = nullptr);
+
+nsresult
+NS_NewTimerWithFuncCallback(nsITimer** aTimer,
+                            nsTimerCallbackFunc aCallback,
+                            void* aClosure,
+                            const mozilla::TimeDuration& aDelay,
+                            uint32_t aType,
+                            const char* aNameString,
+                            nsIEventTarget* aTarget = nullptr);
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult>
+NS_NewTimerWithFuncCallback(nsTimerCallbackFunc aCallback,
+                            void* aClosure,
+                            const mozilla::TimeDuration& aDelay,
+                            uint32_t aType,
+                            const char* aNameString,
+                            nsIEventTarget* aTarget = nullptr);
+
+#define NS_TIMER_CALLBACK_TOPIC "timer-callback"
+
+#ifndef RELEASE_OR_BETA
+#undef NS_DECL_NSITIMERCALLBACK
+#define NS_DECL_NSITIMERCALLBACK                                        \
+  NS_IMETHOD Notify(nsITimer *timer) override;                          \
+  inline void _ensure_GetName_exists(void) {                            \
+    static_assert(std::is_convertible<decltype(this), nsINamed*>::value, \
+                  "nsITimerCallback implementations must also implement nsINamed");     \
+  }
+#endif
+%}
+
+[scriptable, builtinclass, uuid(5482506d-1d21-4d08-b01c-95c87e1295ad)]
+interface nsITimerManager : nsISupports
+{
+  /**
+   * Returns a read-only list of nsITimer objects, implementing only the name,
+   * delay and type attribute getters.
+   * This is meant to be used for tests, to verify that no timer is leftover
+   * at the end of a test. */
+  Array<nsITimer> getTimers();
+};
diff --git a/xpcom/threads/nsMemoryPressure.cpp b/xpcom/threads/nsMemoryPressure.cpp
new file mode 100644
index 0000000000..dbd3a92f79
--- /dev/null
+++ b/xpcom/threads/nsMemoryPressure.cpp
@@ -0,0 +1,104 @@
+/* -*- 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 "nsMemoryPressure.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Services.h"
+
+#include "nsThreadUtils.h"
+#include "nsIObserverService.h"
+
+using namespace mozilla;
+
+const char* const kTopicMemoryPressure = "memory-pressure";
+const char* const kTopicMemoryPressureStop = "memory-pressure-stop";
+const char16_t* const kSubTopicLowMemoryNew = u"low-memory";
+const char16_t* const kSubTopicLowMemoryOngoing = u"low-memory-ongoing";
+
+// This is accessed from any thread through NS_NotifyOfEventualMemoryPressure
+static Atomic<MemoryPressureState, Relaxed> sMemoryPressurePending(
+    MemoryPressureState::NoPressure);
+
+void NS_NotifyOfEventualMemoryPressure(MemoryPressureState aState) {
+  MOZ_ASSERT(aState != MemoryPressureState::None);
+
+  /*
+   * A new memory pressure event erases an ongoing (or stop of) memory pressure,
+   * but an existing "new" memory pressure event takes precedence over a new
+   * "ongoing" or "stop" memory pressure event.
+   */
+  switch (aState) {
+    case MemoryPressureState::None:
+    case MemoryPressureState::LowMemory:
+      sMemoryPressurePending = aState;
+      break;
+    case MemoryPressureState::NoPressure:
+      sMemoryPressurePending.compareExchange(MemoryPressureState::None, aState);
+      break;
+  }
+}
+
+nsresult NS_NotifyOfMemoryPressure(MemoryPressureState aState) {
+  NS_NotifyOfEventualMemoryPressure(aState);
+  nsCOMPtr<nsIRunnable> event =
+      new Runnable("NS_DispatchEventualMemoryPressure");
+  return NS_DispatchToMainThread(event);
+}
+
+void NS_DispatchMemoryPressure() {
+  MOZ_ASSERT(NS_IsMainThread());
+  static MemoryPressureState sMemoryPressureStatus =
+      MemoryPressureState::NoPressure;
+
+  MemoryPressureState mpPending =
+      sMemoryPressurePending.exchange(MemoryPressureState::None);
+  if (mpPending == MemoryPressureState::None) {
+    return;
+  }
+
+  nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+  if (!os) {
+    NS_WARNING("Can't get observer service!");
+    return;
+  }
+
+  switch (mpPending) {
+    case MemoryPressureState::None:
+      MOZ_ASSERT_UNREACHABLE("Already handled this case above.");
+      break;
+    case MemoryPressureState::LowMemory:
+      switch (sMemoryPressureStatus) {
+        case MemoryPressureState::None:
+          MOZ_ASSERT_UNREACHABLE("The internal status should never be None.");
+          break;
+        case MemoryPressureState::NoPressure:
+          sMemoryPressureStatus = MemoryPressureState::LowMemory;
+          os->NotifyObservers(nullptr, kTopicMemoryPressure,
+                              kSubTopicLowMemoryNew);
+          break;
+        case MemoryPressureState::LowMemory:
+          os->NotifyObservers(nullptr, kTopicMemoryPressure,
+                              kSubTopicLowMemoryOngoing);
+          break;
+      }
+      break;
+    case MemoryPressureState::NoPressure:
+      switch (sMemoryPressureStatus) {
+        case MemoryPressureState::None:
+          MOZ_ASSERT_UNREACHABLE("The internal status should never be None.");
+          break;
+        case MemoryPressureState::NoPressure:
+          // Already no pressure.  Do nothing.
+          break;
+        case MemoryPressureState::LowMemory:
+          sMemoryPressureStatus = MemoryPressureState::NoPressure;
+          os->NotifyObservers(nullptr, kTopicMemoryPressureStop, nullptr);
+          break;
+      }
+      break;
+  }
+}
diff --git a/xpcom/threads/nsMemoryPressure.h b/xpcom/threads/nsMemoryPressure.h
new file mode 100644
index 0000000000..5a68b0bce5
--- /dev/null
+++ b/xpcom/threads/nsMemoryPressure.h
@@ -0,0 +1,77 @@
+/* -*- 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 nsMemoryPressure_h__
+#define nsMemoryPressure_h__
+
+#include "nscore.h"
+
+/*
+ * These pre-defined strings are the topic to pass to the observer
+ * service to declare the memory-pressure or lift the memory-pressure.
+ *
+ * 1. Notify kTopicMemoryPressure with kSubTopicLowMemoryNew
+ * New memory pressure deteced
+ * On a new memory pressure, we stop everything to start cleaning
+ * aggresively the memory used, in order to free as much memory as
+ * possible.
+ *
+ * 2. Notify kTopicMemoryPressure with kSubTopicLowMemoryOngoing
+ * Repeated memory pressure.
+ * A repeated memory pressure implies to clean softly recent allocations.
+ * It is supposed to happen after a new memory pressure which already
+ * cleaned aggressivley.  So there is no need to damage the reactivity of
+ * Gecko by stopping the world again.
+ *
+ * In case of conflict with an new memory pressue, the new memory pressure
+ * takes precedence over an ongoing memory pressure.  The reason being
+ * that if no events are processed between 2 notifications (new followed
+ * by ongoing, or ongoing followed by a new) we want to be as aggresive as
+ * possible on the clean-up of the memory.  After all, we are trying to
+ * keep Gecko alive as long as possible.
+ *
+ * 3. Notify kTopicMemoryPressureStop with nullptr
+ * Memory pressure stopped.
+ * We're no longer under acute memory pressure, so we might want to have a
+ * chance of (cautiously) re-enabling some things we previously turned off.
+ * As above, an already enqueued new memory pressure event takes precedence.
+ * The priority ordering between concurrent attempts to queue both stopped
+ * and ongoing memory pressure is currently not defined.
+ */
+extern const char* const kTopicMemoryPressure;
+extern const char* const kTopicMemoryPressureStop;
+extern const char16_t* const kSubTopicLowMemoryNew;
+extern const char16_t* const kSubTopicLowMemoryOngoing;
+
+enum class MemoryPressureState : uint32_t {
+  None,  // For internal use.  Don't use this value.
+  LowMemory,
+  NoPressure,
+};
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event, but if there are no events pending in
+ * the main thread's event queue, the memory pressure event would not be
+ * dispatched until one is enqueued. It is infallible and does not allocate
+ * any memory.
+ *
+ * You may call this function from any thread.
+ */
+void NS_NotifyOfEventualMemoryPressure(MemoryPressureState aState);
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event. We wake up the main thread by adding a
+ * dummy event to its event loop, so, unlike with
+ * NS_NotifyOfEventualMemoryPressure, this memory-pressure event is always
+ * fired relatively quickly, even if the event loop is otherwise empty.
+ *
+ * You may call this function from any thread.
+ */
+nsresult NS_NotifyOfMemoryPressure(MemoryPressureState aState);
+
+#endif  // nsMemoryPressure_h__
diff --git a/xpcom/threads/nsProcess.h b/xpcom/threads/nsProcess.h
new file mode 100644
index 0000000000..95d6748640
--- /dev/null
+++ b/xpcom/threads/nsProcess.h
@@ -0,0 +1,82 @@
+/* -*- 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 _nsPROCESSWIN_H_
+#define _nsPROCESSWIN_H_
+
+#if defined(XP_WIN)
+#  define PROCESSMODEL_WINAPI
+#endif
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Mutex.h"
+#include "nsIProcess.h"
+#include "nsIObserver.h"
+#include "nsMaybeWeakPtr.h"
+#include "nsString.h"
+#ifndef XP_UNIX
+#  include "prproces.h"
+#endif
+#if defined(PROCESSMODEL_WINAPI)
+#  include <windows.h>
+#  include <shellapi.h>
+#endif
+
+#define NS_PROCESS_CID                               \
+  {                                                  \
+    0x7b4eeb20, 0xd781, 0x11d4, {                    \
+      0x8A, 0x83, 0x00, 0x10, 0xa4, 0xe0, 0xc9, 0xca \
+    }                                                \
+  }
+
+class nsIFile;
+
+class nsProcess final : public nsIProcess, public nsIObserver {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIPROCESS
+  NS_DECL_NSIOBSERVER
+
+  nsProcess();
+
+ private:
+  ~nsProcess();
+  PRThread* CreateMonitorThread();
+  static void Monitor(void* aArg);
+  void ProcessComplete();
+  nsresult CopyArgsAndRunProcess(bool aBlocking, const char** aArgs,
+                                 uint32_t aCount, nsIObserver* aObserver,
+                                 bool aHoldWeak);
+  nsresult CopyArgsAndRunProcessw(bool aBlocking, const char16_t** aArgs,
+                                  uint32_t aCount, nsIObserver* aObserver,
+                                  bool aHoldWeak);
+  // The 'args' array is null-terminated.
+  nsresult RunProcess(bool aBlocking, char** aArgs, nsIObserver* aObserver,
+                      bool aHoldWeak, bool aArgsUTF8);
+
+  PRThread* mThread;
+  mozilla::Mutex mLock;
+  bool mShutdown MOZ_GUARDED_BY(mLock);
+  bool mBlocking;
+  bool mStartHidden;
+  bool mNoShell;
+
+  nsCOMPtr<nsIFile> mExecutable;
+  nsString mTargetPath;
+  int32_t mPid;
+  nsMaybeWeakPtr<nsIObserver> mObserver;
+
+  // These members are modified by multiple threads, any accesses should be
+  // protected with mLock.
+  int32_t mExitValue MOZ_GUARDED_BY(mLock);
+#if defined(PROCESSMODEL_WINAPI)
+  HANDLE mProcess MOZ_GUARDED_BY(mLock);
+#elif !defined(XP_UNIX)
+  PRProcess* mProcess MOZ_GUARDED_BY(mLock);
+#endif
+};
+
+#endif
diff --git a/xpcom/threads/nsProcessCommon.cpp b/xpcom/threads/nsProcessCommon.cpp
new file mode 100644
index 0000000000..dbd9993f54
--- /dev/null
+++ b/xpcom/threads/nsProcessCommon.cpp
@@ -0,0 +1,600 @@
+/* -*- 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/. */
+
+/*****************************************************************************
+ *
+ * nsProcess is used to execute new processes and specify if you want to
+ * wait (blocking) or continue (non-blocking).
+ *
+ *****************************************************************************
+ */
+
+#include "mozilla/ArrayUtils.h"
+
+#include "nsCOMPtr.h"
+#include "nsIFile.h"
+#include "nsProcess.h"
+#include "prio.h"
+#include "prenv.h"
+#include "nsCRT.h"
+#include "nsThreadUtils.h"
+#include "nsIObserverService.h"
+#include "nsXULAppAPI.h"
+#include "mozilla/Services.h"
+
+#include <stdlib.h>
+
+#if defined(PROCESSMODEL_WINAPI)
+#  include "nsString.h"
+#  include "nsLiteralString.h"
+#  include "nsReadableUtils.h"
+#  include "mozilla/AssembleCmdLine.h"
+#  include "mozilla/UniquePtrExtensions.h"
+#else
+#  ifdef XP_MACOSX
+#    include <crt_externs.h>
+#    include <spawn.h>
+#  endif
+#  ifdef XP_UNIX
+#    ifndef XP_MACOSX
+#      include "base/process_util.h"
+#    endif
+#    include <sys/wait.h>
+#    include <sys/errno.h>
+#  endif
+#  include <sys/types.h>
+#  include <signal.h>
+#endif
+
+using namespace mozilla;
+
+//-------------------------------------------------------------------//
+// nsIProcess implementation
+//-------------------------------------------------------------------//
+NS_IMPL_ISUPPORTS(nsProcess, nsIProcess, nsIObserver)
+
+// Constructor
+nsProcess::nsProcess()
+    : mThread(nullptr),
+      mLock("nsProcess.mLock"),
+      mShutdown(false),
+      mBlocking(false),
+      mStartHidden(false),
+      mNoShell(false),
+      mPid(-1),
+      mExitValue(-1)
+#if !defined(XP_UNIX)
+      ,
+      mProcess(nullptr)
+#endif
+{
+}
+
+// Destructor
+nsProcess::~nsProcess() = default;
+
+NS_IMETHODIMP
+nsProcess::Init(nsIFile* aExecutable) {
+  if (mExecutable) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (NS_WARN_IF(!aExecutable)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  bool isFile;
+
+  // First make sure the file exists
+  nsresult rv = aExecutable->IsFile(&isFile);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+  if (!isFile) {
+    return NS_ERROR_FAILURE;
+  }
+
+  // Store the nsIFile in mExecutable
+  mExecutable = aExecutable;
+  // Get the path because it is needed by the NSPR process creation
+#ifdef XP_WIN
+  rv = mExecutable->GetTarget(mTargetPath);
+  if (NS_FAILED(rv) || mTargetPath.IsEmpty())
+#endif
+    rv = mExecutable->GetPath(mTargetPath);
+
+  return rv;
+}
+
+void nsProcess::Monitor(void* aArg) {
+  RefPtr<nsProcess> process = dont_AddRef(static_cast<nsProcess*>(aArg));
+
+  if (!process->mBlocking) {
+    NS_SetCurrentThreadName("RunProcess");
+  }
+
+#if defined(PROCESSMODEL_WINAPI)
+  HANDLE processHandle;
+  {
+    // The mutex region cannot include WaitForSingleObject otherwise we'll
+    // block calls such as Kill. So lock on access and store a local.
+    MutexAutoLock lock(process->mLock);
+    processHandle = process->mProcess;
+  }
+
+  DWORD dwRetVal;
+  unsigned long exitCode = -1;
+
+  dwRetVal = WaitForSingleObject(processHandle, INFINITE);
+  if (dwRetVal != WAIT_FAILED) {
+    if (GetExitCodeProcess(processHandle, &exitCode) == FALSE) {
+      exitCode = -1;
+    }
+  }
+
+  // Lock in case Kill or GetExitCode are called during this.
+  {
+    MutexAutoLock lock(process->mLock);
+    CloseHandle(process->mProcess);
+    process->mProcess = nullptr;
+    process->mExitValue = exitCode;
+    if (process->mShutdown) {
+      return;
+    }
+  }
+#else
+#  ifdef XP_UNIX
+  int exitCode = -1;
+  int status = 0;
+  pid_t result;
+  do {
+    result = waitpid(process->mPid, &status, 0);
+  } while (result == -1 && errno == EINTR);
+  if (result == process->mPid) {
+    if (WIFEXITED(status)) {
+      exitCode = WEXITSTATUS(status);
+    } else if (WIFSIGNALED(status)) {
+      exitCode = 256;  // match NSPR's signal exit status
+    }
+  }
+#  else
+  int32_t exitCode = -1;
+  PRProcess* prProcess;
+  {
+    // The mutex region cannot include PR_WaitProcess otherwise we'll
+    // block calls such as Kill. So lock on access and store a local.
+    MutexAutoLock lock(process->mLock);
+    prProcess = process->mProcess;
+  }
+  if (PR_WaitProcess(prProcess, &exitCode) != PR_SUCCESS) {
+    exitCode = -1;
+  }
+#  endif
+
+  // Lock in case Kill or GetExitCode are called during this
+  {
+    MutexAutoLock lock(process->mLock);
+#  if !defined(XP_UNIX)
+    process->mProcess = nullptr;
+#  endif
+    process->mExitValue = exitCode;
+    if (process->mShutdown) {
+      return;
+    }
+  }
+#endif
+
+  // If we ran a background thread for the monitor then notify on the main
+  // thread
+  if (NS_IsMainThread()) {
+    process->ProcessComplete();
+  } else {
+    NS_DispatchToMainThread(NewRunnableMethod(
+        "nsProcess::ProcessComplete", process, &nsProcess::ProcessComplete));
+  }
+}
+
+void nsProcess::ProcessComplete() {
+  if (mThread) {
+    nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "xpcom-shutdown");
+    }
+    PR_JoinThread(mThread);
+    mThread = nullptr;
+  }
+
+  const char* topic;
+  {
+    MutexAutoLock lock(mLock);
+    if (mExitValue != 0) {
+      topic = "process-failed";
+    } else {
+      topic = "process-finished";
+    }
+  }
+
+  mPid = -1;
+  nsCOMPtr<nsIObserver> observer = mObserver.GetValue();
+  mObserver = nullptr;
+
+  if (observer) {
+    observer->Observe(NS_ISUPPORTS_CAST(nsIProcess*, this), topic, nullptr);
+  }
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::Run(bool aBlocking, const char** aArgs, uint32_t aCount) {
+  return CopyArgsAndRunProcess(aBlocking, aArgs, aCount, nullptr, false);
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::RunAsync(const char** aArgs, uint32_t aCount, nsIObserver* aObserver,
+                    bool aHoldWeak) {
+  return CopyArgsAndRunProcess(false, aArgs, aCount, aObserver, aHoldWeak);
+}
+
+nsresult nsProcess::CopyArgsAndRunProcess(bool aBlocking, const char** aArgs,
+                                          uint32_t aCount,
+                                          nsIObserver* aObserver,
+                                          bool aHoldWeak) {
+  // Add one to the aCount for the program name and one for null termination.
+  char** my_argv = nullptr;
+  my_argv = (char**)moz_xmalloc(sizeof(char*) * (aCount + 2));
+
+  my_argv[0] = ToNewUTF8String(mTargetPath);
+
+  for (uint32_t i = 0; i < aCount; ++i) {
+    my_argv[i + 1] = const_cast<char*>(aArgs[i]);
+  }
+
+  my_argv[aCount + 1] = nullptr;
+
+  nsresult rv = RunProcess(aBlocking, my_argv, aObserver, aHoldWeak, false);
+
+  free(my_argv[0]);
+  free(my_argv);
+  return rv;
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::Runw(bool aBlocking, const char16_t** aArgs, uint32_t aCount) {
+  return CopyArgsAndRunProcessw(aBlocking, aArgs, aCount, nullptr, false);
+}
+
+// XXXldb |aArgs| has the wrong const-ness
+NS_IMETHODIMP
+nsProcess::RunwAsync(const char16_t** aArgs, uint32_t aCount,
+                     nsIObserver* aObserver, bool aHoldWeak) {
+  return CopyArgsAndRunProcessw(false, aArgs, aCount, aObserver, aHoldWeak);
+}
+
+nsresult nsProcess::CopyArgsAndRunProcessw(bool aBlocking,
+                                           const char16_t** aArgs,
+                                           uint32_t aCount,
+                                           nsIObserver* aObserver,
+                                           bool aHoldWeak) {
+  // Add one to the aCount for the program name and one for null termination.
+  char** my_argv = nullptr;
+  my_argv = (char**)moz_xmalloc(sizeof(char*) * (aCount + 2));
+
+  my_argv[0] = ToNewUTF8String(mTargetPath);
+
+  for (uint32_t i = 0; i < aCount; i++) {
+    my_argv[i + 1] = ToNewUTF8String(nsDependentString(aArgs[i]));
+  }
+
+  my_argv[aCount + 1] = nullptr;
+
+  nsresult rv = RunProcess(aBlocking, my_argv, aObserver, aHoldWeak, true);
+
+  for (uint32_t i = 0; i <= aCount; ++i) {
+    free(my_argv[i]);
+  }
+  free(my_argv);
+  return rv;
+}
+
+nsresult nsProcess::RunProcess(bool aBlocking, char** aMyArgv,
+                               nsIObserver* aObserver, bool aHoldWeak,
+                               bool aArgsUTF8) {
+  NS_WARNING_ASSERTION(!XRE_IsContentProcess(),
+                       "No launching of new processes in the content process");
+
+  if (NS_WARN_IF(!mExecutable)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  if (NS_WARN_IF(mThread)) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (aObserver) {
+    if (aHoldWeak) {
+      nsresult rv = NS_OK;
+      mObserver = do_GetWeakReference(aObserver, &rv);
+      NS_ENSURE_SUCCESS(rv, rv);
+    } else {
+      mObserver = aObserver;
+    }
+  }
+
+  {
+    MutexAutoLock lock(mLock);
+    mExitValue = -1;
+    mPid = -1;
+  }
+
+#if defined(PROCESSMODEL_WINAPI)
+  BOOL retVal;
+  UniqueFreePtr<wchar_t> cmdLine;
+
+  // |aMyArgv| is null-terminated and always starts with the program path. If
+  // the second slot is non-null then arguments are being passed.
+  if (aMyArgv[1] || mNoShell) {
+    // Pass the executable path as argv[0] to the launched program when calling
+    // CreateProcess().
+    char** argv = mNoShell ? aMyArgv : aMyArgv + 1;
+
+    wchar_t* assembledCmdLine = nullptr;
+    if (assembleCmdLine(argv, &assembledCmdLine,
+                        aArgsUTF8 ? CP_UTF8 : CP_ACP) == -1) {
+      return NS_ERROR_FILE_EXECUTION_FAILED;
+    }
+    cmdLine.reset(assembledCmdLine);
+  }
+
+  // The program name in aMyArgv[0] is always UTF-8
+  NS_ConvertUTF8toUTF16 wideFile(aMyArgv[0]);
+
+  if (mNoShell) {
+    STARTUPINFO startupInfo;
+    ZeroMemory(&startupInfo, sizeof(startupInfo));
+    startupInfo.cb = sizeof(startupInfo);
+    startupInfo.dwFlags = STARTF_USESHOWWINDOW;
+    startupInfo.wShowWindow = mStartHidden ? SW_HIDE : SW_SHOWNORMAL;
+
+    PROCESS_INFORMATION processInfo;
+    retVal = CreateProcess(/* lpApplicationName = */ wideFile.get(),
+                           /* lpCommandLine */ cmdLine.get(),
+                           /* lpProcessAttributes = */ NULL,
+                           /* lpThreadAttributes = */ NULL,
+                           /* bInheritHandles = */ FALSE,
+                           /* dwCreationFlags = */ 0,
+                           /* lpEnvironment = */ NULL,
+                           /* lpCurrentDirectory = */ NULL,
+                           /* lpStartupInfo = */ &startupInfo,
+                           /* lpProcessInformation */ &processInfo);
+
+    if (!retVal) {
+      return NS_ERROR_FILE_EXECUTION_FAILED;
+    }
+
+    CloseHandle(processInfo.hThread);
+
+    // TODO(bug 1763051): assess if we need further work around this locking.
+    MutexAutoLock lock(mLock);
+    mProcess = processInfo.hProcess;
+  } else {
+    SHELLEXECUTEINFOW sinfo;
+    memset(&sinfo, 0, sizeof(SHELLEXECUTEINFOW));
+    sinfo.cbSize = sizeof(SHELLEXECUTEINFOW);
+    sinfo.hwnd = nullptr;
+    sinfo.lpFile = wideFile.get();
+    sinfo.nShow = mStartHidden ? SW_HIDE : SW_SHOWNORMAL;
+
+    /* The SEE_MASK_NO_CONSOLE flag is important to prevent console windows
+     * from appearing. This makes behavior the same on all platforms. The flag
+     * will not have any effect on non-console applications.
+     */
+    sinfo.fMask =
+        SEE_MASK_FLAG_DDEWAIT | SEE_MASK_NO_CONSOLE | SEE_MASK_NOCLOSEPROCESS;
+
+    if (cmdLine) {
+      sinfo.lpParameters = cmdLine.get();
+    }
+
+    retVal = ShellExecuteExW(&sinfo);
+    if (!retVal) {
+      return NS_ERROR_FILE_EXECUTION_FAILED;
+    }
+
+    MutexAutoLock lock(mLock);
+    mProcess = sinfo.hProcess;
+  }
+
+  {
+    MutexAutoLock lock(mLock);
+    mPid = GetProcessId(mProcess);
+  }
+#elif defined(XP_MACOSX)
+  // Note: |aMyArgv| is already null-terminated as required by posix_spawnp.
+  pid_t newPid = 0;
+  int result = posix_spawnp(&newPid, aMyArgv[0], nullptr, nullptr, aMyArgv,
+                            *_NSGetEnviron());
+  mPid = static_cast<int32_t>(newPid);
+
+  if (result != 0) {
+    return NS_ERROR_FAILURE;
+  }
+#elif defined(XP_UNIX)
+  base::LaunchOptions options;
+  std::vector<std::string> argvVec;
+  for (char** arg = aMyArgv; *arg != nullptr; ++arg) {
+    argvVec.push_back(*arg);
+  }
+  pid_t newPid;
+  if (base::LaunchApp(argvVec, std::move(options), &newPid).isOk()) {
+    static_assert(sizeof(pid_t) <= sizeof(int32_t),
+                  "mPid is large enough to hold a pid");
+    mPid = static_cast<int32_t>(newPid);
+  } else {
+    return NS_ERROR_FAILURE;
+  }
+#else
+  {
+    PRProcess* prProcess =
+        PR_CreateProcess(aMyArgv[0], aMyArgv, nullptr, nullptr);
+    if (!prProcess) {
+      return NS_ERROR_FAILURE;
+    }
+    {
+      MutexAutoLock lock(mLock);
+      mProcess = prProcess;
+    }
+    struct MYProcess {
+      uint32_t pid;
+    };
+    MYProcess* ptrProc = (MYProcess*)mProcess;
+    mPid = ptrProc->pid;
+  }
+#endif
+
+  NS_ADDREF_THIS();
+  mBlocking = aBlocking;
+  if (aBlocking) {
+    Monitor(this);
+    MutexAutoLock lock(mLock);
+    if (mExitValue < 0) {
+      return NS_ERROR_FILE_EXECUTION_FAILED;
+    }
+  } else {
+    mThread = CreateMonitorThread();
+    if (!mThread) {
+      NS_RELEASE_THIS();
+      return NS_ERROR_FAILURE;
+    }
+
+    // It isn't a failure if we just can't watch for shutdown
+    nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
+    if (os) {
+      os->AddObserver(this, "xpcom-shutdown", false);
+    }
+  }
+
+  return NS_OK;
+}
+
+// We don't guarantee that monitor threads are joined before Gecko exits, which
+// can cause TSAN to complain about thread leaks. We handle this with a TSAN
+// suppression, and route thread creation through this helper so that the
+// suppression is as narrowly-scoped as possible.
+PRThread* nsProcess::CreateMonitorThread() {
+  return PR_CreateThread(PR_SYSTEM_THREAD, Monitor, this, PR_PRIORITY_NORMAL,
+                         PR_GLOBAL_THREAD, PR_JOINABLE_THREAD, 0);
+}
+
+NS_IMETHODIMP
+nsProcess::GetIsRunning(bool* aIsRunning) {
+  if (mThread) {
+    *aIsRunning = true;
+  } else {
+    *aIsRunning = false;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetStartHidden(bool* aStartHidden) {
+  *aStartHidden = mStartHidden;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::SetStartHidden(bool aStartHidden) {
+  mStartHidden = aStartHidden;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetNoShell(bool* aNoShell) {
+  *aNoShell = mNoShell;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::SetNoShell(bool aNoShell) {
+  mNoShell = aNoShell;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetPid(uint32_t* aPid) {
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+  if (mPid < 0) {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+  *aPid = mPid;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::Kill() {
+  if (!mThread) {
+    return NS_ERROR_FAILURE;
+  }
+
+  {
+    MutexAutoLock lock(mLock);
+#if defined(PROCESSMODEL_WINAPI)
+    if (TerminateProcess(mProcess, 0) == 0) {
+      return NS_ERROR_FAILURE;
+    }
+#elif defined(XP_UNIX)
+    if (kill(mPid, SIGKILL) != 0) {
+      return NS_ERROR_FAILURE;
+    }
+#else
+    if (!mProcess || (PR_KillProcess(mProcess) != PR_SUCCESS)) {
+      return NS_ERROR_FAILURE;
+    }
+#endif
+  }
+
+  // We must null out mThread if we want IsRunning to return false immediately
+  // after this call.
+  nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
+  if (os) {
+    os->RemoveObserver(this, "xpcom-shutdown");
+  }
+  PR_JoinThread(mThread);
+  mThread = nullptr;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::GetExitValue(int32_t* aExitValue) {
+  MutexAutoLock lock(mLock);
+
+  *aExitValue = mExitValue;
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsProcess::Observe(nsISupports* aSubject, const char* aTopic,
+                   const char16_t* aData) {
+  // Shutting down, drop all references
+  if (mThread) {
+    nsCOMPtr<nsIObserverService> os = mozilla::services::GetObserverService();
+    if (os) {
+      os->RemoveObserver(this, "xpcom-shutdown");
+    }
+    mThread = nullptr;
+  }
+
+  mObserver = nullptr;
+
+  MutexAutoLock lock(mLock);
+  mShutdown = true;
+
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsProxyRelease.cpp b/xpcom/threads/nsProxyRelease.cpp
new file mode 100644
index 0000000000..86ed18c8b5
--- /dev/null
+++ b/xpcom/threads/nsProxyRelease.cpp
@@ -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/. */
+
+#include "nsProxyRelease.h"
+#include "nsThreadUtils.h"
+
+namespace detail {
+
+/* static */ nsresult ProxyReleaseChooser<true>::ProxyReleaseISupports(
+    const char* aName, nsIEventTarget* aTarget, nsISupports* aDoomed,
+    bool aAlwaysProxy) {
+  return ::detail::ProxyRelease<nsISupports>(
+      aName, aTarget, dont_AddRef(aDoomed), aAlwaysProxy);
+}
+
+}  // namespace detail
+
+extern "C" {
+
+// This function uses C linkage because it's exposed to Rust to support the
+// `ThreadPtrHolder` wrapper in the `moz_task` crate.
+void NS_ProxyReleaseISupports(const char* aName, nsIEventTarget* aTarget,
+                              nsISupports* aDoomed, bool aAlwaysProxy) {
+  NS_ProxyRelease(aName, aTarget, dont_AddRef(aDoomed), aAlwaysProxy);
+}
+
+}  // extern "C"
diff --git a/xpcom/threads/nsProxyRelease.h b/xpcom/threads/nsProxyRelease.h
new file mode 100644
index 0000000000..c125eae2aa
--- /dev/null
+++ b/xpcom/threads/nsProxyRelease.h
@@ -0,0 +1,380 @@
+/* -*- 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 nsProxyRelease_h__
+#define nsProxyRelease_h__
+
+#include <utility>
+
+#include "MainThreadUtils.h"
+#include "mozilla/Likely.h"
+#include "mozilla/Unused.h"
+#include "nsCOMPtr.h"
+#include "nsIEventTarget.h"
+#include "nsISerialEventTarget.h"
+#include "nsIThread.h"
+#include "nsPrintfCString.h"
+#include "nsThreadUtils.h"
+
+#ifdef XPCOM_GLUE_AVOID_NSPR
+#  error NS_ProxyRelease implementation depends on NSPR.
+#endif
+
+class nsIRunnable;
+
+namespace detail {
+
+template <typename T>
+class ProxyReleaseEvent : public mozilla::CancelableRunnable {
+ public:
+  ProxyReleaseEvent(const char* aName, already_AddRefed<T> aDoomed)
+      : CancelableRunnable(aName), mDoomed(aDoomed.take()) {}
+
+  NS_IMETHOD Run() override {
+    NS_IF_RELEASE(mDoomed);
+    return NS_OK;
+  }
+
+  nsresult Cancel() override { return Run(); }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_IMETHOD GetName(nsACString& aName) override {
+    if (mName) {
+      aName.Append(nsPrintfCString("ProxyReleaseEvent for %s", mName));
+    } else {
+      aName.AssignLiteral("ProxyReleaseEvent");
+    }
+    return NS_OK;
+  }
+#endif
+
+ private:
+  T* MOZ_OWNING_REF mDoomed;
+};
+
+template <typename T>
+nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                      already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+  // Auto-managing release of the pointer.
+  RefPtr<T> doomed = aDoomed;
+  nsresult rv;
+
+  if (!doomed || !aTarget) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  if (!aAlwaysProxy) {
+    bool onCurrentThread = false;
+    rv = aTarget->IsOnCurrentThread(&onCurrentThread);
+    if (NS_SUCCEEDED(rv) && onCurrentThread) {
+      return NS_OK;
+    }
+  }
+
+  nsCOMPtr<nsIRunnable> ev = new ProxyReleaseEvent<T>(aName, doomed.forget());
+
+  rv = aTarget->Dispatch(ev, NS_DISPATCH_NORMAL);
+  if (NS_FAILED(rv)) {
+    NS_WARNING(nsPrintfCString(
+                   "failed to post proxy release event for %s, leaking!", aName)
+                   .get());
+    // It is better to leak the aDoomed object than risk crashing as
+    // a result of deleting it on the wrong thread.
+  }
+  return rv;
+}
+
+template <bool nsISupportsBased>
+struct ProxyReleaseChooser {
+  template <typename T>
+  static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                               already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+    return ::detail::ProxyRelease(aName, aTarget, std::move(aDoomed),
+                                  aAlwaysProxy);
+  }
+};
+
+template <>
+struct ProxyReleaseChooser<true> {
+  // We need an intermediate step for handling classes with ambiguous
+  // inheritance to nsISupports.
+  template <typename T>
+  static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                               already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
+    return ProxyReleaseISupports(aName, aTarget, ToSupports(aDoomed.take()),
+                                 aAlwaysProxy);
+  }
+
+  static nsresult ProxyReleaseISupports(const char* aName,
+                                        nsIEventTarget* aTarget,
+                                        nsISupports* aDoomed,
+                                        bool aAlwaysProxy);
+};
+
+}  // namespace detail
+
+/**
+ * Ensures that the delete of a smart pointer occurs on the target thread.
+ * Note: The doomed object will be leaked if dispatch to the target thread
+ * fails, as releasing it on the current thread may be unsafe
+ *
+ * @param aName
+ *        the labelling name of the runnable involved in the releasing.
+ * @param aTarget
+ *        the target thread where the doomed object should be released.
+ * @param aDoomed
+ *        the doomed object; the object to be released on the target thread.
+ * @param aAlwaysProxy
+ *        normally, if NS_ProxyRelease is called on the target thread, then the
+ *        doomed object will be released directly. However, if this parameter is
+ *        true, then an event will always be posted to the target thread for
+ *        asynchronous release.
+ * @return result of the task which is dispatched to delete the smart pointer
+ *        on the target thread.
+ *        Note: The caller should not attempt to recover from an
+ *        error code returned by trying to perform the final ->Release()
+ *        manually.
+ */
+template <class T>
+inline NS_HIDDEN_(nsresult)
+    NS_ProxyRelease(const char* aName, nsIEventTarget* aTarget,
+                    already_AddRefed<T> aDoomed, bool aAlwaysProxy = false) {
+  return ::detail::ProxyReleaseChooser<
+      std::is_base_of<nsISupports, T>::value>::ProxyRelease(aName, aTarget,
+                                                            std::move(aDoomed),
+                                                            aAlwaysProxy);
+}
+
+/**
+ * Ensures that the delete of a smart pointer occurs on the main thread.
+ *
+ * @param aName
+ *        the labelling name of the runnable involved in the releasing
+ * @param aDoomed
+ *        the doomed object; the object to be released on the main thread.
+ * @param aAlwaysProxy
+ *        normally, if NS_ReleaseOnMainThread is called on the main
+ *        thread, then the doomed object will be released directly. However, if
+ *        this parameter is true, then an event will always be posted to the
+ *        main thread for asynchronous release.
+ */
+template <class T>
+inline NS_HIDDEN_(void)
+    NS_ReleaseOnMainThread(const char* aName, already_AddRefed<T> aDoomed,
+                           bool aAlwaysProxy = false) {
+  RefPtr<T> doomed = aDoomed;
+  if (!doomed) {
+    return;  // Nothing to do.
+  }
+
+  // NS_ProxyRelease treats a null event target as "the current thread".  So a
+  // handle on the main thread is only necessary when we're not already on the
+  // main thread or the release must happen asynchronously.
+  nsCOMPtr<nsIEventTarget> target;
+  if (!NS_IsMainThread() || aAlwaysProxy) {
+    target = mozilla::GetMainThreadSerialEventTarget();
+
+    if (!target) {
+      MOZ_ASSERT_UNREACHABLE("Could not get main thread; leaking an object!");
+      mozilla::Unused << doomed.forget().take();
+      return;
+    }
+  }
+
+  NS_ProxyRelease(aName, target, doomed.forget(), aAlwaysProxy);
+}
+
+/**
+ * Class to safely handle main-thread-only pointers off the main thread.
+ *
+ * Classes like XPCWrappedJS are main-thread-only, which means that it is
+ * forbidden to call methods on instances of these classes off the main thread.
+ * For various reasons (see bug 771074), this restriction applies to
+ * AddRef/Release as well.
