--- a/dom/media/mediasource/TrackBuffersManager.h
+++ b/dom/media/mediasource/TrackBuffersManager.h
@@ -206,17 +206,17 @@ private:
   RefPtr<MediaByteBuffer> mInputBuffer;
   // Buffer full flag as per https://w3c.github.io/media-source/#sourcebuffer-buffer-full-flag.
   // Accessed on both the main thread and the task queue.
   Atomic<bool> mBufferFull;
   bool mFirstInitializationSegmentReceived;
   // Set to true once a new segment is started.
   bool mNewMediaSegmentStarted;
   bool mActiveTrack;
-  MediaContainerType mType;
+  const MediaContainerType mType;
 
   // ContainerParser objects and methods.
   // Those are used to parse the incoming input buffer.
 
   // Recreate the ContainerParser and if aReuseInitData is true then
   // feed it with the previous init segment found.
   void RecreateParser(bool aReuseInitData);
   nsAutoPtr<ContainerParser> mParser;