diff --git a/dom/media/AbstractMediaDecoder.h b/dom/media/AbstractMediaDecoder.h
index 230056fc48c..a2f9b011820 100644
--- a/dom/media/AbstractMediaDecoder.h
+++ b/dom/media/AbstractMediaDecoder.h
@@ -93,6 +93,13 @@ public:
   // Set the media as being seekable or not.
   virtual void SetMediaSeekable(bool aMediaSeekable) = 0;
 
+  void DispatchSetMediaSeekable(bool aMediaSeekable)
+  {
+    nsCOMPtr<nsIRunnable> r = NS_NewRunnableMethodWithArg<bool>(
+      this, &AbstractMediaDecoder::SetMediaSeekable, aMediaSeekable);
+    NS_DispatchToMainThread(r);
+  }
+
   virtual VideoFrameContainer* GetVideoFrameContainer() = 0;
   virtual mozilla::layers::ImageContainer* GetImageContainer() = 0;
 
diff --git a/dom/media/MediaDecoder.cpp b/dom/media/MediaDecoder.cpp
index 69fbf1d8060..3510228e373 100644
--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1110,6 +1110,7 @@ void MediaDecoder::UpdateEstimatedMediaDuration(int64_t aDuration)
 }
 
 void MediaDecoder::SetMediaSeekable(bool aMediaSeekable) {
+  MOZ_ASSERT(NS_IsMainThread());
   ReentrantMonitorAutoEnter mon(GetReentrantMonitor());
   mMediaSeekable = aMediaSeekable;
 }
diff --git a/dom/media/MediaDecoderStateMachine.cpp b/dom/media/MediaDecoderStateMachine.cpp
index 6f74403f485..eb402c7ffa7 100644
--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -1932,7 +1932,7 @@ MediaDecoderStateMachine::OnMetadataRead(MetadataHolder* aMetadata)
 
   // Set mode to PLAYBACK after reading metadata.
   mResource->SetReadMode(MediaCacheStream::MODE_PLAYBACK);
-  mDecoder->SetMediaSeekable(mReader->IsMediaSeekable());
+  mDecoder->DispatchSetMediaSeekable(mReader->IsMediaSeekable());
   mInfo = aMetadata->mInfo;
   mMetadataTags = aMetadata->mTags.forget();
   nsRefPtr<MediaDecoderStateMachine> self = this;
diff --git a/dom/media/ogg/OggReader.cpp b/dom/media/ogg/OggReader.cpp
index 64ee3718613..61b908ed55d 100644
--- a/dom/media/ogg/OggReader.cpp
+++ b/dom/media/ogg/OggReader.cpp
@@ -720,10 +720,7 @@ void OggReader::SetChained(bool aIsChained) {
     ReentrantMonitorAutoEnter mon(mMonitor);
     mIsChained = aIsChained;
   }
-  {
-    ReentrantMonitorAutoEnter mon(mDecoder->GetReentrantMonitor());
-    mDecoder->SetMediaSeekable(false);
-  }
+  mDecoder->DispatchSetMediaSeekable(false);
 }
 
 bool OggReader::ReadOggChain()