--- a/dom/media/MediaDecoderStateMachine.cpp
+++ b/dom/media/MediaDecoderStateMachine.cpp
@@ -483,25 +483,26 @@ bool MediaDecoderStateMachine::HaveEnoug
 bool
 MediaDecoderStateMachine::NeedToDecodeVideo()
 {
   MOZ_ASSERT(OnTaskQueue());
   SAMPLE_LOG("NeedToDecodeVideo() isDec=%d minPrl=%d enufVid=%d",
              IsVideoDecoding(), mMinimizePreroll, HaveEnoughDecodedVideo());
   return IsVideoDecoding() &&
          mState != DECODER_STATE_SEEKING &&
-         ((IsDecodingFirstFrame() && VideoQueue().GetSize() == 0) ||
+         ((!mSentFirstFrameLoadedEvent && VideoQueue().GetSize() == 0) ||
           (!mMinimizePreroll && !HaveEnoughDecodedVideo()));
 }
 
 bool
 MediaDecoderStateMachine::NeedToSkipToNextKeyframe()
 {
   MOZ_ASSERT(OnTaskQueue());
-  if (IsDecodingFirstFrame()) {
+  // Don't skip when we're still decoding first frames.
+  if (!mSentFirstFrameLoadedEvent) {
     return false;
   }
   MOZ_ASSERT(mState == DECODER_STATE_DECODING ||
              mState == DECODER_STATE_BUFFERING ||
              mState == DECODER_STATE_SEEKING);
 
   // Since GetClock() can only be called after starting MediaSink, we return
   // false quickly if it is not started because we won't fall behind playback
@@ -552,17 +553,17 @@ bool
 MediaDecoderStateMachine::NeedToDecodeAudio()
 {
   MOZ_ASSERT(OnTaskQueue());
   SAMPLE_LOG("NeedToDecodeAudio() isDec=%d minPrl=%d enufAud=%d",
              IsAudioDecoding(), mMinimizePreroll, HaveEnoughDecodedAudio());
 
   return IsAudioDecoding() &&
          mState != DECODER_STATE_SEEKING &&
-         ((IsDecodingFirstFrame() && AudioQueue().GetSize() == 0) ||
+         ((!mSentFirstFrameLoadedEvent && AudioQueue().GetSize() == 0) ||
           (!mMinimizePreroll && !HaveEnoughDecodedAudio()));
 }
 
 void
 MediaDecoderStateMachine::OnAudioDecoded(MediaData* aAudioSample)
 {
   MOZ_ASSERT(OnTaskQueue());
   MOZ_ASSERT(mState != DECODER_STATE_SEEKING);
@@ -728,17 +729,17 @@ MediaDecoderStateMachine::OnNotDecoded(M
     }
   }
 }
 
 bool
 MediaDecoderStateMachine::MaybeFinishDecodeFirstFrame()
 {
   MOZ_ASSERT(OnTaskQueue());
-  if (!IsDecodingFirstFrame() ||
+  if (mSentFirstFrameLoadedEvent ||
       (IsAudioDecoding() && AudioQueue().GetSize() == 0) ||
       (IsVideoDecoding() && VideoQueue().GetSize() == 0)) {
     return false;
   }
   FinishDecodeFirstFrame();
   if (!mQueuedSeek.Exists()) {
     return false;
   }
@@ -785,17 +786,17 @@ MediaDecoderStateMachine::OnVideoDecoded
       // arrive, increase the amount of audio we buffer to ensure that we
       // don't run out of audio. This is unnecessary for async readers,
       // since they decode audio and video on different threads so they
       // are unlikely to run out of decoded audio.
       if (mReader->IsAsync()) {
         return;
       }
       TimeDuration decodeTime = TimeStamp::Now() - aDecodeStartTime;
-      if (!IsDecodingFirstFrame() &&
+      if (mSentFirstFrameLoadedEvent &&
           THRESHOLD_FACTOR * DurationToUsecs(decodeTime) > mLowAudioThresholdUsecs &&
           !HasLowUndecodedData())
       {
         mLowAudioThresholdUsecs =
           std::min(THRESHOLD_FACTOR * DurationToUsecs(decodeTime), mAmpleAudioThresholdUsecs);
         mAmpleAudioThresholdUsecs = std::max(THRESHOLD_FACTOR * mLowAudioThresholdUsecs,
                                               mAmpleAudioThresholdUsecs);
         DECODER_LOG("Slow video decode, set mLowAudioThresholdUsecs=%lld mAmpleAudioThresholdUsecs=%lld",
@@ -1498,18 +1499,22 @@ MediaDecoderStateMachine::Seek(SeekTarge
   if (aTarget.IsNextFrame() && !HasVideo()) {
     DECODER_WARN("Ignore a NextFrameSeekTask on a media file without video track.");
     return MediaDecoder::SeekPromise::CreateAndReject(/* aIgnored = */ true, __func__);
   }
 