+ *
+ * This presents a problem for consumers that wish to hold a callback alive
+ * on non-main-thread code. A common example of this is the proxy callback
+ * pattern, where non-main-thread code holds a strong-reference to the callback
+ * object, and dispatches new Runnables (also with a strong reference) to the
+ * main thread in order to execute the callback. This involves several AddRef
+ * and Release calls on the other thread, which is verboten.
+ *
+ * The basic idea of this class is to introduce a layer of indirection.
+ * nsMainThreadPtrHolder is a threadsafe reference-counted class that internally
+ * maintains one strong reference to the main-thread-only object. It must be
+ * instantiated on the main thread (so that the AddRef of the underlying object
+ * happens on the main thread), but consumers may subsequently pass references
+ * to the holder anywhere they please. These references are meant to be opaque
+ * when accessed off-main-thread (assertions enforce this).
+ *
+ * The semantics of RefPtr<nsMainThreadPtrHolder<T>> would be cumbersome, so we
+ * also introduce nsMainThreadPtrHandle<T>, which is conceptually identical to
+ * the above (though it includes various convenience methods). The basic pattern
+ * is as follows.
+ *
+ * // On the main thread:
+ * nsCOMPtr<nsIFooCallback> callback = ...;
+ * nsMainThreadPtrHandle<nsIFooCallback> callbackHandle =
+ *   new nsMainThreadPtrHolder<nsIFooCallback>(callback);
+ * // Pass callbackHandle to structs/classes that might be accessed on other
+ * // threads.
+ *
+ * All structs and classes that might be accessed on other threads should store
+ * an nsMainThreadPtrHandle<T> rather than an nsCOMPtr<T>.
+ */
+template <class T>
+class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHolder final {
+ public:
+  // We can only acquire a pointer on the main thread. We want to fail fast for
+  // threading bugs, so by default we assert if our pointer is used or acquired
+  // off-main-thread. But some consumers need to use the same pointer for
+  // multiple classes, some of which are main-thread-only and some of which
+  // aren't. So we allow them to explicitly disable this strict checking.
+  nsMainThreadPtrHolder(const char* aName, T* aPtr, bool aStrict = true)
+      : mRawPtr(aPtr),
+        mStrict(aStrict)
+#ifndef RELEASE_OR_BETA
+        ,
+        mName(aName)
+#endif
+  {
+    // We can only AddRef our pointer on the main thread, which means that the
+    // holder must be constructed on the main thread.
+    MOZ_ASSERT(!mStrict || NS_IsMainThread());
+    NS_IF_ADDREF(mRawPtr);
+  }
+  nsMainThreadPtrHolder(const char* aName, already_AddRefed<T> aPtr,
+                        bool aStrict = true)
+      : mRawPtr(aPtr.take()),
+        mStrict(aStrict)
+#ifndef RELEASE_OR_BETA
+        ,
+        mName(aName)
+#endif
+  {
+    // Since we don't need to AddRef the pointer, this constructor is safe to
+    // call on any thread.
+  }
+
+  // Copy constructor and operator= deleted. Once constructed, the holder is
+  // immutable.
+  T& operator=(nsMainThreadPtrHolder& aOther) = delete;
+  nsMainThreadPtrHolder(const nsMainThreadPtrHolder& aOther) = delete;
+
+ private:
+  // We can be released on any thread.
+  ~nsMainThreadPtrHolder() {
+    if (NS_IsMainThread()) {
+      NS_IF_RELEASE(mRawPtr);
+    } else if (mRawPtr) {
+      NS_ReleaseOnMainThread(
+#ifdef RELEASE_OR_BETA
+          nullptr,
+#else
+          mName,
+#endif
+          dont_AddRef(mRawPtr));
+    }
+  }
+
+ public:
+  T* get() const {
+    // Nobody should be touching the raw pointer off-main-thread.
+    if (mStrict && MOZ_UNLIKELY(!NS_IsMainThread())) {
+      NS_ERROR("Can't dereference nsMainThreadPtrHolder off main thread");
+      MOZ_CRASH();
+    }
+    return mRawPtr;
+  }
+
+  bool operator==(const nsMainThreadPtrHolder<T>& aOther) const {
+    return mRawPtr == aOther.mRawPtr;
+  }
+  bool operator!() const { return !mRawPtr; }
+
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)
+
+ private:
+  // Our wrapped pointer.
+  T* mRawPtr = nullptr;
+
+  // Whether to strictly enforce thread invariants in this class.
+  bool mStrict = true;
+
+#ifndef RELEASE_OR_BETA
+  const char* mName = nullptr;
+#endif
+};
+
+template <class T>
+class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHandle {
+ public:
+  nsMainThreadPtrHandle() : mPtr(nullptr) {}
+  MOZ_IMPLICIT nsMainThreadPtrHandle(decltype(nullptr)) : mPtr(nullptr) {}
+  explicit nsMainThreadPtrHandle(nsMainThreadPtrHolder<T>* aHolder)
+      : mPtr(aHolder) {}
+  explicit nsMainThreadPtrHandle(
+      already_AddRefed<nsMainThreadPtrHolder<T>> aHolder)
+      : mPtr(aHolder) {}
+  nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) = default;
+  nsMainThreadPtrHandle(nsMainThreadPtrHandle&& aOther) = default;
+  nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) =
+      default;
+  nsMainThreadPtrHandle& operator=(nsMainThreadPtrHandle&& aOther) = default;
+  nsMainThreadPtrHandle& operator=(nsMainThreadPtrHolder<T>* aHolder) {
+    mPtr = aHolder;
+    return *this;
+  }
+
+  // These all call through to nsMainThreadPtrHolder, and thus implicitly
+  // assert that we're on the main thread (if strict). Off-main-thread consumers
+  // must treat these handles as opaque.
+  T* get() const {
+    if (mPtr) {
+      return mPtr.get()->get();
+    }
+    return nullptr;
+  }
+
+  operator T*() const { return get(); }
+  T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return get(); }
+
+  // These are safe to call on other threads with appropriate external locking.
+  bool operator==(const nsMainThreadPtrHandle<T>& aOther) const {
+    if (!mPtr || !aOther.mPtr) {
+      return mPtr == aOther.mPtr;
+    }
+    return *mPtr == *aOther.mPtr;
+  }
+  bool operator!=(const nsMainThreadPtrHandle<T>& aOther) const {
+    return !operator==(aOther);
+  }
+  bool operator==(decltype(nullptr)) const { return mPtr == nullptr; }
+  bool operator!=(decltype(nullptr)) const { return mPtr != nullptr; }
+  bool operator!() const { return !mPtr || !*mPtr; }
+
+ private:
+  RefPtr<nsMainThreadPtrHolder<T>> mPtr;
+};
+
+class nsCycleCollectionTraversalCallback;
+template <typename T>
+void CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
+                              T* aChild, const char* aName, uint32_t aFlags);
+
+template <typename T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    nsMainThreadPtrHandle<T>& aField, const char* aName, uint32_t aFlags = 0) {
+  CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
+}
+
+template <typename T>
+inline void ImplCycleCollectionUnlink(nsMainThreadPtrHandle<T>& aField) {
+  aField = nullptr;
+}
+
+#endif
diff --git a/xpcom/threads/nsThread.cpp b/xpcom/threads/nsThread.cpp
new file mode 100644
index 0000000000..98542e9438
--- /dev/null
+++ b/xpcom/threads/nsThread.cpp
@@ -0,0 +1,1560 @@
+/* -*- 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 "nsThread.h"
+
+#include "base/message_loop.h"
+#include "base/platform_thread.h"
+
+// Chromium's logging can sometimes leak through...
+#ifdef LOG
+#  undef LOG
+#endif
+
+#include "mozilla/ReentrantMonitor.h"
+#include "nsMemoryPressure.h"
+#include "nsThreadManager.h"
+#include "nsIClassInfoImpl.h"
+#include "nsCOMPtr.h"
+#include "nsQueryObject.h"
+#include "pratom.h"
+#include "mozilla/BackgroundHangMonitor.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Logging.h"
+#include "nsIObserverService.h"
+#include "mozilla/IOInterposer.h"
+#include "mozilla/ipc/MessageChannel.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/ProfilerRunnable.h"
+#include "mozilla/SchedulerGroup.h"
+#include "mozilla/Services.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/StaticLocalPtr.h"
+#include "mozilla/StaticPrefs_threads.h"
+#include "mozilla/TaskController.h"
+#include "nsXPCOMPrivate.h"
+#include "mozilla/ChaosMode.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Unused.h"
+#include "mozilla/dom/DocGroup.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "nsThreadSyncDispatch.h"
+#include "nsServiceManagerUtils.h"
+#include "GeckoProfiler.h"
+#include "ThreadEventQueue.h"
+#include "ThreadEventTarget.h"
+#include "ThreadDelay.h"
+
+#include <limits>
+
+#ifdef XP_LINUX
+#  ifdef __GLIBC__
+#    include <gnu/libc-version.h>
+#  endif
+#  include <sys/mman.h>
+#  include <sys/time.h>
+#  include <sys/resource.h>
+#  include <sched.h>
+#  include <stdio.h>
+#endif
+
+#ifdef XP_WIN
+#  include "mozilla/DynamicallyLinkedFunctionPtr.h"
+
+#  include <winbase.h>
+
+using GetCurrentThreadStackLimitsFn = void(WINAPI*)(PULONG_PTR LowLimit,
+                                                    PULONG_PTR HighLimit);
+#endif
+
+#define HAVE_UALARM                                                        \
+  _BSD_SOURCE ||                                                           \
+      (_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) && \
+          !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
+
+#if defined(XP_LINUX) && !defined(ANDROID) && defined(_GNU_SOURCE)
+#  define HAVE_SCHED_SETAFFINITY
+#endif
+
+#ifdef XP_MACOSX
+#  include <mach/mach.h>
+#  include <mach/thread_policy.h>
+#  include <sys/qos.h>
+#endif
+
+#ifdef MOZ_CANARY
+#  include <unistd.h>
+#  include <execinfo.h>
+#  include <signal.h>
+#  include <fcntl.h>
+#  include "nsXULAppAPI.h"
+#endif
+
+using namespace mozilla;
+
+extern void InitThreadLocalVariables();
+
+static LazyLogModule sThreadLog("nsThread");
+#ifdef LOG
+#  undef LOG
+#endif
+#define LOG(args) MOZ_LOG(sThreadLog, mozilla::LogLevel::Debug, args)
+
+NS_DECL_CI_INTERFACE_GETTER(nsThread)
+
+Array<char, nsThread::kRunnableNameBufSize> nsThread::sMainThreadRunnableName;
+
+#ifdef EARLY_BETA_OR_EARLIER
+const uint32_t kTelemetryWakeupCountLimit = 100;
+#endif
+
+//-----------------------------------------------------------------------------
+// Because we do not have our own nsIFactory, we have to implement nsIClassInfo
+// somewhat manually.
+
+class nsThreadClassInfo : public nsIClassInfo {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED  // no mRefCnt
+      NS_DECL_NSICLASSINFO
+
+      nsThreadClassInfo() = default;
+};
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadClassInfo::AddRef() { return 2; }
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadClassInfo::Release() { return 1; }
+NS_IMPL_QUERY_INTERFACE(nsThreadClassInfo, nsIClassInfo)
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetInterfaces(nsTArray<nsIID>& aArray) {
+  return NS_CI_INTERFACE_GETTER_NAME(nsThread)(aArray);
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetScriptableHelper(nsIXPCScriptable** aResult) {
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetContractID(nsACString& aResult) {
+  aResult.SetIsVoid(true);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassDescription(nsACString& aResult) {
+  aResult.SetIsVoid(true);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassID(nsCID** aResult) {
+  *aResult = nullptr;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetFlags(uint32_t* aResult) {
+  *aResult = THREADSAFE;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadClassInfo::GetClassIDNoAlloc(nsCID* aResult) {
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
+//-----------------------------------------------------------------------------
+
+NS_IMPL_ADDREF(nsThread)
+NS_IMPL_RELEASE(nsThread)
+NS_INTERFACE_MAP_BEGIN(nsThread)
+  NS_INTERFACE_MAP_ENTRY(nsIThread)
+  NS_INTERFACE_MAP_ENTRY(nsIThreadInternal)
+  NS_INTERFACE_MAP_ENTRY(nsIEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsISerialEventTarget)
+  NS_INTERFACE_MAP_ENTRY(nsISupportsPriority)
+  NS_INTERFACE_MAP_ENTRY(nsIDirectTaskDispatcher)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIThread)
+  if (aIID.Equals(NS_GET_IID(nsIClassInfo))) {
+    static nsThreadClassInfo sThreadClassInfo;
+    foundInterface = static_cast<nsIClassInfo*>(&sThreadClassInfo);
+  } else
+NS_INTERFACE_MAP_END
+NS_IMPL_CI_INTERFACE_GETTER(nsThread, nsIThread, nsIThreadInternal,
+                            nsIEventTarget, nsISerialEventTarget,
+                            nsISupportsPriority)
+
+//-----------------------------------------------------------------------------
+
+// This event is responsible for notifying nsThread::Shutdown that it is time
+// to call PR_JoinThread. It implements nsICancelableRunnable so that it can
+// run on a DOM Worker thread (where all events must implement
+// nsICancelableRunnable.)
+class nsThreadShutdownAckEvent : public CancelableRunnable {
+ public:
+  explicit nsThreadShutdownAckEvent(NotNull<nsThreadShutdownContext*> aCtx)
+      : CancelableRunnable("nsThreadShutdownAckEvent"),
+        mShutdownContext(aCtx) {}
+  NS_IMETHOD Run() override {
+    mShutdownContext->mTerminatingThread->ShutdownComplete(mShutdownContext);
+    return NS_OK;
+  }
+  nsresult Cancel() override { return Run(); }
+
+ private:
+  virtual ~nsThreadShutdownAckEvent() = default;
+
+  NotNull<RefPtr<nsThreadShutdownContext>> mShutdownContext;
+};
+
+// This event is responsible for setting mShutdownContext
+class nsThreadShutdownEvent : public Runnable {
+ public:
+  nsThreadShutdownEvent(NotNull<nsThread*> aThr,
+                        NotNull<nsThreadShutdownContext*> aCtx)
+      : Runnable("nsThreadShutdownEvent"),
+        mThread(aThr),
+        mShutdownContext(aCtx) {}
+  NS_IMETHOD Run() override {
+    // Creates a cycle between `mThread` and the shutdown context which will be
+    // broken when the thread exits.
+    mThread->mShutdownContext = mShutdownContext;
+    MessageLoop::current()->Quit();
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    // Let's leave a trace that we passed here in the thread's name.
+    nsAutoCString threadName(PR_GetThreadName(PR_GetCurrentThread()));
+    threadName.Append(",SHDRCV"_ns);
+    NS_SetCurrentThreadName(threadName.get());
+#endif
+    return NS_OK;
+  }
+
+ private:
+  NotNull<RefPtr<nsThread>> mThread;
+  NotNull<RefPtr<nsThreadShutdownContext>> mShutdownContext;
+};
+
+//-----------------------------------------------------------------------------
+
+static void SetThreadAffinity(unsigned int cpu) {
+#ifdef HAVE_SCHED_SETAFFINITY
+  cpu_set_t cpus;
+  CPU_ZERO(&cpus);
+  CPU_SET(cpu, &cpus);
+  sched_setaffinity(0, sizeof(cpus), &cpus);
+  // Don't assert sched_setaffinity's return value because it intermittently (?)
+  // fails with EINVAL on Linux x64 try runs.
+#elif defined(XP_MACOSX)
+  // OS X does not provide APIs to pin threads to specific processors, but you
+  // can tag threads as belonging to the same "affinity set" and the OS will try
+  // to run them on the same processor. To run threads on different processors,
+  // tag them as belonging to different affinity sets. Tag 0, the default, means
+  // "no affinity" so let's pretend each CPU has its own tag `cpu+1`.
+  thread_affinity_policy_data_t policy;
+  policy.affinity_tag = cpu + 1;
+  kern_return_t kr = thread_policy_set(
+      mach_thread_self(), THREAD_AFFINITY_POLICY, &policy.affinity_tag, 1);
+  // Setting the thread affinity is not supported on ARM.
+  MOZ_ALWAYS_TRUE(kr == KERN_SUCCESS || kr == KERN_NOT_SUPPORTED);
+#elif defined(XP_WIN)
+  MOZ_ALWAYS_TRUE(SetThreadIdealProcessor(GetCurrentThread(), cpu) !=
+                  (DWORD)-1);
+#endif
+}
+
+static void SetupCurrentThreadForChaosMode() {
+  if (!ChaosMode::isActive(ChaosFeature::ThreadScheduling)) {
+    return;
+  }
+
+#ifdef XP_LINUX
+  // PR_SetThreadPriority doesn't really work since priorities >
+  // PR_PRIORITY_NORMAL can't be set by non-root users. Instead we'll just use
+  // setpriority(2) to set random 'nice values'. In regular Linux this is only
+  // a dynamic adjustment so it still doesn't really do what we want, but tools
+  // like 'rr' can be more aggressive about honoring these values.
+  // Some of these calls may fail due to trying to lower the priority
+  // (e.g. something may have already called setpriority() for this thread).
+  // This makes it hard to have non-main threads with higher priority than the
+  // main thread, but that's hard to fix. Tools like rr can choose to honor the
+  // requested values anyway.
+  // Use just 4 priorities so there's a reasonable chance of any two threads
+  // having equal priority.
+  setpriority(PRIO_PROCESS, 0, ChaosMode::randomUint32LessThan(4));
+#else
+  // We should set the affinity here but NSPR doesn't provide a way to expose
+  // it.
+  uint32_t priority = ChaosMode::randomUint32LessThan(PR_PRIORITY_LAST + 1);
+  PR_SetThreadPriority(PR_GetCurrentThread(), PRThreadPriority(priority));
+#endif
+
+  // Force half the threads to CPU 0 so they compete for CPU
+  if (ChaosMode::randomUint32LessThan(2)) {
+    SetThreadAffinity(0);
+  }
+}
+
+namespace {
+
+struct ThreadInitData {
+  RefPtr<nsThread> thread;
+  nsCString name;
+};
+
+}  // namespace
+
+void nsThread::MaybeRemoveFromThreadList() {
+  nsThreadManager& tm = nsThreadManager::get();
+  OffTheBooksMutexAutoLock mal(tm.ThreadListMutex());
+  if (isInList()) {
+    removeFrom(tm.ThreadList());
+  }
+}
+
+/*static*/
+void nsThread::ThreadFunc(void* aArg) {
+  using mozilla::ipc::BackgroundChild;
+
+  UniquePtr<ThreadInitData> initData(static_cast<ThreadInitData*>(aArg));
+  RefPtr<nsThread>& self = initData->thread;
+
+  MOZ_ASSERT(self->mEventTarget);
+  MOZ_ASSERT(self->mEvents);
+
+  // Note: see the comment in nsThread::Init, where we set these same values.
+  DebugOnly<PRThread*> prev = self->mThread.exchange(PR_GetCurrentThread());
+  MOZ_ASSERT(!prev || prev == PR_GetCurrentThread());
+  self->mEventTarget->SetCurrentThread(self->mThread);
+  SetupCurrentThreadForChaosMode();
+
+  if (!initData->name.IsEmpty()) {
+    NS_SetCurrentThreadName(initData->name.BeginReading());
+  }
+
+  self->InitCommon();
+
+  // Inform the ThreadManager
+  nsThreadManager::get().RegisterCurrentThread(*self);
+
+  mozilla::IOInterposer::RegisterCurrentThread();
+
+  // This must come after the call to nsThreadManager::RegisterCurrentThread(),
+  // because that call is needed to properly set up this thread as an nsThread,
+  // which profiler_register_thread() requires. See bug 1347007.
+  const bool registerWithProfiler = !initData->name.IsEmpty();
+  if (registerWithProfiler) {
+    PROFILER_REGISTER_THREAD(initData->name.BeginReading());
+  }
+
+  {
+    // Scope for MessageLoop.
+    MessageLoop loop(
+#if defined(XP_WIN) || defined(XP_MACOSX)
+        self->mIsUiThread ? MessageLoop::TYPE_MOZILLA_NONMAINUITHREAD
+                          : MessageLoop::TYPE_MOZILLA_NONMAINTHREAD,
+#else
+        MessageLoop::TYPE_MOZILLA_NONMAINTHREAD,
+#endif
+        self);
+
+    // Now, process incoming events...
+    loop.Run();
+
+    self->mEvents->RunShutdownTasks();
+
+    BackgroundChild::CloseForCurrentThread();
+
+    // NB: The main thread does not shut down here!  It shuts down via
+    // nsThreadManager::Shutdown.
+
+    // Do NS_ProcessPendingEvents but with special handling to set
+    // mEventsAreDoomed atomically with the removal of the last event. The key
+    // invariant here is that we will never permit PutEvent to succeed if the
+    // event would be left in the queue after our final call to
+    // NS_ProcessPendingEvents. We also have to keep processing events as long
+    // as we have outstanding mRequestedShutdownContexts.
+    while (true) {
+      // Check and see if we're waiting on any threads.
+      self->WaitForAllAsynchronousShutdowns();
+
+      if (self->mEvents->ShutdownIfNoPendingEvents()) {
+        break;
+      }
+      NS_ProcessPendingEvents(self);
+    }
+  }
+
+  mozilla::IOInterposer::UnregisterCurrentThread();
+
+  // Inform the threadmanager that this thread is going away
+  nsThreadManager::get().UnregisterCurrentThread(*self);
+
+  // The thread should only unregister itself if it was registered above.
+  if (registerWithProfiler) {
+    PROFILER_UNREGISTER_THREAD();
+  }
+
+  NotNull<RefPtr<nsThreadShutdownContext>> context =
+      WrapNotNull(self->mShutdownContext);
+  self->mShutdownContext = nullptr;
+  MOZ_ASSERT(context->mTerminatingThread == self);
+
+  // Take the joining thread from our shutdown context. This may have been
+  // cleared by the joining thread if it decided to cancel waiting on us, in
+  // which case we won't notify our caller, and leak.
+  RefPtr<nsThread> joiningThread;
+  {
+    MutexAutoLock lock(context->mJoiningThreadMutex);
+    joiningThread = context->mJoiningThread.forget();
+    MOZ_RELEASE_ASSERT(joiningThread || context->mThreadLeaked);
+  }
+  if (joiningThread) {
+    // Dispatch shutdown ACK
+    nsCOMPtr<nsIRunnable> event = new nsThreadShutdownAckEvent(context);
+    nsresult dispatch_ack_rv =
+        joiningThread->Dispatch(event, NS_DISPATCH_NORMAL);
+
+    // We do not expect this to ever happen, but If we cannot dispatch
+    // the ack event, someone probably blocks waiting on us and will
+    // crash with a hang later anyways. The best we can do is to tell
+    // the world what happened right here.
+    MOZ_RELEASE_ASSERT(NS_SUCCEEDED(dispatch_ack_rv));
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    // Let's leave a trace that we passed here in the thread's name.
+    nsAutoCString threadName(PR_GetThreadName(PR_GetCurrentThread()));
+    threadName.Append(",SHDACK"_ns);
+    NS_SetCurrentThreadName(threadName.get());
+#endif
+  } else {
+    NS_WARNING(
+        "nsThread exiting after StopWaitingAndLeakThread was called, thread "
+        "resources will be leaked!");
+  }
+
+  // Release any observer of the thread here.
+  self->SetObserver(nullptr);
+
+  // The PRThread will be deleted in PR_JoinThread(), so clear references.
+  self->mThread = nullptr;
+  self->mEventTarget->ClearCurrentThread();
+}
+
+void nsThread::InitCommon() {
+  mThreadId = uint32_t(PlatformThread::CurrentId());
+
+  {
+#if defined(XP_LINUX)
+    pthread_attr_t attr;
+    pthread_attr_init(&attr);
+    pthread_getattr_np(pthread_self(), &attr);
+
+    size_t stackSize;
+    pthread_attr_getstack(&attr, &mStackBase, &stackSize);
+
+    // Glibc prior to 2.27 reports the stack size and base including the guard
+    // region, so we need to compensate for it to get accurate accounting.
+    // Also, this behavior difference isn't guarded by a versioned symbol, so we
+    // actually need to check the runtime glibc version, not the version we were
+    // compiled against.
+    static bool sAdjustForGuardSize = ({
+#  ifdef __GLIBC__
+      unsigned major, minor;
+      sscanf(gnu_get_libc_version(), "%u.%u", &major, &minor) < 2 ||
+          major < 2 || (major == 2 && minor < 27);
+#  else
+      false;
+#  endif
+    });
+    if (sAdjustForGuardSize) {
+      size_t guardSize;
+      pthread_attr_getguardsize(&attr, &guardSize);
+
+      // Note: This assumes that the stack grows down, as is the case on all of
+      // our tier 1 platforms. On platforms where the stack grows up, the
+      // mStackBase adjustment is unnecessary, but doesn't cause any harm other
+      // than under-counting stack memory usage by one page.
+      mStackBase = reinterpret_cast<char*>(mStackBase) + guardSize;
+      stackSize -= guardSize;
+    }
+
+    mStackSize = stackSize;
+
+    // This is a bit of a hack.
+    //
+    // We really do want the NOHUGEPAGE flag on our thread stacks, since we
+    // don't expect any of them to need anywhere near 2MB of space. But setting
+    // it here is too late to have an effect, since the first stack page has
+    // already been faulted in existence, and NSPR doesn't give us a way to set
+    // it beforehand.
+    //
+    // What this does get us, however, is a different set of VM flags on our
+    // thread stacks compared to normal heap memory. Which makes the Linux
+    // kernel report them as separate regions, even when they are adjacent to
+    // heap memory. This allows us to accurately track the actual memory
+    // consumption of our allocated stacks.
+    madvise(mStackBase, stackSize, MADV_NOHUGEPAGE);
+
+    pthread_attr_destroy(&attr);
+#elif defined(XP_WIN)
+    static const StaticDynamicallyLinkedFunctionPtr<
+        GetCurrentThreadStackLimitsFn>
+        sGetStackLimits(L"kernel32.dll", "GetCurrentThreadStackLimits");
+
+    if (sGetStackLimits) {
+      ULONG_PTR stackBottom, stackTop;
+      sGetStackLimits(&stackBottom, &stackTop);
+      mStackBase = reinterpret_cast<void*>(stackBottom);
+      mStackSize = stackTop - stackBottom;
+    }
+#endif
+  }
+
+  InitThreadLocalVariables();
+}
+
+//-----------------------------------------------------------------------------
+
+#ifdef MOZ_CANARY
+int sCanaryOutputFD = -1;
+#endif
+
+nsThread::nsThread(NotNull<SynchronizedEventQueue*> aQueue,
+                   MainThreadFlag aMainThread,
+                   nsIThreadManager::ThreadCreationOptions aOptions)
+    : mEvents(aQueue.get()),
+      mEventTarget(new ThreadEventTarget(
+          mEvents.get(), aMainThread == MAIN_THREAD, aOptions.blockDispatch)),
+      mOutstandingShutdownContexts(0),
+      mShutdownContext(nullptr),
+      mScriptObserver(nullptr),
+      mThreadName("<uninitialized>"),
+      mStackSize(aOptions.stackSize),
+      mNestedEventLoopDepth(0),
+      mShutdownRequired(false),
+      mPriority(PRIORITY_NORMAL),
+      mIsMainThread(aMainThread == MAIN_THREAD),
+      mUseHangMonitor(aMainThread == MAIN_THREAD),
+      mIsUiThread(aOptions.isUiThread),
+      mIsAPoolThreadFree(nullptr),
+      mCanInvokeJS(false),
+#ifdef EARLY_BETA_OR_EARLIER
+      mLastWakeupCheckTime(TimeStamp::Now()),
+#endif
+      mPerformanceCounterState(mNestedEventLoopDepth, mIsMainThread,
+                               aOptions.longTaskLength) {
+#if !(defined(XP_WIN) || defined(XP_MACOSX))
+  MOZ_ASSERT(!mIsUiThread,
+             "Non-main UI threads are only supported on Windows and macOS");
+#endif
+  if (mIsMainThread) {
+    MOZ_ASSERT(!mIsUiThread,
+               "Setting isUIThread is not supported for main threads");
+    mozilla::TaskController::Get()->SetPerformanceCounterState(
+        &mPerformanceCounterState);
+  }
+}
+
+nsThread::nsThread()
+    : mEvents(nullptr),
+      mEventTarget(nullptr),
+      mOutstandingShutdownContexts(0),
+      mShutdownContext(nullptr),
+      mScriptObserver(nullptr),
+      mThreadName("<uninitialized>"),
+      mStackSize(0),
+      mNestedEventLoopDepth(0),
+      mShutdownRequired(false),
+      mPriority(PRIORITY_NORMAL),
+      mIsMainThread(false),
+      mUseHangMonitor(false),
+      mIsUiThread(false),
+      mCanInvokeJS(false),
+#ifdef EARLY_BETA_OR_EARLIER
+      mLastWakeupCheckTime(TimeStamp::Now()),
+#endif
+      mPerformanceCounterState(mNestedEventLoopDepth) {
+  MOZ_ASSERT(!NS_IsMainThread());
+}
+
+nsThread::~nsThread() {
+  NS_ASSERTION(mOutstandingShutdownContexts == 0,
+               "shouldn't be waiting on other threads to shutdown");
+
+  MaybeRemoveFromThreadList();
+}
+
+nsresult nsThread::Init(const nsACString& aName) {
+  MOZ_ASSERT(mEvents);
+  MOZ_ASSERT(mEventTarget);
+  MOZ_ASSERT(!mThread);
+
+  SetThreadNameInternal(aName);
+
+  PRThread* thread = nullptr;
+
+  nsThreadManager& tm = nsThreadManager::get();
+  {
+    OffTheBooksMutexAutoLock lock(tm.ThreadListMutex());
+    if (!tm.AllowNewXPCOMThreadsLocked()) {
+      return NS_ERROR_NOT_INITIALIZED;
+    }
+
+    // We need to fully start the thread while holding the thread list lock, as
+    // the next acquire of the lock could try to shut down this thread (e.g.
+    // during xpcom shutdown), which would hang if `PR_CreateThread` failed.
+
+    UniquePtr<ThreadInitData> initData(
+        new ThreadInitData{this, nsCString(aName)});
+
+    // ThreadFunc is responsible for setting mThread
+    if (!(thread = PR_CreateThread(PR_USER_THREAD, ThreadFunc, initData.get(),
+                                   PR_PRIORITY_NORMAL, PR_GLOBAL_THREAD,
+                                   PR_JOINABLE_THREAD, mStackSize))) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+
+    // The created thread now owns initData, so release our ownership of it.
+    Unused << initData.release();
+
+    // The thread has successfully started, so we can mark it as requiring
+    // shutdown & add it to the thread list.
+    mShutdownRequired = true;
+    tm.ThreadList().insertBack(this);
+  }
+
+  // Note: we set these both here and inside ThreadFunc, to what should be
+  // the same value. This is because calls within ThreadFunc need these values
+  // to be set, and our callers need these values to be set.
+  DebugOnly<PRThread*> prev = mThread.exchange(thread);
+  MOZ_ASSERT(!prev || prev == thread);
+
+  mEventTarget->SetCurrentThread(thread);
+  return NS_OK;
+}
+
+nsresult nsThread::InitCurrentThread() {
+  mThread = PR_GetCurrentThread();
+
+  nsThreadManager& tm = nsThreadManager::get();
+  {
+    OffTheBooksMutexAutoLock lock(tm.ThreadListMutex());
+    // NOTE: We don't check AllowNewXPCOMThreads here, as threads initialized
+    // this way do not need shutdown, so are OK to create after nsThreadManager
+    // shutdown. In addition, the main thread is initialized this way, which
+    // happens before AllowNewXPCOMThreads begins to return true.
+    tm.ThreadList().insertBack(this);
+  }
+
+  SetupCurrentThreadForChaosMode();
+  InitCommon();
+
+  tm.RegisterCurrentThread(*this);
+  return NS_OK;
+}
+
+void nsThread::GetThreadName(nsACString& aNameBuffer) {
+  auto lock = mThreadName.Lock();
+  aNameBuffer = lock.ref();
+}
+
+void nsThread::SetThreadNameInternal(const nsACString& aName) {
+  auto lock = mThreadName.Lock();
+  lock->Assign(aName);
+}
+
+//-----------------------------------------------------------------------------
+// nsIEventTarget
+
+NS_IMETHODIMP
+nsThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+  MOZ_ASSERT(mEventTarget);
+  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
+
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return mEventTarget->Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+nsThread::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) {
+  MOZ_ASSERT(mEventTarget);
+  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
+
+  LOG(("THRD(%p) Dispatch [%p %x]\n", this, /* XXX aEvent */ nullptr, aFlags));
+
+  return mEventTarget->Dispatch(std::move(aEvent), aFlags);
+}
+
+NS_IMETHODIMP
+nsThread::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
+                          uint32_t aDelayMs) {
+  MOZ_ASSERT(mEventTarget);
+  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
+
+  return mEventTarget->DelayedDispatch(std::move(aEvent), aDelayMs);
+}
+
+NS_IMETHODIMP
+nsThread::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  MOZ_ASSERT(mEventTarget);
+  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
+
+  return mEventTarget->RegisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+nsThread::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  MOZ_ASSERT(mEventTarget);
+  NS_ENSURE_TRUE(mEventTarget, NS_ERROR_NOT_IMPLEMENTED);
+
+  return mEventTarget->UnregisterShutdownTask(aTask);
+}
+
+NS_IMETHODIMP
+nsThread::GetRunningEventDelay(TimeDuration* aDelay, TimeStamp* aStart) {
+  if (mIsAPoolThreadFree && *mIsAPoolThreadFree) {
+    // if there are unstarted threads in the pool, a new event to the
+    // pool would not be delayed at all (beyond thread start time)
+    *aDelay = TimeDuration();
+    *aStart = TimeStamp();
+  } else {
+    *aDelay = mLastEventDelay;
+    *aStart = mLastEventStart;
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetRunningEventDelay(TimeDuration aDelay, TimeStamp aStart) {
+  mLastEventDelay = aDelay;
+  mLastEventStart = aStart;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::IsOnCurrentThread(bool* aResult) {
+  if (mEventTarget) {
+    return mEventTarget->IsOnCurrentThread(aResult);
+  }
+  *aResult = PR_GetCurrentThread() == mThread;
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+nsThread::IsOnCurrentThreadInfallible() {
+  // This method is only going to be called if `mThread` is null, which
+  // only happens when the thread has exited the event loop.  Therefore, when
+  // we are called, we can never be on this thread.
+  return false;
+}
+
+//-----------------------------------------------------------------------------
+// nsIThread
+
+NS_IMETHODIMP
+nsThread::GetPRThread(PRThread** aResult) {
+  PRThread* thread = mThread;  // atomic load
+  *aResult = thread;
+  return thread ? NS_OK : NS_ERROR_NOT_AVAILABLE;
+}
+
+NS_IMETHODIMP
+nsThread::GetCanInvokeJS(bool* aResult) {
+  *aResult = mCanInvokeJS;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetCanInvokeJS(bool aCanInvokeJS) {
+  mCanInvokeJS = aCanInvokeJS;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::GetLastLongTaskEnd(TimeStamp* _retval) {
+  *_retval = mPerformanceCounterState.LastLongTaskEnd();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::GetLastLongNonIdleTaskEnd(TimeStamp* _retval) {
+  *_retval = mPerformanceCounterState.LastLongNonIdleTaskEnd();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetNameForWakeupTelemetry(const nsACString& aName) {
+#ifdef EARLY_BETA_OR_EARLIER
+  mNameForWakeupTelemetry = aName;
+#endif
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::AsyncShutdown() {
+  LOG(("THRD(%p) async shutdown\n", this));
+
+  nsCOMPtr<nsIThreadShutdown> shutdown;
+  BeginShutdown(getter_AddRefs(shutdown));
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::BeginShutdown(nsIThreadShutdown** aShutdown) {
+  LOG(("THRD(%p) begin shutdown\n", this));
+
+  MOZ_ASSERT(mEvents);
+  MOZ_ASSERT(mEventTarget);
+  MOZ_ASSERT(mThread != PR_GetCurrentThread());
+  if (NS_WARN_IF(mThread == PR_GetCurrentThread())) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // Prevent multiple calls to this method.
+  if (!mShutdownRequired.compareExchange(true, false)) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  MOZ_ASSERT(mThread);
+
+  RefPtr<nsThread> currentThread = nsThreadManager::get().GetCurrentThread();
+
+  MOZ_DIAGNOSTIC_ASSERT(currentThread->EventQueue(),
+                        "Shutdown() may only be called from an XPCOM thread");
+
+  // Allocate a shutdown context, and record that we're waiting for it.
+  RefPtr<nsThreadShutdownContext> context =
+      new nsThreadShutdownContext(WrapNotNull(this), currentThread);
+
+  ++currentThread->mOutstandingShutdownContexts;
+  nsCOMPtr<nsIRunnable> clearOutstanding = NS_NewRunnableFunction(
+      "nsThread::ClearOutstandingShutdownContext",
+      [currentThread] { --currentThread->mOutstandingShutdownContexts; });
+  context->OnCompletion(clearOutstanding);
+
+  // Set mShutdownContext and wake up the thread in case it is waiting for
+  // events to process.
+  nsCOMPtr<nsIRunnable> event =
+      new nsThreadShutdownEvent(WrapNotNull(this), WrapNotNull(context));
+  if (!mEvents->PutEvent(event.forget(), EventQueuePriority::Normal)) {
+    // We do not expect this to happen. Let's collect some diagnostics.
+    nsAutoCString threadName;
+    GetThreadName(threadName);
+    MOZ_CRASH_UNSAFE_PRINTF("Attempt to shutdown an already dead thread: %s",
+                            threadName.get());
+  }
+
+  // We could still end up with other events being added after the shutdown
+  // task, but that's okay because we process pending events in ThreadFunc
+  // after setting mShutdownContext just before exiting.
+  context.forget(aShutdown);
+  return NS_OK;
+}
+
+void nsThread::ShutdownComplete(NotNull<nsThreadShutdownContext*> aContext) {
+  MOZ_ASSERT(mEvents);
+  MOZ_ASSERT(mEventTarget);
+  MOZ_ASSERT(aContext->mTerminatingThread == this);
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+  {
+    MutexAutoLock lock(aContext->mJoiningThreadMutex);
+
+    // StopWaitingAndLeakThread is explicitely meant to not cause a
+    // nsThreadShutdownAckEvent on the joining thread, which is the only
+    // caller of ShutdownComplete.
