--- a/dom/media/platforms/agnostic/OpusDecoder.cpp
+++ b/dom/media/platforms/agnostic/OpusDecoder.cpp
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "OpusDecoder.h"
 #include "TimeUnits.h"
 #include "VorbisUtils.h"
 #include "VorbisDecoder.h" // For VorbisLayout
 #include "mozilla/Endian.h"
 #include "mozilla/PodOperations.h"
+#include "mozilla/SyncRunnable.h"
 
 #include <stdint.h>
 #include <inttypes.h>  // For PRId64
 
 extern mozilla::LogModule* GetPDMLog();
 #define OPUS_DEBUG(arg, ...) MOZ_LOG(GetPDMLog(), mozilla::LogLevel::Debug, \
     ("OpusDataDecoder(%p)::%s: " arg, this, __func__, ##__VA_ARGS__))
 
@@ -26,16 +27,17 @@ OpusDataDecoder::OpusDataDecoder(const A
   : mInfo(aConfig)
   , mTaskQueue(aTaskQueue)
   , mCallback(aCallback)
   , mOpusDecoder(nullptr)
   , mSkip(0)
   , mDecodedHeader(false)
   , mPaddingDiscarded(false)
   , mFrames(0)
+  , mIsFlushing(false)
 {
 }
 
 OpusDataDecoder::~OpusDataDecoder()
 {
   if (mOpusDecoder) {
     opus_multistream_decoder_destroy(mOpusDecoder);
     mOpusDecoder = nullptr;
@@ -136,16 +138,19 @@ OpusDataDecoder::Input(MediaRawData* aSa
                        this, &OpusDataDecoder::ProcessDecode, aSample));
 
   return NS_OK;
 }
 
 void
 OpusDataDecoder::ProcessDecode(MediaRawData* aSample)
 {
+  if (mIsFlushing) {
+    return;
+  }
   if (DoDecode(aSample) == -1) {
     mCallback->Error();
   } else if(mTaskQueue->IsEmpty()) {
     mCallback->InputExhausted();
   }
 }
 
 int
@@ -308,24 +313,30 @@ OpusDataDecoder::Drain()
 {
   mTaskQueue->Dispatch(NewRunnableMethod(this, &OpusDataDecoder::ProcessDrain));
   return NS_OK;
 }
 
 nsresult
 OpusDataDecoder::Flush()
 {
-  mTaskQueue->Flush();
-  if (mOpusDecoder) {
+  if (!mOpusDecoder) {
+    return NS_OK;
+  }
+  mIsFlushing = true;
+  nsCOMPtr<nsIRunnable> runnable = NS_NewRunnableFunction([this] () {
+    MOZ_ASSERT(mOpusDecoder);
     // Reset the decoder.
     opus_multistream_decoder_ctl(mOpusDecoder, OPUS_RESET_STATE);
     mSkip = mOpusParser->mPreSkip;
     mPaddingDiscarded = false;
     mLastFrameTime.reset();
-  }
+  });
+  SyncRunnable::DispatchToThread(mTaskQueue, runnable);
+  mIsFlushing = false;
   return NS_OK;
 }
 
 /* static */
 bool
 OpusDataDecoder::IsOpus(const nsACString& aMimeType)
 {
   return aMimeType.EqualsLiteral("audio/webm; codecs=opus") ||

--- a/dom/media/platforms/agnostic/OpusDecoder.h
+++ b/dom/media/platforms/agnostic/OpusDecoder.h
@@ -55,12 +55,14 @@ private:
 
   // Opus padding should only be discarded on the final packet.  Once this
   // is set to true, if the reader attempts to decode any further packets it
   // will raise an error so we can indicate that the file is invalid.
   bool mPaddingDiscarded;
   int64_t mFrames;
   Maybe<int64_t> mLastFrameTime;
   uint8_t mMappingTable[MAX_AUDIO_CHANNELS]; // Channel mapping table.
+
+  Atomic<bool> mIsFlushing;
 };
 
 } // namespace mozilla
 #endif