diff --git a/dom/media/mediasource/TrackBuffersManager.cpp b/dom/media/mediasource/TrackBuffersManager.cpp
index 3f8857199fd7..3c9c6c9cc0d2 100644
--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -104,6 +104,7 @@ TrackBuffersManager::TrackBuffersManager(dom::SourceBufferAttributes* aAttribute
   , mEvictionOccurred(false)
   , mMonitor("TrackBuffersManager")
   , mAppendRunning(false)
+  , mSegmentParserLoopRunning(false)
 {
   MOZ_ASSERT(NS_IsMainThread(), "Must be instanciated on the main thread");
 }
@@ -300,7 +301,7 @@ void
 TrackBuffersManager::CompleteResetParserState()
 {
   MOZ_ASSERT(OnTaskQueue());
-  MOZ_DIAGNOSTIC_ASSERT(!mAppendRunning, "Append is running");
+  MOZ_DIAGNOSTIC_ASSERT(!mSegmentParserLoopRunning);
   MSE_DEBUG("");
 
   for (auto& track : GetTracksList()) {
@@ -428,7 +429,6 @@ RefPtr<TrackBuffersManager::RangeRemovalPromise>
 TrackBuffersManager::CodedFrameRemovalWithPromise(TimeInterval aInterval)
 {
   MOZ_ASSERT(OnTaskQueue());
-  MOZ_DIAGNOSTIC_ASSERT(!mAppendRunning, "Logic error: Append in progress");
   bool rv = CodedFrameRemoval(aInterval);
   return RangeRemovalPromise::CreateAndResolve(rv, __func__);
 }
@@ -437,6 +437,7 @@ bool
 TrackBuffersManager::CodedFrameRemoval(TimeInterval aInterval)
 {
   MOZ_ASSERT(OnTaskQueue());
+  MOZ_ASSERT(!mSegmentParserLoopRunning, "Logic error: Append in progress");
   MSE_DEBUG("From %.2fs to %.2f",
             aInterval.mStart.ToSeconds(), aInterval.mEnd.ToSeconds());
 
@@ -575,6 +576,8 @@ TrackBuffersManager::SegmentParserLoop()
 {
   MOZ_ASSERT(OnTaskQueue());
 
+  mSegmentParserLoopRunning = true;
+
   while (true) {
     // 1. If the input buffer is empty, then jump to the need more data step below.
     if (!mInputBuffer || mInputBuffer->IsEmpty()) {
@@ -699,6 +702,7 @@ TrackBuffersManager::NeedMoreData()
   MSE_DEBUG("");
   RestoreCachedVariables();
   mAppendRunning = false;
+  mSegmentParserLoopRunning = false;
   {
     // Wake-up any pending Abort()
     MonitorAutoLock mon(mMonitor);
@@ -712,6 +716,7 @@ TrackBuffersManager::RejectAppend(nsresult aRejectValue, const char* aName)
 {
   MSE_DEBUG("rv=%d", aRejectValue);
   mAppendRunning = false;
+  mSegmentParserLoopRunning = false;
   {
     // Wake-up any pending Abort()
     MonitorAutoLock mon(mMonitor);
diff --git a/dom/media/mediasource/TrackBuffersManager.h b/dom/media/mediasource/TrackBuffersManager.h
index 6188df4d5474..f150e0d49819 100644
--- a/dom/media/mediasource/TrackBuffersManager.h
+++ b/dom/media/mediasource/TrackBuffersManager.h
@@ -350,8 +350,11 @@ private:
   // Monitor to protect following objects accessed across multipple threads.
   // mMonitor is also notified if the value of mAppendRunning becomes false.
   mutable Monitor mMonitor;
-  // Set to true while a BufferAppend is running or is pending.
+  // Set to true while SegmentParserLoop is running.
   Atomic<bool> mAppendRunning;
+  // Set to true while SegmentParserLoop is running.
+  // This is for diagnostic only. Only accessed on the task queue.
+  bool mSegmentParserLoopRunning;
   // Stable audio and video track time ranges.
   media::TimeIntervals mVideoBufferedRanges;
   media::TimeIntervals mAudioBufferedRanges;