+    MOZ_DIAGNOSTIC_ASSERT(!aContext->mThreadLeaked);
+  }
+#endif
+
+  MaybeRemoveFromThreadList();
+
+  // Now, it should be safe to join without fear of dead-locking.
+  PR_JoinThread(aContext->mTerminatingPRThread);
+  MOZ_ASSERT(!mThread);
+
+#ifdef DEBUG
+  nsCOMPtr<nsIThreadObserver> obs = mEvents->GetObserver();
+  MOZ_ASSERT(!obs, "Should have been cleared at shutdown!");
+#endif
+
+  aContext->MarkCompleted();
+}
+
+void nsThread::WaitForAllAsynchronousShutdowns() {
+  // This is the motivating example for why SpinEventLoopUntil
+  // has the template parameter we are providing here.
+  SpinEventLoopUntil<ProcessFailureBehavior::IgnoreAndContinue>(
+      "nsThread::WaitForAllAsynchronousShutdowns"_ns,
+      [&]() { return mOutstandingShutdownContexts == 0; }, this);
+}
+
+NS_IMETHODIMP
+nsThread::Shutdown() {
+  LOG(("THRD(%p) sync shutdown\n", this));
+
+  nsCOMPtr<nsIThreadShutdown> context;
+  nsresult rv = BeginShutdown(getter_AddRefs(context));
+  if (NS_FAILED(rv)) {
+    return NS_OK;  // The thread has already shut down.
+  }
+
+  // If we are going to hang here we want to see the thread's name
+  nsAutoCString threadName;
+  GetThreadName(threadName);
+
+  // Process events on the current thread until we receive a shutdown ACK.
+  // Allows waiting; ensure no locks are held that would deadlock us!
+  SpinEventLoopUntil("nsThread::Shutdown: "_ns + threadName,
+                     [&]() { return context->GetCompleted(); });
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::HasPendingEvents(bool* aResult) {
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  if (mIsMainThread) {
+    *aResult = TaskController::Get()->HasMainThreadPendingTasks();
+  } else {
+    *aResult = mEvents->HasPendingEvent();
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::HasPendingHighPriorityEvents(bool* aResult) {
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  // This function appears to never be called anymore.
+  *aResult = false;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::DispatchToQueue(already_AddRefed<nsIRunnable> aEvent,
+                          EventQueuePriority aQueue) {
+  nsCOMPtr<nsIRunnable> event = aEvent;
+
+  if (NS_WARN_IF(!event)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  if (!mEvents->PutEvent(event.forget(), aQueue)) {
+    NS_WARNING(
+        "An idle event was posted to a thread that will never run it "
+        "(rejected)");
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsThread::SetThreadQoS(nsIThread::QoSPriority aPriority) {
+  if (!StaticPrefs::threads_use_low_power_enabled()) {
+    return NS_OK;
+  }
+  // The approach here is to have a thread set itself for its QoS level,
+  // so we assert if we aren't on the current thread.
+  MOZ_ASSERT(IsOnCurrentThread(), "Can only change the current thread's QoS");
+
+#if defined(XP_MACOSX)
+  // Only arm64 macs may possess heterogeneous cores. On these, we can tell
+  // a thread to set its own QoS status. On intel macs things should behave
+  // normally, and the OS will ignore the QoS state of the thread.
+  if (aPriority == nsIThread::QOS_PRIORITY_LOW) {
+    pthread_set_qos_class_self_np(QOS_CLASS_BACKGROUND, 0);
+  } else if (NS_IsMainThread()) {
+    // MacOS documentation specifies that a main thread should be initialized at
+    // the USER_INTERACTIVE priority, so when we restore thread priorities the
+    // main thread should be setting itself to this.
+    pthread_set_qos_class_self_np(QOS_CLASS_USER_INTERACTIVE, 0);
+  } else {
+    pthread_set_qos_class_self_np(QOS_CLASS_DEFAULT, 0);
+  }
+#endif
+  // Do nothing if an OS-specific implementation is unavailable.
+  return NS_OK;
+}
+
+#ifdef MOZ_CANARY
+void canary_alarm_handler(int signum);
+
+class Canary {
+  // XXX ToDo: support nested loops
+ public:
+  Canary() {
+    if (sCanaryOutputFD > 0 && EventLatencyIsImportant()) {
+      signal(SIGALRM, canary_alarm_handler);
+      ualarm(15000, 0);
+    }
+  }
+
+  ~Canary() {
+    if (sCanaryOutputFD != 0 && EventLatencyIsImportant()) {
+      ualarm(0, 0);
+    }
+  }
+
+  static bool EventLatencyIsImportant() {
+    return NS_IsMainThread() && XRE_IsParentProcess();
+  }
+};
+
+void canary_alarm_handler(int signum) {
+  void* array[30];
+  const char msg[29] = "event took too long to run:\n";
+  // use write to be safe in the signal handler
+  write(sCanaryOutputFD, msg, sizeof(msg));
+  backtrace_symbols_fd(array, backtrace(array, 30), sCanaryOutputFD);
+}
+
+#endif
+
+#define NOTIFY_EVENT_OBSERVERS(observers_, func_, params_)                 \
+  do {                                                                     \
+    if (!observers_.IsEmpty()) {                                           \
+      for (nsCOMPtr<nsIThreadObserver> obs_ : observers_.ForwardRange()) { \
+        obs_->func_ params_;                                               \
+      }                                                                    \
+    }                                                                      \
+  } while (0)
+
+size_t nsThread::ShallowSizeOfIncludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) const {
+  size_t n = 0;
+  if (mShutdownContext) {
+    n += aMallocSizeOf(mShutdownContext);
+  }
+  return aMallocSizeOf(this) + aMallocSizeOf(mThread) + n;
+}
+
+size_t nsThread::SizeOfEventQueues(mozilla::MallocSizeOf aMallocSizeOf) const {
+  size_t n = 0;
+  if (mEventTarget) {
+    // The size of mEvents is reported by mEventTarget.
+    n += mEventTarget->SizeOfIncludingThis(aMallocSizeOf);
+  }
+  return n;
+}
+
+size_t nsThread::SizeOfIncludingThis(
+    mozilla::MallocSizeOf aMallocSizeOf) const {
+  return ShallowSizeOfIncludingThis(aMallocSizeOf) +
+         SizeOfEventQueues(aMallocSizeOf);
+}
+
+NS_IMETHODIMP
+nsThread::ProcessNextEvent(bool aMayWait, bool* aResult) {
+  MOZ_ASSERT(mEvents);
+  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
+
+  LOG(("THRD(%p) ProcessNextEvent [%u %u]\n", this, aMayWait,
+       mNestedEventLoopDepth));
+
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  // The toplevel event loop normally blocks waiting for the next event, but
+  // if we're trying to shut this thread down, we must exit the event loop
+  // when the event queue is empty. This only applys to the toplevel event
+  // loop! Nested event loops (e.g. during sync dispatch) are waiting for
+  // some state change and must be able to block even if something has
+  // requested shutdown of the thread. Otherwise we'll just busywait as we
+  // endlessly look for an event, fail to find one, and repeat the nested
+  // event loop since its state change hasn't happened yet.
+  bool reallyWait = aMayWait && (mNestedEventLoopDepth > 0 || !ShuttingDown());
+
+  Maybe<dom::AutoNoJSAPI> noJSAPI;
+
+  if (mUseHangMonitor && reallyWait) {
+    BackgroundHangMonitor().NotifyWait();
+  }
+
+  if (mIsMainThread) {
+    DoMainThreadSpecificProcessing();
+  }
+
+  ++mNestedEventLoopDepth;
+
+  // We only want to create an AutoNoJSAPI on threads that actually do DOM
+  // stuff (including workers).  Those are exactly the threads that have an
+  // mScriptObserver.
+  bool callScriptObserver = !!mScriptObserver;
+  if (callScriptObserver) {
+    noJSAPI.emplace();
+    mScriptObserver->BeforeProcessTask(reallyWait);
+  }
+
+  DrainDirectTasks();
+
+#ifdef EARLY_BETA_OR_EARLIER
+  // Need to capture mayWaitForWakeup state before OnProcessNextEvent,
+  // since on the main thread OnProcessNextEvent ends up waiting for the new
+  // events.
+  bool mayWaitForWakeup = reallyWait && !mEvents->HasPendingEvent();
+#endif
+
+  nsCOMPtr<nsIThreadObserver> obs = mEvents->GetObserverOnThread();
+  if (obs) {
+    obs->OnProcessNextEvent(this, reallyWait);
+  }
+
+  NOTIFY_EVENT_OBSERVERS(EventQueue()->EventObservers(), OnProcessNextEvent,
+                         (this, reallyWait));
+
+  DrainDirectTasks();
+
+#ifdef MOZ_CANARY
+  Canary canary;
+#endif
+  nsresult rv = NS_OK;
+
+  {
+    // Scope for |event| to make sure that its destructor fires while
+    // mNestedEventLoopDepth has been incremented, since that destructor can
+    // also do work.
+    nsCOMPtr<nsIRunnable> event;
+    bool usingTaskController = mIsMainThread;
+    if (usingTaskController) {
+      event = TaskController::Get()->GetRunnableForMTTask(reallyWait);
+    } else {
+      event = mEvents->GetEvent(reallyWait, &mLastEventDelay);
+    }
+
+    *aResult = (event.get() != nullptr);
+
+    if (event) {
+#ifdef EARLY_BETA_OR_EARLIER
+      if (mayWaitForWakeup && mThread) {
+        ++mWakeupCount;
+        if (mWakeupCount == kTelemetryWakeupCountLimit) {
+          TimeStamp now = TimeStamp::Now();
+          double ms = (now - mLastWakeupCheckTime).ToMilliseconds();
+          if (ms < 0) {
+            ms = 0;
+          }
+          const char* name = !mNameForWakeupTelemetry.IsEmpty()
+                                 ? mNameForWakeupTelemetry.get()
+                                 : PR_GetThreadName(mThread);
+          if (!name) {
+            name = mIsMainThread ? "MainThread" : "(nameless thread)";
+          }
+          nsDependentCString key(name);
+          Telemetry::Accumulate(Telemetry::THREAD_WAKEUP, key,
+                                static_cast<uint32_t>(ms));
+          mLastWakeupCheckTime = now;
+          mWakeupCount = 0;
+        }
+      }
+#endif
+
+      LOG(("THRD(%p) running [%p]\n", this, event.get()));
+
+      Maybe<LogRunnable::Run> log;
+
+      if (!usingTaskController) {
+        log.emplace(event);
+      }
+
+      // Delay event processing to encourage whoever dispatched this event
+      // to run.
+      DelayForChaosMode(ChaosFeature::TaskRunning, 1000);
+
+      mozilla::TimeStamp now = mozilla::TimeStamp::Now();
+
+      if (mUseHangMonitor) {
+        BackgroundHangMonitor().NotifyActivity();
+      }
+
+      Maybe<PerformanceCounterState::Snapshot> snapshot;
+      if (!usingTaskController) {
+        snapshot.emplace(mPerformanceCounterState.RunnableWillRun(now, false));
+      }
+
+      mLastEventStart = now;
+
+      if (!usingTaskController) {
+        AUTO_PROFILE_FOLLOWING_RUNNABLE(event);
+        event->Run();
+      } else {
+        // Avoid generating "Runnable" profiler markers for the
+        // "TaskController::ExecutePendingMTTasks" runnables created
+        // by TaskController, which already adds "Runnable" markers
+        // when executing tasks.
+        event->Run();
+      }
+
+      if (usingTaskController) {
+        *aResult = TaskController::Get()->MTTaskRunnableProcessedTask();
+      } else {
+        mPerformanceCounterState.RunnableDidRun(EmptyCString(),
+                                                std::move(snapshot.ref()));
+      }
+
+      // To cover the event's destructor code inside the LogRunnable span.
+      event = nullptr;
+    } else {
+      mLastEventDelay = TimeDuration();
+      mLastEventStart = TimeStamp();
+      if (aMayWait) {
+        MOZ_ASSERT(ShuttingDown(),
+                   "This should only happen when shutting down");
+        rv = NS_ERROR_UNEXPECTED;
+      }
+    }
+  }
+
+  DrainDirectTasks();
+
+  NOTIFY_EVENT_OBSERVERS(EventQueue()->EventObservers(), AfterProcessNextEvent,
+                         (this, *aResult));
+
+  if (obs) {
+    obs->AfterProcessNextEvent(this, *aResult);
+  }
+
+  // In case some EventObserver dispatched some direct tasks; process them
+  // now.
+  DrainDirectTasks();
+
+  if (callScriptObserver) {
+    if (mScriptObserver) {
+      mScriptObserver->AfterProcessTask(mNestedEventLoopDepth);
+    }
+    noJSAPI.reset();
+  }
+
+  --mNestedEventLoopDepth;
+
+  return rv;
+}
+
+//-----------------------------------------------------------------------------
+// nsISupportsPriority
+
+NS_IMETHODIMP
+nsThread::GetPriority(int32_t* aPriority) {
+  *aPriority = mPriority;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetPriority(int32_t aPriority) {
+  if (NS_WARN_IF(!mThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  // NSPR defines the following four thread priorities:
+  //   PR_PRIORITY_LOW
+  //   PR_PRIORITY_NORMAL
+  //   PR_PRIORITY_HIGH
+  //   PR_PRIORITY_URGENT
+  // We map the priority values defined on nsISupportsPriority to these
+  // values.
+
+  mPriority = aPriority;
+
+  PRThreadPriority pri;
+  if (mPriority <= PRIORITY_HIGHEST) {
+    pri = PR_PRIORITY_URGENT;
+  } else if (mPriority < PRIORITY_NORMAL) {
+    pri = PR_PRIORITY_HIGH;
+  } else if (mPriority > PRIORITY_NORMAL) {
+    pri = PR_PRIORITY_LOW;
+  } else {
+    pri = PR_PRIORITY_NORMAL;
+  }
+  // If chaos mode is active, retain the randomly chosen priority
+  if (!ChaosMode::isActive(ChaosFeature::ThreadScheduling)) {
+    PR_SetThreadPriority(mThread, pri);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::AdjustPriority(int32_t aDelta) {
+  return SetPriority(mPriority + aDelta);
+}
+
+//-----------------------------------------------------------------------------
+// nsIThreadInternal
+
+NS_IMETHODIMP
+nsThread::GetObserver(nsIThreadObserver** aObs) {
+  MOZ_ASSERT(mEvents);
+  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
+
+  nsCOMPtr<nsIThreadObserver> obs = mEvents->GetObserver();
+  obs.forget(aObs);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::SetObserver(nsIThreadObserver* aObs) {
+  MOZ_ASSERT(mEvents);
+  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
+
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  mEvents->SetObserver(aObs);
+  return NS_OK;
+}
+
+uint32_t nsThread::RecursionDepth() const {
+  MOZ_ASSERT(PR_GetCurrentThread() == mThread);
+  return mNestedEventLoopDepth;
+}
+
+NS_IMETHODIMP
+nsThread::AddObserver(nsIThreadObserver* aObserver) {
+  MOZ_ASSERT(mEvents);
+  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
+
+  if (NS_WARN_IF(!aObserver)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  EventQueue()->AddObserver(aObserver);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThread::RemoveObserver(nsIThreadObserver* aObserver) {
+  MOZ_ASSERT(mEvents);
+  NS_ENSURE_TRUE(mEvents, NS_ERROR_NOT_IMPLEMENTED);
+
+  if (NS_WARN_IF(PR_GetCurrentThread() != mThread)) {
+    return NS_ERROR_NOT_SAME_THREAD;
+  }
+
+  EventQueue()->RemoveObserver(aObserver);
+
+  return NS_OK;
+}
+
+void nsThread::SetScriptObserver(
+    mozilla::CycleCollectedJSContext* aScriptObserver) {
+  if (!aScriptObserver) {
+    mScriptObserver = nullptr;
+    return;
+  }
+
+  MOZ_ASSERT(!mScriptObserver);
+  mScriptObserver = aScriptObserver;
+}
+
+void NS_DispatchMemoryPressure();
+
+void nsThread::DoMainThreadSpecificProcessing() const {
+  MOZ_ASSERT(mIsMainThread);
+
+  ipc::CancelCPOWs();
+
+  // Fire a memory pressure notification, if one is pending.
+  if (!ShuttingDown()) {
+    NS_DispatchMemoryPressure();
+  }
+}
+
+//-----------------------------------------------------------------------------
+// nsIDirectTaskDispatcher
+
+NS_IMETHODIMP
+nsThread::DispatchDirectTask(already_AddRefed<nsIRunnable> aEvent) {
+  if (!IsOnCurrentThread()) {
+    return NS_ERROR_FAILURE;
+  }
+  mDirectTasks.AddTask(std::move(aEvent));
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsThread::DrainDirectTasks() {
+  if (!IsOnCurrentThread()) {
+    return NS_ERROR_FAILURE;
+  }
+  mDirectTasks.DrainTasks();
+  return NS_OK;
+}
+
+NS_IMETHODIMP nsThread::HaveDirectTasks(bool* aValue) {
+  if (!IsOnCurrentThread()) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aValue = mDirectTasks.HaveTasks();
+  return NS_OK;
+}
+
+NS_IMPL_ISUPPORTS(nsThreadShutdownContext, nsIThreadShutdown)
+
+NS_IMETHODIMP
+nsThreadShutdownContext::OnCompletion(nsIRunnable* aEvent) {
+  if (mCompleted) {
+    aEvent->Run();
+  } else {
+    mCompletionCallbacks.AppendElement(aEvent);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadShutdownContext::GetCompleted(bool* aCompleted) {
+  *aCompleted = mCompleted;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadShutdownContext::StopWaitingAndLeakThread() {
+  // Take the joining thread from `mJoiningThread` so that the terminating
+  // thread won't try to dispatch nsThreadShutdownAckEvent to us anymore.
+  RefPtr<nsThread> joiningThread;
+  {
+    MutexAutoLock lock(mJoiningThreadMutex);
+    if (!mJoiningThread) {
+      // Shutdown is already being resolved, so there's nothing for us to do.
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    joiningThread = mJoiningThread.forget();
+    mThreadLeaked = true;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(joiningThread->IsOnCurrentThread());
+
+  MarkCompleted();
+
+  return NS_OK;
+}
+
+void nsThreadShutdownContext::MarkCompleted() {
+  MOZ_ASSERT(!mCompleted);
+  mCompleted = true;
+  nsTArray<nsCOMPtr<nsIRunnable>> callbacks(std::move(mCompletionCallbacks));
+  for (auto& callback : callbacks) {
+    callback->Run();
+  }
+}
+
+namespace mozilla {
+PerformanceCounterState::Snapshot PerformanceCounterState::RunnableWillRun(
+    TimeStamp aNow, bool aIsIdleRunnable) {
+  if (mIsMainThread && IsNestedRunnable()) {
+    // Flush out any accumulated time that should be accounted to the
+    // current runnable before we start running a nested runnable.  Don't
+    // do this for non-mainthread threads that may be running their own
+    // event loops, like SocketThread.
+    MaybeReportAccumulatedTime("nested runnable"_ns, aNow);
+  }
+
+  Snapshot snapshot(mCurrentEventLoopDepth, mCurrentRunnableIsIdleRunnable);
+
+  mCurrentEventLoopDepth = mNestedEventLoopDepth;
+  mCurrentRunnableIsIdleRunnable = aIsIdleRunnable;
+  mCurrentTimeSliceStart = aNow;
+
+  return snapshot;
+}
+
+void PerformanceCounterState::RunnableDidRun(const nsCString& aName,
+                                             Snapshot&& aSnapshot) {
+  // First thing: Restore our mCurrentEventLoopDepth so we can use
+  // IsNestedRunnable().
+  mCurrentEventLoopDepth = aSnapshot.mOldEventLoopDepth;
+
+  // We may not need the current timestamp; don't bother computing it if we
+  // don't.
+  TimeStamp now;
+  if (mLongTaskLength.isSome() || IsNestedRunnable()) {
+    now = TimeStamp::Now();
+  }
+  if (mLongTaskLength.isSome()) {
+    MaybeReportAccumulatedTime(aName, now);
+  }
+
+  // And now restore the rest of our state.
+  mCurrentRunnableIsIdleRunnable = aSnapshot.mOldIsIdleRunnable;
+  if (IsNestedRunnable()) {
+    // Reset mCurrentTimeSliceStart to right now, so our parent runnable's
+    // next slice can be properly accounted for.
+    mCurrentTimeSliceStart = now;
+  } else {
+    // We are done at the outermost level; we are no longer in a timeslice.
+    mCurrentTimeSliceStart = TimeStamp();
+  }
+}
+
+void PerformanceCounterState::MaybeReportAccumulatedTime(const nsCString& aName,
+                                                         TimeStamp aNow) {
+  MOZ_ASSERT(mCurrentTimeSliceStart,
+             "How did we get here if we're not in a timeslice?");
+  if (!mLongTaskLength.isSome()) {
+    return;
+  }
+
+  TimeDuration duration = aNow - mCurrentTimeSliceStart;
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  if (mIsMainThread && duration.ToMilliseconds() > LONGTASK_TELEMETRY_MS) {
+    Telemetry::Accumulate(Telemetry::EVENT_LONGTASK, aName,
+                          duration.ToMilliseconds());
+  }
+#endif
+
+  // Long tasks only matter on the main thread.
+  if (duration.ToMilliseconds() >= mLongTaskLength.value()) {
+    // Idle events (gc...) don't *really* count here
+    if (!mCurrentRunnableIsIdleRunnable) {
+      mLastLongNonIdleTaskEnd = aNow;
+    }
+    mLastLongTaskEnd = aNow;
+
+    if (profiler_thread_is_being_profiled_for_markers()) {
+      struct LongTaskMarker {
+        static constexpr Span<const char> MarkerTypeName() {
+          return MakeStringSpan("MainThreadLongTask");
+        }
+        static void StreamJSONMarkerData(
+            baseprofiler::SpliceableJSONWriter& aWriter) {
+          aWriter.StringProperty("category", "LongTask");
+        }
+        static MarkerSchema MarkerTypeDisplay() {
+          using MS = MarkerSchema;
+          MS schema{MS::Location::MarkerChart, MS::Location::MarkerTable};
+          schema.AddKeyLabelFormatSearchable("category", "Type",
+                                             MS::Format::String,
+                                             MS::Searchable::Searchable);
+          return schema;
+        }
+      };
+
+      profiler_add_marker(mCurrentRunnableIsIdleRunnable
+                              ? ProfilerString8View("LongIdleTask")
+                              : ProfilerString8View("LongTask"),
+                          geckoprofiler::category::OTHER,
+                          MarkerTiming::Interval(mCurrentTimeSliceStart, aNow),
+                          LongTaskMarker{});
+    }
+  }
+}
+
+}  // namespace mozilla
diff --git a/xpcom/threads/nsThread.h b/xpcom/threads/nsThread.h
new file mode 100644
index 0000000000..5b177f0c0c
--- /dev/null
+++ b/xpcom/threads/nsThread.h
@@ -0,0 +1,364 @@
+/* -*- 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 nsThread_h__
+#define nsThread_h__
+
+#include "MainThreadUtils.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/DataMutex.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtr.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsIEventTarget.h"
+#include "nsISerialEventTarget.h"
+#include "nsISupportsPriority.h"
+#include "nsIThread.h"
+#include "nsIThreadInternal.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+class CycleCollectedJSContext;
+class DelayedRunnable;
+class SynchronizedEventQueue;
+class ThreadEventQueue;
+class ThreadEventTarget;
+
+template <typename T, size_t Length>
+class Array;
+}  // namespace mozilla
+
+using mozilla::NotNull;
+
+class nsIRunnable;
+class nsThreadShutdownContext;
+
+// See https://www.w3.org/TR/longtasks
+#define W3_LONGTASK_BUSY_WINDOW_MS 50
+
+// Time a Runnable executes before we accumulate telemetry on it
+#define LONGTASK_TELEMETRY_MS 30
+
+// A class for managing performance counter state.
+namespace mozilla {
+class PerformanceCounterState {
+ public:
+  explicit PerformanceCounterState(
+      const uint32_t& aNestedEventLoopDepthRef, bool aIsMainThread = false,
+      const Maybe<uint32_t>& aLongTaskLength = Nothing())
+      : mNestedEventLoopDepth(aNestedEventLoopDepthRef),
+        mIsMainThread(aIsMainThread),
+        mLongTaskLength(aLongTaskLength),
+        // Does it really make sense to initialize these to "now" when we
+        // haven't run any tasks?
+        mLastLongTaskEnd(TimeStamp::Now()),
+        mLastLongNonIdleTaskEnd(mLastLongTaskEnd) {}
+
+  class Snapshot {
+   public:
+    Snapshot(uint32_t aOldEventLoopDepth, bool aOldIsIdleRunnable)
+        : mOldEventLoopDepth(aOldEventLoopDepth),
+          mOldIsIdleRunnable(aOldIsIdleRunnable) {}
+
+    Snapshot(const Snapshot&) = default;
+    Snapshot(Snapshot&&) = default;
+
+   private:
+    friend class PerformanceCounterState;
+
+    const uint32_t mOldEventLoopDepth;
+    const bool mOldIsIdleRunnable;
+  };
+
+  // Notification that a runnable is about to run.  This captures a snapshot of
+  // our current state before we reset to prepare for the new runnable.  This
+  // muast be called after mNestedEventLoopDepth has been incremented for the
+  // runnable execution.  The performance counter passed in should be the one
+  // for the relevant runnable and may be null.  aIsIdleRunnable should be true
+  // if and only if the runnable has idle priority.
+  Snapshot RunnableWillRun(TimeStamp aNow, bool aIsIdleRunnable);
+
+  // Notification that a runnable finished executing.  This must be passed the
+  // snapshot that RunnableWillRun returned for the same runnable.  This must be
+  // called before mNestedEventLoopDepth is decremented after the runnable's
+  // execution.
+  void RunnableDidRun(const nsCString& aName, Snapshot&& aSnapshot);
+
+  const TimeStamp& LastLongTaskEnd() const { return mLastLongTaskEnd; }
+  const TimeStamp& LastLongNonIdleTaskEnd() const {
+    return mLastLongNonIdleTaskEnd;
+  }
+
+ private:
+  // Called to report accumulated time, as needed, when we're about to run a
+  // runnable or just finished running one.
+  void MaybeReportAccumulatedTime(const nsCString& aName, TimeStamp aNow);
+
+  // Whether the runnable we are about to run, or just ran, is a nested
+  // runnable, in the sense that there is some other runnable up the stack
+  // spinning the event loop.  This must be called before we change our
+  // mCurrentEventLoopDepth (when about to run a new event) or after we restore
+  // it (after we ran one).
+  bool IsNestedRunnable() const {
+    return mNestedEventLoopDepth > mCurrentEventLoopDepth;
+  }
+
+  // The event loop depth of the currently running runnable.  Set to the max
+  // value of a uint32_t when there is no runnable running, so when starting to
+  // run a toplevel (not nested) runnable IsNestedRunnable() will test false.
+  uint32_t mCurrentEventLoopDepth = std::numeric_limits<uint32_t>::max();
+
+  // A reference to the nsThread's mNestedEventLoopDepth, so we can
+  // see what it is right now.
+  const uint32_t& mNestedEventLoopDepth;
+
+  // A boolean that indicates whether the currently running runnable is an idle
+  // runnable.  Only has a useful value between RunnableWillRun() being called
+  // and RunnableDidRun() returning.
+  bool mCurrentRunnableIsIdleRunnable = false;
+
+  // Whether we're attached to the mainthread nsThread.
+  const bool mIsMainThread;
+
+  // what is considered a LongTask (in ms)
+  const Maybe<uint32_t> mLongTaskLength;
+
+  // The timestamp from which time to be accounted for should be measured.  This
+  // can be the start of a runnable running or the end of a nested runnable
+  // running.
+  TimeStamp mCurrentTimeSliceStart;
+
+  // Information about when long tasks last ended.
+  TimeStamp mLastLongTaskEnd;
+  TimeStamp mLastLongNonIdleTaskEnd;
+};
+}  // namespace mozilla
+
+// A native thread
+class nsThread : public nsIThreadInternal,
+                 public nsISupportsPriority,
+                 public nsIDirectTaskDispatcher,
+                 private mozilla::LinkedListElement<nsThread> {
+  friend mozilla::LinkedList<nsThread>;
+  friend mozilla::LinkedListElement<nsThread>;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+  NS_DECL_NSITHREAD
+  NS_DECL_NSITHREADINTERNAL
+  NS_DECL_NSISUPPORTSPRIORITY
+  NS_DECL_NSIDIRECTTASKDISPATCHER
+
+  enum MainThreadFlag { MAIN_THREAD, NOT_MAIN_THREAD };
+
+  nsThread(NotNull<mozilla::SynchronizedEventQueue*> aQueue,
+           MainThreadFlag aMainThread,
+           nsIThreadManager::ThreadCreationOptions aOptions);
+
+ private:
+  nsThread();
+
+ public:
+  // Initialize this as a named wrapper for a new PRThread.
+  nsresult Init(const nsACString& aName);
+
+  // Initialize this as a wrapper for the current PRThread.
+  nsresult InitCurrentThread();
+
+  // Get this thread's name, thread-safe.
+  void GetThreadName(nsACString& aNameBuffer);
+
+  // Set this thread's name. Consider using
+  // NS_SetCurrentThreadName if you are not sure.
+  void SetThreadNameInternal(const nsACString& aName);
+
+ private:
+  // Initializes the mThreadId and stack base/size members, and adds the thread
+  // to the ThreadList().
+  void InitCommon();
+
+ public:
+  // The PRThread corresponding to this thread.
+  PRThread* GetPRThread() const { return mThread; }
+
+  const void* StackBase() const { return mStackBase; }
+  size_t StackSize() const { return mStackSize; }
+
+  uint32_t ThreadId() const { return mThreadId; }
+
+  // If this flag is true, then the nsThread was created using
+  // nsIThreadManager::NewThread.
+  bool ShutdownRequired() { return mShutdownRequired; }
+
+  // Lets GetRunningEventDelay() determine if the pool this is part
+  // of has an unstarted thread
+  void SetPoolThreadFreePtr(mozilla::Atomic<bool, mozilla::Relaxed>* aPtr) {
+    mIsAPoolThreadFree = aPtr;
+  }
+
+  void SetScriptObserver(mozilla::CycleCollectedJSContext* aScriptObserver);
+
+  uint32_t RecursionDepth() const;
+
+  void ShutdownComplete(NotNull<nsThreadShutdownContext*> aContext);
+
+  void WaitForAllAsynchronousShutdowns();
+
+  static const uint32_t kRunnableNameBufSize = 1000;
+  static mozilla::Array<char, kRunnableNameBufSize> sMainThreadRunnableName;
+
+  mozilla::SynchronizedEventQueue* EventQueue() { return mEvents.get(); }
+
+  bool ShuttingDown() const { return mShutdownContext != nullptr; }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  // Returns the size of this object, its PRThread, and its shutdown contexts,
+  // but excluding its event queues.
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  size_t SizeOfEventQueues(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  void SetUseHangMonitor(bool aValue) {
+    MOZ_ASSERT(IsOnCurrentThread());
+    mUseHangMonitor = aValue;
+  }
+
+ private:
+  void DoMainThreadSpecificProcessing() const;
+
+ protected:
+  friend class nsThreadShutdownEvent;
+
+  virtual ~nsThread();
+
+  static void ThreadFunc(void* aArg);
+
+  // Helper
+  already_AddRefed<nsIThreadObserver> GetObserver() {
+    nsIThreadObserver* obs;
+    nsThread::GetObserver(&obs);
+    return already_AddRefed<nsIThreadObserver>(obs);
+  }
+
+  already_AddRefed<nsThreadShutdownContext> ShutdownInternal(bool aSync);
+
+  friend class nsThreadManager;
+  friend class nsThreadPool;
+
+  void MaybeRemoveFromThreadList();
+
+  // Whether or not these members have a value determines whether the nsThread
+  // is treated as a full XPCOM thread or as a thin wrapper.
+  //
+  // For full nsThreads, they will always contain valid pointers. For thin
+  // wrappers around non-XPCOM threads, they will be null, and event dispatch
+  // methods which rely on them will fail (and assert) if called.
+  RefPtr<mozilla::SynchronizedEventQueue> mEvents;
+  RefPtr<mozilla::ThreadEventTarget> mEventTarget;
+
+  // The number of outstanding nsThreadShutdownContext started by this thread.
+  // The thread will not be allowed to exit until this number reaches 0.
+  uint32_t mOutstandingShutdownContexts;
+  // The shutdown context for ourselves.
+  RefPtr<nsThreadShutdownContext> mShutdownContext;
+
+  mozilla::CycleCollectedJSContext* mScriptObserver;
+
+  // Our name.
+  mozilla::DataMutex<nsCString> mThreadName;
+
+  void* mStackBase = nullptr;
+  uint32_t mStackSize;
+  uint32_t mThreadId;
+
+  uint32_t mNestedEventLoopDepth;
+
+  mozilla::Atomic<bool> mShutdownRequired;
+
+  int8_t mPriority;
+
+  const bool mIsMainThread;
+  bool mUseHangMonitor;
+  const bool mIsUiThread;
+  mozilla::Atomic<bool, mozilla::Relaxed>* mIsAPoolThreadFree;
+
+  // Set to true if this thread creates a JSRuntime.
+  bool mCanInvokeJS;
+
+  // The time the currently running event spent in event queues, and
+  // when it started running.  If no event is running, they are
+  // TimeDuration() & TimeStamp().
+  mozilla::TimeDuration mLastEventDelay;
+  mozilla::TimeStamp mLastEventStart;
+
+#ifdef EARLY_BETA_OR_EARLIER
+  nsCString mNameForWakeupTelemetry;
+  mozilla::TimeStamp mLastWakeupCheckTime;
+  uint32_t mWakeupCount = 0;
+#endif
+
+  mozilla::PerformanceCounterState mPerformanceCounterState;
+
+  mozilla::SimpleTaskQueue mDirectTasks;
+};
+
+class nsThreadShutdownContext final : public nsIThreadShutdown {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSITHREADSHUTDOWN
+
+ private:
+  friend class nsThread;
+  friend class nsThreadShutdownEvent;
+  friend class nsThreadShutdownAckEvent;
+
+  nsThreadShutdownContext(NotNull<nsThread*> aTerminatingThread,
+                          nsThread* aJoiningThread)
+      : mTerminatingThread(aTerminatingThread),
+        mTerminatingPRThread(aTerminatingThread->GetPRThread()),
+        mJoiningThreadMutex("nsThreadShutdownContext::mJoiningThreadMutex"),
+        mJoiningThread(aJoiningThread) {}
+
+  ~nsThreadShutdownContext() = default;
+
+  // Must be called on the joining thread.
+  void MarkCompleted();
+
+  // NB: This may be the last reference.
+  NotNull<RefPtr<nsThread>> const mTerminatingThread;
+  PRThread* const mTerminatingPRThread;
+
+  // May only be accessed on the joining thread.
+  bool mCompleted = false;
+  nsTArray<nsCOMPtr<nsIRunnable>> mCompletionCallbacks;
+
+  // The thread waiting for this thread to shut down. Will either be cleared by
+  // the joining thread if `StopWaitingAndLeakThread` is called or by the
+  // terminating thread upon exiting and notifying the joining thread.
+  mozilla::Mutex mJoiningThreadMutex;
+  RefPtr<nsThread> mJoiningThread MOZ_GUARDED_BY(mJoiningThreadMutex);
+  bool mThreadLeaked MOZ_GUARDED_BY(mJoiningThreadMutex) = false;
+};
+
+#if defined(XP_UNIX) && !defined(ANDROID) && !defined(DEBUG) && HAVE_UALARM && \
+    defined(_GNU_SOURCE)
+#  define MOZ_CANARY
+
+extern int sCanaryOutputFD;
+#endif
+
+#endif  // nsThread_h__
diff --git a/xpcom/threads/nsThreadManager.cpp b/xpcom/threads/nsThreadManager.cpp
new file mode 100644
index 0000000000..805be6ee64
--- /dev/null
+++ b/xpcom/threads/nsThreadManager.cpp
@@ -0,0 +1,841 @@
+/* -*- 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 "nsThreadManager.h"
+#include "nsThread.h"
+#include "nsThreadPool.h"
+#include "nsThreadUtils.h"
+#include "nsIClassInfoImpl.h"
+#include "nsExceptionHandler.h"
+#include "nsTArray.h"
+#include "nsXULAppAPI.h"
+#include "nsExceptionHandler.h"
+#include "mozilla/AbstractThread.h"
+#include "mozilla/AppShutdown.h"
+#include "mozilla/ClearOnShutdown.h"
+#include "mozilla/CycleCollectedJSContext.h"  // nsAutoMicroTask
+#include "mozilla/EventQueue.h"
+#include "mozilla/InputTaskManager.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/NeverDestroyed.h"
+#include "mozilla/Preferences.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/ThreadEventQueue.h"
+#include "mozilla/ThreadLocal.h"
+#include "TaskController.h"
+#include "ThreadEventTarget.h"
+#ifdef MOZ_CANARY
+#  include <fcntl.h>
+#  include <unistd.h>
+#endif
+
+#include "MainThreadIdlePeriod.h"
+
+using namespace mozilla;
+
+static MOZ_THREAD_LOCAL(bool) sTLSIsMainThread;
+
+bool NS_IsMainThreadTLSInitialized() { return sTLSIsMainThread.initialized(); }
+
+class BackgroundEventTarget final : public nsIEventTarget,
+                                    public TaskQueueTracker {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+
+  BackgroundEventTarget() = default;
+
+  nsresult Init();
+
+  already_AddRefed<TaskQueue> CreateBackgroundTaskQueue(const char* aName);
+
+  void BeginShutdown(nsTArray<RefPtr<ShutdownPromise>>&);
+  void FinishShutdown();
+
+ private:
+  ~BackgroundEventTarget() = default;
+
+  nsCOMPtr<nsIThreadPool> mPool;
+  nsCOMPtr<nsIThreadPool> mIOPool;
+};
+
+NS_IMPL_ISUPPORTS(BackgroundEventTarget, nsIEventTarget, TaskQueueTracker)
+
+nsresult BackgroundEventTarget::Init() {
+  nsCOMPtr<nsIThreadPool> pool(new nsThreadPool());
+  NS_ENSURE_TRUE(pool, NS_ERROR_FAILURE);
+
+  nsresult rv = pool->SetName("BackgroundThreadPool"_ns);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Use potentially more conservative stack size.
