diff --git a/dom/media/webm/WebMBufferedParser.cpp b/dom/media/webm/WebMBufferedParser.cpp
index 62333ab96cbd..c0e37c18838f 100644
--- a/dom/media/webm/WebMBufferedParser.cpp
+++ b/dom/media/webm/WebMBufferedParser.cpp
@@ -313,7 +313,7 @@ bool WebMBufferedState::CalculateBufferedForRange(int64_t aStartOffset,
                                                   int64_t aEndOffset,
                                                   uint64_t* aStartTime,
                                                   uint64_t* aEndTime) {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
 
   // Find the first WebMTimeDataOffset at or after aStartOffset.
   uint32_t start = mTimeMapping.IndexOfFirstElementGt(aStartOffset - 1,
@@ -360,7 +360,7 @@ bool WebMBufferedState::CalculateBufferedForRange(int64_t aStartOffset,
 }
 
 bool WebMBufferedState::GetOffsetForTime(uint64_t aTime, int64_t* aOffset) {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
 
   if (mTimeMapping.IsEmpty()) {
     return false;
@@ -412,7 +412,7 @@ void WebMBufferedState::NotifyDataArrived(const unsigned char* aBuffer,
   }
 
   {
-    ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+    MutexAutoLock lock(mMutex);
     mRangeParsers[idx].Append(aBuffer, aLength, mTimeMapping);
   }
 
@@ -432,12 +432,12 @@ void WebMBufferedState::NotifyDataArrived(const unsigned char* aBuffer,
     return;
   }
 
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
   mLastBlockOffset = mRangeParsers.LastElement().mBlockEndOffset;
 }
 
 void WebMBufferedState::Reset() {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
   mRangeParsers.Clear();
   mTimeMapping.Clear();
 }
@@ -500,13 +500,13 @@ int64_t WebMBufferedState::GetInitEndOffset() {
 }
 
 int64_t WebMBufferedState::GetLastBlockOffset() {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
 
   return mLastBlockOffset;
 }
 
 bool WebMBufferedState::GetStartTime(uint64_t* aTime) {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
 
   if (mTimeMapping.IsEmpty()) {
     return false;
@@ -523,7 +523,7 @@ bool WebMBufferedState::GetStartTime(uint64_t* aTime) {
 
 bool WebMBufferedState::GetNextKeyframeTime(uint64_t aTime,
                                             uint64_t* aKeyframeTime) {
-  ReentrantMonitorAutoEnter mon(mReentrantMonitor);
+  MutexAutoLock lock(mMutex);
   int64_t offset = 0;
   bool rv = GetOffsetForTime(aTime, &offset);
   if (!rv) {
diff --git a/dom/media/webm/WebMBufferedParser.h b/dom/media/webm/WebMBufferedParser.h
index 66de893b530d..8bce578320ff 100644
--- a/dom/media/webm/WebMBufferedParser.h
+++ b/dom/media/webm/WebMBufferedParser.h
@@ -8,7 +8,7 @@
 
 #  include "nsISupportsImpl.h"
 #  include "nsTArray.h"
-#  include "mozilla/ReentrantMonitor.h"
+#  include "mozilla/Mutex.h"
 #  include "MediaResource.h"
 
 namespace mozilla {
@@ -261,8 +261,7 @@ class WebMBufferedState final {
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(WebMBufferedState)
 
  public:
-  WebMBufferedState()
-      : mReentrantMonitor("WebMBufferedState"), mLastBlockOffset(-1) {
+  WebMBufferedState() : mMutex("WebMBufferedState"), mLastBlockOffset(-1) {
     MOZ_COUNT_CTOR(WebMBufferedState);
   }
 
@@ -296,13 +295,13 @@ class WebMBufferedState final {
   MOZ_COUNTED_DTOR(WebMBufferedState)
 
   // Synchronizes access to the mTimeMapping array and mLastBlockOffset.
-  ReentrantMonitor mReentrantMonitor;
+  Mutex mMutex;
 
   // Sorted (by offset) map of data offsets to timecodes.  Populated
   // on the main thread as data is received and parsed by WebMBufferedParsers.
-  nsTArray<WebMTimeDataOffset> mTimeMapping GUARDED_BY(mReentrantMonitor);
+  nsTArray<WebMTimeDataOffset> mTimeMapping GUARDED_BY(mMutex);
   // The last complete block parsed. -1 if not set.
-  int64_t mLastBlockOffset GUARDED_BY(mReentrantMonitor);
+  int64_t mLastBlockOffset GUARDED_BY(mMutex);
 
   // Sorted (by offset) live parser instances.  Main thread only.
   nsTArray<WebMBufferedParser> mRangeParsers;