diff --git a/dom/media/MediaFormatReader.cpp b/dom/media/MediaFormatReader.cpp
index 6c65699e638d..f7d06bfcc844 100644
--- a/dom/media/MediaFormatReader.cpp
+++ b/dom/media/MediaFormatReader.cpp
@@ -96,8 +96,7 @@ MediaFormatReader::Shutdown()
       mAudio.RejectPromise(CANCELED, __func__);
     }
     mAudio.mInitPromise.DisconnectIfExists();
-    mAudio.mDecoder->Shutdown();
-    mAudio.mDecoder = nullptr;
+    mAudio.ShutdownDecoder();
   }
   if (mAudio.mTrackDemuxer) {
     mAudio.ResetDemuxer();
@@ -117,8 +116,7 @@ MediaFormatReader::Shutdown()
       mVideo.RejectPromise(CANCELED, __func__);
     }
     mVideo.mInitPromise.DisconnectIfExists();
-    mVideo.mDecoder->Shutdown();
-    mVideo.mDecoder = nullptr;
+    mVideo.ShutdownDecoder();
   }
   if (mVideo.mTrackDemuxer) {
     mVideo.ResetDemuxer();
@@ -381,6 +379,8 @@ MediaFormatReader::EnsureDecoderCreated(TrackType aTrack)
 
   decoder.mDecoderInitialized = false;
 
+  MonitorAutoLock mon(decoder.mMonitor);
+
   switch (aTrack) {
     case TrackType::kAudioTrack:
       decoder.mDecoder =
@@ -406,6 +406,11 @@ MediaFormatReader::EnsureDecoderCreated(TrackType aTrack)
     default:
       break;
   }
+  if (decoder.mDecoder ) {
+    decoder.mDescription = decoder.mDecoder->GetDescriptionName();
+  } else {
+    decoder.mDescription = "error creating decoder";
+  }
   return decoder.mDecoder != nullptr;
 }
 
@@ -429,13 +434,14 @@ MediaFormatReader::EnsureDecoderInitialized(TrackType aTrack)
                 auto& decoder = self->GetDecoderData(aTrack);
                 decoder.mInitPromise.Complete();
                 decoder.mDecoderInitialized = true;
+                MonitorAutoLock mon(decoder.mMonitor);
+                decoder.mDescription = decoder.mDecoder->GetDescriptionName();
                 self->ScheduleUpdate(aTrack);
               },
               [self, aTrack] (MediaDataDecoder::DecoderFailureReason aResult) {
                 auto& decoder = self->GetDecoderData(aTrack);
                 decoder.mInitPromise.Complete();
-                decoder.mDecoder->Shutdown();
-                decoder.mDecoder = nullptr;
+                decoder.ShutdownDecoder();
                 self->NotifyError(aTrack);
               }));
   return false;
@@ -465,8 +471,7 @@ MediaFormatReader::DisableHardwareAcceleration()
   if (HasVideo() && !mHardwareAccelerationDisabled) {
     mHardwareAccelerationDisabled = true;
     Flush(TrackInfo::kVideoTrack);
-    mVideo.mDecoder->Shutdown();
-    mVideo.mDecoder = nullptr;
+    mVideo.ShutdownDecoder();
     if (!EnsureDecoderCreated(TrackType::kVideoTrack)) {
       LOG("Unable to re-create decoder, aborting");
       NotifyError(TrackInfo::kVideoTrack);
@@ -919,8 +924,7 @@ MediaFormatReader::HandleDemuxedSamples(TrackType aTrack,
       // Flush will clear our array of queued samples. So make a copy now.
       nsTArray<RefPtr<MediaRawData>> samples{decoder.mQueuedSamples};
       Flush(aTrack);
-      decoder.mDecoder->Shutdown();
-      decoder.mDecoder = nullptr;
+      decoder.ShutdownDecoder();
       if (sample->mKeyframe) {
         decoder.mQueuedSamples.AppendElements(Move(samples));
         NotifyDecodingRequested(aTrack);
@@ -1604,11 +1608,8 @@ void MediaFormatReader::ReleaseMediaResources()
   if (mVideoFrameContainer) {
     mVideoFrameContainer->ClearCurrentFrame();
   }
-  if (mVideo.mDecoder) {
-    mVideo.mInitPromise.DisconnectIfExists();
-    mVideo.mDecoder->Shutdown();
-    mVideo.mDecoder = nullptr;
-  }
+  mVideo.mInitPromise.DisconnectIfExists();
+  mVideo.ShutdownDecoder();
 }
 
