diff --git a/dom/media/MediaTaskQueue.cpp b/dom/media/MediaTaskQueue.cpp
index f5dae5c0ae20..11231fb31150 100644
--- a/dom/media/MediaTaskQueue.cpp
+++ b/dom/media/MediaTaskQueue.cpp
@@ -106,8 +106,16 @@ private:
 void
 MediaTaskQueue::SyncDispatch(TemporaryRef<nsIRunnable> aRunnable) {
   NS_WARNING("MediaTaskQueue::SyncDispatch is dangerous and deprecated. Stop using this!");
-  RefPtr<MediaTaskQueueSyncRunnable> task(new MediaTaskQueueSyncRunnable(aRunnable));
-  Dispatch(task);
+  nsRefPtr<MediaTaskQueueSyncRunnable> task(new MediaTaskQueueSyncRunnable(aRunnable));
+
+  // Tail dispatchers don't interact nicely with sync dispatch. We require that
+  // nothing is already in the tail dispatcher, and then sidestep it for this
+  // task.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+  nsRefPtr<MediaTaskQueueSyncRunnable> taskRef = task;
+  Dispatch(taskRef.forget(), AssertDispatchSuccess, TailDispatch);
+
   task->WaitUntilDone();
 }
 
@@ -121,6 +129,11 @@ MediaTaskQueue::AwaitIdle()
 void
 MediaTaskQueue::AwaitIdleLocked()
 {
+  // Make the there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+
   mQueueMonitor.AssertCurrentThreadOwns();
   MOZ_ASSERT(mIsRunning || mTasks.empty());
   while (mIsRunning) {
@@ -131,6 +144,11 @@ MediaTaskQueue::AwaitIdleLocked()
 void
 MediaTaskQueue::AwaitShutdownAndIdle()
 {
+  // Make the there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+
   MonitorAutoLock mon(mQueueMonitor);
   while (!mIsShutdown) {
     mQueueMonitor.Wait();
@@ -176,6 +194,11 @@ FlushableMediaTaskQueue::FlushAndDispatch(TemporaryRef<nsIRunnable> aRunnable)
 void
 FlushableMediaTaskQueue::FlushLocked()
 {
+  // Make the there are no tasks for this queue waiting in the caller's tail
+  // dispatcher.
+  MOZ_ASSERT_IF(AbstractThread::GetCurrent(),
+                !AbstractThread::GetCurrent()->TailDispatcher().HasTasksFor(this));
+
   mQueueMonitor.AssertCurrentThreadOwns();
   MOZ_ASSERT(mIsFlushing);
 
diff --git a/dom/media/TaskDispatcher.h b/dom/media/TaskDispatcher.h
index ea6a3efc1858..be11b1a100ca 100644
--- a/dom/media/TaskDispatcher.h
+++ b/dom/media/TaskDispatcher.h
@@ -44,6 +44,8 @@ public:
   virtual void AddTask(AbstractThread* aThread,
                        already_AddRefed<nsIRunnable> aRunnable,
                        AbstractThread::DispatchFailureHandling aFailureHandling = AbstractThread::AssertDispatchSuccess) = 0;
+
+  virtual bool HasTasksFor(AbstractThread* aThread) = 0;
 };
 
 /*
@@ -88,6 +90,8 @@ public:
     }
   }
 
+  bool HasTasksFor(AbstractThread* aThread) override { return !!GetTaskGroup(aThread); }
+
 private:
 
   struct PerThreadTaskGroup
@@ -131,16 +135,27 @@ private:
 
   PerThreadTaskGroup& EnsureTaskGroup(AbstractThread* aThread)
   {
-    for (size_t i = 0; i < mTaskGroups.Length(); ++i) {
-      if (mTaskGroups[i]->mThread == aThread) {
-        return *mTaskGroups[i];
-      }
+    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;
+  }
+
   // Task groups, organized by thread.
   nsTArray<UniquePtr<PerThreadTaskGroup>> mTaskGroups;