--- a/dom/media/MediaFormatReader.cpp
+++ b/dom/media/MediaFormatReader.cpp
@@ -647,20 +647,19 @@ MediaFormatReader::DecoderFactory::DoIni
 // This ensure that the reader's taskqueue will never blocked while a demuxer
 // is itself blocked attempting to access the MediaCache or the MediaResource.
 class MediaFormatReader::DemuxerProxy
 {
   using TrackType = TrackInfo::TrackType;
   class Wrapper;
 
 public:
-  explicit DemuxerProxy(MediaDataDemuxer* aDemuxer, AbstractThread* aMainThread)
+  explicit DemuxerProxy(MediaDataDemuxer* aDemuxer)
     : mTaskQueue(new AutoTaskQueue(
-                   GetMediaThreadPool(MediaThreadType::PLATFORM_DECODER),
-                   aMainThread))
+                   GetMediaThreadPool(MediaThreadType::PLATFORM_DECODER)))
     , mData(new Data(aDemuxer))
   {
     MOZ_COUNT_CTOR(DemuxerProxy);
   }
 
   ~DemuxerProxy()
   {
     MOZ_COUNT_DTOR(DemuxerProxy);
@@ -987,19 +986,17 @@ MediaFormatReader::DemuxerProxy::NotifyD
 MediaFormatReader::MediaFormatReader(AbstractMediaDecoder* aDecoder,
                                      MediaDataDemuxer* aDemuxer,
                                      VideoFrameContainer* aVideoFrameContainer)
   : MediaDecoderReader(aDecoder)
   , mAudio(this, MediaData::AUDIO_DATA,
            Preferences::GetUint("media.audio-max-decode-error", 3))
   , mVideo(this, MediaData::VIDEO_DATA,
            Preferences::GetUint("media.video-max-decode-error", 2))
-  , mDemuxer(new DemuxerProxy(aDemuxer, aDecoder
-                                        ? aDecoder->AbstractMainThread()
-                                        : AbstractThread::MainThread()))
+  , mDemuxer(new DemuxerProxy(aDemuxer))
   , mDemuxerInitDone(false)
   , mLastReportedNumDecodedFrames(0)
   , mPreviousDecodedKeyframeTime_us(sNoPreviousDecodedKeyframe)
   , mInitDone(false)
   , mTrackDemuxersMayBlock(false)
   , mSeekScheduled(false)
   , mVideoFrameContainer(aVideoFrameContainer)
   , mDecoderFactory(new DecoderFactory(this))

--- a/dom/media/mediasource/AutoTaskQueue.h
+++ b/dom/media/mediasource/AutoTaskQueue.h
@@ -13,21 +13,19 @@
 
 namespace mozilla {
 
 // A convenience TaskQueue not requiring explicit shutdown.
 class AutoTaskQueue : public AbstractThread
 {
 public:
   explicit AutoTaskQueue(already_AddRefed<SharedThreadPool> aPool,
-                         AbstractThread* aAbstractMainThread,
                          bool aSupportsTailDispatch = false)
   : AbstractThread(aSupportsTailDispatch)
   , mTaskQueue(new TaskQueue(Move(aPool), aSupportsTailDispatch))
-  , mAbstractMainThread(aAbstractMainThread)
   {}
 
   TaskDispatcher& TailDispatcher() override
   {
     return mTaskQueue->TailDispatcher();
   }
 
   void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
@@ -47,17 +45,16 @@ public:
   bool IsCurrentThreadIn() override { return mTaskQueue->IsCurrentThreadIn(); }
 
 private:
   ~AutoTaskQueue()
   {
     RefPtr<TaskQueue> taskqueue = mTaskQueue;
     nsCOMPtr<nsIRunnable> task =
       NS_NewRunnableFunction([taskqueue]() { taskqueue->BeginShutdown(); });
-    mAbstractMainThread->Dispatch(task.forget());
+    SystemGroup::Dispatch("~AutoTaskQueue", TaskCategory::Other, task.forget());
   }
   RefPtr<TaskQueue> mTaskQueue;
-  const RefPtr<AbstractThread> mAbstractMainThread;
 };
 
 } // namespace mozilla
 
 #endif

--- a/dom/media/mediasource/MediaSourceDemuxer.cpp
+++ b/dom/media/mediasource/MediaSourceDemuxer.cpp
@@ -17,17 +17,16 @@
 namespace mozilla {
 
 typedef TrackInfo::TrackType TrackType;
 using media::TimeUnit;
 using media::TimeIntervals;
 
 MediaSourceDemuxer::MediaSourceDemuxer(AbstractThread* aAbstractMainThread)
   : mTaskQueue(new AutoTaskQueue(GetMediaThreadPool(MediaThreadType::PLAYBACK),
-                                 aAbstractMainThread,
                                  /* aSupportsTailDispatch = */ false))
   , mMonitor("MediaSourceDemuxer")
 {
   MOZ_ASSERT(NS_IsMainThread());
 }
 
 // Due to inaccuracies in determining buffer end
 // frames (Bug 1065207). This value is based on videos seen in the wild.