diff --git a/dom/media/MediaCache.cpp b/dom/media/MediaCache.cpp
index 07b0ada01609..4aab66779380 100644
--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -2243,11 +2243,10 @@ MediaCacheStream::SetPlaybackRate(uint32_t aBytesPerSecond)
 }
 
 nsresult
-MediaCacheStream::Seek(int32_t aWhence, int64_t aOffset)
+MediaCacheStream::SeekInternal(int32_t aWhence, int64_t aOffset)
 {
-  NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
+  mMediaCache->GetReentrantMonitor().AssertCurrentThreadIn();
 
-  ReentrantMonitorAutoEnter mon(mMediaCache->GetReentrantMonitor());
   if (mClosed)
     return NS_ERROR_FAILURE;
 
@@ -2301,11 +2300,10 @@ MediaCacheStream::Tell()
 }
 
 nsresult
-MediaCacheStream::Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes)
+MediaCacheStream::ReadInternal(char* aBuffer, uint32_t aCount, uint32_t* aBytes)
 {
-  NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
+  mMediaCache->GetReentrantMonitor().AssertCurrentThreadIn();
 
-  ReentrantMonitorAutoEnter mon(mMediaCache->GetReentrantMonitor());
   if (mClosed)
     return NS_ERROR_FAILURE;
 
@@ -2374,7 +2372,7 @@ MediaCacheStream::Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes)
       }
 
       // No data has been read yet, so block
-      mon.Wait();
+      mMediaCache->GetReentrantMonitor().Wait();
       if (mClosed) {
         // We may have successfully read some data, but let's just throw
         // that out.
@@ -2419,9 +2417,9 @@ MediaCacheStream::ReadAt(int64_t aOffset, char* aBuffer,
   NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
 
   ReentrantMonitorAutoEnter mon(mMediaCache->GetReentrantMonitor());
-  nsresult rv = Seek(nsISeekableStream::NS_SEEK_SET, aOffset);
+  nsresult rv = SeekInternal(nsISeekableStream::NS_SEEK_SET, aOffset);
   if (NS_FAILED(rv)) return rv;
-  return Read(aBuffer, aCount, aBytes);
+  return ReadInternal(aBuffer, aCount, aBytes);
 }
 
 nsresult
diff --git a/dom/media/MediaCache.h b/dom/media/MediaCache.h
index 0fd451252c85..c7f303277905 100644
--- a/dom/media/MediaCache.h
+++ b/dom/media/MediaCache.h
@@ -335,17 +335,12 @@ public:
   // These methods must be called on a different thread from the main
   // thread. They should always be called on the same thread for a given
   // stream.
-  // This can fail when aWhence is NS_SEEK_END and no stream length
-  // is known.
-  nsresult Seek(int32_t aWhence, int64_t aOffset);
   int64_t Tell();
+  // Seeks to aOffset in the stream then performs a Read operation.
   // *aBytes gets the number of bytes that were actually read. This can
   // be less than aCount. If the first byte of data is not in the cache,
   // this will block until the data is available or the stream is
   // closed, otherwise it won't block.
-  nsresult Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes);
-  // Seeks to aOffset in the stream then performs a Read operation. See
-  // 'Read' for argument and return details.
   nsresult ReadAt(int64_t aOffset, char* aBuffer,
                   uint32_t aCount, uint32_t* aBytes);
 
@@ -439,6 +434,15 @@ private:
   // Update mPrincipal given that data has been received from aPrincipal
   bool UpdatePrincipal(nsIPrincipal* aPrincipal);
 
+  // This can fail when aWhence is NS_SEEK_END and no stream length
+  // is known.
+  nsresult SeekInternal(int32_t aWhence, int64_t aOffset);
+  // *aBytes gets the number of bytes that were actually read. This can
+  // be less than aCount. If the first byte of data is not in the cache,
+  // this will block until the data is available or the stream is
+  // closed, otherwise it won't block.
+  nsresult ReadInternal(char* aBuffer, uint32_t aCount, uint32_t* aBytes);
+
   // Instance of MediaCache to use with this MediaCacheStream.
   RefPtr<MediaCache> mMediaCache;