+  rv = pool->SetThreadStackSize(nsIThreadManager::kThreadPoolStackSize);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Thread limit of 2 makes deadlock during synchronous dispatch less likely.
+  rv = pool->SetThreadLimit(2);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = pool->SetIdleThreadLimit(1);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Leave threads alive for up to 5 minutes
+  rv = pool->SetIdleThreadTimeout(300000);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Initialize the background I/O event target.
+  nsCOMPtr<nsIThreadPool> ioPool(new nsThreadPool());
+  NS_ENSURE_TRUE(pool, NS_ERROR_FAILURE);
+
+  // The io pool spends a lot of its time blocking on io, so we want to offload
+  // these jobs on a lower priority if available.
+  rv = ioPool->SetQoSForThreads(nsIThread::QOS_PRIORITY_LOW);
+  NS_ENSURE_SUCCESS(
+      rv, rv);  // note: currently infallible, keeping this for brevity.
+
+  rv = ioPool->SetName("BgIOThreadPool"_ns);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Use potentially more conservative stack size.
+  rv = ioPool->SetThreadStackSize(nsIThreadManager::kThreadPoolStackSize);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Thread limit of 4 makes deadlock during synchronous dispatch less likely.
+  rv = ioPool->SetThreadLimit(4);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = ioPool->SetIdleThreadLimit(1);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Leave threads alive for up to 5 minutes
+  rv = ioPool->SetIdleThreadTimeout(300000);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  pool.swap(mPool);
+  ioPool.swap(mIOPool);
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP_(bool)
+BackgroundEventTarget::IsOnCurrentThreadInfallible() {
+  return mPool->IsOnCurrentThread() || mIOPool->IsOnCurrentThread();
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::IsOnCurrentThread(bool* aValue) {
+  bool value = false;
+  if (NS_SUCCEEDED(mPool->IsOnCurrentThread(&value)) && value) {
+    *aValue = value;
+    return NS_OK;
+  }
+  return mIOPool->IsOnCurrentThread(aValue);
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::Dispatch(already_AddRefed<nsIRunnable> aRunnable,
+                                uint32_t aFlags) {
+  // We need to be careful here, because if an event is getting dispatched here
+  // from within TaskQueue::Runner::Run, it will be dispatched with
+  // NS_DISPATCH_AT_END, but we might not be running the event on the same
+  // pool, depending on which pool we were on and the dispatch flags.  If we
+  // dispatch an event with NS_DISPATCH_AT_END to the wrong pool, the pool
+  // may not process the event in a timely fashion, which can lead to deadlock.
+  uint32_t flags = aFlags & ~NS_DISPATCH_EVENT_MAY_BLOCK;
+  bool mayBlock = bool(aFlags & NS_DISPATCH_EVENT_MAY_BLOCK);
+  nsCOMPtr<nsIThreadPool>& pool = mayBlock ? mIOPool : mPool;
+
+  // If we're already running on the pool we want to dispatch to, we can
+  // unconditionally add NS_DISPATCH_AT_END to indicate that we shouldn't spin
+  // up a new thread.
+  //
+  // Otherwise, we should remove NS_DISPATCH_AT_END so we don't run into issues
+  // like those in the above comment.
+  if (pool->IsOnCurrentThread()) {
+    flags |= NS_DISPATCH_AT_END;
+  } else {
+    flags &= ~NS_DISPATCH_AT_END;
+  }
+
+  return pool->Dispatch(std::move(aRunnable), flags);
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::DispatchFromScript(nsIRunnable* aRunnable,
+                                          uint32_t aFlags) {
+  nsCOMPtr<nsIRunnable> runnable(aRunnable);
+  return Dispatch(runnable.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::DelayedDispatch(already_AddRefed<nsIRunnable> aRunnable,
+                                       uint32_t) {
+  nsCOMPtr<nsIRunnable> dropRunnable(aRunnable);
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::RegisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+BackgroundEventTarget::UnregisterShutdownTask(nsITargetShutdownTask* aTask) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+void BackgroundEventTarget::BeginShutdown(
+    nsTArray<RefPtr<ShutdownPromise>>& promises) {
+  auto queues = GetAllTrackedTaskQueues();
+  for (auto& queue : queues) {
+    promises.AppendElement(queue->BeginShutdown());
+  }
+}
+
+void BackgroundEventTarget::FinishShutdown() {
+  mPool->Shutdown();
+  mIOPool->Shutdown();
+}
+
+already_AddRefed<TaskQueue> BackgroundEventTarget::CreateBackgroundTaskQueue(
+    const char* aName) {
+  return TaskQueue::Create(do_AddRef(this), aName).forget();
+}
+
+extern "C" {
+// This uses the C language linkage because it's exposed to Rust
+// via the xpcom/rust/moz_task crate.
+bool NS_IsMainThread() { return sTLSIsMainThread.get(); }
+}
+
+void NS_SetMainThread() {
+  if (!sTLSIsMainThread.init()) {
+    MOZ_CRASH();
+  }
+  sTLSIsMainThread.set(true);
+  MOZ_ASSERT(NS_IsMainThread());
+  // We initialize the SerialEventTargetGuard's TLS here for simplicity as it
+  // needs to be initialized around the same time you would initialize
+  // sTLSIsMainThread.
+  SerialEventTargetGuard::InitTLS();
+  nsThreadPool::InitTLS();
+}
+
+#ifdef DEBUG
+
+namespace mozilla {
+
+void AssertIsOnMainThread() { MOZ_ASSERT(NS_IsMainThread(), "Wrong thread!"); }
+
+}  // namespace mozilla
+
+#endif
+
+//-----------------------------------------------------------------------------
+
+/* static */
+void nsThreadManager::ReleaseThread(void* aData) {
+  static_cast<nsThread*>(aData)->Release();
+}
+
+// statically allocated instance
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadManager::AddRef() { return 2; }
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsThreadManager::Release() { return 1; }
+NS_IMPL_CLASSINFO(nsThreadManager, nullptr,
+                  nsIClassInfo::THREADSAFE | nsIClassInfo::SINGLETON,
+                  NS_THREADMANAGER_CID)
+NS_IMPL_QUERY_INTERFACE_CI(nsThreadManager, nsIThreadManager)
+NS_IMPL_CI_INTERFACE_GETTER(nsThreadManager, nsIThreadManager)
+
+//-----------------------------------------------------------------------------
+
+/*static*/ nsThreadManager& nsThreadManager::get() {
+  static NeverDestroyed<nsThreadManager> sInstance;
+  return *sInstance;
+}
+
+nsThreadManager::nsThreadManager()
+    : mCurThreadIndex(0),
+      mMutex("nsThreadManager::mMutex"),
+      mState(State::eUninit) {}
+
+nsThreadManager::~nsThreadManager() = default;
+
+nsresult nsThreadManager::Init() {
+  // Child processes need to initialize the thread manager before they
+  // initialize XPCOM in order to set up the crash reporter. This leads to
+  // situations where we get initialized twice.
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    if (mState > State::eUninit) {
+      return NS_OK;
+    }
+  }
+
+  if (PR_NewThreadPrivateIndex(&mCurThreadIndex, ReleaseThread) == PR_FAILURE) {
+    return NS_ERROR_FAILURE;
+  }
+
+#ifdef MOZ_CANARY
+  const int flags = O_WRONLY | O_APPEND | O_CREAT | O_NONBLOCK;
+  const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
+  char* env_var_flag = getenv("MOZ_KILL_CANARIES");
+  sCanaryOutputFD =
+      env_var_flag
+          ? (env_var_flag[0] ? open(env_var_flag, flags, mode) : STDERR_FILENO)
+          : 0;
+#endif
+
+  TaskController::Initialize();
+
+  // Initialize idle handling.
+  nsCOMPtr<nsIIdlePeriod> idlePeriod = new MainThreadIdlePeriod();
+  TaskController::Get()->SetIdleTaskManager(
+      new IdleTaskManager(idlePeriod.forget()));
+
+  // Create main thread queue that forwards events to TaskController and
+  // construct main thread.
+  UniquePtr<EventQueue> queue = MakeUnique<EventQueue>(true);
+
+  RefPtr<ThreadEventQueue> synchronizedQueue =
+      new ThreadEventQueue(std::move(queue), true);
+
+  mMainThread =
+      new nsThread(WrapNotNull(synchronizedQueue), nsThread::MAIN_THREAD,
+                   {0, false, false, Some(W3_LONGTASK_BUSY_WINDOW_MS)});
+
+  nsresult rv = mMainThread->InitCurrentThread();
+  if (NS_FAILED(rv)) {
+    mMainThread = nullptr;
+    return rv;
+  }
+#ifdef MOZ_MEMORY
+  jemalloc_set_main_thread();
+#endif
+
+  // Init AbstractThread.
+  AbstractThread::InitTLS();
+  AbstractThread::InitMainThread();
+
+  // Initialize the background event target.
+  RefPtr<BackgroundEventTarget> target(new BackgroundEventTarget());
+
+  rv = target->Init();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+
+    mBackgroundEventTarget = std::move(target);
+
+    mState = State::eActive;
+  }
+
+  return NS_OK;
+}
+
+void nsThreadManager::ShutdownNonMainThreads() {
+  MOZ_ASSERT(NS_IsMainThread(), "shutdown not called from main thread");
+
+  // Empty the main thread event queue before we begin shutting down threads.
+  NS_ProcessPendingEvents(mMainThread);
+
+  mMainThread->mEvents->RunShutdownTasks();
+
+  RefPtr<BackgroundEventTarget> backgroundEventTarget;
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mState == State::eActive, "shutdown called multiple times");
+    backgroundEventTarget = mBackgroundEventTarget;
+  }
+
+  nsTArray<RefPtr<ShutdownPromise>> promises;
+  backgroundEventTarget->BeginShutdown(promises);
+
+  bool taskQueuesShutdown = false;
+  // It's fine to capture everything by reference in the Then handler since it
+  // runs before we exit the nested event loop, thanks to the SpinEventLoopUntil
+  // below.
+  ShutdownPromise::All(mMainThread, promises)->Then(mMainThread, __func__, [&] {
+    backgroundEventTarget->FinishShutdown();
+    taskQueuesShutdown = true;
+  });
+
+  // Wait for task queues to shutdown, so we don't shut down the underlying
+  // threads of the background event target in the block below, thereby
+  // preventing the task queues from emptying, preventing the shutdown promises
+  // from resolving, and prevent anything checking `taskQueuesShutdown` from
+  // working.
+  mozilla::SpinEventLoopUntil(
+      "nsThreadManager::Shutdown"_ns, [&]() { return taskQueuesShutdown; },
+      mMainThread);
+
+  {
+    // Prevent new nsThreads from being created, and collect a list of threads
+    // which need to be shut down.
+    //
+    // We don't prevent new thread creation until we've shut down background
+    // task queues, to ensure that they are able to start thread pool threads
+    // for shutdown tasks.
+    nsTArray<RefPtr<nsThread>> threadsToShutdown;
+    {
+      OffTheBooksMutexAutoLock lock(mMutex);
+      mState = State::eShutdown;
+
+      for (auto* thread : mThreadList) {
+        if (thread->ShutdownRequired()) {
+          threadsToShutdown.AppendElement(thread);
+        }
+      }
+    }
+
+    // It's tempting to walk the list of threads here and tell them each to stop
+    // accepting new events, but that could lead to badness if one of those
+    // threads is stuck waiting for a response from another thread.  To do it
+    // right, we'd need some way to interrupt the threads.
+    //
+    // Instead, we process events on the current thread while waiting for
+    // threads to shutdown.  This means that we have to preserve a mostly
+    // functioning world until such time as the threads exit.
+
+    // As we're going to be waiting for all asynchronous shutdowns below, we
+    // can begin asynchronously shutting down all XPCOM threads here, rather
+    // than shutting each thread down one-at-a-time.
+    for (const auto& thread : threadsToShutdown) {
+      thread->AsyncShutdown();
+    }
+  }
+
+  // NB: It's possible that there are events in the queue that want to *start*
+  // an asynchronous shutdown. But we have already started async shutdown of
+  // the threads above, so there's no need to worry about them. We only have to
+  // wait for all in-flight asynchronous thread shutdowns to complete.
+  mMainThread->WaitForAllAsynchronousShutdowns();
+
+  // There are no more background threads at this point.
+}
+
+void nsThreadManager::ShutdownMainThread() {
+#ifdef DEBUG
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mState == State::eShutdown, "Must have called BeginShutdown");
+  }
+#endif
+
+  // Do NS_ProcessPendingEvents but with special handling to set
+  // mEventsAreDoomed atomically with the removal of the last event. This means
+  // that PutEvent cannot succeed if the event would be left in the main thread
+  // queue after our final call to NS_ProcessPendingEvents.
+  // See comments in `nsThread::ThreadFunc` for a more detailed explanation.
+  while (true) {
+    if (mMainThread->mEvents->ShutdownIfNoPendingEvents()) {
+      break;
+    }
+    NS_ProcessPendingEvents(mMainThread);
+  }
+
+  // Normally thread shutdown clears the observer for the thread, but since the
+  // main thread is special we do it manually here after we're sure all events
+  // have been processed.
+  mMainThread->SetObserver(nullptr);
+
+  OffTheBooksMutexAutoLock lock(mMutex);
+  mBackgroundEventTarget = nullptr;
+}
+
+void nsThreadManager::ReleaseMainThread() {
+#ifdef DEBUG
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mState == State::eShutdown, "Must have called BeginShutdown");
+    MOZ_ASSERT(!mBackgroundEventTarget, "Must have called ShutdownMainThread");
+  }
+#endif
+  MOZ_ASSERT(mMainThread);
+
+  // Release main thread object.
+  mMainThread = nullptr;
+
+  // Remove the TLS entry for the main thread.
+  PR_SetThreadPrivate(mCurThreadIndex, nullptr);
+}
+
+void nsThreadManager::RegisterCurrentThread(nsThread& aThread) {
+  MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+
+  aThread.AddRef();  // for TLS entry
+  PR_SetThreadPrivate(mCurThreadIndex, &aThread);
+
+#ifdef DEBUG
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    MOZ_ASSERT(aThread.isInList(),
+               "Thread was not added to the thread list before registering!");
+  }
+#endif
+}
+
+void nsThreadManager::UnregisterCurrentThread(nsThread& aThread) {
+  MOZ_ASSERT(aThread.GetPRThread() == PR_GetCurrentThread(), "bad aThread");
+
+  PR_SetThreadPrivate(mCurThreadIndex, nullptr);
+  // Ref-count balanced via ReleaseThread
+}
+
+// Not to be used for MainThread!
+nsThread* nsThreadManager::CreateCurrentThread(SynchronizedEventQueue* aQueue) {
+  // Make sure we don't have an nsThread yet.
+  MOZ_ASSERT(!PR_GetThreadPrivate(mCurThreadIndex));
+
+  if (!AllowNewXPCOMThreads()) {
+    return nullptr;
+  }
+
+  RefPtr<nsThread> thread = new nsThread(
+      WrapNotNull(aQueue), nsThread::NOT_MAIN_THREAD, {.stackSize = 0});
+  if (NS_FAILED(thread->InitCurrentThread())) {
+    return nullptr;
+  }
+
+  return thread.get();  // reference held in TLS
+}
+
+nsresult nsThreadManager::DispatchToBackgroundThread(nsIRunnable* aEvent,
+                                                     uint32_t aDispatchFlags) {
+  RefPtr<BackgroundEventTarget> backgroundTarget;
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    if (!AllowNewXPCOMThreadsLocked() || !mBackgroundEventTarget) {
+      return NS_ERROR_FAILURE;
+    }
+    backgroundTarget = mBackgroundEventTarget;
+  }
+
+  return backgroundTarget->Dispatch(aEvent, aDispatchFlags);
+}
+
+already_AddRefed<TaskQueue> nsThreadManager::CreateBackgroundTaskQueue(
+    const char* aName) {
+  RefPtr<BackgroundEventTarget> backgroundTarget;
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    if (!AllowNewXPCOMThreadsLocked() || !mBackgroundEventTarget) {
+      return nullptr;
+    }
+    backgroundTarget = mBackgroundEventTarget;
+  }
+
+  return backgroundTarget->CreateBackgroundTaskQueue(aName);
+}
+
+nsThread* nsThreadManager::GetCurrentThread() {
+  // read thread local storage
+  void* data = PR_GetThreadPrivate(mCurThreadIndex);
+  if (data) {
+    return static_cast<nsThread*>(data);
+  }
+
+  // Keep this function working early during startup or late during shutdown on
+  // the main thread.
+  if (!AllowNewXPCOMThreads() || NS_IsMainThread()) {
+    return nullptr;
+  }
+
+  // OK, that's fine.  We'll dynamically create one :-)
+  //
+  // We assume that if we're implicitly creating a thread here that it doesn't
+  // want an event queue. Any thread which wants an event queue should
+  // explicitly create its nsThread wrapper.
+  //
+  // nsThread::InitCurrentThread() will check AllowNewXPCOMThreads, and return
+  // an error if we're too late in shutdown to create new XPCOM threads.
+  RefPtr<nsThread> thread = new nsThread();
+  if (NS_FAILED(thread->InitCurrentThread())) {
+    return nullptr;
+  }
+
+  return thread.get();  // reference held in TLS
+}
+
+bool nsThreadManager::IsNSThread() const {
+  {
+    OffTheBooksMutexAutoLock lock(mMutex);
+    if (mState == State::eUninit) {
+      return false;
+    }
+  }
+  if (auto* thread = (nsThread*)PR_GetThreadPrivate(mCurThreadIndex)) {
+    return thread->EventQueue();
+  }
+  return false;
+}
+
+NS_IMETHODIMP
+nsThreadManager::NewNamedThread(
+    const nsACString& aName, nsIThreadManager::ThreadCreationOptions aOptions,
+    nsIThread** aResult) {
+  // Note: can be called from arbitrary threads
+
+  [[maybe_unused]] TimeStamp startTime = TimeStamp::Now();
+
+  RefPtr<ThreadEventQueue> queue =
+      new ThreadEventQueue(MakeUnique<EventQueue>());
+  RefPtr<nsThread> thr =
+      new nsThread(WrapNotNull(queue), nsThread::NOT_MAIN_THREAD, aOptions);
+
+  // Note: nsThread::Init() will check AllowNewXPCOMThreads, and return an
+  // error if we're too late in shutdown to create new XPCOM threads. If we
+  // aren't, the thread will be synchronously added to mThreadList.
+  nsresult rv = thr->Init(aName);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
+  PROFILER_MARKER_TEXT(
+      "NewThread", OTHER,
+      MarkerOptions(MarkerStack::Capture(),
+                    MarkerTiming::IntervalUntilNowFrom(startTime)),
+      aName);
+  if (!NS_IsMainThread()) {
+    PROFILER_MARKER_TEXT(
+        "NewThread (non-main thread)", OTHER,
+        MarkerOptions(MarkerStack::Capture(), MarkerThreadId::MainThread(),
+                      MarkerTiming::IntervalUntilNowFrom(startTime)),
+        aName);
+  }
+
+  thr.forget(aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetMainThread(nsIThread** aResult) {
+  // Keep this functioning during Shutdown
+  if (!mMainThread) {
+    if (!NS_IsMainThread()) {
+      NS_WARNING(
+          "Called GetMainThread but there isn't a main thread and "
+          "we're not the main thread.");
+    }
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  NS_ADDREF(*aResult = mMainThread);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetCurrentThread(nsIThread** aResult) {
+  // Keep this functioning during Shutdown
+  if (!mMainThread) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  *aResult = GetCurrentThread();
+  if (!*aResult) {
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+  NS_ADDREF(*aResult);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::SpinEventLoopUntil(const nsACString& aVeryGoodReasonToDoThis,
+                                    nsINestedEventLoopCondition* aCondition) {
+  return SpinEventLoopUntilInternal(aVeryGoodReasonToDoThis, aCondition,
+                                    ShutdownPhase::NotInShutdown);
+}
+
+NS_IMETHODIMP
+nsThreadManager::SpinEventLoopUntilOrQuit(
+    const nsACString& aVeryGoodReasonToDoThis,
+    nsINestedEventLoopCondition* aCondition) {
+  return SpinEventLoopUntilInternal(aVeryGoodReasonToDoThis, aCondition,
+                                    ShutdownPhase::AppShutdownConfirmed);
+}
+
+// statics from SpinEventLoopUntil.h
+AutoNestedEventLoopAnnotation* AutoNestedEventLoopAnnotation::sCurrent =
+    nullptr;
+StaticMutex AutoNestedEventLoopAnnotation::sStackMutex;
+
+// static from SpinEventLoopUntil.h
+void AutoNestedEventLoopAnnotation::AnnotateXPCOMSpinEventLoopStack(
+    const nsACString& aStack) {
+  if (aStack.Length() > 0) {
+    nsCString prefixedStack(XRE_GetProcessTypeString());
+    prefixedStack += ": "_ns + aStack;
+    CrashReporter::AnnotateCrashReport(
+        CrashReporter::Annotation::XPCOMSpinEventLoopStack, prefixedStack);
+  } else {
+    CrashReporter::AnnotateCrashReport(
+        CrashReporter::Annotation::XPCOMSpinEventLoopStack, ""_ns);
+  }
+}
+
+nsresult nsThreadManager::SpinEventLoopUntilInternal(
+    const nsACString& aVeryGoodReasonToDoThis,
+    nsINestedEventLoopCondition* aCondition,
+    ShutdownPhase aShutdownPhaseToCheck) {
+  // XXX: We would want to AssertIsOnMainThread(); but that breaks some GTest.
+  nsCOMPtr<nsINestedEventLoopCondition> condition(aCondition);
+  nsresult rv = NS_OK;
+
+  if (!mozilla::SpinEventLoopUntil(aVeryGoodReasonToDoThis, [&]() -> bool {
+        // Check if an ongoing shutdown reached our limits.
+        if (aShutdownPhaseToCheck > ShutdownPhase::NotInShutdown &&
+            AppShutdown::GetCurrentShutdownPhase() >= aShutdownPhaseToCheck) {
+          return true;
+        }
+
+        bool isDone = false;
+        rv = condition->IsDone(&isDone);
+        // JS failure should be unusual, but we need to stop and propagate
+        // the error back to the caller.
+        if (NS_FAILED(rv)) {
+          return true;
+        }
+
+        return isDone;
+      })) {
+    // We stopped early for some reason, which is unexpected.
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  // If we exited when the condition told us to, we need to return whether
+  // the condition encountered failure when executing.
+  return rv;
+}
+
+NS_IMETHODIMP
+nsThreadManager::SpinEventLoopUntilEmpty() {
+  nsIThread* thread = NS_GetCurrentThread();
+
+  while (NS_HasPendingEvents(thread)) {
+    (void)NS_ProcessNextEvent(thread, false);
+  }
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::GetMainThreadEventTarget(nsIEventTarget** aTarget) {
+  nsCOMPtr<nsIEventTarget> target = GetMainThreadSerialEventTarget();
+  target.forget(aTarget);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadManager::DispatchToMainThread(nsIRunnable* aEvent, uint32_t aPriority,
+                                      uint8_t aArgc) {
+  // Note: C++ callers should instead use NS_DispatchToMainThread.
+  MOZ_ASSERT(NS_IsMainThread());
+
+  // Keep this functioning during Shutdown
+  if (NS_WARN_IF(!mMainThread)) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+  // If aPriority wasn't explicitly passed, that means it should be treated as
+  // PRIORITY_NORMAL.
+  if (aArgc > 0 && aPriority != nsIRunnablePriority::PRIORITY_NORMAL) {
+    nsCOMPtr<nsIRunnable> event(aEvent);
+    return mMainThread->DispatchFromScript(
+        new PrioritizableRunnable(event.forget(), aPriority), 0);
+  }
+  return mMainThread->DispatchFromScript(aEvent, 0);
+}
+
+class AutoMicroTaskWrapperRunnable final : public Runnable {
+ public:
+  explicit AutoMicroTaskWrapperRunnable(nsIRunnable* aEvent)
+      : Runnable("AutoMicroTaskWrapperRunnable"), mEvent(aEvent) {
+    MOZ_ASSERT(aEvent);
+  }
+
+ private:
+  ~AutoMicroTaskWrapperRunnable() = default;
+
+  NS_IMETHOD Run() override {
+    nsAutoMicroTask mt;
+
+    return mEvent->Run();
+  }
+
+  RefPtr<nsIRunnable> mEvent;
+};
+
+NS_IMETHODIMP
+nsThreadManager::DispatchToMainThreadWithMicroTask(nsIRunnable* aEvent,
+                                                   uint32_t aPriority,
+                                                   uint8_t aArgc) {
+  RefPtr<AutoMicroTaskWrapperRunnable> runnable =
+      new AutoMicroTaskWrapperRunnable(aEvent);
+
+  return DispatchToMainThread(runnable, aPriority, aArgc);
+}
+
+void nsThreadManager::EnableMainThreadEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  InputTaskManager::Get()->EnableInputEventPrioritization();
+}
+
+void nsThreadManager::FlushInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  InputTaskManager::Get()->FlushInputEventPrioritization();
+}
+
+void nsThreadManager::SuspendInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  InputTaskManager::Get()->SuspendInputEventPrioritization();
+}
+
+void nsThreadManager::ResumeInputEventPrioritization() {
+  MOZ_ASSERT(NS_IsMainThread());
+  InputTaskManager::Get()->ResumeInputEventPrioritization();
+}
+
+// static
+bool nsThreadManager::MainThreadHasPendingHighPriorityEvents() {
+  MOZ_ASSERT(NS_IsMainThread());
+  bool retVal = false;
+  if (get().mMainThread) {
+    get().mMainThread->HasPendingHighPriorityEvents(&retVal);
+  }
+  return retVal;
+}
+
+NS_IMETHODIMP
+nsThreadManager::IdleDispatchToMainThread(nsIRunnable* aEvent,
+                                          uint32_t aTimeout) {
+  // Note: C++ callers should instead use NS_DispatchToThreadQueue or
+  // NS_DispatchToCurrentThreadQueue.
+  MOZ_ASSERT(NS_IsMainThread());
+
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  if (aTimeout) {
+    return NS_DispatchToThreadQueue(event.forget(), aTimeout, mMainThread,
+                                    EventQueuePriority::Idle);
+  }
+
+  return NS_DispatchToThreadQueue(event.forget(), mMainThread,
+                                  EventQueuePriority::Idle);
+}
+
+NS_IMETHODIMP
+nsThreadManager::DispatchDirectTaskToCurrentThread(nsIRunnable* aEvent) {
+  NS_ENSURE_STATE(aEvent);
+  nsCOMPtr<nsIRunnable> runnable = aEvent;
+  return GetCurrentThread()->DispatchDirectTask(runnable.forget());
+}
+
+bool nsThreadManager::AllowNewXPCOMThreads() {
+  mozilla::OffTheBooksMutexAutoLock lock(mMutex);
+  return AllowNewXPCOMThreadsLocked();
+}
diff --git a/xpcom/threads/nsThreadManager.h b/xpcom/threads/nsThreadManager.h
new file mode 100644
index 0000000000..46e061a1ad
--- /dev/null
+++ b/xpcom/threads/nsThreadManager.h
@@ -0,0 +1,156 @@
+/* -*- 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 nsThreadManager_h__
+#define nsThreadManager_h__
+
+#include "nsIThreadManager.h"
+#include "nsThread.h"
+#include "mozilla/ShutdownPhase.h"
+
+class nsIRunnable;
+class nsIThread;
+
+namespace mozilla {
+class IdleTaskManager;
+class SynchronizedEventQueue;
+class TaskQueue;
+
+template <typename T>
+class NeverDestroyed;
+}  // namespace mozilla
+
+class BackgroundEventTarget;
+
+class nsThreadManager : public nsIThreadManager {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSITHREADMANAGER
+
+  static nsThreadManager& get();
+
+  nsresult Init();
+
+  // Shutdown all threads other than the main thread.  This function should only
+  // be called on the main thread of the application process.
+  void ShutdownNonMainThreads();
+
+  // Finish shutting down all threads. This function must be called after
+  // ShutdownNonMainThreads and will delete the BackgroundEventTarget and
+  // take the main thread event target out of commission, but without
+  // releasing the underlying nsThread object.
+  void ShutdownMainThread();
+
+  // Release the underlying main thread nsThread object.
+  void ReleaseMainThread();
+
+  // Called by nsThread to inform the ThreadManager it exists.  This method
+  // must be called when the given thread is the current thread.
+  void RegisterCurrentThread(nsThread& aThread);
+
+  // Called by nsThread to inform the ThreadManager it is going away.  This
+  // method must be called when the given thread is the current thread.
+  void UnregisterCurrentThread(nsThread& aThread);
+
+  // Returns the current thread.  Returns null if OOM or if ThreadManager isn't
+  // initialized.  Creates the nsThread if one does not exist yet.
+  nsThread* GetCurrentThread();
+
+  // Returns true iff the currently running thread has an nsThread associated
+  // with it (ie; whether this is a thread that we can dispatch runnables to).
+  bool IsNSThread() const;
+
+  // CreateCurrentThread sets up an nsThread for the current thread. It uses the
+  // event queue and main thread flags passed in. It should only be called once
+  // for the current thread. After it returns, GetCurrentThread() will return
+  // the thread that was created. GetCurrentThread() will also create a thread
+  // (lazily), but it doesn't allow the queue or main-thread attributes to be
+  // specified.
+  nsThread* CreateCurrentThread(mozilla::SynchronizedEventQueue* aQueue);
+
+  nsresult DispatchToBackgroundThread(nsIRunnable* aEvent,
+                                      uint32_t aDispatchFlags);
+
+  already_AddRefed<mozilla::TaskQueue> CreateBackgroundTaskQueue(
+      const char* aName);
+
+  ~nsThreadManager();
+
+  void EnableMainThreadEventPrioritization();
+  void FlushInputEventPrioritization();
+  void SuspendInputEventPrioritization();
+  void ResumeInputEventPrioritization();
+
+  static bool MainThreadHasPendingHighPriorityEvents();
+
+  nsIThread* GetMainThreadWeak() { return mMainThread; }
+
+  // Low level methods for interacting with the global thread list. Be very
+  // careful when holding `ThreadListMutex()` as no new threads can be started
+  // while it is held.
+  mozilla::OffTheBooksMutex& ThreadListMutex() MOZ_RETURN_CAPABILITY(mMutex) {
+    return mMutex;
+  }
+
+  bool AllowNewXPCOMThreads() MOZ_EXCLUDES(mMutex);
+  bool AllowNewXPCOMThreadsLocked() MOZ_REQUIRES(mMutex) {
+    return mState == State::eActive;
+  }
+
+  mozilla::LinkedList<nsThread>& ThreadList() MOZ_REQUIRES(mMutex) {
+    return mThreadList;
+  }
+
+ private:
+  friend class mozilla::NeverDestroyed<nsThreadManager>;
+
+  nsThreadManager();
+
+  nsresult SpinEventLoopUntilInternal(
+      const nsACString& aVeryGoodReasonToDoThis,
+      nsINestedEventLoopCondition* aCondition,
+      mozilla::ShutdownPhase aShutdownPhaseToCheck);
+
+  static void ReleaseThread(void* aData);
+
+  enum class State : uint8_t {
+    // The thread manager has yet to be initialized.
+    eUninit,
+    // The thread manager is active, and operating normally.
+    eActive,
+    // The thread manager is in XPCOM shutdown. New calls to NS_NewNamedThread
+    // will fail, as all XPCOM threads are required to be shutting down.
+    eShutdown,
+  };
+
+  unsigned mCurThreadIndex;  // thread-local-storage index
+  RefPtr<nsThread> mMainThread;
+
+  mutable mozilla::OffTheBooksMutex mMutex;
+
+  // Current state in the thread manager's lifecycle. See docs above.
+  State mState MOZ_GUARDED_BY(mMutex);
+
+  // Global list of active nsThread instances, including both explicitly and
+  // implicitly created threads.
+  //
+  // NOTE: New entries to this list _may_ be added after mAllowNewThreads has
+  // been cleared, but only for implicitly created thread wrappers which are
+  // not shut down during XPCOM shutdown.
+  mozilla::LinkedList<nsThread> mThreadList MOZ_GUARDED_BY(mMutex);
+
+  // Shared event target used for background runnables.
+  RefPtr<BackgroundEventTarget> mBackgroundEventTarget MOZ_GUARDED_BY(mMutex);
+};
+
+#define NS_THREADMANAGER_CID                         \
+  { /* 7a4204c6-e45a-4c37-8ebb-6709a22c917c */       \
+    0x7a4204c6, 0xe45a, 0x4c37, {                    \
+      0x8e, 0xbb, 0x67, 0x09, 0xa2, 0x2c, 0x91, 0x7c \
+    }                                                \
+  }
+
+#endif  // nsThreadManager_h__
diff --git a/xpcom/threads/nsThreadPool.cpp b/xpcom/threads/nsThreadPool.cpp
new file mode 100644
index 0000000000..c2ef022035
--- /dev/null
+++ b/xpcom/threads/nsThreadPool.cpp
@@ -0,0 +1,610 @@
+/* -*- 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 "nsThreadPool.h"
+
+#include "nsCOMArray.h"
+#include "ThreadDelay.h"
+#include "nsThreadManager.h"
+#include "nsThread.h"
+#include "nsThreadUtils.h"
+#include "prinrval.h"
+#include "mozilla/Logging.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ProfilerRunnable.h"
+#include "mozilla/SchedulerGroup.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/SpinEventLoopUntil.h"
+#include "nsThreadSyncDispatch.h"
+
+#include <mutex>
+
+using namespace mozilla;
+
+static LazyLogModule sThreadPoolLog("nsThreadPool");
+#ifdef LOG
+#  undef LOG
+#endif
+#define LOG(args) MOZ_LOG(sThreadPoolLog, mozilla::LogLevel::Debug, args)
+
+static MOZ_THREAD_LOCAL(nsThreadPool*) gCurrentThreadPool;
+
+void nsThreadPool::InitTLS() { gCurrentThreadPool.infallibleInit(); }
+
+// DESIGN:
+//  o  Allocate anonymous threads.
+//  o  Use nsThreadPool::Run as the main routine for each thread.
+//  o  Each thread waits on the event queue's monitor, checking for
+//     pending events and rescheduling itself as an idle thread.
+
+#define DEFAULT_THREAD_LIMIT 4
+#define DEFAULT_IDLE_THREAD_LIMIT 1
+#define DEFAULT_IDLE_THREAD_TIMEOUT PR_SecondsToInterval(60)
+
+NS_IMPL_ISUPPORTS_INHERITED(nsThreadPool, Runnable, nsIThreadPool,
+                            nsIEventTarget)
+
+nsThreadPool* nsThreadPool::GetCurrentThreadPool() {
+  return gCurrentThreadPool.get();
+}
+
+nsThreadPool::nsThreadPool()
+    : Runnable("nsThreadPool"),
+      mMutex("[nsThreadPool.mMutex]"),
+      mEventsAvailable(mMutex, "[nsThreadPool.mEventsAvailable]"),
+      mThreadLimit(DEFAULT_THREAD_LIMIT),
+      mIdleThreadLimit(DEFAULT_IDLE_THREAD_LIMIT),
+      mIdleThreadTimeout(DEFAULT_IDLE_THREAD_TIMEOUT),
+      mIdleCount(0),
+      mQoSPriority(nsIThread::QOS_PRIORITY_NORMAL),
+      mStackSize(nsIThreadManager::DEFAULT_STACK_SIZE),
+      mShutdown(false),
+      mRegressiveMaxIdleTime(false),
+      mIsAPoolThreadFree(true) {
+  LOG(("THRD-P(%p) constructor!!!\n", this));
+}
+
+nsThreadPool::~nsThreadPool() {
+  // Threads keep a reference to the nsThreadPool until they return from Run()
+  // after removing themselves from mThreads.
+  MOZ_ASSERT(mThreads.IsEmpty());
+}
+
+nsresult nsThreadPool::PutEvent(nsIRunnable* aEvent) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return PutEvent(event.forget(), 0);
+}
+
+nsresult nsThreadPool::PutEvent(already_AddRefed<nsIRunnable> aEvent,
+                                uint32_t aFlags) {
+  // Avoid spawning a new thread while holding the event queue lock...
+
+  bool spawnThread = false;
+  uint32_t stackSize = 0;
+  nsCString name;
+  {
+    MutexAutoLock lock(mMutex);
+
+    if (NS_WARN_IF(mShutdown)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    LOG(("THRD-P(%p) put [%d %d %d]\n", this, mIdleCount, mThreads.Count(),
+         mThreadLimit));
+    MOZ_ASSERT(mIdleCount <= (uint32_t)mThreads.Count(), "oops");
+
+    // Make sure we have a thread to service this event.
+    if (mThreads.Count() < (int32_t)mThreadLimit &&
+        !(aFlags & NS_DISPATCH_AT_END) &&
+        // Spawn a new thread if we don't have enough idle threads to serve
+        // pending events immediately.
+        mEvents.Count(lock) >= mIdleCount) {
+      spawnThread = true;
+    }
+
+    nsCOMPtr<nsIRunnable> event(aEvent);
+    LogRunnable::LogDispatch(event);
+    mEvents.PutEvent(event.forget(), EventQueuePriority::Normal, lock);
+    mEventsAvailable.Notify();
+    stackSize = mStackSize;
+    name = mName;
+  }
+
+  auto delay = MakeScopeExit([&]() {
+    // Delay to encourage the receiving task to run before we do work.