 bool
@@ -1666,12 +1667,25 @@ void
 MediaFormatReader::GetMozDebugReaderData(nsAString& aString)
 {
   nsAutoCString result;
+  const char* audioName = "unavailable";
+  const char* videoName = audioName;
+
+  if (HasAudio()) {
+    MonitorAutoLock mon(mAudio.mMonitor);
+    audioName = mAudio.mDescription;
+  }
+  if (HasVideo()) {
+    MonitorAutoLock mon(mVideo.mMonitor);
+    videoName = mVideo.mDescription;
+  }
+
+  result += nsPrintfCString("audio decoder: %s\n", audioName);
+  result += nsPrintfCString("audio frames decoded: %lld\n",
+                            mAudio.mNumSamplesOutputTotal);
+  result += nsPrintfCString("video decoder: %s\n", videoName);
   result += nsPrintfCString("hardware video decoding: %s\n",
                             VideoIsHardwareAccelerated() ? "enabled" : "disabled");
-  result += nsPrintfCString("audio frames decoded: %lld (skipped:%lld)\n"
-                            "video frames decoded: %lld (skipped:%lld)\n",
-                            mAudio.mNumSamplesOutputTotal,
-                            mAudio.mNumSamplesSkippedTotal,
+  result += nsPrintfCString("video frames decoded: %lld (skipped:%lld)\n",
                             mVideo.mNumSamplesOutputTotal,
                             mVideo.mNumSamplesSkippedTotal);
   aString += NS_ConvertUTF8toUTF16(result);
diff --git a/dom/media/MediaFormatReader.h b/dom/media/MediaFormatReader.h
index 30207b59fc4e..4fe53ed24be3 100644
--- a/dom/media/MediaFormatReader.h
+++ b/dom/media/MediaFormatReader.h
@@ -10,6 +10,7 @@
 #include "mozilla/Atomics.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/TaskQueue.h"
+#include "mozilla/Monitor.h"
 
 #include "MediaDataDemuxer.h"
 #include "MediaDecoderReader.h"
@@ -213,6 +214,8 @@ private:
                 uint32_t aDecodeAhead)
       : mOwner(aOwner)
       , mType(aType)
+      , mMonitor("DecoderData")
+      , mDescription("shutdown")
       , mDecodeAhead(aDecodeAhead)
       , mUpdateScheduled(false)
       , mDemuxEOS(false)
@@ -240,14 +243,27 @@ private:
     // Disambiguate Audio vs Video.
     MediaData::Type mType;
     RefPtr<MediaTrackDemuxer> mTrackDemuxer;
-    // The platform decoder.
-    RefPtr<MediaDataDecoder> mDecoder;
     // TaskQueue on which decoder can choose to decode.
     // Only non-null up until the decoder is created.
     RefPtr<FlushableTaskQueue> mTaskQueue;
     // Callback that receives output and error notifications from the decoder.
     nsAutoPtr<DecoderCallback> mCallback;
 
+    // Monitor protecting mDescription and mDecoder.
+    Monitor mMonitor;
+    // The platform decoder.
+    RefPtr<MediaDataDecoder> mDecoder;
+    const char* mDescription;
+    void ShutdownDecoder()
+    {
+      MonitorAutoLock mon(mMonitor);
+      if (mDecoder) {
+        mDecoder->Shutdown();
+      }
+      mDescription = "shutdown";
+      mDecoder = nullptr;
+    }
+
     // Only accessed from reader's task queue.
     uint32_t mDecodeAhead;
     bool mUpdateScheduled;