--- a/dom/media/mediasource/MediaSource.cpp
+++ b/dom/media/mediasource/MediaSource.cpp
@@ -531,26 +531,23 @@ MediaSource::DurationChange(double aNewD
   // 2. If new duration is less than the highest starting presentation timestamp
   // of any buffered coded frames for all SourceBuffer objects in sourceBuffers,
   // then throw an InvalidStateError exception and abort these steps.
   if (aNewDuration < mSourceBuffers->HighestStartTime()) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return;
   }
 
-  // 3. Update duration to new duration.
-  // 4. If a user agent is unable to partially render audio frames or text cues
-  // that start before and end after the duration, then run the following steps:
-  // Update new duration to the highest end time reported by the buffered
-  // attribute across all SourceBuffer objects in sourceBuffers.
-  //   1. Update new duration to the highest end time reported by the buffered
-  //      attribute across all SourceBuffer objects in sourceBuffers.
-  //   2. Update duration to new duration.
+  // 3. Let highest end time be the largest track buffer ranges end time across
+  // all the track buffers across all SourceBuffer objects in sourceBuffers.
+  double highestEndTime = mSourceBuffers->HighestEndTime();
+  // 4. If new duration is less than highest end time, then
+  //    4.1 Update new duration to equal highest end time.
   aNewDuration =
-    std::max(aNewDuration, mSourceBuffers->GetHighestBufferedEndTime());
+    std::max(aNewDuration, highestEndTime);
 
   // 5. Update the media duration to new duration and run the HTMLMediaElement
   // duration change algorithm.
   mDecoder->SetMediaSourceDuration(aNewDuration);
 }
 
 void
 MediaSource::GetMozDebugReaderData(nsAString& aString)

--- a/dom/media/mediasource/SourceBuffer.cpp
+++ b/dom/media/mediasource/SourceBuffer.cpp
@@ -549,16 +549,25 @@ double
 SourceBuffer::HighestStartTime()
 {
   MOZ_ASSERT(NS_IsMainThread());
   return mTrackBuffersManager
          ? mTrackBuffersManager->HighestStartTime().ToSeconds()
          : 0.0;
 }
 
+double
+SourceBuffer::HighestEndTime()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  return mTrackBuffersManager
+         ? mTrackBuffersManager->HighestEndTime().ToSeconds()
+         : 0.0;
+}
+
 NS_IMPL_CYCLE_COLLECTION_CLASS(SourceBuffer)
 
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(SourceBuffer)
   // Tell the TrackBuffer to end its current SourceBufferResource.
   TrackBuffersManager* manager = tmp->mTrackBuffersManager;
   if (manager) {
     manager->Detach();
   }

--- a/dom/media/mediasource/SourceBuffer.h
+++ b/dom/media/mediasource/SourceBuffer.h
@@ -113,16 +113,17 @@ public:
     return mMediaSource != nullptr;
   }
 
   void Ended();
 
   double GetBufferedStart();
   double GetBufferedEnd();
   double HighestStartTime();
+  double HighestEndTime();
 
   // Runs the range removal algorithm as defined by the MSE spec.
   void RangeRemoval(double aStart, double aEnd);
 
   bool IsActive() const
   {
     return mActive;
   }

--- a/dom/media/mediasource/SourceBufferList.cpp
+++ b/dom/media/mediasource/SourceBufferList.cpp
@@ -186,16 +186,28 @@ SourceBufferList::HighestStartTime()
   double highestStartTime = 0;
   for (auto& sourceBuffer : mSourceBuffers) {
     highestStartTime =
       std::max(sourceBuffer->HighestStartTime(), highestStartTime);
   }
   return highestStartTime;
 }
 
+double
+SourceBufferList::HighestEndTime()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  double highestEndTime = 0;
+  for (auto& sourceBuffer : mSourceBuffers) {
+    highestEndTime =
+      std::max(sourceBuffer->HighestEndTime(), highestEndTime);
+  }
+  return highestEndTime;
+}
+
 JSObject*
 SourceBufferList::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aGivenProto)
 {
   return SourceBufferListBinding::Wrap(aCx, this, aGivenProto);
 }
 
 NS_IMPL_CYCLE_COLLECTION_INHERITED(SourceBufferList, DOMEventTargetHelper,
                                    mMediaSource, mSourceBuffers)