+    DelayForChaosMode(ChaosFeature::TaskDispatching, 1000);
+  });
+
+  LOG(("THRD-P(%p) put [spawn=%d]\n", this, spawnThread));
+  if (!spawnThread) {
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIThread> thread;
+  nsresult rv = NS_NewNamedThread(
+      mThreadNaming.GetNextThreadName(name), getter_AddRefs(thread), nullptr,
+      {.stackSize = stackSize, .blockDispatch = true});
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  bool killThread = false;
+  {
+    MutexAutoLock lock(mMutex);
+    if (mShutdown) {
+      killThread = true;
+    } else if (mThreads.Count() < (int32_t)mThreadLimit) {
+      mThreads.AppendObject(thread);
+      if (mThreads.Count() >= (int32_t)mThreadLimit) {
+        mIsAPoolThreadFree = false;
+      }
+    } else {
+      // Someone else may have also been starting a thread
+      killThread = true;  // okay, we don't need this thread anymore
+    }
+  }
+  LOG(("THRD-P(%p) put [%p kill=%d]\n", this, thread.get(), killThread));
+  if (killThread) {
+    // We never dispatched any events to the thread, so we can shut it down
+    // asynchronously without worrying about anything.
+    ShutdownThread(thread);
+  } else {
+    thread->Dispatch(this, NS_DISPATCH_IGNORE_BLOCK_DISPATCH);
+  }
+
+  return NS_OK;
+}
+
+void nsThreadPool::ShutdownThread(nsIThread* aThread) {
+  LOG(("THRD-P(%p) shutdown async [%p]\n", this, aThread));
+
+  // This is either called by a threadpool thread that is out of work, or
+  // a thread that attempted to create a threadpool thread and raced in
+  // such a way that the newly created thread is no longer necessary.
+  // In the first case, we must go to another thread to shut aThread down
+  // (because it is the current thread).  In the second case, we cannot
+  // synchronously shut down the current thread (because then Dispatch() would
+  // spin the event loop, and that could blow up the world), and asynchronous
+  // shutdown requires this thread have an event loop (and it may not, see bug
+  // 10204784).  The simplest way to cover all cases is to asynchronously
+  // shutdown aThread from the main thread.
+  SchedulerGroup::Dispatch(NewRunnableMethod(
+      "nsIThread::AsyncShutdown", aThread, &nsIThread::AsyncShutdown));
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetQoSForThreads(nsIThread::QoSPriority aPriority) {
+  MutexAutoLock lock(mMutex);
+  mQoSPriority = aPriority;
+
+  // We don't notify threads here to observe the change, because we don't want
+  // to create spurious wakeups during idle. Rather, we want threads to simply
+  // observe the change on their own if they wake up to do some task.
+
+  return NS_OK;
+}
+
+// This event 'runs' for the lifetime of the worker thread.  The actual
+// eventqueue is mEvents, and is shared by all the worker threads.  This
+// means that the set of threads together define the delay seen by a new
+// event sent to the pool.
+//
+// To model the delay experienced by the pool, we can have each thread in
+// the pool report 0 if it's idle OR if the pool is below the threadlimit;
+// or otherwise the current event's queuing delay plus current running
+// time.
+//
+// To reconstruct the delays for the pool, the profiler can look at all the
+// threads that are part of a pool (pools have defined naming patterns that
+// can be user to connect them).  If all threads have delays at time X,
+// that means that all threads saturated at that point and any event
+// dispatched to the pool would get a delay.
+//
+// The delay experienced by an event dispatched when all pool threads are
+// busy is based on the calculations shown in platform.cpp.  Run that
+// algorithm for each thread in the pool, and the delay at time X is the
+// longest value for time X of any of the threads, OR the time from X until
+// any one of the threads reports 0 (i.e. it's not busy), whichever is
+// shorter.
+
+// In order to record this when the profiler samples threads in the pool,
+// each thread must (effectively) override GetRunnningEventDelay, by
+// resetting the mLastEventDelay/Start values in the nsThread when we start
+// to run an event (or when we run out of events to run).  Note that handling
+// the shutdown of a thread may be a little tricky.
+
+NS_IMETHODIMP
+nsThreadPool::Run() {
+  nsCOMPtr<nsIThread> current;
+  nsThreadManager::get().GetCurrentThread(getter_AddRefs(current));
+
+  bool shutdownThreadOnExit = false;
+  bool exitThread = false;
+  bool wasIdle = false;
+  TimeStamp idleSince;
+  nsIThread::QoSPriority threadPriority = nsIThread::QOS_PRIORITY_NORMAL;
+
+  // This thread is an nsThread created below with NS_NewNamedThread()
+  static_cast<nsThread*>(current.get())
+      ->SetPoolThreadFreePtr(&mIsAPoolThreadFree);
+
+  nsCOMPtr<nsIThreadPoolListener> listener;
+  {
+    MutexAutoLock lock(mMutex);
+    listener = mListener;
+    LOG(("THRD-P(%p) enter %s\n", this, mName.BeginReading()));
+
+    // Go ahead and check for thread priority. If priority is normal, do nothing
+    // because threads are created with default priority.
+    if (threadPriority != mQoSPriority) {
+      current->SetThreadQoS(threadPriority);
+      threadPriority = mQoSPriority;
+    }
+  }
+
+  if (listener) {
+    listener->OnThreadCreated();
+  }
+
+  MOZ_ASSERT(!gCurrentThreadPool.get());
+  gCurrentThreadPool.set(this);
+
+  do {
+    nsCOMPtr<nsIRunnable> event;
+    TimeDuration delay;
+    {
+      MutexAutoLock lock(mMutex);
+
+      // Before getting the next event, we can adjust priority as needed.
+      if (threadPriority != mQoSPriority) {
+        current->SetThreadQoS(threadPriority);
+        threadPriority = mQoSPriority;
+      }
+
+      event = mEvents.GetEvent(lock, &delay);
+      if (!event) {
+        TimeStamp now = TimeStamp::Now();
+        uint32_t idleTimeoutDivider =
+            (mIdleCount && mRegressiveMaxIdleTime) ? mIdleCount : 1;
+        TimeDuration timeout = TimeDuration::FromMilliseconds(
+            static_cast<double>(mIdleThreadTimeout) / idleTimeoutDivider);
+
+        // If we are shutting down, then don't keep any idle threads.
+        if (mShutdown) {
+          exitThread = true;
+        } else {
+          if (wasIdle) {
+            // if too many idle threads or idle for too long, then bail.
+            if (mIdleCount > mIdleThreadLimit ||
+                (mIdleThreadTimeout != UINT32_MAX &&
+                 (now - idleSince) >= timeout)) {
+              exitThread = true;
+            }
+          } else {
+            // if would be too many idle threads...
+            if (mIdleCount == mIdleThreadLimit) {
+              exitThread = true;
+            } else {
+              ++mIdleCount;
+              idleSince = now;
+              wasIdle = true;
+            }
+          }
+        }
+
+        if (exitThread) {
+          if (wasIdle) {
+            --mIdleCount;
+          }
+          shutdownThreadOnExit = mThreads.RemoveObject(current);
+
+          // keep track if there are threads available to start
+          mIsAPoolThreadFree = (mThreads.Count() < (int32_t)mThreadLimit);
+        } else {
+          current->SetRunningEventDelay(TimeDuration(), TimeStamp());
+
+          AUTO_PROFILER_LABEL("nsThreadPool::Run::Wait", IDLE);
+
+          TimeDuration delta = timeout - (now - idleSince);
+          LOG(("THRD-P(%p) %s waiting [%f]\n", this, mName.BeginReading(),
+               delta.ToMilliseconds()));
+          mEventsAvailable.Wait(delta);
+          LOG(("THRD-P(%p) done waiting\n", this));
+        }
+      } else if (wasIdle) {
+        wasIdle = false;
+        --mIdleCount;
+      }
+    }
+    if (event) {
+      if (MOZ_LOG_TEST(sThreadPoolLog, mozilla::LogLevel::Debug)) {
+        MutexAutoLock lock(mMutex);
+        LOG(("THRD-P(%p) %s running [%p]\n", this, mName.BeginReading(),
+             event.get()));
+      }
+
+      // Delay event processing to encourage whoever dispatched this event
+      // to run.
+      DelayForChaosMode(ChaosFeature::TaskRunning, 1000);
+
+      if (profiler_thread_is_being_profiled(
+              ThreadProfilingFeatures::Sampling)) {
+        // We'll handle the case of unstarted threads available
+        // when we sample.
+        current->SetRunningEventDelay(delay, TimeStamp::Now());
+      }
+
+      LogRunnable::Run log(event);
+      AUTO_PROFILE_FOLLOWING_RUNNABLE(event);
+      event->Run();
+      // To cover the event's destructor code in the LogRunnable span
+      event = nullptr;
+    }
+  } while (!exitThread);
+
+  if (listener) {
+    listener->OnThreadShuttingDown();
+  }
+
+  MOZ_ASSERT(gCurrentThreadPool.get() == this);
+  gCurrentThreadPool.set(nullptr);
+
+  if (shutdownThreadOnExit) {
+    ShutdownThread(current);
+  }
+
+  LOG(("THRD-P(%p) leave\n", this));
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return Dispatch(event.forget(), aFlags);
+}
+
+NS_IMETHODIMP
+nsThreadPool::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) {
+  LOG(("THRD-P(%p) dispatch [%p %x]\n", this, /* XXX aEvent*/ nullptr, aFlags));
+
+  if (NS_WARN_IF(mShutdown)) {
+    nsCOMPtr<nsIRunnable> event(aEvent);
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  NS_ASSERTION(aFlags == NS_DISPATCH_NORMAL || aFlags == NS_DISPATCH_AT_END,
+               "unexpected dispatch flags");
+  PutEvent(std::move(aEvent), aFlags);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsThreadPool::RegisterShutdownTask(nsITargetShutdownTask*) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP
+nsThreadPool::UnregisterShutdownTask(nsITargetShutdownTask*) {
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
+NS_IMETHODIMP_(bool)
+nsThreadPool::IsOnCurrentThreadInfallible() {
+  return gCurrentThreadPool.get() == this;
+}
+
+NS_IMETHODIMP
+nsThreadPool::IsOnCurrentThread(bool* aResult) {
+  MutexAutoLock lock(mMutex);
+  if (NS_WARN_IF(mShutdown)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  *aResult = IsOnCurrentThreadInfallible();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::Shutdown() { return ShutdownWithTimeout(-1); }
+
+NS_IMETHODIMP
+nsThreadPool::ShutdownWithTimeout(int32_t aTimeoutMs) {
+  nsCOMArray<nsIThread> threads;
+  nsCOMPtr<nsIThreadPoolListener> listener;
+  {
+    MutexAutoLock lock(mMutex);
+    if (mShutdown) {
+      return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
+    }
+    mShutdown = true;
+    mEventsAvailable.NotifyAll();
+
+    threads.AppendObjects(mThreads);
+    mThreads.Clear();
+
+    // Swap in a null listener so that we release the listener at the end of
+    // this method. The listener will be kept alive as long as the other threads
+    // that were created when it was set.
+    mListener.swap(listener);
+  }
+
+  nsTArray<nsCOMPtr<nsIThreadShutdown>> contexts;
+  for (int32_t i = 0; i < threads.Count(); ++i) {
+    nsCOMPtr<nsIThreadShutdown> context;
+    if (NS_SUCCEEDED(threads[i]->BeginShutdown(getter_AddRefs(context)))) {
+      contexts.AppendElement(std::move(context));
+    }
+  }
+
+  // Start a timer which will stop waiting & leak the thread, forcing
+  // onCompletion to be called when it expires.
+  nsCOMPtr<nsITimer> timer;
+  if (aTimeoutMs >= 0) {
+    NS_NewTimerWithCallback(
+        getter_AddRefs(timer),
+        [&](nsITimer*) {
+          for (auto& context : contexts) {
+            context->StopWaitingAndLeakThread();
+          }
+        },
+        aTimeoutMs, nsITimer::TYPE_ONE_SHOT,
+        "nsThreadPool::ShutdownWithTimeout");
+  }
+
+  // Start a counter and register a callback to decrement outstandingThreads
+  // when the threads finish exiting. We'll spin an event loop until
+  // outstandingThreads reaches 0.
+  uint32_t outstandingThreads = contexts.Length();
+  RefPtr onCompletion = NS_NewCancelableRunnableFunction(
+      "nsThreadPool thread completion", [&] { --outstandingThreads; });
+  for (auto& context : contexts) {
+    context->OnCompletion(onCompletion);
+  }
+
+  mozilla::SpinEventLoopUntil("nsThreadPool::ShutdownWithTimeout"_ns,
+                              [&] { return outstandingThreads == 0; });
+
+  if (timer) {
+    timer->Cancel();
+  }
+  onCompletion->Cancel();
+
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetThreadLimit(uint32_t* aValue) {
+  MutexAutoLock lock(mMutex);
+  *aValue = mThreadLimit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetThreadLimit(uint32_t aValue) {
+  MutexAutoLock lock(mMutex);
+  LOG(("THRD-P(%p) thread limit [%u]\n", this, aValue));
+  mThreadLimit = aValue;
+  if (mIdleThreadLimit > mThreadLimit) {
+    mIdleThreadLimit = mThreadLimit;
+  }
+
+  if (static_cast<uint32_t>(mThreads.Count()) > mThreadLimit) {
+    mEventsAvailable
+        .NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetIdleThreadLimit(uint32_t* aValue) {
+  MutexAutoLock lock(mMutex);
+  *aValue = mIdleThreadLimit;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetIdleThreadLimit(uint32_t aValue) {
+  MutexAutoLock lock(mMutex);
+  LOG(("THRD-P(%p) idle thread limit [%u]\n", this, aValue));
+  mIdleThreadLimit = aValue;
+  if (mIdleThreadLimit > mThreadLimit) {
+    mIdleThreadLimit = mThreadLimit;
+  }
+
+  // Do we need to kill some idle threads?
+  if (mIdleCount > mIdleThreadLimit) {
+    mEventsAvailable
+        .NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetIdleThreadTimeout(uint32_t* aValue) {
+  MutexAutoLock lock(mMutex);
+  *aValue = mIdleThreadTimeout;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetIdleThreadTimeout(uint32_t aValue) {
+  MutexAutoLock lock(mMutex);
+  uint32_t oldTimeout = mIdleThreadTimeout;
+  mIdleThreadTimeout = aValue;
+
+  // Do we need to notify any idle threads that their sleep time has shortened?
+  if (mIdleThreadTimeout < oldTimeout && mIdleCount > 0) {
+    mEventsAvailable
+        .NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetIdleThreadTimeoutRegressive(bool* aValue) {
+  MutexAutoLock lock(mMutex);
+  *aValue = mRegressiveMaxIdleTime;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetIdleThreadTimeoutRegressive(bool aValue) {
+  MutexAutoLock lock(mMutex);
+  bool oldRegressive = mRegressiveMaxIdleTime;
+  mRegressiveMaxIdleTime = aValue;
+
+  // Would setting regressive timeout effect idle threads?
+  if (mRegressiveMaxIdleTime > oldRegressive && mIdleCount > 1) {
+    mEventsAvailable
+        .NotifyAll();  // wake up threads so they observe this change
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetThreadStackSize(uint32_t* aValue) {
+  MutexAutoLock lock(mMutex);
+  *aValue = mStackSize;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetThreadStackSize(uint32_t aValue) {
+  MutexAutoLock lock(mMutex);
+  mStackSize = aValue;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::GetListener(nsIThreadPoolListener** aListener) {
+  MutexAutoLock lock(mMutex);
+  NS_IF_ADDREF(*aListener = mListener);
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetListener(nsIThreadPoolListener* aListener) {
+  nsCOMPtr<nsIThreadPoolListener> swappedListener(aListener);
+  {
+    MutexAutoLock lock(mMutex);
+    mListener.swap(swappedListener);
+  }
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsThreadPool::SetName(const nsACString& aName) {
+  MutexAutoLock lock(mMutex);
+  if (mThreads.Count()) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+  mName = aName;
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsThreadPool.h b/xpcom/threads/nsThreadPool.h
new file mode 100644
index 0000000000..b10a2f5265
--- /dev/null
+++ b/xpcom/threads/nsThreadPool.h
@@ -0,0 +1,68 @@
+/* -*- 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 nsThreadPool_h__
+#define nsThreadPool_h__
+
+#include "nsIThreadPool.h"
+#include "nsIRunnable.h"
+#include "nsCOMArray.h"
+#include "nsCOMPtr.h"
+#include "nsThreadUtils.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/Mutex.h"
+
+class nsIThread;
+
+class nsThreadPool final : public mozilla::Runnable, public nsIThreadPool {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIEVENTTARGET_FULL
+  NS_DECL_NSITHREADPOOL
+  NS_DECL_NSIRUNNABLE
+
+  nsThreadPool();
+
+  static void InitTLS();
+  static nsThreadPool* GetCurrentThreadPool();
+
+ private:
+  ~nsThreadPool();
+
+  void ShutdownThread(nsIThread* aThread);
+  nsresult PutEvent(nsIRunnable* aEvent);
+  nsresult PutEvent(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags);
+
+  mozilla::Mutex mMutex;
+  nsCOMArray<nsIThread> mThreads MOZ_GUARDED_BY(mMutex);
+  mozilla::CondVar mEventsAvailable MOZ_GUARDED_BY(mMutex);
+  mozilla::EventQueue mEvents MOZ_GUARDED_BY(mMutex);
+  uint32_t mThreadLimit MOZ_GUARDED_BY(mMutex);
+  uint32_t mIdleThreadLimit MOZ_GUARDED_BY(mMutex);
+  uint32_t mIdleThreadTimeout MOZ_GUARDED_BY(mMutex);
+  uint32_t mIdleCount MOZ_GUARDED_BY(mMutex);
+  nsIThread::QoSPriority mQoSPriority MOZ_GUARDED_BY(mMutex);
+  uint32_t mStackSize MOZ_GUARDED_BY(mMutex);
+  nsCOMPtr<nsIThreadPoolListener> mListener MOZ_GUARDED_BY(mMutex);
+  mozilla::Atomic<bool, mozilla::Relaxed> mShutdown;
+  bool mRegressiveMaxIdleTime MOZ_GUARDED_BY(mMutex);
+  mozilla::Atomic<bool, mozilla::Relaxed> mIsAPoolThreadFree;
+  // set once before we start threads
+  nsCString mName MOZ_GUARDED_BY(mMutex);
+  nsThreadPoolNaming mThreadNaming;  // all data inside this is atomic
+};
+
+#define NS_THREADPOOL_CID                            \
+  { /* 547ec2a8-315e-4ec4-888e-6e4264fe90eb */       \
+    0x547ec2a8, 0x315e, 0x4ec4, {                    \
+      0x88, 0x8e, 0x6e, 0x42, 0x64, 0xfe, 0x90, 0xeb \
+    }                                                \
+  }
+
+#endif  // nsThreadPool_h__
diff --git a/xpcom/threads/nsThreadSyncDispatch.h b/xpcom/threads/nsThreadSyncDispatch.h
new file mode 100644
index 0000000000..1673453f9d
--- /dev/null
+++ b/xpcom/threads/nsThreadSyncDispatch.h
@@ -0,0 +1,65 @@
+/* -*- 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 nsThreadSyncDispatch_h_
+#define nsThreadSyncDispatch_h_
+
+#include "mozilla/Atomics.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/SpinEventLoopUntil.h"
+
+#include "nsThreadUtils.h"
+#include "LeakRefPtr.h"
+
+class nsThreadSyncDispatch : public mozilla::Runnable {
+ public:
+  nsThreadSyncDispatch(already_AddRefed<nsIEventTarget> aOrigin,
+                       already_AddRefed<nsIRunnable>&& aTask)
+      : Runnable("nsThreadSyncDispatch"),
+        mOrigin(aOrigin),
+        mSyncTask(std::move(aTask)),
+        mIsPending(true) {}
+
+  bool IsPending() {
+    // This is an atomic acquire on the origin thread.
+    return mIsPending;
+  }
+
+  void SpinEventLoopUntilComplete(const nsACString& aVeryGoodReasonToDoThis) {
+    mozilla::SpinEventLoopUntil(aVeryGoodReasonToDoThis,
+                                [&]() -> bool { return !IsPending(); });
+  }
+
+ private:
+  NS_IMETHOD Run() override {
+    if (nsCOMPtr<nsIRunnable> task = mSyncTask.take()) {
+      MOZ_ASSERT(!mSyncTask);
+
+      mozilla::DebugOnly<nsresult> result = task->Run();
+      MOZ_ASSERT(NS_SUCCEEDED(result), "task in sync dispatch should not fail");
+
+      // We must release the task here to ensure that when the original
+      // thread is unblocked, this task has been released.
+      task = nullptr;
+
+      // This is an atomic release on the target thread.
+      mIsPending = false;
+
+      // unblock the origin thread
+      mOrigin->Dispatch(this, NS_DISPATCH_IGNORE_BLOCK_DISPATCH);
+    }
+
+    return NS_OK;
+  }
+
+  nsCOMPtr<nsIEventTarget> mOrigin;
+  // The task is leaked by default when Run() is not called, because
+  // otherwise we may release it in an incorrect thread.
+  mozilla::LeakRefPtr<nsIRunnable> mSyncTask;
+  mozilla::Atomic<bool, mozilla::ReleaseAcquire> mIsPending;
+};
+
+#endif  // nsThreadSyncDispatch_h_
diff --git a/xpcom/threads/nsThreadUtils.cpp b/xpcom/threads/nsThreadUtils.cpp
new file mode 100644
index 0000000000..68d36605b3
--- /dev/null
+++ b/xpcom/threads/nsThreadUtils.cpp
@@ -0,0 +1,769 @@
+/* -*- 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 "nsThreadUtils.h"
+
+#include "chrome/common/ipc_message.h"  // for IPC::Message
+#include "LeakRefPtr.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Likely.h"
+#include "mozilla/TaskQueue.h"
+#include "mozilla/TimeStamp.h"
+#include "nsComponentManagerUtils.h"
+#include "nsExceptionHandler.h"
+#include "nsIEventTarget.h"
+#include "nsITimer.h"
+#include "nsString.h"
+#include "nsThreadSyncDispatch.h"
+#include "nsTimerImpl.h"
+#include "prsystem.h"
+
+#include "nsThreadManager.h"
+#include "nsThreadPool.h"
+#include "TaskController.h"
+
+#ifdef XP_WIN
+#  include <windows.h>
+#elif defined(XP_MACOSX)
+#  include <sys/resource.h>
+#endif
+
+#if defined(ANDROID)
+#  include <sys/prctl.h>
+#endif
+
+static mozilla::LazyLogModule sEventDispatchAndRunLog("events");
+#ifdef LOG1
+#  undef LOG1
+#endif
+#define LOG1(args) \
+  MOZ_LOG(sEventDispatchAndRunLog, mozilla::LogLevel::Error, args)
+#define LOG1_ENABLED() \
+  MOZ_LOG_TEST(sEventDispatchAndRunLog, mozilla::LogLevel::Error)
+
+using namespace mozilla;
+
+#ifndef XPCOM_GLUE_AVOID_NSPR
+
+NS_IMPL_ISUPPORTS(IdlePeriod, nsIIdlePeriod)
+
+NS_IMETHODIMP
+IdlePeriod::GetIdlePeriodHint(TimeStamp* aIdleDeadline) {
+  *aIdleDeadline = TimeStamp();
+  return NS_OK;
+}
+
+// NS_IMPL_NAMED_* relies on the mName field, which is not present on
+// release or beta. Instead, fall back to using "Runnable" for all
+// runnables.
+#  ifndef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+NS_IMPL_ISUPPORTS(Runnable, nsIRunnable)
+#  else
+NS_IMPL_NAMED_ADDREF(Runnable, mName)
+NS_IMPL_NAMED_RELEASE(Runnable, mName)
+NS_IMPL_QUERY_INTERFACE(Runnable, nsIRunnable, nsINamed)
+#  endif
+
+NS_IMETHODIMP
+Runnable::Run() {
+  // Do nothing
+  return NS_OK;
+}
+
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+NS_IMETHODIMP
+Runnable::GetName(nsACString& aName) {
+  if (mName) {
+    aName.AssignASCII(mName);
+  } else {
+    aName.Truncate();
+  }
+  return NS_OK;
+}
+#  endif
+
+NS_IMPL_ISUPPORTS_INHERITED(DiscardableRunnable, Runnable,
+                            nsIDiscardableRunnable)
+
+NS_IMPL_ISUPPORTS_INHERITED(CancelableRunnable, DiscardableRunnable,
+                            nsICancelableRunnable)
+
+void CancelableRunnable::OnDiscard() {
+  // Tasks that implement Cancel() can be safely cleaned up if it turns out
+  // that the task will not run.
+  (void)NS_WARN_IF(NS_FAILED(Cancel()));
+}
+
+NS_IMPL_ISUPPORTS_INHERITED(IdleRunnable, DiscardableRunnable, nsIIdleRunnable)
+
+NS_IMPL_ISUPPORTS_INHERITED(CancelableIdleRunnable, CancelableRunnable,
+                            nsIIdleRunnable)
+
+NS_IMPL_ISUPPORTS_INHERITED(PrioritizableRunnable, Runnable,
+                            nsIRunnablePriority)
+
+PrioritizableRunnable::PrioritizableRunnable(
+    already_AddRefed<nsIRunnable>&& aRunnable, uint32_t aPriority)
+    // Real runnable name is managed by overridding the GetName function.
+    : Runnable("PrioritizableRunnable"),
+      mRunnable(std::move(aRunnable)),
+      mPriority(aPriority) {
+#  if DEBUG
+  nsCOMPtr<nsIRunnablePriority> runnablePrio = do_QueryInterface(mRunnable);
+  MOZ_ASSERT(!runnablePrio);
+#  endif
+}
+
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+NS_IMETHODIMP
+PrioritizableRunnable::GetName(nsACString& aName) {
+  // Try to get a name from the underlying runnable.
+  nsCOMPtr<nsINamed> named = do_QueryInterface(mRunnable);
+  if (named) {
+    named->GetName(aName);
+  }
+  return NS_OK;
+}
+#  endif
+
+NS_IMETHODIMP
+PrioritizableRunnable::Run() {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
+  return mRunnable->Run();
+}
+
+NS_IMETHODIMP
+PrioritizableRunnable::GetPriority(uint32_t* aPriority) {
+  *aPriority = mPriority;
+  return NS_OK;
+}
+
+already_AddRefed<nsIRunnable> mozilla::CreateRenderBlockingRunnable(
+    already_AddRefed<nsIRunnable>&& aRunnable) {
+  nsCOMPtr<nsIRunnable> runnable = new PrioritizableRunnable(
+      std::move(aRunnable), nsIRunnablePriority::PRIORITY_RENDER_BLOCKING);
+  return runnable.forget();
+}
+
+NS_IMPL_ISUPPORTS_INHERITED(PrioritizableCancelableRunnable, CancelableRunnable,
+                            nsIRunnablePriority)
+
+NS_IMETHODIMP
+PrioritizableCancelableRunnable::GetPriority(uint32_t* aPriority) {
+  *aPriority = mPriority;
+  return NS_OK;
+}
+
+#endif  // XPCOM_GLUE_AVOID_NSPR
+
+//-----------------------------------------------------------------------------
+
+nsresult NS_NewNamedThread(const nsACString& aName, nsIThread** aResult,
+                           nsIRunnable* aInitialEvent,
+                           nsIThreadManager::ThreadCreationOptions aOptions) {
+  nsCOMPtr<nsIRunnable> event = aInitialEvent;
+  return NS_NewNamedThread(aName, aResult, event.forget(), aOptions);
+}
+
+nsresult NS_NewNamedThread(const nsACString& aName, nsIThread** aResult,
+                           already_AddRefed<nsIRunnable> aInitialEvent,
+                           nsIThreadManager::ThreadCreationOptions aOptions) {
+  nsCOMPtr<nsIRunnable> event = std::move(aInitialEvent);
+  nsCOMPtr<nsIThread> thread;
+  nsresult rv = nsThreadManager::get().nsThreadManager::NewNamedThread(
+      aName, aOptions, getter_AddRefs(thread));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    return rv;
+  }
+
+  if (event) {
+    rv = thread->Dispatch(event.forget(), NS_DISPATCH_IGNORE_BLOCK_DISPATCH);
+    if (NS_WARN_IF(NS_FAILED(rv))) {
+      return rv;
+    }
+  }
+
+  *aResult = nullptr;
+  thread.swap(*aResult);
+  return NS_OK;
+}
+
+nsresult NS_GetCurrentThread(nsIThread** aResult) {
+  return nsThreadManager::get().nsThreadManager::GetCurrentThread(aResult);
+}
+
+nsresult NS_GetMainThread(nsIThread** aResult) {
+  return nsThreadManager::get().nsThreadManager::GetMainThread(aResult);
+}
+
+nsresult NS_DispatchToCurrentThread(already_AddRefed<nsIRunnable>&& aEvent) {
+  nsresult rv;
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  // XXX: Consider using GetCurrentSerialEventTarget() to support TaskQueues.
+  nsISerialEventTarget* thread = NS_GetCurrentThread();
+  if (!thread) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  // To keep us from leaking the runnable if dispatch method fails,
+  // we grab the reference on failures and release it.
+  nsIRunnable* temp = event.get();
+  rv = thread->Dispatch(event.forget(), NS_DISPATCH_NORMAL);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    // Dispatch() leaked the reference to the event, but due to caller's
+    // assumptions, we shouldn't leak here. And given we are on the same
+    // thread as the dispatch target, it's mostly safe to do it here.
+    NS_RELEASE(temp);
+  }
+  return rv;
+}
+
+// It is common to call NS_DispatchToCurrentThread with a newly
+// allocated runnable with a refcount of zero. To keep us from leaking
+// the runnable if the dispatch method fails, we take a death grip.
+nsresult NS_DispatchToCurrentThread(nsIRunnable* aEvent) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return NS_DispatchToCurrentThread(event.forget());
+}
+
+nsresult NS_DispatchToMainThread(already_AddRefed<nsIRunnable>&& aEvent,
+                                 uint32_t aDispatchFlags) {
+  LeakRefPtr<nsIRunnable> event(std::move(aEvent));
+  nsCOMPtr<nsIThread> thread;
+  nsresult rv = NS_GetMainThread(getter_AddRefs(thread));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    NS_ASSERTION(false,
+                 "Failed NS_DispatchToMainThread() in shutdown; leaking");
+    // NOTE: if you stop leaking here, adjust Promise::MaybeReportRejected(),
+    // which assumes a leak here, or split into leaks and no-leaks versions
+    return rv;
+  }
+  return thread->Dispatch(event.take(), aDispatchFlags);
+}
+
+// In the case of failure with a newly allocated runnable with a
+// refcount of zero, we intentionally leak the runnable, because it is
+// likely that the runnable is being dispatched to the main thread
+// because it owns main thread only objects, so it is not safe to
+// release them here.
+nsresult NS_DispatchToMainThread(nsIRunnable* aEvent, uint32_t aDispatchFlags) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return NS_DispatchToMainThread(event.forget(), aDispatchFlags);
+}
+
+nsresult NS_DelayedDispatchToCurrentThread(
+    already_AddRefed<nsIRunnable>&& aEvent, uint32_t aDelayMs) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+
+  // XXX: Consider using GetCurrentSerialEventTarget() to support TaskQueues.
+  nsISerialEventTarget* thread = NS_GetCurrentThread();
+  if (!thread) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  return thread->DelayedDispatch(event.forget(), aDelayMs);
+}
+
+nsresult NS_DispatchToThreadQueue(already_AddRefed<nsIRunnable>&& aEvent,
+                                  nsIThread* aThread,
+                                  EventQueuePriority aQueue) {
+  nsresult rv;
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  NS_ENSURE_TRUE(event, NS_ERROR_INVALID_ARG);
+  if (!aThread) {
+    return NS_ERROR_UNEXPECTED;
+  }
+  // To keep us from leaking the runnable if dispatch method fails,
+  // we grab the reference on failures and release it.
+  nsIRunnable* temp = event.get();
+  rv = aThread->DispatchToQueue(event.forget(), aQueue);
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    // Dispatch() leaked the reference to the event, but due to caller's
+    // assumptions, we shouldn't leak here. And given we are on the same
+    // thread as the dispatch target, it's mostly safe to do it here.
+    NS_RELEASE(temp);
+  }
+
+  return rv;
+}
+
+nsresult NS_DispatchToCurrentThreadQueue(already_AddRefed<nsIRunnable>&& aEvent,
+                                         EventQueuePriority aQueue) {
+  return NS_DispatchToThreadQueue(std::move(aEvent), NS_GetCurrentThread(),
+                                  aQueue);
+}
+
+extern nsresult NS_DispatchToMainThreadQueue(
+    already_AddRefed<nsIRunnable>&& aEvent, EventQueuePriority aQueue) {
+  nsCOMPtr<nsIThread> mainThread;
+  nsresult rv = NS_GetMainThread(getter_AddRefs(mainThread));
+  if (NS_SUCCEEDED(rv)) {
+    return NS_DispatchToThreadQueue(std::move(aEvent), mainThread, aQueue);
+  }
+  return rv;
+}
+
+class IdleRunnableWrapper final : public Runnable,
+                                  public nsIDiscardableRunnable,
+                                  public nsIIdleRunnable {
+ public:
+  explicit IdleRunnableWrapper(already_AddRefed<nsIRunnable>&& aEvent)
+      : Runnable("IdleRunnableWrapper"),
+        mRunnable(std::move(aEvent)),
+        mDiscardable(do_QueryInterface(mRunnable)) {}
+
+  NS_DECL_ISUPPORTS_INHERITED
+
+  NS_IMETHOD Run() override {
+    if (!mRunnable) {
+      return NS_OK;
+    }
+    CancelTimer();
+    // Don't clear mDiscardable because that would cause QueryInterface to
+    // change behavior during the lifetime of an instance.
+    nsCOMPtr<nsIRunnable> runnable = std::move(mRunnable);
+    return runnable->Run();
+  }
+
+  // nsIDiscardableRunnable
+  void OnDiscard() override {
+    if (!mRunnable) {
+      // Run() was already called from TimedOut().
+      return;
+    }
+    mDiscardable->OnDiscard();
+    mRunnable = nullptr;
+  }
+
+  static void TimedOut(nsITimer* aTimer, void* aClosure) {
+    RefPtr<IdleRunnableWrapper> runnable =
+        static_cast<IdleRunnableWrapper*>(aClosure);
+    LogRunnable::Run log(runnable);
+    runnable->Run();
+    runnable = nullptr;
+  }
+
+  void SetTimer(uint32_t aDelay, nsIEventTarget* aTarget) override {
+    MOZ_ASSERT(aTarget);
+    MOZ_ASSERT(!mTimer);
+    NS_NewTimerWithFuncCallback(getter_AddRefs(mTimer), TimedOut, this, aDelay,
+                                nsITimer::TYPE_ONE_SHOT,
+                                "IdleRunnableWrapper::SetTimer", aTarget);
+  }
+
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_IMETHOD GetName(nsACString& aName) override {
+    aName.AssignLiteral("IdleRunnableWrapper");
+    if (nsCOMPtr<nsINamed> named = do_QueryInterface(mRunnable)) {
+      nsAutoCString name;
+      named->GetName(name);
+      if (!name.IsEmpty()) {
+        aName.AppendLiteral(" for ");
+        aName.Append(name);
+      }
+    }
+    return NS_OK;
+  }
+#endif
+
+ private:
+  ~IdleRunnableWrapper() { CancelTimer(); }
+
+  void CancelTimer() {
+    if (mTimer) {
+      mTimer->Cancel();
+    }
+  }
+
+  nsCOMPtr<nsITimer> mTimer;
+  nsCOMPtr<nsIRunnable> mRunnable;
+  nsCOMPtr<nsIDiscardableRunnable> mDiscardable;
+};
+
+NS_IMPL_ADDREF_INHERITED(IdleRunnableWrapper, Runnable)
+NS_IMPL_RELEASE_INHERITED(IdleRunnableWrapper, Runnable)
+
+NS_INTERFACE_MAP_BEGIN(IdleRunnableWrapper)
+  NS_INTERFACE_MAP_ENTRY(nsIIdleRunnable)
+  NS_INTERFACE_MAP_ENTRY_CONDITIONAL(nsIDiscardableRunnable, mDiscardable)
+NS_INTERFACE_MAP_END_INHERITING(Runnable)
+
+extern nsresult NS_DispatchToThreadQueue(already_AddRefed<nsIRunnable>&& aEvent,
+                                         uint32_t aTimeout, nsIThread* aThread,
+                                         EventQueuePriority aQueue) {
+  nsCOMPtr<nsIRunnable> event(std::move(aEvent));
+  NS_ENSURE_TRUE(event, NS_ERROR_INVALID_ARG);
+  MOZ_ASSERT(aQueue == EventQueuePriority::Idle ||
+             aQueue == EventQueuePriority::DeferredTimers);
+  if (!aThread) {
+    return NS_ERROR_UNEXPECTED;
+  }
+
+  nsCOMPtr<nsIIdleRunnable> idleEvent = do_QueryInterface(event);
+
+  if (!idleEvent) {
+    idleEvent = new IdleRunnableWrapper(event.forget());
+    event = do_QueryInterface(idleEvent);
+    MOZ_DIAGNOSTIC_ASSERT(event);
+  }
+  idleEvent->SetTimer(aTimeout, aThread);
+
+  nsresult rv = NS_DispatchToThreadQueue(event.forget(), aThread, aQueue);
+  if (NS_SUCCEEDED(rv)) {
+    // This is intended to bind with the "DISP" log made from inside
+    // NS_DispatchToThreadQueue for the `event`. There is no possibly to inject
+    // another "DISP" for a different event on this thread.
