--- a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp
+++ b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp
@@ -27,16 +27,17 @@ StaticMutex FFmpegDataDecoder<LIBAV_VER>
                                                   AVCodecID aCodecID)
   : mLib(aLib)
   , mCallback(aCallback)
   , mCodecContext(nullptr)
   , mFrame(NULL)
   , mExtraData(nullptr)
   , mCodecID(aCodecID)
   , mTaskQueue(aTaskQueue)
+  , mIsFlushing(false)
 {
   MOZ_ASSERT(aLib);
   MOZ_COUNT_CTOR(FFmpegDataDecoder);
 }
 
 FFmpegDataDecoder<LIBAV_VER>::~FFmpegDataDecoder()
 {
   MOZ_COUNT_DTOR(FFmpegDataDecoder);
@@ -110,16 +111,19 @@ FFmpegDataDecoder<LIBAV_VER>::Shutdown()
   }
   return NS_OK;
 }
 
 void
 FFmpegDataDecoder<LIBAV_VER>::ProcessDecode(MediaRawData* aSample)
 {
   MOZ_ASSERT(mTaskQueue->IsCurrentThreadIn());
+  if (mIsFlushing) {
+    return;
+  }
   switch (DoDecode(aSample)) {
     case DecodeResult::DECODE_ERROR:
       mCallback->Error();
       break;
     default:
       if (mTaskQueue->IsEmpty()) {
         mCallback->InputExhausted();
       }
@@ -133,20 +137,21 @@ FFmpegDataDecoder<LIBAV_VER>::Input(Medi
     this, &FFmpegDataDecoder::ProcessDecode, aSample));
   return NS_OK;
 }
 
 nsresult
 FFmpegDataDecoder<LIBAV_VER>::Flush()
 {
   MOZ_ASSERT(mCallback->OnReaderTaskQueue());
-  mTaskQueue->Flush();
+  mIsFlushing = true;
   nsCOMPtr<nsIRunnable> runnable =
     NewRunnableMethod(this, &FFmpegDataDecoder<LIBAV_VER>::ProcessFlush);
   SyncRunnable::DispatchToThread(mTaskQueue, runnable);
+  mIsFlushing = false;
   return NS_OK;
 }
 
 nsresult
 FFmpegDataDecoder<LIBAV_VER>::Drain()
 {
   MOZ_ASSERT(mCallback->OnReaderTaskQueue());
   nsCOMPtr<nsIRunnable> runnable =

--- a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h
+++ b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h
@@ -63,13 +63,16 @@ protected:
 
 private:
   void ProcessDecode(MediaRawData* aSample);
   virtual DecodeResult DoDecode(MediaRawData* aSample) = 0;
   virtual void ProcessDrain() = 0;
 
   static StaticMutex sMonitor;
   const RefPtr<FlushableTaskQueue> mTaskQueue;
+  // Set/cleared on reader thread calling Flush() to indicate that output is
+  // not required and so input samples on mTaskQueue need not be processed.
+  Atomic<bool> mIsFlushing;
 };
 
 } // namespace mozilla
 
 #endif // __FFmpegDataDecoder_h__