--- a/xpcom/threads/TaskQueue.cpp
+++ b/xpcom/threads/TaskQueue.cpp
@@ -1,20 +1,72 @@
 /* -*- 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 "nsIEventTarget.h"
 #include "nsThreadUtils.h"
 
 namespace mozilla {
 
+class TaskQueue::EventTargetWrapper final : public nsIEventTarget
+{
+  RefPtr<TaskQueue> mTaskQueue;
+
+  ~EventTargetWrapper()
+  {
+  }
+
+public:
+  explicit EventTargetWrapper(TaskQueue* aTaskQueue)
+    : mTaskQueue(aTaskQueue)
+  {
+    MOZ_ASSERT(mTaskQueue);
+  }
+
+  NS_IMETHOD
+  DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> ref = aEvent;
+    return Dispatch(ref.forget(), aFlags);
+  }
+
+  NS_IMETHOD
+  Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags) override
+  {
+    nsCOMPtr<nsIRunnable> runnable = aEvent;
+    MonitorAutoLock mon(mTaskQueue->mQueueMonitor);
+    return mTaskQueue->DispatchLocked(/* passed by ref */runnable,
+                                      AbortIfFlushing,
+                                      DontAssertDispatchSuccess,
+                                      NormalDispatch);
+  }
+
+  NS_IMETHOD
+  DelayedDispatch(already_AddRefed<nsIRunnable>, uint32_t aFlags) override
+  {
+    return NS_ERROR_NOT_IMPLEMENTED;
+  }
+
+  NS_IMETHOD
+  IsOnCurrentThread(bool* aResult) override
+  {
+    *aResult = mTaskQueue->IsCurrentThreadIn();
+    return NS_OK;
+  }
+
+  NS_DECL_THREADSAFE_ISUPPORTS
+};
+
+NS_IMPL_ISUPPORTS(TaskQueue::EventTargetWrapper, nsIEventTarget)
+
 TaskQueue::TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                      bool aRequireTailDispatch)
   : AbstractThread(aRequireTailDispatch)
   , mTarget(aTarget)
   , mQueueMonitor("TaskQueue::Queue")
   , mTailDispatcher(nullptr)
   , mIsRunning(false)
   , mIsShutdown(false)
@@ -138,16 +190,23 @@ TaskQueue::IsEmpty()
 bool
 TaskQueue::IsCurrentThreadIn()
 {
   bool in = NS_GetCurrentThread() == mRunningThread;
   MOZ_ASSERT(in == (GetCurrent() == this));
   return in;
 }
 
+already_AddRefed<nsIEventTarget>
+TaskQueue::WrapAsEventTarget()
+{
+  nsCOMPtr<nsIEventTarget> ref = new EventTargetWrapper(this);
+  return ref.forget();
+}
+
 nsresult
 TaskQueue::Runner::Run()
 {
   RefPtr<nsIRunnable> event;
   {
     MonitorAutoLock mon(mQueue->mQueueMonitor);
     MOZ_ASSERT(mQueue->mIsRunning);
     if (mQueue->mTasks.size() == 0) {

--- a/xpcom/threads/TaskQueue.h
+++ b/xpcom/threads/TaskQueue.h
@@ -24,17 +24,36 @@ namespace mozilla {
 
 typedef MozPromise<bool, bool, false> ShutdownPromise;
 
 // 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.
-class TaskQueue : public AbstractThread {
+//
+// 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.
+class TaskQueue : public AbstractThread
+{
+  class EventTargetWrapper;
+
 public:
   explicit TaskQueue(already_AddRefed<nsIEventTarget> aTarget,
                      bool aSupportsTailDispatch = false);
 
   TaskDispatcher& TailDispatcher() override;
 
   TaskQueue* AsTaskQueue() override { return this; }
 
@@ -71,16 +90,20 @@ public:
   void AwaitShutdownAndIdle();
 
   bool IsEmpty();
 
   // Returns true if the current thread is currently running a Runnable in
   // the task queue.
   bool IsCurrentThreadIn() override;
 
+  // Create a new nsIEventTarget wrapper object that dispatches to this
+  // TaskQueue.
+  already_AddRefed<nsIEventTarget> WrapAsEventTarget();
+
 protected:
   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();