+    LOG1(("TIMEOUT %u", aTimeout));
+  }
+
+  return rv;
+}
+
+extern nsresult NS_DispatchToCurrentThreadQueue(
+    already_AddRefed<nsIRunnable>&& aEvent, uint32_t aTimeout,
+    EventQueuePriority aQueue) {
+  return NS_DispatchToThreadQueue(std::move(aEvent), aTimeout,
+                                  NS_GetCurrentThread(), aQueue);
+}
+
+#ifndef XPCOM_GLUE_AVOID_NSPR
+nsresult NS_ProcessPendingEvents(nsIThread* aThread, PRIntervalTime aTimeout) {
+  nsresult rv = NS_OK;
+
+  if (!aThread) {
+    aThread = NS_GetCurrentThread();
+    if (NS_WARN_IF(!aThread)) {
+      return NS_ERROR_UNEXPECTED;
+    }
+  }
+
+  PRIntervalTime start = PR_IntervalNow();
+  for (;;) {
+    bool processedEvent;
+    rv = aThread->ProcessNextEvent(false, &processedEvent);
+    if (NS_FAILED(rv) || !processedEvent) {
+      break;
+    }
+    if (PR_IntervalNow() - start > aTimeout) {
+      break;
+    }
+  }
+  return rv;
+}
+#endif  // XPCOM_GLUE_AVOID_NSPR
+
+inline bool hasPendingEvents(nsIThread* aThread) {
+  bool val;
+  return NS_SUCCEEDED(aThread->HasPendingEvents(&val)) && val;
+}
+
+bool NS_HasPendingEvents(nsIThread* aThread) {
+  if (!aThread) {
+    aThread = NS_GetCurrentThread();
+    if (NS_WARN_IF(!aThread)) {
+      return false;
+    }
+  }
+  return hasPendingEvents(aThread);
+}
+
+bool NS_ProcessNextEvent(nsIThread* aThread, bool aMayWait) {
+  if (!aThread) {
+    aThread = NS_GetCurrentThread();
+    if (NS_WARN_IF(!aThread)) {
+      return false;
+    }
+  }
+  bool val;
+  return NS_SUCCEEDED(aThread->ProcessNextEvent(aMayWait, &val)) && val;
+}
+
+void NS_SetCurrentThreadName(const char* aName) {
+  PR_SetCurrentThreadName(aName);
+#if defined(ANDROID) && defined(DEBUG)
+  // Check nspr does the right thing on Android.
+  char buffer[16] = {'\0'};
+  prctl(PR_GET_NAME, buffer);
+  MOZ_ASSERT(0 == strncmp(buffer, aName, 15));
+#endif
+  if (nsThreadManager::get().IsNSThread()) {
+    nsThread* thread = nsThreadManager::get().GetCurrentThread();
+    thread->SetThreadNameInternal(nsDependentCString(aName));
+  }
+}
+
+nsIThread* NS_GetCurrentThread() {
+  return nsThreadManager::get().GetCurrentThread();
+}
+
+nsIThread* NS_GetCurrentThreadNoCreate() {
+  if (nsThreadManager::get().IsNSThread()) {
+    return NS_GetCurrentThread();
+  }
+  return nullptr;
+}
+
+// nsThreadPoolNaming
+nsCString nsThreadPoolNaming::GetNextThreadName(const nsACString& aPoolName) {
+  nsCString name(aPoolName);
+  name.AppendLiteral(" #");
+  name.AppendInt(++mCounter, 10);  // The counter is declared as atomic
+  return name;
+}
+
+nsresult NS_DispatchBackgroundTask(already_AddRefed<nsIRunnable> aEvent,
+                                   uint32_t aDispatchFlags) {
+  nsCOMPtr<nsIRunnable> event(aEvent);
+  return nsThreadManager::get().DispatchToBackgroundThread(event,
+                                                           aDispatchFlags);
+}
+
+// nsAutoLowPriorityIO
+nsAutoLowPriorityIO::nsAutoLowPriorityIO() {
+#if defined(XP_WIN)
+  lowIOPrioritySet =
+      SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_BEGIN);
+#elif defined(XP_MACOSX)
+  oldPriority = getiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_THREAD);
+  lowIOPrioritySet =
+      oldPriority != -1 &&
+      setiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_THREAD, IOPOL_THROTTLE) != -1;
+#else
+  lowIOPrioritySet = false;
+#endif
+}
+
+nsAutoLowPriorityIO::~nsAutoLowPriorityIO() {
+#if defined(XP_WIN)
+  if (MOZ_LIKELY(lowIOPrioritySet)) {
+    // On Windows the old thread priority is automatically restored
+    SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_END);
+  }
+#elif defined(XP_MACOSX)
+  if (MOZ_LIKELY(lowIOPrioritySet)) {
+    setiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_THREAD, oldPriority);
+  }
+#endif
+}
+
+namespace mozilla {
+
+nsISerialEventTarget* GetCurrentSerialEventTarget() {
+  if (nsISerialEventTarget* current =
+          SerialEventTargetGuard::GetCurrentSerialEventTarget()) {
+    return current;
+  }
+
+  MOZ_DIAGNOSTIC_ASSERT(!nsThreadPool::GetCurrentThreadPool(),
+                        "Call to GetCurrentSerialEventTarget() from thread "
+                        "pool without an active TaskQueue");
+
+  nsCOMPtr<nsIThread> thread;
+  nsresult rv = NS_GetCurrentThread(getter_AddRefs(thread));
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+
+  return thread;
+}
+
+nsISerialEventTarget* GetMainThreadSerialEventTarget() {
+  return static_cast<nsThread*>(nsThreadManager::get().GetMainThreadWeak());
+}
+
+size_t GetNumberOfProcessors() {
+#if defined(XP_LINUX) && defined(MOZ_SANDBOX)
+  static const PRInt32 procs = PR_GetNumberOfProcessors();
+#else
+  PRInt32 procs = PR_GetNumberOfProcessors();
+#endif
+  MOZ_ASSERT(procs > 0);
+  return static_cast<size_t>(procs);
+}
+
+template <typename T>
+void LogTaskBase<T>::LogDispatch(T* aEvent) {
+  LOG1(("DISP %p", aEvent));
+}
+template <typename T>
+void LogTaskBase<T>::LogDispatch(T* aEvent, void* aContext) {
+  LOG1(("DISP %p (%p)", aEvent, aContext));
+}
+
+template <>
+void LogTaskBase<IPC::Message>::LogDispatchWithPid(IPC::Message* aEvent,
+                                                   int32_t aPid) {
+  if (aEvent->seqno() && aPid > 0) {
+    LOG1(("SEND %p %d %d", aEvent, aEvent->seqno(), aPid));
+  }
+}
+
+template <typename T>
+LogTaskBase<T>::Run::Run(T* aEvent, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  // Logging address of this RAII so that we can use it to identify the DONE log
+  // while not keeping any ref to the event that could be invalid at the dtor
+  // time.
+  LOG1(("EXEC %p %p", aEvent, this));
+}
+template <typename T>
+LogTaskBase<T>::Run::Run(T* aEvent, void* aContext, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  LOG1(("EXEC %p (%p) %p", aEvent, aContext, this));
+}
+
+template <>
+LogTaskBase<nsIRunnable>::Run::Run(nsIRunnable* aEvent, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  if (!LOG1_ENABLED()) {
+    return;
+  }
+
+  nsCOMPtr<nsINamed> named(do_QueryInterface(aEvent));
+  if (!named) {
+    LOG1(("EXEC %p %p", aEvent, this));
+    return;
+  }
+
+  nsAutoCString name;
+  named->GetName(name);
+  LOG1(("EXEC %p %p [%s]", aEvent, this, name.BeginReading()));
+}
+
+template <>
+LogTaskBase<Task>::Run::Run(Task* aTask, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  if (!LOG1_ENABLED()) {
+    return;
+  }
+
+  nsAutoCString name;
+  if (!aTask->GetName(name)) {
+    LOG1(("EXEC %p %p", aTask, this));
+    return;
+  }
+
+  LOG1(("EXEC %p %p [%s]", aTask, this, name.BeginReading()));
+}
+
+template <>
+LogTaskBase<IPC::Message>::Run::Run(IPC::Message* aMessage, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  LOG1(("RECV %p %p %d [%s]", aMessage, this, aMessage->seqno(),
+        aMessage->name()));
+}
+
+template <>
+LogTaskBase<nsTimerImpl>::Run::Run(nsTimerImpl* aEvent, bool aWillRunAgain)
+    : mWillRunAgain(aWillRunAgain) {
+  // The name of the timer will be logged when running it on the target thread.
+  // Logging it here (on the `Timer` thread) would be redundant.
+  LOG1(("EXEC %p %p [nsTimerImpl]", aEvent, this));
+}
+
+template <typename T>
+LogTaskBase<T>::Run::~Run() {
+  LOG1((mWillRunAgain ? "INTERRUPTED %p" : "DONE %p", this));
+}
+
+template class LogTaskBase<nsIRunnable>;
+template class LogTaskBase<MicroTaskRunnable>;
+template class LogTaskBase<IPC::Message>;
+template class LogTaskBase<nsTimerImpl>;
+template class LogTaskBase<Task>;
+template class LogTaskBase<PresShell>;
+template class LogTaskBase<dom::FrameRequestCallback>;
+
+MOZ_THREAD_LOCAL(nsISerialEventTarget*)
+SerialEventTargetGuard::sCurrentThreadTLS;
+void SerialEventTargetGuard::InitTLS() {
+  MOZ_ASSERT(NS_IsMainThread());
+  if (!sCurrentThreadTLS.init()) {
+    MOZ_CRASH();
+  }
+}
+
+}  // namespace mozilla
+
+bool nsIEventTarget::IsOnCurrentThread() {
+  if (mThread) {
+    return mThread == PR_GetCurrentThread();
+  }
+  return IsOnCurrentThreadInfallible();
+}
+
+extern "C" {
+// These functions use the C language linkage because they're exposed to Rust
+// via the xpcom/rust/moz_task crate, which wraps them in safe Rust functions
+// that enable Rust code to get/create threads and dispatch runnables on them.
+
+nsresult NS_GetCurrentThreadRust(nsIThread** aResult) {
+  return NS_GetCurrentThread(aResult);
+}
+
+nsresult NS_GetMainThreadRust(nsIThread** aResult) {
+  return NS_GetMainThread(aResult);
+}
+
+// NS_NewNamedThread's aStackSize parameter has the default argument
+// nsIThreadManager::DEFAULT_STACK_SIZE, but we can't omit default arguments
+// when calling a C++ function from Rust, and we can't access
+// nsIThreadManager::DEFAULT_STACK_SIZE in Rust to pass it explicitly,
+// since it is defined in a %{C++ ... %} block within nsIThreadManager.idl.
+// So we indirect through this function.
+nsresult NS_NewNamedThreadWithDefaultStackSize(const nsACString& aName,
+                                               nsIThread** aResult,
+                                               nsIRunnable* aEvent) {
+  return NS_NewNamedThread(aName, aResult, aEvent);
+}
+
+bool NS_IsOnCurrentThread(nsIEventTarget* aTarget) {
+  return aTarget->IsOnCurrentThread();
+}
+
+nsresult NS_DispatchBackgroundTask(nsIRunnable* aEvent,
+                                   uint32_t aDispatchFlags) {
+  return nsThreadManager::get().DispatchToBackgroundThread(aEvent,
+                                                           aDispatchFlags);
+}
+
+nsresult NS_CreateBackgroundTaskQueue(const char* aName,
+                                      nsISerialEventTarget** aTarget) {
+  nsCOMPtr<nsISerialEventTarget> target =
+      nsThreadManager::get().CreateBackgroundTaskQueue(aName);
+  if (!target) {
+    return NS_ERROR_FAILURE;
+  }
+
+  target.forget(aTarget);
+  return NS_OK;
+}
+
+}  // extern "C"
+
+nsresult NS_DispatchAndSpinEventLoopUntilComplete(
+    const nsACString& aVeryGoodReasonToDoThis, nsIEventTarget* aEventTarget,
+    already_AddRefed<nsIRunnable> aEvent) {
+  // NOTE: Get the current thread specifically, as `SpinEventLoopUntil` can
+  // only spin that event target's loop. The reply will specify
+  // NS_DISPATCH_IGNORE_BLOCK_DISPATCH to ensure the reply is received even if
+  // the caller is a threadpool thread.
+  nsCOMPtr<nsIThread> current = NS_GetCurrentThread();
+  if (NS_WARN_IF(!current)) {
+    return NS_ERROR_NOT_AVAILABLE;
+  }
+
+  RefPtr<nsThreadSyncDispatch> wrapper =
+      new nsThreadSyncDispatch(current.forget(), std::move(aEvent));
+  nsresult rv = aEventTarget->Dispatch(do_AddRef(wrapper));
+  if (NS_WARN_IF(NS_FAILED(rv))) {
+    // FIXME: Consider avoiding leaking the `nsThreadSyncDispatch` as well by
+    // using a fallible version of `Dispatch` once that is added.
+    return rv;
+  }
+
+  wrapper->SpinEventLoopUntilComplete(aVeryGoodReasonToDoThis);
+  return NS_OK;
+}
diff --git a/xpcom/threads/nsThreadUtils.h b/xpcom/threads/nsThreadUtils.h
new file mode 100644
index 0000000000..72041da295
--- /dev/null
+++ b/xpcom/threads/nsThreadUtils.h
@@ -0,0 +1,1925 @@
+/* -*- 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 nsThreadUtils_h__
+#define nsThreadUtils_h__
+
+#include <type_traits>
+#include <tuple>
+#include <utility>
+
+#include "MainThreadUtils.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/AbstractThread.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Likely.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ThreadLocal.h"
+#include "mozilla/TimeStamp.h"
+
+#include "nsCOMPtr.h"
+#include "nsICancelableRunnable.h"
+#include "nsIDiscardableRunnable.h"
+#include "nsIIdlePeriod.h"
+#include "nsIIdleRunnable.h"
+#include "nsINamed.h"
+#include "nsIRunnable.h"
+#include "nsIThreadManager.h"
+#include "nsITimer.h"
+#include "nsString.h"
+#include "prinrval.h"
+#include "prthread.h"
+
+class MessageLoop;
+class nsIThread;
+
+//-----------------------------------------------------------------------------
+// These methods are alternatives to the methods on nsIThreadManager, provided
+// for convenience.
+
+/**
+ * Create a new thread, and optionally provide an initial event for the thread.
+ *
+ * @param aName
+ *   The name of the thread.
+ * @param aResult
+ *   The resulting nsIThread object.
+ * @param aInitialEvent
+ *   The initial event to run on this thread.  This parameter may be null.
+ * @param aOptions
+ *   Options used to configure thread creation.
+ *   Options are documented in nsIThreadManager.idl.
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   Indicates that the given name is not unique.
+ */
+
+extern nsresult NS_NewNamedThread(
+    const nsACString& aName, nsIThread** aResult,
+    nsIRunnable* aInitialEvent = nullptr,
+    nsIThreadManager::ThreadCreationOptions aOptions = {});
+
+extern nsresult NS_NewNamedThread(
+    const nsACString& aName, nsIThread** aResult,
+    already_AddRefed<nsIRunnable> aInitialEvent,
+    nsIThreadManager::ThreadCreationOptions aOptions = {});
+
+template <size_t LEN>
+inline nsresult NS_NewNamedThread(
+    const char (&aName)[LEN], nsIThread** aResult,
+    already_AddRefed<nsIRunnable> aInitialEvent,
+    nsIThreadManager::ThreadCreationOptions aOptions = {}) {
+  static_assert(LEN <= 16, "Thread name must be no more than 16 characters");
+  return NS_NewNamedThread(nsDependentCString(aName, LEN - 1), aResult,
+                           std::move(aInitialEvent), aOptions);
+}
+
+template <size_t LEN>
+inline nsresult NS_NewNamedThread(
+    const char (&aName)[LEN], nsIThread** aResult,
+    nsIRunnable* aInitialEvent = nullptr,
+    nsIThreadManager::ThreadCreationOptions aOptions = {}) {
+  nsCOMPtr<nsIRunnable> event = aInitialEvent;
+  static_assert(LEN <= 16, "Thread name must be no more than 16 characters");
+  return NS_NewNamedThread(nsDependentCString(aName, LEN - 1), aResult,
+                           event.forget(), aOptions);
+}
+
+/**
+ * Get a reference to the current thread, creating it if it does not exist yet.
+ *
+ * @param aResult
+ *   The resulting nsIThread object.
+ */
+extern nsresult NS_GetCurrentThread(nsIThread** aResult);
+
+/**
+ * Dispatch the given event to the current thread.
+ *
+ * @param aEvent
+ *   The event to dispatch.
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ */
+extern nsresult NS_DispatchToCurrentThread(nsIRunnable* aEvent);
+extern nsresult NS_DispatchToCurrentThread(
+    already_AddRefed<nsIRunnable>&& aEvent);
+
+/**
+ * Dispatch the given event to the main thread.
+ *
+ * @param aEvent
+ *   The event to dispatch.
+ * @param aDispatchFlags
+ *   The flags to pass to the main thread's dispatch method.
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ */
+extern nsresult NS_DispatchToMainThread(
+    nsIRunnable* aEvent, uint32_t aDispatchFlags = NS_DISPATCH_NORMAL);
+extern nsresult NS_DispatchToMainThread(
+    already_AddRefed<nsIRunnable>&& aEvent,
+    uint32_t aDispatchFlags = NS_DISPATCH_NORMAL);
+
+extern nsresult NS_DelayedDispatchToCurrentThread(
+    already_AddRefed<nsIRunnable>&& aEvent, uint32_t aDelayMs);
+
+/**
+ * Dispatch the given event to the specified queue of the current thread.
+ *
+ * @param aEvent The event to dispatch.
+ * @param aQueue The event queue for the thread to use
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ * @returns NS_ERROR_UNEXPECTED
+ *   If the thread is shutting down.
+ */
+extern nsresult NS_DispatchToCurrentThreadQueue(
+    already_AddRefed<nsIRunnable>&& aEvent, mozilla::EventQueuePriority aQueue);
+
+/**
+ * Dispatch the given event to the specified queue of the main thread.
+ *
+ * @param aEvent The event to dispatch.
+ * @param aQueue The event queue for the thread to use
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ * @returns NS_ERROR_UNEXPECTED
+ *   If the thread is shutting down.
+ */
+extern nsresult NS_DispatchToMainThreadQueue(
+    already_AddRefed<nsIRunnable>&& aEvent, mozilla::EventQueuePriority aQueue);
+
+/**
+ * Dispatch the given event to an idle queue of the current thread.
+ *
+ * @param aEvent The event to dispatch. If the event implements
+ *   nsIIdleRunnable, it will receive a call on
+ *   nsIIdleRunnable::SetTimer when dispatched, with the value of
+ *   aTimeout.
+ *
+ * @param aTimeout The time in milliseconds until the event should be
+ *   moved from an idle queue to the regular queue, if it hasn't been
+ *   executed. If aEvent is also an nsIIdleRunnable, it is expected
+ *   that it should handle the timeout itself, after a call to
+ *   nsIIdleRunnable::SetTimer.
+ *
+ * @param aQueue
+ *   The event queue for the thread to use.  Must be an idle queue
+ *   (Idle or DeferredTimers)
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ * @returns NS_ERROR_UNEXPECTED
+ *   If the thread is shutting down.
+ */
+extern nsresult NS_DispatchToCurrentThreadQueue(
+    already_AddRefed<nsIRunnable>&& aEvent, uint32_t aTimeout,
+    mozilla::EventQueuePriority aQueue);
+
+/**
+ * Dispatch the given event to a queue of a thread.
+ *
+ * @param aEvent The event to dispatch.
+ * @param aThread The target thread for the dispatch.
+ * @param aQueue The event queue for the thread to use.
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ * @returns NS_ERROR_UNEXPECTED
+ *   If the thread is shutting down.
+ */
+extern nsresult NS_DispatchToThreadQueue(already_AddRefed<nsIRunnable>&& aEvent,
+                                         nsIThread* aThread,
+                                         mozilla::EventQueuePriority aQueue);
+
+/**
+ * Dispatch the given event to an idle queue of a thread.
+ *
+ * @param aEvent The event to dispatch. If the event implements
+ *   nsIIdleRunnable, it will receive a call on
+ *   nsIIdleRunnable::SetTimer when dispatched, with the value of
+ *   aTimeout.
+ *
+ * @param aTimeout The time in milliseconds until the event should be
+ *   moved from an idle queue to the regular queue, if it hasn't been
+ *   executed. If aEvent is also an nsIIdleRunnable, it is expected
+ *   that it should handle the timeout itself, after a call to
+ *   nsIIdleRunnable::SetTimer.
+ *
+ * @param aThread The target thread for the dispatch.
+ *
+ * @param aQueue
+ *   The event queue for the thread to use.  Must be an idle queue
+ *   (Idle or DeferredTimers)
+ *
+ * @returns NS_ERROR_INVALID_ARG
+ *   If event is null.
+ * @returns NS_ERROR_UNEXPECTED
+ *   If the thread is shutting down.
+ */
+extern nsresult NS_DispatchToThreadQueue(already_AddRefed<nsIRunnable>&& aEvent,
+                                         uint32_t aTimeout, nsIThread* aThread,
+                                         mozilla::EventQueuePriority aQueue);
+
+#ifndef XPCOM_GLUE_AVOID_NSPR
+/**
+ * Process all pending events for the given thread before returning.  This
+ * method simply calls ProcessNextEvent on the thread while HasPendingEvents
+ * continues to return true and the time spent in NS_ProcessPendingEvents
+ * does not exceed the given timeout value.
+ *
+ * @param aThread
+ *   The thread object for which to process pending events.  If null, then
+ *   events will be processed for the current thread.
+ * @param aTimeout
+ *   The maximum number of milliseconds to spend processing pending events.
+ *   Events are not pre-empted to honor this timeout.  Rather, the timeout
+ *   value is simply used to determine whether or not to process another event.
+ *   Pass PR_INTERVAL_NO_TIMEOUT to specify no timeout.
+ */
+extern nsresult NS_ProcessPendingEvents(
+    nsIThread* aThread, PRIntervalTime aTimeout = PR_INTERVAL_NO_TIMEOUT);
+#endif
+
+/**
+ * Shortcut for nsIThread::HasPendingEvents.
+ *
+ * It is an error to call this function when the given thread is not the
+ * current thread.  This function will return false if called from some
+ * other thread.
+ *
+ * @param aThread
+ *   The current thread or null.
+ *
+ * @returns
+ *   A boolean value that if "true" indicates that there are pending events
+ *   in the current thread's event queue.
+ */
+extern bool NS_HasPendingEvents(nsIThread* aThread = nullptr);
+
+/**
+ * Shortcut for nsIThread::ProcessNextEvent.
+ *
+ * It is an error to call this function when the given thread is not the
+ * current thread.  This function will simply return false if called
+ * from some other thread.
+ *
+ * @param aThread
+ *   The current thread or null.
+ * @param aMayWait
+ *   A boolean parameter that if "true" indicates that the method may block
+ *   the calling thread to wait for a pending event.
+ *
+ * @returns
+ *   A boolean value that if "true" indicates that an event from the current
+ *   thread's event queue was processed.
+ */
+extern bool NS_ProcessNextEvent(nsIThread* aThread = nullptr,
+                                bool aMayWait = true);
+
+/**
+ * Returns true if we're in the compositor thread.
+ *
+ * We declare this here because the headers required to invoke
+ * CompositorThreadHolder::IsInCompositorThread() also pull in a bunch of system
+ * headers that #define various tokens in a way that can break the build.
+ */
+extern bool NS_IsInCompositorThread();
+
+extern bool NS_IsInCanvasThreadOrWorker();
+
+extern bool NS_IsInVRThread();
+
+//-----------------------------------------------------------------------------
+// Helpers that work with nsCOMPtr:
+
+inline already_AddRefed<nsIThread> do_GetCurrentThread() {
+  nsIThread* thread = nullptr;
+  NS_GetCurrentThread(&thread);
+  return already_AddRefed<nsIThread>(thread);
+}
+
+inline already_AddRefed<nsIThread> do_GetMainThread() {
+  nsIThread* thread = nullptr;
+  NS_GetMainThread(&thread);
+  return already_AddRefed<nsIThread>(thread);
+}
+
+//-----------------------------------------------------------------------------
+
+// Fast access to the current thread.  Will create an nsIThread if one does not
+// exist already!  Do not release the returned pointer!  If you want to use this
+// pointer from some other thread, then you will need to AddRef it.  Otherwise,
+// you should only consider this pointer valid from code running on the current
+// thread.
+extern nsIThread* NS_GetCurrentThread();
+
+// Exactly the same as NS_GetCurrentThread, except it will not create an
+// nsThread if one does not exist yet. This is useful in cases where you have
+// code that runs on threads that may or may not not be driven by an nsThread
+// event loop, and wish to avoid inadvertently creating a superfluous nsThread.
+extern nsIThread* NS_GetCurrentThreadNoCreate();
+
+/**
+ * Set the name of the current thread. Prefer this function over
+ * PR_SetCurrentThreadName() if possible. The name will also be included in the
+ * crash report.
+ *
+ * @param aName
+ *   Name of the thread. A C language null-terminated string.
+ */
+extern void NS_SetCurrentThreadName(const char* aName);
+
+//-----------------------------------------------------------------------------
+
+#ifndef XPCOM_GLUE_AVOID_NSPR
+
+namespace mozilla {
+
+// This class is designed to be subclassed.
+class IdlePeriod : public nsIIdlePeriod {
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIIDLEPERIOD
+
+  IdlePeriod() = default;
+
+ protected:
+  virtual ~IdlePeriod() = default;
+
+ private:
+  IdlePeriod(const IdlePeriod&) = delete;
+  IdlePeriod& operator=(const IdlePeriod&) = delete;
+  IdlePeriod& operator=(const IdlePeriod&&) = delete;
+};
+
+// Cancelable runnable methods implement nsICancelableRunnable, and
+// Idle and IdleWithTimer also nsIIdleRunnable.
+enum class RunnableKind { Standard, Cancelable, Idle, IdleWithTimer };
+
+// Implementing nsINamed on Runnable bloats vtables for the hundreds of
+// Runnable subclasses that we have, so we want to avoid that overhead
+// when we're not using nsINamed for anything.
+#  ifndef RELEASE_OR_BETA
+#    define MOZ_COLLECTING_RUNNABLE_TELEMETRY
+#  endif
+
+// This class is designed to be subclassed.
+class Runnable : public nsIRunnable
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+    ,
+                 public nsINamed
+#  endif
+{
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIRUNNABLE
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_DECL_NSINAMED
+#  endif
+
+  Runnable() = delete;
+
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  explicit Runnable(const char* aName) : mName(aName) {}
+#  else
+  explicit Runnable(const char* aName) {}
+#  endif
+
+ protected:
+  virtual ~Runnable() = default;
+
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  const char* mName = nullptr;
+#  endif
+
+ private:
+  Runnable(const Runnable&) = delete;
+  Runnable& operator=(const Runnable&) = delete;
+  Runnable& operator=(const Runnable&&) = delete;
+};
+
+// This is a base class for tasks that might not be run, such as those that may
+// be dispatched to workers.
+// The owner of an event target will call either Run() or OnDiscard()
+// exactly once.
+// Derived classes should override Run().  An OnDiscard() override may
+// provide cleanup when Run() will not be called.
+class DiscardableRunnable : public Runnable, public nsIDiscardableRunnable {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  // nsIDiscardableRunnable
+  void OnDiscard() override {}
+
+  DiscardableRunnable() = delete;
+  explicit DiscardableRunnable(const char* aName) : Runnable(aName) {}
+
+ protected:
+  virtual ~DiscardableRunnable() = default;
+
+ private:
+  DiscardableRunnable(const DiscardableRunnable&) = delete;
+  DiscardableRunnable& operator=(const DiscardableRunnable&) = delete;
+  DiscardableRunnable& operator=(const DiscardableRunnable&&) = delete;
+};
+
+// This class is designed to be subclassed.
+// Derived classes should override Run() and Cancel() to provide that
+// calling Run() after Cancel() is a no-op.
+class CancelableRunnable : public DiscardableRunnable,
+                           public nsICancelableRunnable {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  // nsIDiscardableRunnable
+  void OnDiscard() override;
+  // nsICancelableRunnable
+  virtual nsresult Cancel() override = 0;
+
+  CancelableRunnable() = delete;
+  explicit CancelableRunnable(const char* aName) : DiscardableRunnable(aName) {}
+
+ protected:
+  virtual ~CancelableRunnable() = default;
+
+ private:
+  CancelableRunnable(const CancelableRunnable&) = delete;
+  CancelableRunnable& operator=(const CancelableRunnable&) = delete;
+  CancelableRunnable& operator=(const CancelableRunnable&&) = delete;
+};
+
+// This class is designed to be subclassed.
+class IdleRunnable : public DiscardableRunnable, public nsIIdleRunnable {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+
+  explicit IdleRunnable(const char* aName) : DiscardableRunnable(aName) {}
+
+ protected:
+  virtual ~IdleRunnable() = default;
+
+ private:
+  IdleRunnable(const IdleRunnable&) = delete;
+  IdleRunnable& operator=(const IdleRunnable&) = delete;
+  IdleRunnable& operator=(const IdleRunnable&&) = delete;
+};
+
+// This class is designed to be subclassed.
+class CancelableIdleRunnable : public CancelableRunnable,
+                               public nsIIdleRunnable {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+
+  CancelableIdleRunnable() : CancelableRunnable("CancelableIdleRunnable") {}
+  explicit CancelableIdleRunnable(const char* aName)
+      : CancelableRunnable(aName) {}
+
+ protected:
+  virtual ~CancelableIdleRunnable() = default;
+
+ private:
+  CancelableIdleRunnable(const CancelableIdleRunnable&) = delete;
+  CancelableIdleRunnable& operator=(const CancelableIdleRunnable&) = delete;
+  CancelableIdleRunnable& operator=(const CancelableIdleRunnable&&) = delete;
+};
+
+// This class is designed to be a wrapper of a real runnable to support event
+// prioritizable.
+class PrioritizableRunnable : public Runnable, public nsIRunnablePriority {
+ public:
+  PrioritizableRunnable(already_AddRefed<nsIRunnable>&& aRunnable,
+                        uint32_t aPriority);
+
+#  ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  NS_IMETHOD GetName(nsACString& aName) override;
+#  endif
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIRUNNABLE
+  NS_DECL_NSIRUNNABLEPRIORITY
+
+ protected:
+  virtual ~PrioritizableRunnable() = default;
+
+  nsCOMPtr<nsIRunnable> mRunnable;
+  uint32_t mPriority;
+};
+
+class PrioritizableCancelableRunnable : public CancelableRunnable,
+                                        public nsIRunnablePriority {
+ public:
+  PrioritizableCancelableRunnable(uint32_t aPriority, const char* aName)
+      : CancelableRunnable(aName), mPriority(aPriority) {}
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_NSIRUNNABLEPRIORITY
+
+ protected:
+  virtual ~PrioritizableCancelableRunnable() = default;
+
+  const uint32_t mPriority;
+};
+
+extern already_AddRefed<nsIRunnable> CreateRenderBlockingRunnable(
+    already_AddRefed<nsIRunnable>&& aRunnable);
+
+namespace detail {
+
+// An event that can be used to call a C++11 functions or function objects,
+// including lambdas. The function must have no required arguments, and must
+// return void.
+template <typename StoredFunction>
+class RunnableFunction : public Runnable {
+ public:
+  template <typename F>
+  explicit RunnableFunction(const char* aName, F&& aFunction)
+      : Runnable(aName), mFunction(std::forward<F>(aFunction)) {}
+
+  NS_IMETHOD Run() override {
+    static_assert(std::is_void_v<decltype(mFunction())>,
+                  "The lambda must return void!");
+    mFunction();
+    return NS_OK;
+  }
+
+ private:
+  StoredFunction mFunction;
+};
+
+// Type alias for NS_NewRunnableFunction
+template <typename Function>
+using RunnableFunctionImpl =
+    // Make sure we store a non-reference in nsRunnableFunction.
+    typename detail::RunnableFunction<std::remove_reference_t<Function>>;
+}  // namespace detail
+
+namespace detail {
+
+template <typename CVRemoved>
+struct IsRefcountedSmartPointerHelper : std::false_type {};
+
+template <typename Pointee>
+struct IsRefcountedSmartPointerHelper<RefPtr<Pointee>> : std::true_type {};
+
+template <typename Pointee>
+struct IsRefcountedSmartPointerHelper<nsCOMPtr<Pointee>> : std::true_type {};
+
+}  // namespace detail
+
+template <typename T>
+struct IsRefcountedSmartPointer
+    : detail::IsRefcountedSmartPointerHelper<std::remove_cv_t<T>> {};
+
+namespace detail {
+
+template <typename T, typename CVRemoved>
+struct RemoveSmartPointerHelper {
+  typedef T Type;
+};
+
+template <typename T, typename Pointee>
+struct RemoveSmartPointerHelper<T, RefPtr<Pointee>> {
+  typedef Pointee Type;
+};
+
+template <typename T, typename Pointee>
+struct RemoveSmartPointerHelper<T, nsCOMPtr<Pointee>> {
+  typedef Pointee Type;
+};
+
+}  // namespace detail
+
+template <typename T>
+struct RemoveSmartPointer
+    : detail::RemoveSmartPointerHelper<T, std::remove_cv_t<T>> {};
+
+namespace detail {
+
+template <typename T, typename CVRemoved>
+struct RemoveRawOrSmartPointerHelper {
+  typedef T Type;
+};
+
+template <typename T, typename Pointee>
+struct RemoveRawOrSmartPointerHelper<T, Pointee*> {
+  typedef Pointee Type;
+};
+
+template <typename T, typename Pointee>
+struct RemoveRawOrSmartPointerHelper<T, RefPtr<Pointee>> {
+  typedef Pointee Type;
+};
+
+template <typename T, typename Pointee>
+struct RemoveRawOrSmartPointerHelper<T, nsCOMPtr<Pointee>> {
+  typedef Pointee Type;
+};
+
+}  // namespace detail
+
+template <typename T>
+struct RemoveRawOrSmartPointer
+    : detail::RemoveRawOrSmartPointerHelper<T, std::remove_cv_t<T>> {};
+
+}  // namespace mozilla
+
+inline nsISupports* ToSupports(mozilla::Runnable* p) {
+  return static_cast<nsIRunnable*>(p);
+}
+
+template <typename Function>
+already_AddRefed<mozilla::Runnable> NS_NewRunnableFunction(
+    const char* aName, Function&& aFunction) {
+  // We store a non-reference in RunnableFunction, but still forward aFunction
+  // to move if possible.
+  return do_AddRef(new mozilla::detail::RunnableFunctionImpl<Function>(
+      aName, std::forward<Function>(aFunction)));
+}
+
+// Creates a new object implementing nsIRunnable and nsICancelableRunnable,
+// which runs a given function on Run and clears the stored function object on a
+// call to `Cancel` (and thus destroys all objects it holds).
+template <typename Function>
+already_AddRefed<mozilla::CancelableRunnable> NS_NewCancelableRunnableFunction(
+    const char* aName, Function&& aFunc) {
+  class FuncCancelableRunnable final : public mozilla::CancelableRunnable {
+   public:
+    static_assert(
+        std::is_void_v<
+            decltype(std::declval<std::remove_reference_t<Function>>()())>);
+
+    NS_INLINE_DECL_REFCOUNTING_INHERITED(FuncCancelableRunnable,
+                                         CancelableRunnable)
+
+    explicit FuncCancelableRunnable(const char* aName, Function&& aFunc)
+        : CancelableRunnable{aName},
+          mFunc{mozilla::Some(std::forward<Function>(aFunc))} {}
+
+    NS_IMETHOD Run() override {
+      if (mFunc) {
+        (*mFunc)();
+      }
+
+      return NS_OK;
+    }
+
+    nsresult Cancel() override {
+      mFunc.reset();
+      return NS_OK;
+    }
+
+   private:
+    ~FuncCancelableRunnable() = default;
+
+    mozilla::Maybe<std::remove_reference_t<Function>> mFunc;
+  };
+
+  return mozilla::MakeAndAddRef<FuncCancelableRunnable>(
+      aName, std::forward<Function>(aFunc));
+}
+
+namespace mozilla {
+namespace detail {
+
+template <RunnableKind Kind>
+class TimerBehaviour {
+ public:
+  nsITimer* GetTimer() { return nullptr; }
+  void CancelTimer() {}
+
+ protected:
+  ~TimerBehaviour() = default;
+};
+
+template <>
+class TimerBehaviour<RunnableKind::IdleWithTimer> {
+ public:
+  nsITimer* GetTimer() {
+    if (!mTimer) {
+      mTimer = NS_NewTimer();
+    }
+
+    return mTimer;
+  }
+
+  void CancelTimer() {
+    if (mTimer) {
+      mTimer->Cancel();
+    }
+  }
+
+ protected:
+  ~TimerBehaviour() { CancelTimer(); }
+
+ private:
+  nsCOMPtr<nsITimer> mTimer;
+};
+
+}  // namespace detail
+}  // namespace mozilla
+
+// An event that can be used to call a method on a class.  The class type must
+// support reference counting. This event supports Revoke for use
+// with nsRevocableEventPtr.
+template <class ClassType, typename ReturnType = void, bool Owning = true,
+          mozilla::RunnableKind Kind = mozilla::RunnableKind::Standard>
+class nsRunnableMethod
+    : public std::conditional_t<
+          Kind == mozilla::RunnableKind::Standard, mozilla::Runnable,
+          std::conditional_t<Kind == mozilla::RunnableKind::Cancelable,
+                             mozilla::CancelableRunnable,
+                             mozilla::CancelableIdleRunnable>>,
+      protected mozilla::detail::TimerBehaviour<Kind> {
+  using BaseType = std::conditional_t<
+      Kind == mozilla::RunnableKind::Standard, mozilla::Runnable,
+      std::conditional_t<Kind == mozilla::RunnableKind::Cancelable,
+                         mozilla::CancelableRunnable,
+                         mozilla::CancelableIdleRunnable>>;
+
+ public:
+  nsRunnableMethod(const char* aName) : BaseType(aName) {}
+
+  virtual void Revoke() = 0;
+
+  // These ReturnTypeEnforcer classes disallow return types that
+  // we know are not safe. The default ReturnTypeEnforcer compiles just fine but
+  // already_AddRefed will not.
+  template <typename OtherReturnType>
+  class ReturnTypeEnforcer {
+   public:
+    typedef int ReturnTypeIsSafe;
+  };
+
+  template <class T>
+  class ReturnTypeEnforcer<already_AddRefed<T>> {
+    // No ReturnTypeIsSafe makes this illegal!
+  };
+
+  // Make sure this return type is safe.
