--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -886,16 +886,19 @@ private:
 class MediaDecoderStateMachine::CompletedState
   : public MediaDecoderStateMachine::StateObject
 {
 public:
   explicit CompletedState(Master* aPtr) : StateObject(aPtr) {}
 
   void Enter()
   {
+    // We've decoded all samples. We don't need decoders anymore.
+    Reader()->ReleaseResources();
+
     mMaster->ScheduleStateMachine();
   }
 
   void Exit() override
   {
     mSentPlaybackEndedEvent = false;
   }