--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -58,16 +58,18 @@ static const uint64_t ESTIMATED_DURATION
 
 LazyLogModule gMediaDecoderLog("MediaDecoder");
 #define DECODER_LOG(x, ...) \
   MOZ_LOG(gMediaDecoderLog, LogLevel::Debug, ("Decoder=%p " x, this, ##__VA_ARGS__))
 
 #define DUMP_LOG(x, ...) \
   NS_DebugBreak(NS_DEBUG_WARNING, nsPrintfCString("Decoder=%p " x, this, ##__VA_ARGS__).get(), nullptr, nullptr, -1)
 
+#define NS_DispatchToMainThread(...) CompileError_UseAbstractMainThreadInstead
+
 static const char*
 ToPlayStateStr(MediaDecoder::PlayState aState)
 {
   switch (aState) {
     case MediaDecoder::PLAY_STATE_START:    return "START";
     case MediaDecoder::PLAY_STATE_LOADING:  return "LOADING";
     case MediaDecoder::PLAY_STATE_PAUSED:   return "PAUSED";
     case MediaDecoder::PLAY_STATE_PLAYING:  return "PLAYING";
@@ -149,16 +151,17 @@ MediaDecoder::ResourceCallback::Resource
 }
 
 void
 MediaDecoder::ResourceCallback::Connect(MediaDecoder* aDecoder)
 {
   MOZ_ASSERT(NS_IsMainThread());
   mDecoder = aDecoder;
   mTimer = do_CreateInstance("@mozilla.org/timer;1");
+  mTimer->SetTarget(mAbstractMainThread->AsEventTarget());
 }
 
 void
 MediaDecoder::ResourceCallback::Disconnect()
 {
   MOZ_ASSERT(NS_IsMainThread());
   if (mDecoder) {
     mDecoder = nullptr;
@@ -1650,18 +1653,18 @@ MediaMemoryTracker::CollectReports(nsIHa
   //     resources memory and finish the asynchronous memory report.
   RefPtr<MediaDecoder::ResourceSizes> resourceSizes =
       new MediaDecoder::ResourceSizes(MediaMemoryTracker::MallocSizeOf);
 
   nsCOMPtr<nsIHandleReportCallback> handleReport = aHandleReport;
   nsCOMPtr<nsISupports> data = aData;
 
   resourceSizes->Promise()->Then(
-      // Non-DocGroup version of AbstractThread::MainThread is fine for memory
-      // report.
+      // Don't use SystemGroup::AbstractMainThreadFor() for
+      // handleReport->Callback() will run scripts.
       AbstractThread::MainThread(),
       __func__,
       [handleReport, data] (size_t size) {
         handleReport->Callback(
             EmptyCString(), NS_LITERAL_CSTRING("explicit/media/resources"),
             KIND_HEAP, UNITS_BYTES, size,
             NS_LITERAL_CSTRING("Memory used by media resources including "
                                "streaming buffers, caches, etc."),
@@ -1810,17 +1813,17 @@ MediaDecoder::DumpDebugInfo()
 
   if (!GetStateMachine()) {
     DUMP_LOG("%s", str.get());
     return;
   }
 
   RefPtr<MediaDecoder> self = this;
   GetStateMachine()->RequestDebugInfo()->Then(
-    AbstractThread::MainThread(), __func__,
+    SystemGroup::AbstractMainThreadFor(TaskCategory::Other), __func__,
     [this, self, str] (const nsACString& aString) {
       DUMP_LOG("%s", str.get());
       DUMP_LOG("%s", aString.Data());
     },
     [this, self, str] () {
       DUMP_LOG("%s", str.get());
     });
 }
@@ -1831,17 +1834,17 @@ MediaDecoder::RequestDebugInfo()
   MOZ_DIAGNOSTIC_ASSERT(!IsShutdown());
 
   auto str = GetDebugInfo();
   if (!GetStateMachine()) {
     return DebugInfoPromise::CreateAndResolve(str, __func__);
   }
 
   return GetStateMachine()->RequestDebugInfo()->Then(
-    AbstractThread::MainThread(), __func__,
+    SystemGroup::AbstractMainThreadFor(TaskCategory::Other), __func__,
     [str] (const nsACString& aString) {
       nsCString result = str + nsCString("\n") + aString;
       return DebugInfoPromise::CreateAndResolve(result, __func__);
     },
     [str] () {
       return DebugInfoPromise::CreateAndResolve(str, __func__);
     });
 }
@@ -1867,8 +1870,9 @@ MediaMemoryTracker::~MediaMemoryTracker(
 {
   UnregisterWeakMemoryReporter(this);
 }
 
 } // namespace mozilla
 
 // avoid redefined macro in unified build
 #undef DECODER_LOG
+#undef NS_DispatchToMainThread