+  typedef typename ReturnTypeEnforcer<ReturnType>::ReturnTypeIsSafe check;
+};
+
+template <class ClassType, bool Owning>
+struct nsRunnableMethodReceiver {
+  RefPtr<ClassType> mObj;
+  explicit nsRunnableMethodReceiver(ClassType* aObj) : mObj(aObj) {}
+  explicit nsRunnableMethodReceiver(RefPtr<ClassType>&& aObj)
+      : mObj(std::move(aObj)) {}
+  ~nsRunnableMethodReceiver() { Revoke(); }
+  ClassType* Get() const { return mObj.get(); }
+  void Revoke() { mObj = nullptr; }
+};
+
+template <class ClassType>
+struct nsRunnableMethodReceiver<ClassType, false> {
+  ClassType* MOZ_NON_OWNING_REF mObj;
+  explicit nsRunnableMethodReceiver(ClassType* aObj) : mObj(aObj) {}
+  ClassType* Get() const { return mObj; }
+  void Revoke() { mObj = nullptr; }
+};
+
+static inline constexpr bool IsIdle(mozilla::RunnableKind aKind) {
+  return aKind == mozilla::RunnableKind::Idle ||
+         aKind == mozilla::RunnableKind::IdleWithTimer;
+}
+
+template <typename PtrType, typename Method, bool Owning,
+          mozilla::RunnableKind Kind>
+struct nsRunnableMethodTraits;
+
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind, typename... As>
+struct nsRunnableMethodTraits<PtrType, R (C::*)(As...), Owning, Kind> {
+  typedef typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind, typename... As>
+struct nsRunnableMethodTraits<PtrType, R (C::*)(As...) const, Owning, Kind> {
+  typedef const typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type
+      class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+
+#  ifdef NS_HAVE_STDCALL
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind, typename... As>
+struct nsRunnableMethodTraits<PtrType, R (__stdcall C::*)(As...), Owning,
+                              Kind> {
+  typedef typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind>
+struct nsRunnableMethodTraits<PtrType, R (NS_STDCALL C::*)(), Owning, Kind> {
+  typedef typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind, typename... As>
+struct nsRunnableMethodTraits<PtrType, R (__stdcall C::*)(As...) const, Owning,
+                              Kind> {
+  typedef const typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type
+      class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+
+template <typename PtrType, class C, typename R, bool Owning,
+          mozilla::RunnableKind Kind>
+struct nsRunnableMethodTraits<PtrType, R (NS_STDCALL C::*)() const, Owning,
+                              Kind> {
+  typedef const typename mozilla::RemoveRawOrSmartPointer<PtrType>::Type
+      class_type;
+  static_assert(std::is_base_of<C, class_type>::value,
+                "Stored class must inherit from method's class");
+  typedef R return_type;
+  typedef nsRunnableMethod<C, R, Owning, Kind> base_type;
+  static const bool can_cancel = Kind == mozilla::RunnableKind::Cancelable;
+};
+#  endif
+
+// IsParameterStorageClass<T>::value is true if T is a parameter-storage class
+// that will be recognized by NS_New[NonOwning]RunnableMethodWithArg[s] to
+// force a specific storage&passing strategy (instead of inferring one,
+// see ParameterStorage).
+// When creating a new storage class, add a specialization for it to be
+// recognized.
+template <typename T>
+struct IsParameterStorageClass : public std::false_type {};
+
+// StoreXPassByY structs used to inform nsRunnableMethodArguments how to
+// store arguments, and how to pass them to the target method.
+
+template <typename T>
+struct StoreCopyPassByValue {
+  using stored_type = std::decay_t<T>;
+  typedef stored_type passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByValue(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByValue<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StoreCopyPassByConstLRef {
+  using stored_type = std::decay_t<T>;
+  typedef const stored_type& passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByConstLRef(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByConstLRef<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StoreCopyPassByLRef {
+  using stored_type = std::decay_t<T>;
+  typedef stored_type& passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByLRef(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByLRef<S>> : public std::true_type {
+};
+
+template <typename T>
+struct StoreCopyPassByRRef {
+  using stored_type = std::decay_t<T>;
+  typedef stored_type&& passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByRRef(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return std::move(m); }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByRRef<S>> : public std::true_type {
+};
+
+template <typename T>
+struct StoreRefPassByLRef {
+  typedef T& stored_type;
+  typedef T& passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreRefPassByLRef(A& a) : m(a) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreRefPassByLRef<S>> : public std::true_type {
+};
+
+template <typename T>
+struct StoreConstRefPassByConstLRef {
+  typedef const T& stored_type;
+  typedef const T& passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreConstRefPassByConstLRef(const A& a) : m(a) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreConstRefPassByConstLRef<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StoreRefPtrPassByPtr {
+  typedef RefPtr<T> stored_type;
+  typedef T* passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreRefPtrPassByPtr(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return m.get(); }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreRefPtrPassByPtr<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StorePtrPassByPtr {
+  typedef T* stored_type;
+  typedef T* passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StorePtrPassByPtr(A a) : m(a) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StorePtrPassByPtr<S>> : public std::true_type {};
+
+template <typename T>
+struct StoreConstPtrPassByConstPtr {
+  typedef const T* stored_type;
+  typedef const T* passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreConstPtrPassByConstPtr(A a) : m(a) {}
+  passed_type PassAsParameter() { return m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreConstPtrPassByConstPtr<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StoreCopyPassByConstPtr {
+  typedef T stored_type;
+  typedef const T* passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByConstPtr(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return &m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByConstPtr<S>>
+    : public std::true_type {};
+
+template <typename T>
+struct StoreCopyPassByPtr {
+  typedef T stored_type;
+  typedef T* passed_type;
+  stored_type m;
+  template <typename A>
+  MOZ_IMPLICIT StoreCopyPassByPtr(A&& a) : m(std::forward<A>(a)) {}
+  passed_type PassAsParameter() { return &m; }
+};
+template <typename S>
+struct IsParameterStorageClass<StoreCopyPassByPtr<S>> : public std::true_type {
+};
+
+namespace detail {
+
+template <typename>
+struct SFINAE1True : std::true_type {};
+
+template <class T>
+static auto HasRefCountMethodsTest(int)
+    -> SFINAE1True<decltype(std::declval<T>().AddRef(),
+                            std::declval<T>().Release())>;
+template <class>
+static auto HasRefCountMethodsTest(long) -> std::false_type;
+
+template <class T>
+struct HasRefCountMethods : decltype(HasRefCountMethodsTest<T>(0)) {};
+
+template <typename TWithoutPointer>
+struct NonnsISupportsPointerStorageClass
+    : std::conditional<
+          std::is_const_v<TWithoutPointer>,
+          StoreConstPtrPassByConstPtr<std::remove_const_t<TWithoutPointer>>,
+          StorePtrPassByPtr<TWithoutPointer>> {
+  using Type = typename NonnsISupportsPointerStorageClass::conditional::type;
+};
+
+template <typename TWithoutPointer>
+struct PointerStorageClass
+    : std::conditional<
+          HasRefCountMethods<TWithoutPointer>::value,
+          StoreRefPtrPassByPtr<TWithoutPointer>,
+          typename NonnsISupportsPointerStorageClass<TWithoutPointer>::Type> {
+  using Type = typename PointerStorageClass::conditional::type;
+};
+
+template <typename TWithoutRef>
+struct LValueReferenceStorageClass
+    : std::conditional<
+          std::is_const_v<TWithoutRef>,
+          StoreConstRefPassByConstLRef<std::remove_const_t<TWithoutRef>>,
+          StoreRefPassByLRef<TWithoutRef>> {
+  using Type = typename LValueReferenceStorageClass::conditional::type;
+};
+
+template <typename T>
+struct SmartPointerStorageClass
+    : std::conditional<
+          mozilla::IsRefcountedSmartPointer<T>::value,
+          StoreRefPtrPassByPtr<typename mozilla::RemoveSmartPointer<T>::Type>,
+          StoreCopyPassByConstLRef<T>> {
+  using Type = typename SmartPointerStorageClass::conditional::type;
+};
+
+template <typename T>
+struct NonLValueReferenceStorageClass
+    : std::conditional<std::is_rvalue_reference_v<T>,
+                       StoreCopyPassByRRef<std::remove_reference_t<T>>,
+                       typename SmartPointerStorageClass<T>::Type> {
+  using Type = typename NonLValueReferenceStorageClass::conditional::type;
+};
+
+template <typename T>
+struct NonPointerStorageClass
+    : std::conditional<std::is_lvalue_reference_v<T>,
+                       typename LValueReferenceStorageClass<
+                           std::remove_reference_t<T>>::Type,
+                       typename NonLValueReferenceStorageClass<T>::Type> {
+  using Type = typename NonPointerStorageClass::conditional::type;
+};
+
+template <typename T>
+struct NonParameterStorageClass
+    : std::conditional<
+          std::is_pointer_v<T>,
+          typename PointerStorageClass<std::remove_pointer_t<T>>::Type,
+          typename NonPointerStorageClass<T>::Type> {
+  using Type = typename NonParameterStorageClass::conditional::type;
+};
+
+// Choose storage&passing strategy based on preferred storage type:
+// - If IsParameterStorageClass<T>::value is true, use as-is.
+// - RC*       -> StoreRefPtrPassByPtr<RC>       :Store RefPtr<RC>, pass RC*
+//   ^^ RC quacks like a ref-counted type (i.e., has AddRef and Release methods)
+// - const T*  -> StoreConstPtrPassByConstPtr<T> :Store const T*, pass const T*
+// - T*        -> StorePtrPassByPtr<T>           :Store T*, pass T*.
+// - const T&  -> StoreConstRefPassByConstLRef<T>:Store const T&, pass const T&.
+// - T&        -> StoreRefPassByLRef<T>          :Store T&, pass T&.
+// - T&&       -> StoreCopyPassByRRef<T>         :Store T, pass std::move(T).
+// - RefPtr<T>, nsCOMPtr<T>
+//             -> StoreRefPtrPassByPtr<T>        :Store RefPtr<T>, pass T*
+// - Other T   -> StoreCopyPassByConstLRef<T>    :Store T, pass const T&.
+// Other available explicit options:
+// -              StoreCopyPassByValue<T>        :Store T, pass T.
+// -              StoreCopyPassByLRef<T>         :Store T, pass T& (of copy!)
+// -              StoreCopyPassByConstPtr<T>     :Store T, pass const T*
+// -              StoreCopyPassByPtr<T>          :Store T, pass T* (of copy!)
+// Or create your own class with PassAsParameter() method, optional
+// clean-up in destructor, and with associated IsParameterStorageClass<>.
+template <typename T>
+struct ParameterStorage
+    : std::conditional<IsParameterStorageClass<T>::value, T,
+                       typename NonParameterStorageClass<T>::Type> {
+  using Type = typename ParameterStorage::conditional::type;
+};
+
+template <class T>
+static auto HasSetDeadlineTest(int)
+    -> SFINAE1True<decltype(std::declval<T>().SetDeadline(
+        std::declval<mozilla::TimeStamp>()))>;
+
+template <class T>
+static auto HasSetDeadlineTest(long) -> std::false_type;
+
+template <class T>
+struct HasSetDeadline : decltype(HasSetDeadlineTest<T>(0)) {};
+
+template <class T>
+std::enable_if_t<::detail::HasSetDeadline<T>::value> SetDeadlineImpl(
+    T* aObj, mozilla::TimeStamp aTimeStamp) {
+  aObj->SetDeadline(aTimeStamp);
+}
+
+template <class T>
+std::enable_if_t<!::detail::HasSetDeadline<T>::value> SetDeadlineImpl(
+    T* aObj, mozilla::TimeStamp aTimeStamp) {}
+} /* namespace detail */
+
+namespace mozilla {
+namespace detail {
+
+// struct used to store arguments and later apply them to a method.
+template <typename... Ts>
+struct RunnableMethodArguments final {
+  std::tuple<typename ::detail::ParameterStorage<Ts>::Type...> mArguments;
+  template <typename... As>
+  explicit RunnableMethodArguments(As&&... aArguments)
+      : mArguments(std::forward<As>(aArguments)...) {}
+  template <class C, typename M>
+  decltype(auto) apply(C* o, M m) {
+    return std::apply(
+        [&o, m](auto&&... args) {
+          return ((*o).*m)(args.PassAsParameter()...);
+        },
+        mArguments);
+  }
+};
+
+template <typename PtrType, typename Method, bool Owning, RunnableKind Kind,
+          typename... Storages>
+class RunnableMethodImpl final
+    : public ::nsRunnableMethodTraits<PtrType, Method, Owning,
+                                      Kind>::base_type {
+  typedef typename ::nsRunnableMethodTraits<PtrType, Method, Owning, Kind>
+      Traits;
+
+  typedef typename Traits::class_type ClassType;
+  typedef typename Traits::base_type BaseType;
+  ::nsRunnableMethodReceiver<ClassType, Owning> mReceiver;
+  Method mMethod;
+  RunnableMethodArguments<Storages...> mArgs;
+  using BaseType::CancelTimer;
+  using BaseType::GetTimer;
+
+ private:
+  virtual ~RunnableMethodImpl() { Revoke(); };
+  static void TimedOut(nsITimer* aTimer, void* aClosure) {
+    static_assert(IsIdle(Kind), "Don't use me!");
+    RefPtr<CancelableIdleRunnable> r =
+        static_cast<CancelableIdleRunnable*>(aClosure);
+    r->SetDeadline(TimeStamp());
+    r->Run();
+    r->Cancel();
+  }
+
+ public:
+  template <typename ForwardedPtrType, typename... Args>
+  explicit RunnableMethodImpl(const char* aName, ForwardedPtrType&& aObj,
+                              Method aMethod, Args&&... aArgs)
+      : BaseType(aName),
+        mReceiver(std::forward<ForwardedPtrType>(aObj)),
+        mMethod(aMethod),
+        mArgs(std::forward<Args>(aArgs)...) {
+    static_assert(sizeof...(Storages) == sizeof...(Args),
+                  "Storages and Args should have equal sizes");
+  }
+
+  NS_IMETHOD Run() {
+    CancelTimer();
+
+    if (MOZ_LIKELY(mReceiver.Get())) {
+      mArgs.apply(mReceiver.Get(), mMethod);
+    }
+
+    return NS_OK;
+  }
+
+  nsresult Cancel() {
+    static_assert(Kind >= RunnableKind::Cancelable, "Don't use me!");
+    Revoke();
+    return NS_OK;
+  }
+
+  void Revoke() {
+    CancelTimer();
+    mReceiver.Revoke();
+  }
+
+  void SetDeadline(TimeStamp aDeadline) {
+    if (MOZ_LIKELY(mReceiver.Get())) {
+      ::detail::SetDeadlineImpl(mReceiver.Get(), aDeadline);
+    }
+  }
+
+  void SetTimer(uint32_t aDelay, nsIEventTarget* aTarget) {
+    MOZ_ASSERT(aTarget);
+
+    if (nsCOMPtr<nsITimer> timer = GetTimer()) {
+      timer->Cancel();
+      timer->SetTarget(aTarget);
+      timer->InitWithNamedFuncCallback(TimedOut, this, aDelay,
+                                       nsITimer::TYPE_ONE_SHOT,
+                                       "detail::RunnableMethodImpl::SetTimer");
+    }
+  }
+};
+
+// Type aliases for NewRunnableMethod.
+template <typename PtrType, typename Method>
+using OwningRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      true, RunnableKind::Standard>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using OwningRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, true,
+                       RunnableKind::Standard, Storages...>;
+
+// Type aliases for NewCancelableRunnableMethod.
+template <typename PtrType, typename Method>
+using CancelableRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      true,
+                                      RunnableKind::Cancelable>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using CancelableRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, true,
+                       RunnableKind::Cancelable, Storages...>;
+
+// Type aliases for NewIdleRunnableMethod.
+template <typename PtrType, typename Method>
+using IdleRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      true, RunnableKind::Idle>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using IdleRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, true,
+                       RunnableKind::Idle, Storages...>;
+
+// Type aliases for NewIdleRunnableMethodWithTimer.
+template <typename PtrType, typename Method>
+using IdleRunnableMethodWithTimer =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      true,
+                                      RunnableKind::IdleWithTimer>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using IdleRunnableMethodWithTimerImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, true,
+                       RunnableKind::IdleWithTimer, Storages...>;
+
+// Type aliases for NewNonOwningRunnableMethod.
+template <typename PtrType, typename Method>
+using NonOwningRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      false, RunnableKind::Standard>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using NonOwningRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, false,
+                       RunnableKind::Standard, Storages...>;
+
+// Type aliases for NonOwningCancelableRunnableMethod
+template <typename PtrType, typename Method>
+using NonOwningCancelableRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      false,
+                                      RunnableKind::Cancelable>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using NonOwningCancelableRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, false,
+                       RunnableKind::Cancelable, Storages...>;
+
+// Type aliases for NonOwningIdleRunnableMethod
+template <typename PtrType, typename Method>
+using NonOwningIdleRunnableMethod =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      false, RunnableKind::Idle>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using NonOwningIdleRunnableMethodImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, false,
+                       RunnableKind::Idle, Storages...>;
+
+// Type aliases for NewIdleRunnableMethodWithTimer.
+template <typename PtrType, typename Method>
+using NonOwningIdleRunnableMethodWithTimer =
+    typename ::nsRunnableMethodTraits<std::remove_reference_t<PtrType>, Method,
+                                      false,
+                                      RunnableKind::IdleWithTimer>::base_type;
+template <typename PtrType, typename Method, typename... Storages>
+using NonOwningIdleRunnableMethodWithTimerImpl =
+    RunnableMethodImpl<std::remove_reference_t<PtrType>, Method, false,
+                       RunnableKind::IdleWithTimer, Storages...>;
+
+}  // namespace detail
+
+// NewRunnableMethod and friends
+//
+// Very often in Gecko, you'll find yourself in a situation where you want
+// to invoke a method (with or without arguments) asynchronously.  You
+// could write a small helper class inheriting from nsRunnable to handle
+// all these details, or you could let NewRunnableMethod take care of all
+// those details for you.
+//
+// The simplest use of NewRunnableMethod looks like:
+//
+//   nsCOMPtr<nsIRunnable> event =
+//     mozilla::NewRunnableMethod("description", myObject,
+//                                &MyClass::HandleEvent);
+//   NS_DispatchToCurrentThread(event);
+//
+// Statically enforced constraints:
+//  - myObject must be of (or implicitly convertible to) type MyClass
+//  - MyClass must define AddRef and Release methods
+//
+// The "description" string should specify a human-readable name for the
+// runnable; the provided string is used by various introspection tools
+// in the browser.
+//
+// The created runnable will take a strong reference to `myObject`.  For
+// non-refcounted objects, or refcounted objects with unusual refcounting
+// requirements, and if and only if you are 110% certain that `myObject`
+// will live long enough, you can use NewNonOwningRunnableMethod instead,
+// which will, as its name implies, take a non-owning reference.  If you
+// find yourself having to use this function, you should accompany your use
+// with a proof comment describing why the runnable will not lead to
+// use-after-frees.
+//
+// (If you find yourself writing contorted code to Release() an object
+// asynchronously on a different thread, you should use the
+// NS_ProxyRelease function.)
+//
+// Invoking a method with arguments takes a little more care.  The
+// natural extension of the above:
+//
+//   nsCOMPtr<nsIRunnable> event =
+//     mozilla::NewRunnableMethod("description", myObject,
+//                                &MyClass::HandleEvent,
+//                                arg1, arg2, ...);
+//
+// can lead to security hazards (e.g. passing in raw pointers to refcounted
+// objects and storing those raw pointers in the runnable).  We therefore
+// require you to specify the storage types used by the runnable, just as
+// you would if you were writing out the class by hand:
+//
+//   nsCOMPtr<nsIRunnable> event =
+//     mozilla::NewRunnableMethod<RefPtr<T>, nsTArray<U>>
+//         ("description", myObject, &MyClass::HandleEvent, arg1, arg2);
+//
+// Please note that you do not have to pass the same argument type as you
+// specify in the template arguments.  For example, if you want to transfer
+// ownership to a runnable, you can write:
+//
+//   RefPtr<T> ptr = ...;
+//   nsTArray<U> array = ...;
+//   nsCOMPtr<nsIRunnable> event =
+//     mozilla::NewRunnableMethod<RefPtr<T>, nsTArray<U>>
+//         ("description", myObject, &MyClass::DoSomething,
+//          std::move(ptr), std::move(array));
+//
+// and there will be no extra AddRef/Release traffic, or copying of the array.
+//
+// Each type that you specify as a template argument to NewRunnableMethod
+// comes with its own style of storage in the runnable and its own style
+// of argument passing to the invoked method.  See the comment for
+// ParameterStorage above for more details.
+//
+// If you need to customize the storage type and/or argument passing type,
+// you can write your own class to use as a template argument to
+// NewRunnableMethod.  If you find yourself having to do that frequently,
+// please file a bug in Core::XPCOM about adding the custom type to the
+// core code in this file, and/or for custom rules for ParameterStorage
+// to select that strategy.
+//
+// For places that require you to use cancelable runnables, such as
+// workers, there's also NewCancelableRunnableMethod and its non-owning
+// counterpart.  The runnables returned by these methods additionally
+// implement nsICancelableRunnable.
+//
+// Finally, all of the functions discussed above have additional overloads
+// that do not take a `const char*` as their first parameter; you may see
+// these in older code.  The `const char*` overload is preferred and
+// should be used in new code exclusively.
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::OwningRunnableMethod<PtrType, Method>>
+NewRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod) {
+  return do_AddRef(new detail::OwningRunnableMethodImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::CancelableRunnableMethod<PtrType, Method>>
+NewCancelableRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod) {
+  return do_AddRef(new detail::CancelableRunnableMethodImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::IdleRunnableMethod<PtrType, Method>>
+NewIdleRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod) {
+  return do_AddRef(new detail::IdleRunnableMethodImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::IdleRunnableMethodWithTimer<PtrType, Method>>
+NewIdleRunnableMethodWithTimer(const char* aName, PtrType&& aPtr,
+                               Method aMethod) {
+  return do_AddRef(new detail::IdleRunnableMethodWithTimerImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::NonOwningRunnableMethod<PtrType, Method>>
+NewNonOwningRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod) {
+  return do_AddRef(new detail::NonOwningRunnableMethodImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::NonOwningCancelableRunnableMethod<PtrType, Method>>
+NewNonOwningCancelableRunnableMethod(const char* aName, PtrType&& aPtr,
+                                     Method aMethod) {
+  return do_AddRef(
+      new detail::NonOwningCancelableRunnableMethodImpl<PtrType, Method>(
+          aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::NonOwningIdleRunnableMethod<PtrType, Method>>
+NewNonOwningIdleRunnableMethod(const char* aName, PtrType&& aPtr,
+                               Method aMethod) {
+  return do_AddRef(new detail::NonOwningIdleRunnableMethodImpl<PtrType, Method>(
+      aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+template <typename PtrType, typename Method>
+already_AddRefed<detail::NonOwningIdleRunnableMethodWithTimer<PtrType, Method>>
+NewNonOwningIdleRunnableMethodWithTimer(const char* aName, PtrType&& aPtr,
+                                        Method aMethod) {
+  return do_AddRef(
+      new detail::NonOwningIdleRunnableMethodWithTimerImpl<PtrType, Method>(
+          aName, std::forward<PtrType>(aPtr), aMethod));
+}
+
+// Similar to NewRunnableMethod. Call like so:
+// nsCOMPtr<nsIRunnable> event =
+//   NewRunnableMethod<Types,...>(myObject, &MyClass::HandleEvent, myArg1,...);
+// 'Types' are the stored type for each argument, see ParameterStorage for
+// details.
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::OwningRunnableMethod<PtrType, Method>>
+NewRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod,
+                  Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::OwningRunnableMethodImpl<PtrType, Method, Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::NonOwningRunnableMethod<PtrType, Method>>
+NewNonOwningRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod,
+                           Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::NonOwningRunnableMethodImpl<PtrType, Method, Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::CancelableRunnableMethod<PtrType, Method>>
+NewCancelableRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod,
+                            Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::CancelableRunnableMethodImpl<PtrType, Method, Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::NonOwningCancelableRunnableMethod<PtrType, Method>>
+NewNonOwningCancelableRunnableMethod(const char* aName, PtrType&& aPtr,
+                                     Method aMethod, Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::NonOwningCancelableRunnableMethodImpl<PtrType, Method,
+                                                        Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::IdleRunnableMethod<PtrType, Method>>
+NewIdleRunnableMethod(const char* aName, PtrType&& aPtr, Method aMethod,
+                      Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::IdleRunnableMethodImpl<PtrType, Method, Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+template <typename... Storages, typename PtrType, typename Method,
+          typename... Args>
+already_AddRefed<detail::NonOwningIdleRunnableMethod<PtrType, Method>>
+NewNonOwningIdleRunnableMethod(const char* aName, PtrType&& aPtr,
+                               Method aMethod, Args&&... aArgs) {
+  static_assert(sizeof...(Storages) == sizeof...(Args),
+                "<Storages...> size should be equal to number of arguments");
+  return do_AddRef(
+      new detail::NonOwningIdleRunnableMethodImpl<PtrType, Method, Storages...>(
+          aName, std::forward<PtrType>(aPtr), aMethod,
+          std::forward<Args>(aArgs)...));
+}
+
+}  // namespace mozilla
+
+#endif  // XPCOM_GLUE_AVOID_NSPR
+
+// This class is designed to be used when you have an event class E that has a
+// pointer back to resource class R.  If R goes away while E is still pending,
+// then it is important to "revoke" E so that it does not try use R after R has
+// been destroyed.  nsRevocableEventPtr makes it easy for R to manage such
+// situations:
+//
+//   class R;
+//
+//   class E : public mozilla::Runnable {
+//   public:
+//     void Revoke() {
+//       mResource = nullptr;
+//     }
+//   private:
+//     R *mResource;
+//   };
+//
+//   class R {
+//   public:
+//     void EventHandled() {
+//       mEvent.Forget();
+//     }
+//   private:
+//     nsRevocableEventPtr<E> mEvent;
+//   };
+//
+//   void R::PostEvent() {
+//     // Make sure any pending event is revoked.
+//     mEvent->Revoke();
+//
+//     nsCOMPtr<nsIRunnable> event = new E();
+//     if (NS_SUCCEEDED(NS_DispatchToCurrentThread(event))) {
+//       // Keep pointer to event so we can revoke it.
+//       mEvent = event;
+//     }
+//   }
+//
+//   NS_IMETHODIMP E::Run() {
+//     if (!mResource)
+//       return NS_OK;
+//     ...
+//     mResource->EventHandled();
+//     return NS_OK;
+//   }
+//
+template <class T>
+class nsRevocableEventPtr {
+ public:
+  nsRevocableEventPtr() : mEvent(nullptr) {}
+  ~nsRevocableEventPtr() { Revoke(); }
+
+  const nsRevocableEventPtr& operator=(RefPtr<T>&& aEvent) {
+    if (mEvent != aEvent) {
+      Revoke();
+      mEvent = std::move(aEvent);
+    }
+    return *this;
+  }
+
+  void Revoke() {
+    if (mEvent) {
+      mEvent->Revoke();
+      mEvent = nullptr;
+    }
+  }
+
+  void Forget() { mEvent = nullptr; }
+  bool IsPending() { return mEvent != nullptr; }
+  T* get() { return mEvent; }
+
+ private:
+  // Not implemented
+  nsRevocableEventPtr(const nsRevocableEventPtr&);
+  nsRevocableEventPtr& operator=(const nsRevocableEventPtr&);
+
+  RefPtr<T> mEvent;
+};
+
+template <class T>
+inline already_AddRefed<T> do_AddRef(nsRevocableEventPtr<T>& aObj) {
+  return do_AddRef(aObj.get());
+}
+
+/**
+ * A simple helper to suffix thread pool name
+ * with incremental numbers.
+ */
+class nsThreadPoolNaming {
+ public:
+  nsThreadPoolNaming() = default;
+
+  /**
+   * Returns a thread name as "<aPoolName> #<n>" and increments the counter.
+   */
+  nsCString GetNextThreadName(const nsACString& aPoolName);
+
+  template <size_t LEN>
+  nsCString GetNextThreadName(const char (&aPoolName)[LEN]) {
+    return GetNextThreadName(nsDependentCString(aPoolName, LEN - 1));
+  }
+
+ private:
+  mozilla::Atomic<uint32_t> mCounter{0};
+
+  nsThreadPoolNaming(const nsThreadPoolNaming&) = delete;
+  void operator=(const nsThreadPoolNaming&) = delete;
+};
+
+/**
+ * Thread priority in most operating systems affect scheduling, not IO.  This
+ * helper is used to set the current thread to low IO priority for the lifetime
+ * of the created object.  You can only use this low priority IO setting within
+ * the context of the current thread.
+ */
+class MOZ_STACK_CLASS nsAutoLowPriorityIO {
+ public:
+  nsAutoLowPriorityIO();
+  ~nsAutoLowPriorityIO();
+
+ private:
+  bool lowIOPrioritySet;
+#if defined(XP_MACOSX)
+  int oldPriority;
+#endif
+};
+
+void NS_SetMainThread();
+
+// Used only on cooperatively scheduled "main" threads. Causes the thread to be
+// considered a main thread and also causes GetCurrentVirtualThread to return
+// aVirtualThread.
+void NS_SetMainThread(PRThread* aVirtualThread);
+
+// Used only on cooperatively scheduled "main" threads. Causes the thread to no
+// longer be considered a main thread. Also causes GetCurrentVirtualThread() to
+// return a unique value.
+void NS_UnsetMainThread();
+
+/**
+ * Return the expiration time of the next timer to run on the current
+ * thread.  If that expiration time is greater than aDefault, then
+ * return aDefault.  aSearchBound specifies a maximum number of timers
+ * to examine to find a timer on the current thread.  If no timer that
+ * will run on the current thread is found after examining
+ * aSearchBound timers, return the highest seen expiration time as a
+ * best effort guess.
+ *
+ * Timers with either the type nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY or
+ * nsITIMER::TYPE_REPEATING_SLACK_LOW_PRIORITY will be skipped when
+ * searching for the next expiration time.  This enables timers to
+ * have lower priority than callbacks dispatched from
+ * nsIThread::IdleDispatch.
+ */
+extern mozilla::TimeStamp NS_GetTimerDeadlineHintOnCurrentThread(
+    mozilla::TimeStamp aDefault, uint32_t aSearchBound);
+
+/**
+ * Dispatches the given event to a background thread.  The primary benefit of
+ * this API is that you do not have to manage the lifetime of your own thread
+ * for running your own events; the thread manager will take care of the
+ * background thread's lifetime.  Not having to manage your own thread also
+ * means less resource usage, as the underlying implementation here can manage
+ * spinning up and shutting down threads appropriately.
+ *
+ * NOTE: there is no guarantee that events dispatched via these APIs are run
+ * serially, in dispatch order; several dispatched events may run in parallel.
+ * If you depend on serial execution of dispatched events, you should use
+ * NS_CreateBackgroundTaskQueue instead, and dispatch events to the returned
+ * event target.
+ */
+extern nsresult NS_DispatchBackgroundTask(
+    already_AddRefed<nsIRunnable> aEvent,
+    uint32_t aDispatchFlags = NS_DISPATCH_NORMAL);
+extern "C" nsresult NS_DispatchBackgroundTask(
+    nsIRunnable* aEvent, uint32_t aDispatchFlags = NS_DISPATCH_NORMAL);
+
+/**
+ * Obtain a new serial event target that dispatches runnables to a background
+ * thread.  In many cases, this is a straight replacement for creating your
+ * own, private thread, and is generally preferred to creating your own,
+ * private thread.
+ */
+extern "C" nsresult NS_CreateBackgroundTaskQueue(
+    const char* aName, nsISerialEventTarget** aTarget);
+
+/**
+ * Dispatch the given runnable to the given event target, spinning the current
+ * thread's event loop until the runnable has finished executing.
+ *
+ * This is roughly equivalent to the previously-supported `NS_DISPATCH_SYNC`
+ * flag.
+ */
+extern nsresult NS_DispatchAndSpinEventLoopUntilComplete(
+    const nsACString& aVeryGoodReasonToDoThis, nsIEventTarget* aEventTarget,
+    already_AddRefed<nsIRunnable> aEvent);
+
+// Predeclaration for logging function below
+namespace IPC {
+class Message;
+class MessageReader;
+class MessageWriter;
+}  // namespace IPC
+
+class nsTimerImpl;
+
+namespace mozilla {
+
+// RAII class that will set the TLS entry to return the currently running
+// nsISerialEventTarget.
+// It should be used from inner event loop implementation.
+class SerialEventTargetGuard {
+ public:
+  explicit SerialEventTargetGuard(nsISerialEventTarget* aThread)
+      : mLastCurrentThread(sCurrentThreadTLS.get()) {
+    Set(aThread);
+  }
+
+  ~SerialEventTargetGuard() { sCurrentThreadTLS.set(mLastCurrentThread); }
+
+  static void InitTLS();
+  static nsISerialEventTarget* GetCurrentSerialEventTarget() {
+    return sCurrentThreadTLS.get();
+  }
+
+ protected:
+  friend class ::MessageLoop;
+  static void Set(nsISerialEventTarget* aThread) {
+    MOZ_ASSERT(aThread->IsOnCurrentThread());
+    sCurrentThreadTLS.set(aThread);
+  }
+
+ private:
+  static MOZ_THREAD_LOCAL(nsISerialEventTarget*) sCurrentThreadTLS;
+  nsISerialEventTarget* mLastCurrentThread;
+};
+
+// Get the serial event target corresponding to the currently executing task
+// queue or thread. This method will assert if called on a thread pool without
+// an active task queue.
+//
+// This function should generally be preferred over NS_GetCurrentThread since it
+// will return a more useful answer when called from a task queue running on a
+// thread pool or on a non-xpcom thread which accepts runnable dispatches.
+//
+// NOTE: The returned nsISerialEventTarget may not accept runnable dispatches
+// (e.g. if it corresponds to a non-xpcom thread), however it may still be used
+// to check if you're on the given thread/queue using IsOnCurrentThread().
+
+nsISerialEventTarget* GetCurrentSerialEventTarget();
+
+// Get a weak reference to a serial event target which can be used to dispatch
+// runnables to the main thread.
+//
+// NOTE: While this is currently a weak pointer to the nsIThread* returned from
+// NS_GetMainThread(), this may change in the future.
+
+nsISerialEventTarget* GetMainThreadSerialEventTarget();
+
+// Returns the number of CPUs, like PR_GetNumberOfProcessors, except
+// that it can return a cached value on platforms where sandboxing
+// would prevent reading the current value (currently Linux).  CPU
+// hotplugging is uncommon, so this is unlikely to make a difference
+// in practice.
+size_t GetNumberOfProcessors();
+
+/**
+ * A helper class to log tasks dispatch and run with "MOZ_LOG=events:1".  The
+ * output is more machine readable and creates a link between dispatch and run.
+ *
+ * Usage example for the concrete template type nsIRunnable.
+ * To log a dispatch, which means putting an event to a queue:
+ *   LogRunnable::LogDispatch(event);
+ *   theQueue.putEvent(event);
+ *
+ * To log execution (running) of the event:
+ *   nsCOMPtr<nsIRunnable> event = theQueue.popEvent();
+ *   {
+ *     LogRunnable::Run log(event);
+ *     event->Run();
+ *     event = null;  // to include the destructor code in the span
+ *   }
+ *
+ * The class is a template so that we can support various specific super-types
+ * of tasks in the future.  We can't use void* because it may cast differently
+ * and tracking the pointer in logs would then be impossible.
+ */
+template <typename T>
+class LogTaskBase {
+ public:
+  LogTaskBase() = delete;
+
+  // Adds a simple log about dispatch of this runnable.
+  static void LogDispatch(T* aEvent);
+  // The `aContext` pointer adds another uniqe identifier, nothing more
+  static void LogDispatch(T* aEvent, void* aContext);
+
+  // Logs dispatch of the message and along that also the PID of the target
+  // proccess, purposed for uniquely identifying IPC messages.
+  static void LogDispatchWithPid(T* aEvent, int32_t aPid);
+
+  // This is designed to surround a call to `Run()` or any code representing
+  // execution of the task body.
+  // The constructor adds a simple log about start of the runnable execution and
+  // the destructor adds a log about ending the execution.
+  class MOZ_RAII Run {
+   public:
+    Run() = delete;
+    explicit Run(T* aEvent, bool aWillRunAgain = false);
+    explicit Run(T* aEvent, void* aContext, bool aWillRunAgain = false);
+    ~Run();
+
+    // When this is called, the log in this RAII dtor will only say
+    // "interrupted" expecting that the event will run again.
+    void WillRunAgain() { mWillRunAgain = true; }
+
+   private:
+    bool mWillRunAgain = false;
+  };
+};
+
+class MicroTaskRunnable;
+class Task;  // TaskController
+class PresShell;
+namespace dom {
+class FrameRequestCallback;
+}  // namespace dom
+
+// Specialized methods must be explicitly predeclared.
+template <>
+LogTaskBase<nsIRunnable>::Run::Run(nsIRunnable* aEvent, bool aWillRunAgain);
+template <>
+LogTaskBase<Task>::Run::Run(Task* aTask, bool aWillRunAgain);
+template <>
+void LogTaskBase<IPC::Message>::LogDispatchWithPid(IPC::Message* aEvent,
+                                                   int32_t aPid);
+template <>
+LogTaskBase<IPC::Message>::Run::Run(IPC::Message* aMessage, bool aWillRunAgain);
+template <>
+LogTaskBase<nsTimerImpl>::Run::Run(nsTimerImpl* aEvent, bool aWillRunAgain);
+
+typedef LogTaskBase<nsIRunnable> LogRunnable;
+typedef LogTaskBase<MicroTaskRunnable> LogMicroTaskRunnable;
+typedef LogTaskBase<IPC::Message> LogIPCMessage;
+typedef LogTaskBase<nsTimerImpl> LogTimerEvent;
+typedef LogTaskBase<Task> LogTask;
+typedef LogTaskBase<PresShell> LogPresShellObserver;
+typedef LogTaskBase<dom::FrameRequestCallback> LogFrameRequestCallback;
+// If you add new types don't forget to add:
+// `template class LogTaskBase<YourType>;` to nsThreadUtils.cpp
+
+}  // namespace mozilla
+
+#endif  // nsThreadUtils_h__
diff --git a/xpcom/threads/nsTimerImpl.cpp b/xpcom/threads/nsTimerImpl.cpp
new file mode 100644
index 0000000000..b350e9c0a9
--- /dev/null
+++ b/xpcom/threads/nsTimerImpl.cpp
@@ -0,0 +1,822 @@
+/* -*- 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 "nsTimerImpl.h"
+
+#include <utility>
+
+#include "TimerThread.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/ProfilerLabels.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/Try.h"
+#include "nsThreadManager.h"
+#include "nsThreadUtils.h"
+#include "pratom.h"
+
+#ifdef XP_WIN
+#  include <process.h>
+#  ifndef getpid
+#    define getpid _getpid
+#  endif
+#else
+#  include <unistd.h>
+#endif
+
+using mozilla::Atomic;
+using mozilla::LogLevel;
+using mozilla::MakeRefPtr;
+using mozilla::MutexAutoLock;
+using mozilla::TimeDuration;
+using mozilla::TimeStamp;
+
+// Holds the timer thread and manages all interactions with it
+// under a locked mutex. This wrapper is not destroyed until after
+// nsThreadManager shutdown to ensure we don't UAF during an offthread access to
+// the timer thread.
