--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -1435,18 +1435,16 @@ SeekingState::SeekCompleted()
 
   // We want to resolve the seek request prior finishing the first frame
   // to ensure that the seeked event is fired prior loadeded.
   mSeekJob.Resolve(__func__);
 
   // Notify FirstFrameLoaded now if we haven't since we've decoded some data
   // for readyState to transition to HAVE_CURRENT_DATA and fire 'loadeddata'.
   if (!mMaster->mSentFirstFrameLoadedEvent) {
-    // Only MSE can start seeking before finishing decoding first frames.
-    MOZ_ASSERT(Reader()->ForceZeroStartTime());
     mMaster->FinishDecodeFirstFrame();
   }
 
   // Ensure timestamps are up to date.
   if (!mSeekJob.mTarget.IsVideoOnly()) {
     // Don't update playback position for video-only seek.
     // Otherwise we might have |newCurrentTime > mMediaSink->GetPosition()|
     // and fail the assertion in GetClock() since we didn't stop MediaSink.