--- a/dom/media/mediasource/SourceBufferList.h
+++ b/dom/media/mediasource/SourceBufferList.h
@@ -81,16 +81,17 @@ public:
   // Append a SourceBuffer to the list. No event is fired.
   void AppendSimple(SourceBuffer* aSourceBuffer);
 
   // Remove all SourceBuffers from mSourceBuffers.
   //  No event is fired and no action is performed on the sourcebuffers.
   void ClearSimple();
 
   double HighestStartTime();
+  double HighestEndTime();
 
 private:
   ~SourceBufferList();
 
   friend class AsyncEventRunner<SourceBufferList>;
   void DispatchSimpleEvent(const char* aName);
   void QueueAsyncSimpleEvent(const char* aName);
 

--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -295,31 +295,28 @@ TrackBuffersManager::EvictData(const Tim
 
   return EvictDataResult::NO_DATA_EVICTED;
 }
 
 TimeIntervals
 TrackBuffersManager::Buffered()
 {
   MSE_DEBUG("");
-  MonitorAutoLock mon(mMonitor);
   // http://w3c.github.io/media-source/index.html#widl-SourceBuffer-buffered
   // 2. Let highest end time be the largest track buffer ranges end time across all the track buffers managed by this SourceBuffer object.
-  TimeUnit highestEndTime;
+  TimeUnit highestEndTime = HighestEndTime();
 
+  MonitorAutoLock mon(mMonitor);
   nsTArray<TimeIntervals*> tracks;
   if (HasVideo()) {
     tracks.AppendElement(&mVideoBufferedRanges);
   }
   if (HasAudio()) {
     tracks.AppendElement(&mAudioBufferedRanges);
   }
-  for (auto trackRanges : tracks) {
-    highestEndTime = std::max(trackRanges->GetEnd(), highestEndTime);
-  }
 
   // 3. Let intersection ranges equal a TimeRange object containing a single range from 0 to highest end time.
   TimeIntervals intersection{TimeInterval(TimeUnit::FromSeconds(0), highestEndTime)};
 
   // 4. For each track buffer managed by this SourceBuffer, run the following steps:
   //   1. Let track ranges equal the track buffer ranges for the current track buffer.
   for (auto trackRanges : tracks) {
     // 2. If readyState is "ended", then set the end time on the last range in track ranges to highest end time.
@@ -1912,16 +1909,35 @@ TrackBuffersManager::HighestStartTime()
   TimeUnit highestStartTime;
   for (auto& track : GetTracksList()) {
     highestStartTime =
       std::max(track->mHighestStartTimestamp, highestStartTime);
   }
   return highestStartTime;
 }
 
+TimeUnit
+TrackBuffersManager::HighestEndTime()
+{
+  MonitorAutoLock mon(mMonitor);
+  TimeUnit highestEndTime;
+
+  nsTArray<TimeIntervals*> tracks;
+  if (HasVideo()) {
+    tracks.AppendElement(&mVideoBufferedRanges);
+  }
+  if (HasAudio()) {
+    tracks.AppendElement(&mAudioBufferedRanges);
+  }
+  for (auto trackRanges : tracks) {
+    highestEndTime = std::max(trackRanges->GetEnd(), highestEndTime);
+  }
+  return highestEndTime;
+}
+
 const TrackBuffersManager::TrackBuffer&
 TrackBuffersManager::GetTrackBuffer(TrackInfo::TrackType aTrack)
 {
   MOZ_ASSERT(OnTaskQueue());
   return GetTracksData(aTrack).mBuffers.LastElement();
 }
 
 uint32_t TrackBuffersManager::FindSampleIndex(const TrackBuffer& aTrackBuffer,

--- a/dom/media/mediasource/TrackBuffersManager.h
+++ b/dom/media/mediasource/TrackBuffersManager.h
@@ -118,16 +118,17 @@ public:
   // and if still more space is needed remove from the end.
   EvictDataResult EvictData(const media::TimeUnit& aPlaybackTime, int64_t aSize);
 
   // Returns the buffered range currently managed.
   // This may be called on any thread.
   // Buffered must conform to http://w3c.github.io/media-source/index.html#widl-SourceBuffer-buffered
   media::TimeIntervals Buffered();
   media::TimeUnit HighestStartTime();
+  media::TimeUnit HighestEndTime();
 
   // Return the size of the data managed by this SourceBufferContentManager.
   int64_t GetSize() const;
 
   // Indicate that the MediaSource parent object got into "ended" state.
   void Ended();
 
   // The parent SourceBuffer is about to be destroyed.