+class TimerThreadWrapper {
+ public:
+  constexpr TimerThreadWrapper() : mThread(nullptr){};
+  ~TimerThreadWrapper() = default;
+
+  nsresult Init();
+  void Shutdown();
+
+  nsresult AddTimer(nsTimerImpl* aTimer, const MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(aTimer->mMutex);
+  nsresult RemoveTimer(nsTimerImpl* aTimer, const MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(aTimer->mMutex);
+  TimeStamp FindNextFireTimeForCurrentThread(TimeStamp aDefault,
+                                             uint32_t aSearchBound);
+  uint32_t AllowedEarlyFiringMicroseconds();
+  nsresult GetTimers(nsTArray<RefPtr<nsITimer>>& aRetVal);
+
+ private:
+  static mozilla::StaticMutex sMutex;
+  TimerThread* mThread MOZ_GUARDED_BY(sMutex);
+};
+
+mozilla::StaticMutex TimerThreadWrapper::sMutex;
+
+nsresult TimerThreadWrapper::Init() {
+  mozilla::StaticMutexAutoLock lock(sMutex);
+  mThread = new TimerThread();
+
+  NS_ADDREF(mThread);
+
+  return NS_OK;
+}
+
+void TimerThreadWrapper::Shutdown() {
+  RefPtr<TimerThread> thread;
+
+  {
+    mozilla::StaticMutexAutoLock lock(sMutex);
+    if (!mThread) {
+      return;
+    }
+    thread = mThread;
+  }
+  // Shutdown calls |nsTimerImpl::Cancel| which needs to make a call into
+  // |RemoveTimer|. This can't be done under the lock.
+  thread->Shutdown();
+
+  {
+    mozilla::StaticMutexAutoLock lock(sMutex);
+    NS_RELEASE(mThread);
+  }
+}
+
+nsresult TimerThreadWrapper::AddTimer(nsTimerImpl* aTimer,
+                                      const MutexAutoLock& aProofOfLock) {
+  mozilla::StaticMutexAutoLock lock(sMutex);
+  if (mThread) {
+    return mThread->AddTimer(aTimer, aProofOfLock);
+  }
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
+nsresult TimerThreadWrapper::RemoveTimer(nsTimerImpl* aTimer,
+                                         const MutexAutoLock& aProofOfLock) {
+  mozilla::StaticMutexAutoLock lock(sMutex);
+  if (mThread) {
+    return mThread->RemoveTimer(aTimer, aProofOfLock);
+  }
+  return NS_ERROR_NOT_AVAILABLE;
+}
+
+TimeStamp TimerThreadWrapper::FindNextFireTimeForCurrentThread(
+    TimeStamp aDefault, uint32_t aSearchBound) {
+  mozilla::StaticMutexAutoLock lock(sMutex);
+  return mThread
+             ? mThread->FindNextFireTimeForCurrentThread(aDefault, aSearchBound)
+             : TimeStamp();
+}
+
+uint32_t TimerThreadWrapper::AllowedEarlyFiringMicroseconds() {
+  mozilla::StaticMutexAutoLock lock(sMutex);
+  return mThread ? mThread->AllowedEarlyFiringMicroseconds() : 0;
+}
+
+nsresult TimerThreadWrapper::GetTimers(nsTArray<RefPtr<nsITimer>>& aRetVal) {
+  RefPtr<TimerThread> thread;
+  {
+    mozilla::StaticMutexAutoLock lock(sMutex);
+    if (!mThread) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+    thread = mThread;
+  }
+  return thread->GetTimers(aRetVal);
+}
+
+static TimerThreadWrapper gThreadWrapper;
+
+// This module prints info about the precision of timers.
+static mozilla::LazyLogModule sTimerLog("nsTimerImpl");
+
+mozilla::LogModule* GetTimerLog() { return sTimerLog; }
+
+TimeStamp NS_GetTimerDeadlineHintOnCurrentThread(TimeStamp aDefault,
+                                                 uint32_t aSearchBound) {
+  return gThreadWrapper.FindNextFireTimeForCurrentThread(aDefault,
+                                                         aSearchBound);
+}
+
+already_AddRefed<nsITimer> NS_NewTimer() { return NS_NewTimer(nullptr); }
+
+already_AddRefed<nsITimer> NS_NewTimer(nsIEventTarget* aTarget) {
+  return nsTimer::WithEventTarget(aTarget).forget();
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithObserver(
+    nsIObserver* aObserver, uint32_t aDelay, uint32_t aType,
+    nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithObserver(getter_AddRefs(timer), aObserver, aDelay,
+                                  aType, aTarget));
+  return std::move(timer);
+}
+nsresult NS_NewTimerWithObserver(nsITimer** aTimer, nsIObserver* aObserver,
+                                 uint32_t aDelay, uint32_t aType,
+                                 nsIEventTarget* aTarget) {
+  auto timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->Init(aObserver, aDelay, aType));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithCallback(
+    nsITimerCallback* aCallback, uint32_t aDelay, uint32_t aType,
+    nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithCallback(getter_AddRefs(timer), aCallback, aDelay,
+                                  aType, aTarget));
+  return std::move(timer);
+}
+nsresult NS_NewTimerWithCallback(nsITimer** aTimer, nsITimerCallback* aCallback,
+                                 uint32_t aDelay, uint32_t aType,
+                                 nsIEventTarget* aTarget) {
+  auto timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->InitWithCallback(aCallback, aDelay, aType));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithCallback(
+    nsITimerCallback* aCallback, const TimeDuration& aDelay, uint32_t aType,
+    nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithCallback(getter_AddRefs(timer), aCallback, aDelay,
+                                  aType, aTarget));
+  return std::move(timer);
+}
+nsresult NS_NewTimerWithCallback(nsITimer** aTimer, nsITimerCallback* aCallback,
+                                 const TimeDuration& aDelay, uint32_t aType,
+                                 nsIEventTarget* aTarget) {
+  auto timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->InitHighResolutionWithCallback(aCallback, aDelay, aType));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithCallback(
+    std::function<void(nsITimer*)>&& aCallback, uint32_t aDelay, uint32_t aType,
+    const char* aNameString, nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithCallback(getter_AddRefs(timer), std::move(aCallback),
+                                  aDelay, aType, aNameString, aTarget));
+  return timer;
+}
+nsresult NS_NewTimerWithCallback(nsITimer** aTimer,
+                                 std::function<void(nsITimer*)>&& aCallback,
+                                 uint32_t aDelay, uint32_t aType,
+                                 const char* aNameString,
+                                 nsIEventTarget* aTarget) {
+  return NS_NewTimerWithCallback(aTimer, std::move(aCallback),
+                                 TimeDuration::FromMilliseconds(aDelay), aType,
+                                 aNameString, aTarget);
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithCallback(
+    std::function<void(nsITimer*)>&& aCallback, const TimeDuration& aDelay,
+    uint32_t aType, const char* aNameString, nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithCallback(getter_AddRefs(timer), std::move(aCallback),
+                                  aDelay, aType, aNameString, aTarget));
+  return timer;
+}
+nsresult NS_NewTimerWithCallback(nsITimer** aTimer,
+                                 std::function<void(nsITimer*)>&& aCallback,
+                                 const TimeDuration& aDelay, uint32_t aType,
+                                 const char* aNameString,
+                                 nsIEventTarget* aTarget) {
+  RefPtr<nsTimer> timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->InitWithClosureCallback(std::move(aCallback), aDelay, aType,
+                                         aNameString));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithFuncCallback(
+    nsTimerCallbackFunc aCallback, void* aClosure, uint32_t aDelay,
+    uint32_t aType, const char* aNameString, nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithFuncCallback(getter_AddRefs(timer), aCallback,
+                                      aClosure, aDelay, aType, aNameString,
+                                      aTarget));
+  return std::move(timer);
+}
+nsresult NS_NewTimerWithFuncCallback(nsITimer** aTimer,
+                                     nsTimerCallbackFunc aCallback,
+                                     void* aClosure, uint32_t aDelay,
+                                     uint32_t aType, const char* aNameString,
+                                     nsIEventTarget* aTarget) {
+  auto timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->InitWithNamedFuncCallback(aCallback, aClosure, aDelay, aType,
+                                           aNameString));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+mozilla::Result<nsCOMPtr<nsITimer>, nsresult> NS_NewTimerWithFuncCallback(
+    nsTimerCallbackFunc aCallback, void* aClosure, const TimeDuration& aDelay,
+    uint32_t aType, const char* aNameString, nsIEventTarget* aTarget) {
+  nsCOMPtr<nsITimer> timer;
+  MOZ_TRY(NS_NewTimerWithFuncCallback(getter_AddRefs(timer), aCallback,
+                                      aClosure, aDelay, aType, aNameString,
+                                      aTarget));
+  return std::move(timer);
+}
+nsresult NS_NewTimerWithFuncCallback(nsITimer** aTimer,
+                                     nsTimerCallbackFunc aCallback,
+                                     void* aClosure, const TimeDuration& aDelay,
+                                     uint32_t aType, const char* aNameString,
+                                     nsIEventTarget* aTarget) {
+  auto timer = nsTimer::WithEventTarget(aTarget);
+
+  MOZ_TRY(timer->InitHighResolutionWithNamedFuncCallback(
+      aCallback, aClosure, aDelay, aType, aNameString));
+  timer.forget(aTimer);
+  return NS_OK;
+}
+
+// This module prints info about which timers are firing, which is useful for
+// wakeups for the purposes of power profiling. Set the following environment
+// variable before starting the browser.
+//
+//   MOZ_LOG=TimerFirings:4
+//
+// Then a line will be printed for every timer that fires.
+//
+// If you redirect this output to a file called "out", you can then
+// post-process it with a command something like the following.
+//
+//   cat out | grep timer | sort | uniq -c | sort -r -n
+//
+// This will show how often each unique line appears, with the most common ones
+// first.
+//
+// More detailed docs are here:
+// https://developer.mozilla.org/en-US/docs/Mozilla/Performance/TimerFirings_logging
+//
+static mozilla::LazyLogModule sTimerFiringsLog("TimerFirings");
+
+static mozilla::LogModule* GetTimerFiringsLog() { return sTimerFiringsLog; }
+
+#include <math.h>
+
+/* static */
+mozilla::StaticMutex nsTimerImpl::sDeltaMutex;
+/* static */
+double nsTimerImpl::sDeltaSumSquared MOZ_GUARDED_BY(nsTimerImpl::sDeltaMutex) =
+    0;
+/* static */
+double nsTimerImpl::sDeltaSum MOZ_GUARDED_BY(nsTimerImpl::sDeltaMutex) = 0;
+/* static */
+double nsTimerImpl::sDeltaNum MOZ_GUARDED_BY(nsTimerImpl::sDeltaMutex) = 0;
+
+static void myNS_MeanAndStdDev(double n, double sumOfValues,
+                               double sumOfSquaredValues, double* meanResult,
+                               double* stdDevResult) {
+  double mean = 0.0, var = 0.0, stdDev = 0.0;
+  if (n > 0.0 && sumOfValues >= 0) {
+    mean = sumOfValues / n;
+    double temp = (n * sumOfSquaredValues) - (sumOfValues * sumOfValues);
+    if (temp < 0.0 || n <= 1) {
+      var = 0.0;
+    } else {
+      var = temp / (n * (n - 1));
+    }
+    // for some reason, Windows says sqrt(0.0) is "-1.#J" (?!) so do this:
+    stdDev = var != 0.0 ? sqrt(var) : 0.0;
+  }
+  *meanResult = mean;
+  *stdDevResult = stdDev;
+}
+
+NS_IMPL_QUERY_INTERFACE(nsTimer, nsITimer)
+NS_IMPL_ADDREF(nsTimer)
+
+NS_IMPL_ISUPPORTS(nsTimerManager, nsITimerManager)
+
+NS_IMETHODIMP nsTimerManager::GetTimers(nsTArray<RefPtr<nsITimer>>& aRetVal) {
+  return gThreadWrapper.GetTimers(aRetVal);
+}
+
+NS_IMETHODIMP_(MozExternalRefCountType)
+nsTimer::Release(void) {
+  nsrefcnt count = --mRefCnt;
+  NS_LOG_RELEASE(this, count, "nsTimer");
+
+  if (count == 1) {
+    // Last ref, in nsTimerImpl::mITimer. Make sure the cycle is broken.
+    mImpl->CancelImpl(true);
+  } else if (count == 0) {
+    delete this;
+  }
+
+  return count;
+}
+
+nsTimerImpl::nsTimerImpl(nsITimer* aTimer, nsIEventTarget* aTarget)
+    : mEventTarget(aTarget),
+      mIsInTimerThread(false),
+      mType(0),
+      mGeneration(0),
+      mITimer(aTimer),
+      mMutex("nsTimerImpl::mMutex"),
+      mCallback(UnknownCallback{}),
+      mFiring(0) {
+  // XXX some code creates timers during xpcom shutdown, when threads are no
+  // longer available, so we cannot turn this on yet.
+  // MOZ_ASSERT(mEventTarget);
+}
+
+// static
+nsresult nsTimerImpl::Startup() { return gThreadWrapper.Init(); }
+
+void nsTimerImpl::Shutdown() {
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    mozilla::StaticMutexAutoLock lock(sDeltaMutex);
+    double mean = 0, stddev = 0;
+    myNS_MeanAndStdDev(sDeltaNum, sDeltaSum, sDeltaSumSquared, &mean, &stddev);
+
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("sDeltaNum = %f, sDeltaSum = %f, sDeltaSumSquared = %f\n",
+             sDeltaNum, sDeltaSum, sDeltaSumSquared));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("mean: %fms, stddev: %fms\n", mean, stddev));
+  }
+
+  gThreadWrapper.Shutdown();
+}
+
+nsresult nsTimerImpl::InitCommon(const TimeDuration& aDelay, uint32_t aType,
+                                 Callback&& newCallback,
+                                 const MutexAutoLock& aProofOfLock) {
+  if (!mEventTarget) {
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  gThreadWrapper.RemoveTimer(this, aProofOfLock);
+
+  // If we have an existing callback, using `swap` ensures it's destroyed after
+  // the mutex is unlocked in our caller.
+  std::swap(mCallback, newCallback);
+  ++mGeneration;
+
+  mType = (uint8_t)aType;
+  mDelay = aDelay;
+  mTimeout = TimeStamp::Now() + mDelay;
+
+  return gThreadWrapper.AddTimer(this, aProofOfLock);
+}
+
+nsresult nsTimerImpl::InitWithNamedFuncCallback(nsTimerCallbackFunc aFunc,
+                                                void* aClosure, uint32_t aDelay,
+                                                uint32_t aType,
+                                                const char* aName) {
+  return InitHighResolutionWithNamedFuncCallback(
+      aFunc, aClosure, TimeDuration::FromMilliseconds(aDelay), aType, aName);
+}
+
+nsresult nsTimerImpl::InitHighResolutionWithNamedFuncCallback(
+    nsTimerCallbackFunc aFunc, void* aClosure, const TimeDuration& aDelay,
+    uint32_t aType, const char* aName) {
+  if (NS_WARN_IF(!aFunc)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb{FuncCallback{aFunc, aClosure, aName}};
+
+  MutexAutoLock lock(mMutex);
+  return InitCommon(aDelay, aType, std::move(cb), lock);
+}
+
+nsresult nsTimerImpl::InitWithCallback(nsITimerCallback* aCallback,
+                                       uint32_t aDelayInMs, uint32_t aType) {
+  return InitHighResolutionWithCallback(
+      aCallback, TimeDuration::FromMilliseconds(aDelayInMs), aType);
+}
+
+nsresult nsTimerImpl::InitHighResolutionWithCallback(
+    nsITimerCallback* aCallback, const TimeDuration& aDelay, uint32_t aType) {
+  if (NS_WARN_IF(!aCallback)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  // Goes out of scope after the unlock, prevents deadlock
+  Callback cb{nsCOMPtr{aCallback}};
+
+  MutexAutoLock lock(mMutex);
+  return InitCommon(aDelay, aType, std::move(cb), lock);
+}
+
+nsresult nsTimerImpl::Init(nsIObserver* aObserver, uint32_t aDelayInMs,
+                           uint32_t aType) {
+  if (NS_WARN_IF(!aObserver)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb{nsCOMPtr{aObserver}};
+
+  MutexAutoLock lock(mMutex);
+  return InitCommon(TimeDuration::FromMilliseconds(aDelayInMs), aType,
+                    std::move(cb), lock);
+}
+
+nsresult nsTimerImpl::InitWithClosureCallback(
+    std::function<void(nsITimer*)>&& aCallback, const TimeDuration& aDelay,
+    uint32_t aType, const char* aNameString) {
+  if (NS_WARN_IF(!aCallback)) {
+    return NS_ERROR_INVALID_ARG;
+  }
+
+  Callback cb{ClosureCallback{std::move(aCallback), aNameString}};
+
+  MutexAutoLock lock(mMutex);
+  return InitCommon(aDelay, aType, std::move(cb), lock);
+}
+
+nsresult nsTimerImpl::Cancel() {
+  CancelImpl(false);
+  return NS_OK;
+}
+
+void nsTimerImpl::CancelImpl(bool aClearITimer) {
+  Callback cbTrash{UnknownCallback{}};
+  RefPtr<nsITimer> timerTrash;
+
+  {
+    MutexAutoLock lock(mMutex);
+    gThreadWrapper.RemoveTimer(this, lock);
+
+    // The swap ensures our callback isn't dropped until after the mutex is
+    // unlocked.
+    std::swap(cbTrash, mCallback);
+    ++mGeneration;
+
+    // Don't clear this if we're firing; once Fire returns, we'll get this call
+    // again.
+    if (aClearITimer && !mFiring) {
+      MOZ_RELEASE_ASSERT(
+          mITimer,
+          "mITimer was nulled already! "
+          "This indicates that someone has messed up the refcount on nsTimer!");
+      timerTrash.swap(mITimer);
+    }
+  }
+}
+
+nsresult nsTimerImpl::SetDelay(uint32_t aDelay) {
+  MutexAutoLock lock(mMutex);
+  if (GetCallback().is<UnknownCallback>() && !IsRepeating()) {
+    // This may happen if someone tries to re-use a one-shot timer
+    // by re-setting delay instead of reinitializing the timer.
+    NS_ERROR(
+        "nsITimer->SetDelay() called when the "
+        "one-shot timer is not set up.");
+    return NS_ERROR_NOT_INITIALIZED;
+  }
+
+  bool reAdd = false;
+  reAdd = NS_SUCCEEDED(gThreadWrapper.RemoveTimer(this, lock));
+
+  mDelay = TimeDuration::FromMilliseconds(aDelay);
+  mTimeout = TimeStamp::Now() + mDelay;
+
+  if (reAdd) {
+    gThreadWrapper.AddTimer(this, lock);
+  }
+
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetDelay(uint32_t* aDelay) {
+  MutexAutoLock lock(mMutex);
+  *aDelay = mDelay.ToMilliseconds();
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::SetType(uint32_t aType) {
+  MutexAutoLock lock(mMutex);
+  mType = (uint8_t)aType;
+  // XXX if this is called, we should change the actual type.. this could effect
+  // repeating timers.  we need to ensure in Fire() that if mType has changed
+  // during the callback that we don't end up with the timer in the queue twice.
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetType(uint32_t* aType) {
+  MutexAutoLock lock(mMutex);
+  *aType = mType;
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetClosure(void** aClosure) {
+  MutexAutoLock lock(mMutex);
+  if (GetCallback().is<FuncCallback>()) {
+    *aClosure = GetCallback().as<FuncCallback>().mClosure;
+  } else {
+    *aClosure = nullptr;
+  }
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetCallback(nsITimerCallback** aCallback) {
+  MutexAutoLock lock(mMutex);
+  if (GetCallback().is<InterfaceCallback>()) {
+    NS_IF_ADDREF(*aCallback = GetCallback().as<InterfaceCallback>());
+  } else {
+    *aCallback = nullptr;
+  }
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetTarget(nsIEventTarget** aTarget) {
+  MutexAutoLock lock(mMutex);
+  NS_IF_ADDREF(*aTarget = mEventTarget);
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::SetTarget(nsIEventTarget* aTarget) {
+  MutexAutoLock lock(mMutex);
+  if (NS_WARN_IF(!mCallback.is<UnknownCallback>())) {
+    return NS_ERROR_ALREADY_INITIALIZED;
+  }
+
+  if (aTarget) {
+    mEventTarget = aTarget;
+  } else {
+    mEventTarget = mozilla::GetCurrentSerialEventTarget();
+  }
+  return NS_OK;
+}
+
+nsresult nsTimerImpl::GetAllowedEarlyFiringMicroseconds(uint32_t* aValueOut) {
+  *aValueOut = gThreadWrapper.AllowedEarlyFiringMicroseconds();
+  return NS_OK;
+}
+
+void nsTimerImpl::Fire(int32_t aGeneration) {
+  uint8_t oldType;
+  uint32_t oldDelay;
+  TimeStamp oldTimeout;
+  Callback callbackDuringFire{UnknownCallback{}};
+  nsCOMPtr<nsITimer> timer;
+
+  {
+    // Don't fire callbacks or fiddle with refcounts when the mutex is locked.
+    // If some other thread Cancels/Inits after this, they're just too late.
+    MutexAutoLock lock(mMutex);
+    if (aGeneration != mGeneration) {
+      // This timer got rescheduled or cancelled before we fired, so ignore this
+      // firing
+      return;
+    }
+
+    // We modify mTimeout, so we must not be in the current TimerThread's
+    // mTimers list.
+    MOZ_ASSERT(!mIsInTimerThread);
+
+    ++mFiring;
+    callbackDuringFire = mCallback;
+    oldType = mType;
+    oldDelay = mDelay.ToMilliseconds();
+    oldTimeout = mTimeout;
+    // Ensure that the nsITimer does not unhook from the nsTimerImpl during
+    // Fire; this will cause null pointer crashes if the user of the timer drops
+    // its reference, and then uses the nsITimer* passed in the callback.
+    timer = mITimer;
+  }
+
+  AUTO_PROFILER_LABEL("nsTimerImpl::Fire", OTHER);
+
+  TimeStamp fireTime;
+  if (MOZ_LOG_TEST(GetTimerLog(), LogLevel::Debug)) {
+    fireTime = TimeStamp::Now();
+    TimeDuration delta = fireTime - oldTimeout;
+    int32_t d = delta.ToMilliseconds();  // delta in ms
+    {
+      mozilla::StaticMutexAutoLock lock(sDeltaMutex);
+      sDeltaSum += abs(d);
+      sDeltaSumSquared += double(d) * double(d);
+      sDeltaNum++;
+    }
+
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("[this=%p] expected delay time %4ums\n", this, oldDelay));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("[this=%p] actual delay time   %4dms\n", this, oldDelay + d));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("[this=%p] (mType is %d)       -------\n", this, oldType));
+    MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+            ("[this=%p]     delta           %4dms\n", this, d));
+  }
+
+  if (MOZ_LOG_TEST(GetTimerFiringsLog(), LogLevel::Debug)) {
+    LogFiring(callbackDuringFire, oldType, oldDelay);
+  }
+
+  callbackDuringFire.match(
+      [](const UnknownCallback&) {},
+      [&](const InterfaceCallback& i) { i->Notify(timer); },
+      [&](const ObserverCallback& o) {
+        o->Observe(timer, NS_TIMER_CALLBACK_TOPIC, nullptr);
+      },
+      [&](const FuncCallback& f) { f.mFunc(timer, f.mClosure); },
+      [&](const ClosureCallback& c) { c.mFunc(timer); });
+
+  TimeStamp now = TimeStamp::Now();
+
+  MutexAutoLock lock(mMutex);
+  if (aGeneration == mGeneration) {
+    if (IsRepeating()) {
+      // Repeating timer has not been re-init or canceled; reschedule
+      if (IsSlack()) {
+        mTimeout = now + mDelay;
+      } else {
+        if (mDelay) {
+          // If we are late enough finishing the callback that we have missed
+          // some firings, do not attempt to play catchup, just get back on the
+          // cadence we're supposed to maintain.
+          unsigned missedFirings =
+              static_cast<unsigned>((now - mTimeout) / mDelay);
+          mTimeout += mDelay * (missedFirings + 1);
+        } else {
+          // Can we stop allowing repeating timers with delay 0?
+          mTimeout = now;
+        }
+      }
+      gThreadWrapper.AddTimer(this, lock);
+    } else {
+      // Non-repeating timer that has not been re-scheduled. Clear.
+      // XXX(nika): Other callsites seem to go to some effort to avoid
+      // destroying mCallback when it's held. Why not this one?
+      mCallback = mozilla::AsVariant(UnknownCallback{});
+    }
+  }
+
+  --mFiring;
+
+  MOZ_LOG(GetTimerLog(), LogLevel::Debug,
+          ("[this=%p] Took %fms to fire timer callback\n", this,
+           (now - fireTime).ToMilliseconds()));
+}
+
+// See the big comment above GetTimerFiringsLog() to understand this code.
+void nsTimerImpl::LogFiring(const Callback& aCallback, uint8_t aType,
+                            uint32_t aDelay) {
+  const char* typeStr;
+  switch (aType) {
+    case nsITimer::TYPE_ONE_SHOT:
+      typeStr = "ONE_SHOT  ";
+      break;
+    case nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY:
+      typeStr = "ONE_LOW   ";
+      break;
+    case nsITimer::TYPE_REPEATING_SLACK:
+      typeStr = "SLACK     ";
+      break;
+    case nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY:
+      typeStr = "SLACK_LOW ";
+      break;
+    case nsITimer::TYPE_REPEATING_PRECISE: /* fall through */
+    case nsITimer::TYPE_REPEATING_PRECISE_CAN_SKIP:
+      typeStr = "PRECISE   ";
+      break;
+    default:
+      MOZ_CRASH("bad type");
+  }
+
+  aCallback.match(
+      [&](const UnknownCallback&) {
+        MOZ_LOG(
+            GetTimerFiringsLog(), LogLevel::Debug,
+            ("[%d]     ??? timer (%s, %5d ms)\n", getpid(), typeStr, aDelay));
+      },
+      [&](const InterfaceCallback& i) {
+        MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+                ("[%d]   iface timer (%s %5d ms): %p\n", getpid(), typeStr,
+                 aDelay, i.get()));
+      },
+      [&](const ObserverCallback& o) {
+        MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+                ("[%d]     obs timer (%s %5d ms): %p\n", getpid(), typeStr,
+                 aDelay, o.get()));
+      },
+      [&](const FuncCallback& f) {
+        MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+                ("[%d]      fn timer (%s %5d ms): %s\n", getpid(), typeStr,
+                 aDelay, f.mName));
+      },
+      [&](const ClosureCallback& c) {
+        MOZ_LOG(GetTimerFiringsLog(), LogLevel::Debug,
+                ("[%d] closure timer (%s %5d ms): %s\n", getpid(), typeStr,
+                 aDelay, c.mName));
+      });
+}
+
+void nsTimerImpl::GetName(nsACString& aName,
+                          const MutexAutoLock& aProofOfLock) {
+  GetCallback().match(
+      [&](const UnknownCallback&) { aName.AssignLiteral("Canceled_timer"); },
+      [&](const InterfaceCallback& i) {
+        if (nsCOMPtr<nsINamed> named = do_QueryInterface(i)) {
+          named->GetName(aName);
+        } else {
+          aName.AssignLiteral("Anonymous_interface_timer");
+        }
+      },
+      [&](const ObserverCallback& o) {
+        if (nsCOMPtr<nsINamed> named = do_QueryInterface(o)) {
+          named->GetName(aName);
+        } else {
+          aName.AssignLiteral("Anonymous_observer_timer");
+        }
+      },
+      [&](const FuncCallback& f) { aName.Assign(f.mName); },
+      [&](const ClosureCallback& c) { aName.Assign(c.mName); });
+}
+
+nsresult nsTimerImpl::GetName(nsACString& aName) {
+  MutexAutoLock lock(mMutex);
+  GetName(aName, lock);
+  return NS_OK;
+}
+
+nsTimer::~nsTimer() = default;
+
+size_t nsTimer::SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) {
+  return aMallocSizeOf(this);
+}
+
+/* static */
+RefPtr<nsTimer> nsTimer::WithEventTarget(nsIEventTarget* aTarget) {
+  if (!aTarget) {
+    aTarget = mozilla::GetCurrentSerialEventTarget();
+  }
+  return do_AddRef(new nsTimer(aTarget));
+}
+
+/* static */
+nsresult nsTimer::XPCOMConstructor(REFNSIID aIID, void** aResult) {
+  *aResult = nullptr;
+  auto timer = WithEventTarget(nullptr);
+
+  return timer->QueryInterface(aIID, aResult);
+}
diff --git a/xpcom/threads/nsTimerImpl.h b/xpcom/threads/nsTimerImpl.h
new file mode 100644
index 0000000000..49f1fd00d5
--- /dev/null
+++ b/xpcom/threads/nsTimerImpl.h
@@ -0,0 +1,231 @@
+/* -*- 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 nsTimerImpl_h___
+#define nsTimerImpl_h___
+
+#include "nsITimer.h"
+#include "nsIEventTarget.h"
+#include "nsIObserver.h"
+
+#include "nsCOMPtr.h"
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Variant.h"
+#include "mozilla/Logging.h"
+
+extern mozilla::LogModule* GetTimerLog();
+
+#define NS_TIMER_CID                                 \
+  { /* 5ff24248-1dd2-11b2-8427-fbab44f29bc8 */       \
+    0x5ff24248, 0x1dd2, 0x11b2, {                    \
+      0x84, 0x27, 0xfb, 0xab, 0x44, 0xf2, 0x9b, 0xc8 \
+    }                                                \
+  }
+
+class nsIObserver;
+
+namespace mozilla {
+class LogModule;
+}
+
+// TimerThread, nsTimerEvent, and nsTimer have references to these. nsTimer has
+// a separate lifecycle so we can Cancel() the underlying timer when the user of
+// the nsTimer has let go of its last reference.
+class nsTimerImpl {
+  ~nsTimerImpl() {
+    MOZ_ASSERT(!mIsInTimerThread);
+
+    // The nsITimer interface requires that its users keep a reference to the
+    // timers they use while those timers are initialized but have not yet
+    // fired. If this assert ever fails, it is a bug in the code that created
+    // and used the timer.
+    //
+    // Further, note that this should never fail even with a misbehaving user,
+    // because nsTimer::Release checks for a refcount of 1 with an armed timer
+    // (a timer whose only reference is from the timer thread) and when it hits
+    // this will remove the timer from the timer thread and thus destroy the
+    // last reference, preventing this situation from occurring.
+    MOZ_ASSERT(
+        mCallback.is<UnknownCallback>() || mEventTarget->IsOnCurrentThread(),
+        "Must not release mCallback off-target without canceling");
+  }
+
+ public:
+  typedef mozilla::TimeStamp TimeStamp;
+
+  nsTimerImpl(nsITimer* aTimer, nsIEventTarget* aTarget);
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsTimerImpl)
+  NS_DECL_NON_VIRTUAL_NSITIMER
+
+  static nsresult Startup();
+  static void Shutdown();
+
+  void SetDelayInternal(uint32_t aDelay, TimeStamp aBase = TimeStamp::Now());
+  void CancelImpl(bool aClearITimer);
+
+  void Fire(int32_t aGeneration);
+
+  int32_t GetGeneration() { return mGeneration; }
+
+  struct UnknownCallback {};
+
+  using InterfaceCallback = nsCOMPtr<nsITimerCallback>;
+
+  using ObserverCallback = nsCOMPtr<nsIObserver>;
+
+  /// A raw function pointer and its closed-over state, along with its name for
+  /// logging purposes.
+  struct FuncCallback {
+    nsTimerCallbackFunc mFunc;
+    void* mClosure;
+    const char* mName;
+  };
+
+  /// A callback defined by an owned closure and its name for logging purposes.
+  struct ClosureCallback {
+    std::function<void(nsITimer*)> mFunc;
+    const char* mName;
+  };
+
+  using Callback =
+      mozilla::Variant<UnknownCallback, InterfaceCallback, ObserverCallback,
+                       FuncCallback, ClosureCallback>;
+
+  nsresult InitCommon(const mozilla::TimeDuration& aDelay, uint32_t aType,
+                      Callback&& newCallback,
+                      const mozilla::MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(mMutex);
+
+  Callback& GetCallback() MOZ_REQUIRES(mMutex) {
+    mMutex.AssertCurrentThreadOwns();
+    return mCallback;
+  }
+
+  bool IsRepeating() const {
+    static_assert(nsITimer::TYPE_ONE_SHOT < nsITimer::TYPE_REPEATING_SLACK,
+                  "invalid ordering of timer types!");
+    static_assert(
+        nsITimer::TYPE_REPEATING_SLACK < nsITimer::TYPE_REPEATING_PRECISE,
+        "invalid ordering of timer types!");
+    static_assert(nsITimer::TYPE_REPEATING_PRECISE <
+                      nsITimer::TYPE_REPEATING_PRECISE_CAN_SKIP,
+                  "invalid ordering of timer types!");
+    return mType >= nsITimer::TYPE_REPEATING_SLACK &&
+           mType < nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY;
+  }
+
+  bool IsLowPriority() const {
+    return mType == nsITimer::TYPE_ONE_SHOT_LOW_PRIORITY ||
+           mType == nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY;
+  }
+
+  bool IsSlack() const {
+    return mType == nsITimer::TYPE_REPEATING_SLACK ||
+           mType == nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY;
+  }
+
+  void GetName(nsACString& aName, const mozilla::MutexAutoLock& aProofOfLock)
+      MOZ_REQUIRES(mMutex);
+
+  bool IsInTimerThread() const { return mIsInTimerThread; }
+  void SetIsInTimerThread(bool aIsInTimerThread) {
+    mIsInTimerThread = aIsInTimerThread;
+  }
+
+  nsCOMPtr<nsIEventTarget> mEventTarget;
+
+  void LogFiring(const Callback& aCallback, uint8_t aType, uint32_t aDelay);
+
+  nsresult InitWithClosureCallback(std::function<void(nsITimer*)>&& aCallback,
+                                   const mozilla::TimeDuration& aDelay,
+                                   uint32_t aType, const char* aNameString);
+
+  // Is this timer currently referenced from a TimerThread::Entry?
+  // Note: It is cleared before the Entry is destroyed.  Take() also sets it to
+  // false, to indicate it's no longer in the TimerThread's list. This Take()
+  // call is NOT made under the nsTimerImpl's mutex (all other
+  // SetIsInTimerThread calls are under the mutex).  However, ALL accesses to
+  // mIsInTimerThread are under the TimerThread's Monitor lock, so consistency
+  // is guaranteed by that.
+  bool mIsInTimerThread;
+
+  // These members are set by the initiating thread, when the timer's type is
+  // changed and during the period where it fires on that thread.
+  uint8_t mType;
+
+  // The generation number of this timer, re-generated each time the timer is
+  // initialized so one-shot timers can be canceled and re-initialized by the
+  // arming thread without any bad race conditions.
+  // Updated only after this timer has been removed from the timer thread.
+  int32_t mGeneration;
+
+  mozilla::TimeDuration mDelay MOZ_GUARDED_BY(mMutex);
+  // Never updated while in the TimerThread's timer list.  Only updated
+  // before adding to that list or during nsTimerImpl::Fire(), when it has
+  // been removed from the TimerThread's list.  TimerThread can access
+  // mTimeout of any timer in the list safely
+  mozilla::TimeStamp mTimeout;
+
+  RefPtr<nsITimer> mITimer MOZ_GUARDED_BY(mMutex);
+  mozilla::Mutex mMutex;
+  Callback mCallback MOZ_GUARDED_BY(mMutex);
+  // Counter because in rare cases we can Fire reentrantly
+  unsigned int mFiring MOZ_GUARDED_BY(mMutex);
+
+  static mozilla::StaticMutex sDeltaMutex;
+  static double sDeltaSum MOZ_GUARDED_BY(sDeltaMutex);
+  static double sDeltaSumSquared MOZ_GUARDED_BY(sDeltaMutex);
+  static double sDeltaNum MOZ_GUARDED_BY(sDeltaMutex);
+};
+
+class nsTimer final : public nsITimer {
+  explicit nsTimer(nsIEventTarget* aTarget)
+      : mImpl(new nsTimerImpl(this, aTarget)) {}
+
+  virtual ~nsTimer();
+
+ public:
+  friend class TimerThread;
+  friend class nsTimerEvent;
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_FORWARD_SAFE_NSITIMER(mImpl);
+
+  // NOTE: This constructor is not exposed on `nsITimer` as NS_FORWARD_SAFE_
+  // does not support forwarding rvalue references.
+  nsresult InitWithClosureCallback(std::function<void(nsITimer*)>&& aCallback,
+                                   const mozilla::TimeDuration& aDelay,
+                                   uint32_t aType, const char* aNameString) {
+    return mImpl ? mImpl->InitWithClosureCallback(std::move(aCallback), aDelay,
+                                                  aType, aNameString)
+                 : NS_ERROR_NULL_POINTER;
+  }
+
+  // Create a timer targeting the given target.  nullptr indicates that the
+  // current thread should be used as the timer's target.
+  static RefPtr<nsTimer> WithEventTarget(nsIEventTarget* aTarget);
+
+  static nsresult XPCOMConstructor(REFNSIID aIID, void** aResult);
+
+ private:
+  // nsTimerImpl holds a strong ref to us. When our refcount goes to 1, we will
+  // null this to break the cycle.
+  RefPtr<nsTimerImpl> mImpl;
+};
+
+class nsTimerManager final : public nsITimerManager {
+ public:
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSITIMERMANAGER
+ private:
+  ~nsTimerManager() = default;
+};
+
+#endif /* nsTimerImpl_h___ */