   MOZ_ASSERT(mState > DECODER_STATE_DECODING_METADATA,
                "We should have got duration already");
 
-  if (mState < DECODER_STATE_DECODING ||
-      (IsDecodingFirstFrame() && !mReader->ForceZeroStartTime())) {
+  // Can't seek until the start time is known.
+  bool hasStartTime = mSentFirstFrameLoadedEvent || mReader->ForceZeroStartTime();
+  // Can't seek when state is WAIT_FOR_CDM or DORMANT.
+  bool stateAllowed = mState >= DECODER_STATE_DECODING;
+
+  if (!stateAllowed || !hasStartTime) {
     DECODER_LOG("Seek() Not Enough Data to continue at this stage, queuing seek");
     mQueuedSeek.RejectIfExists(__func__);
     mQueuedSeek.mTarget = aTarget;
     return mQueuedSeek.mPromise.Ensure(__func__);
   }
   mQueuedSeek.RejectIfExists(__func__);
 
   DECODER_LOG("Changed state to SEEKING (to %lld)", aTarget.GetTime().ToMicroseconds());
@@ -1893,18 +1898,18 @@ bool MediaDecoderStateMachine::HasLowUnd
 {
   MOZ_ASSERT(OnTaskQueue());
   return HasLowUndecodedData(mLowDataThresholdUsecs);
 }
 
 bool MediaDecoderStateMachine::HasLowUndecodedData(int64_t aUsecs)
 {
   MOZ_ASSERT(OnTaskQueue());
-  NS_ASSERTION(mState >= DECODER_STATE_DECODING && !IsDecodingFirstFrame(),
-               "Must have loaded first frame for mBuffered to be valid");
+  MOZ_ASSERT(mState >= DECODER_STATE_DECODING && mSentFirstFrameLoadedEvent,
+             "Must have loaded first frame for mBuffered to be valid");
 
   // If we don't have a duration, mBuffered is probably not going to have
   // a useful buffered range. Return false here so that we don't get stuck in
   // buffering mode for live streams.
   if (Duration().IsInfinite()) {
     return false;
   }
 
@@ -2262,19 +2267,18 @@ nsresult MediaDecoderStateMachine::RunSt
   switch (mState) {
     case DECODER_STATE_SHUTDOWN:
     case DECODER_STATE_DORMANT:
     case DECODER_STATE_WAIT_FOR_CDM:
     case DECODER_STATE_DECODING_METADATA:
       return NS_OK;
 
     case DECODER_STATE_DECODING: {
-      if (IsDecodingFirstFrame()) {
-        // We haven't completed decoding our first frames, we can't start
-        // playback yet.
+      // Can't start playback until having decoded first frames.
+      if (!mSentFirstFrameLoadedEvent) {
         return NS_OK;
       }
       if (mPlayState != MediaDecoder::PLAY_STATE_PLAYING && IsPlaying())
       {
         // We're playing, but the element/decoder is in paused state. Stop
         // playing!
         StopPlayback();
       }
@@ -2481,17 +2485,17 @@ MediaDecoderStateMachine::UpdatePlayback
 }
 
 void MediaDecoderStateMachine::UpdateNextFrameStatus()
 {
   MOZ_ASSERT(OnTaskQueue());
 
   MediaDecoderOwner::NextFrameStatus status;
   const char* statusString;
-  if (mState <= DECODER_STATE_WAIT_FOR_CDM || IsDecodingFirstFrame()) {
+  if (mState < DECODER_STATE_DECODING || !mSentFirstFrameLoadedEvent) {
     status = MediaDecoderOwner::NEXT_FRAME_UNAVAILABLE;
     statusString = "NEXT_FRAME_UNAVAILABLE";
   } else if (IsBuffering()) {
     status = MediaDecoderOwner::NEXT_FRAME_UNAVAILABLE_BUFFERING;
     statusString = "NEXT_FRAME_UNAVAILABLE_BUFFERING";
   } else if (IsSeeking()) {
     status = MediaDecoderOwner::NEXT_FRAME_UNAVAILABLE_SEEKING;
     statusString = "NEXT_FRAME_UNAVAILABLE_SEEKING";