--- a/dom/media/MediaFormatReader.cpp
+++ b/dom/media/MediaFormatReader.cpp
@@ -38,16 +38,18 @@ using mozilla::layers::LayerManager;
 using mozilla::layers::LayersBackend;
 
 static mozilla::LazyLogModule sFormatDecoderLog("MediaFormatReader");
 mozilla::LazyLogModule gMediaDemuxerLog("MediaDemuxer");
 
 #define LOG(arg, ...) MOZ_LOG(sFormatDecoderLog, mozilla::LogLevel::Debug, ("MediaFormatReader(%p)::%s: " arg, this, __func__, ##__VA_ARGS__))
 #define LOGV(arg, ...) MOZ_LOG(sFormatDecoderLog, mozilla::LogLevel::Verbose, ("MediaFormatReader(%p)::%s: " arg, this, __func__, ##__VA_ARGS__))
 
+#define NS_DispatchToMainThread(...) CompileError_UseAbstractMainThreadInstead
+
 namespace mozilla {
 
 /**
  * This is a singleton which controls the number of decoders that can be
  * created concurrently. Before calling PDMFactory::CreateDecoder(), Alloc()
  * must be called to get a token object as a permission to create a decoder.
  * The token should stay alive until Shutdown() is called on the decoder.
  * The destructor of the token will restore the decoder count so it is available
@@ -111,21 +113,23 @@ private:
 
   GlobalAllocPolicy& mPolicy; // reference to a singleton object.
 };
 
 GlobalAllocPolicy::GlobalAllocPolicy()
   : mMonitor("DecoderAllocPolicy::mMonitor")
   , mDecoderLimit(MediaPrefs::MediaDecoderLimit())
 {
-  // Non DocGroup-version AbstractThread::MainThread is fine for
-  // ClearOnShutdown().
-  AbstractThread::MainThread()->Dispatch(NS_NewRunnableFunction([this] () {
-    ClearOnShutdown(this, ShutdownPhase::ShutdownThreads);
-  }));
+  SystemGroup::Dispatch(
+    "GlobalAllocPolicy::ClearOnShutdown",
+    TaskCategory::Other,
+    NS_NewRunnableFunction([this] () {
+      ClearOnShutdown(this, ShutdownPhase::ShutdownThreads);
+    })
+  );
 }
 
 GlobalAllocPolicy::~GlobalAllocPolicy()
 {
   while (!mPromises.empty()) {
     RefPtr<PromisePrivate> p = mPromises.front().forget();
     mPromises.pop();
     p->Reject(true, __func__);
@@ -1331,19 +1335,20 @@ MediaFormatReader::OnDemuxerInitDone(nsr
       mAudio.mTrackDemuxer = nullptr;
     }
   }
 
   UniquePtr<EncryptionInfo> crypto = mDemuxer->GetCrypto();
   if (mDecoder && crypto && crypto->IsEncrypted()) {
     // Try and dispatch 'encrypted'. Won't go if ready state still HAVE_NOTHING.
     for (uint32_t i = 0; i < crypto->mInitDatas.Length(); i++) {
-      NS_DispatchToMainThread(
+      nsCOMPtr<nsIRunnable> r =
         new DispatchKeyNeededEvent(mDecoder, crypto->mInitDatas[i].mInitData,
-                                   crypto->mInitDatas[i].mType));
+                                   crypto->mInitDatas[i].mType);
+      mDecoder->AbstractMainThread()->Dispatch(r.forget());
     }
     mInfo.mCrypto = *crypto;
   }
 
   int64_t videoDuration = HasVideo() ? mInfo.mVideo.mDuration : 0;
   int64_t audioDuration = HasAudio() ? mInfo.mAudio.mDuration : 0;
 
   int64_t duration = std::max(videoDuration, audioDuration);
@@ -3044,8 +3049,10 @@ MediaFormatReader::OnFirstDemuxFailed(Tr
 
   auto& decoder = GetDecoderData(aType);
   MOZ_ASSERT(decoder.mFirstDemuxedSampleTime.isNothing());
   decoder.mFirstDemuxedSampleTime.emplace(TimeUnit::FromInfinity());
   MaybeResolveMetadataPromise();
 }
 
 } // namespace mozilla
+
+#undef NS_DispatchToMainThread