diff --git a/dom/media/MediaDecoder.h b/dom/media/MediaDecoder.h
index 5b72e8bf1113..9e2813bc07cb 100644
--- a/dom/media/MediaDecoder.h
+++ b/dom/media/MediaDecoder.h
@@ -242,6 +242,9 @@ public:
   // Return true if the decoder has reached the end of playback.
   bool IsEnded() const;
 
+  // True if we are playing a MediaSource object.
+  virtual bool IsMSE() const { return false; }
+
   // Return true if the MediaDecoderOwner's error attribute is not null.
   // Must be called before Shutdown().
   bool OwnerHasError() const;
diff --git a/dom/media/MediaDecoderStateMachine.cpp b/dom/media/MediaDecoderStateMachine.cpp
index 2f1d4555c925..1f5e0814927d 100644
--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -521,6 +521,13 @@ public:
 
   void Enter();
 
+  void Exit() override
+  {
+    // mPendingSeek is either moved in MaybeFinishDecodeFirstFrame()
+    // or should be rejected here before transition to SHUTDOWN.
+    mPendingSeek.RejectIfExists(__func__);
+  }
+
   State GetState() const override
   {
     return DECODER_STATE_DECODING_FIRSTFRAME;
@@ -592,10 +599,24 @@ public:
     MOZ_ASSERT(false, "Shouldn't have suspended video decoding.");
   }
 
+  RefPtr<MediaDecoder::SeekPromise> HandleSeek(SeekTarget aTarget) override
+  {
+    if (mMaster->mIsMSE) {
+      return StateObject::HandleSeek(aTarget);
+    }
+    // Delay seek request until decoding first frames for non-MSE media.
+    SLOG("Not Enough Data to seek at this stage, queuing seek");
+    mPendingSeek.RejectIfExists(__func__);
+    mPendingSeek.mTarget.emplace(aTarget);
+    return mPendingSeek.mPromise.Ensure(__func__);
+  }
+
 private:
   // Notify FirstFrameLoaded if having decoded first frames and
   // transition to SEEKING if there is any pending seek, or DECODING otherwise.
   void MaybeFinishDecodeFirstFrame();
+
+  SeekJob mPendingSeek;
 };
 
 /**
@@ -2099,7 +2120,11 @@ DecodingFirstFrameState::MaybeFinishDecodeFirstFrame()
   }
 
   mMaster->FinishDecodeFirstFrame();
-  SetState<DecodingState>();
+  if (mPendingSeek.Exists()) {
+    SetSeekingState(Move(mPendingSeek), EventVisibility::Observable);
+  } else {
+    SetState<DecodingState>();
+  }
 }
 
 void
@@ -2547,6 +2572,7 @@ MediaDecoderStateMachine::MediaDecoderStateMachine(MediaDecoder* aDecoder,
   mVideoDecodeSuspendTimer(mTaskQueue),
   mOutputStreamManager(new OutputStreamManager()),
   mResource(aDecoder->GetResource()),
+  mIsMSE(aDecoder->IsMSE()),
   INIT_MIRROR(mBuffered, TimeIntervals()),
   INIT_MIRROR(mEstimatedDuration, NullableTimeUnit()),
   INIT_MIRROR(mExplicitDuration, Maybe<double>()),
diff --git a/dom/media/MediaDecoderStateMachine.h b/dom/media/MediaDecoderStateMachine.h
index 7dd0c4e0b141..46b4d2a1af14 100644
--- a/dom/media/MediaDecoderStateMachine.h
+++ b/dom/media/MediaDecoderStateMachine.h
@@ -693,6 +693,8 @@ private:
   RefPtr<CDMProxy> mCDMProxy;
   MozPromiseRequestHolder<MediaDecoder::CDMProxyPromise> mCDMProxyPromise;
 
+  const bool mIsMSE;
+
 private:
   // The buffered range. Mirrored from the decoder thread.
   Mirror<media::TimeIntervals> mBuffered;
diff --git a/dom/media/mediasource/MediaSourceDecoder.h b/dom/media/mediasource/MediaSourceDecoder.h
index 420f7208010f..8139d35989f0 100644
--- a/dom/media/mediasource/MediaSourceDecoder.h
+++ b/dom/media/mediasource/MediaSourceDecoder.h
@@ -76,6 +76,8 @@ public:
 
   MediaEventSource<void>* WaitingForKeyEvent() override;
 
+  bool IsMSE() const override { return true; }
+
 private:
   void DoSetMediaSourceDuration(double aDuration);
   media::TimeInterval ClampIntervalToEnd(const media::TimeInterval& aInterval);