diff --git a/dom/media/MediaCache.cpp b/dom/media/MediaCache.cpp
index c101ca670a91..dc41c20213e6 100644
--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -1884,19 +1884,16 @@ MediaCacheStream::NotifyDataReceived(int64_t aSize, const char* aData)
   // This might happen off the main thread.
   MOZ_DIAGNOSTIC_ASSERT(!mClosed);
 
-  // Update principals before putting the data in the cache. This is important,
-  // we want to make sure all principals are updated before any consumer
-  // can see the new data.
-  // We do this without holding the cache monitor, in case the client wants
-  // to do something that takes a lock.
+  ReentrantMonitorAutoEnter mon(mMediaCache->GetReentrantMonitor());
   {
+    // Need to acquire the monitor because this code might run
+    // off the main thread.
     MediaCache::ResourceStreamIterator iter(mMediaCache, mResourceID);
     while (MediaCacheStream* stream = iter.Next()) {
       stream->mClient->CacheClientUpdatePrincipal();
     }
   }
 
-  ReentrantMonitorAutoEnter mon(mMediaCache->GetReentrantMonitor());
   int64_t size = aSize;
   const char* data = aData;
 
diff --git a/dom/media/MediaResource.cpp b/dom/media/MediaResource.cpp
index f22ea09ad2c9..8c9fee9d041a 100644
--- a/dom/media/MediaResource.cpp
+++ b/dom/media/MediaResource.cpp
@@ -889,12 +889,6 @@ ChannelMediaResource::UpdatePrincipal()
 void
 ChannelMediaResource::CacheClientUpdatePrincipal()
 {
-  // This might happen off the main thread.
-  if (NS_IsMainThread()) {
-    UpdatePrincipal();
-    return;
-  }
-
   SystemGroup::Dispatch(
     TaskCategory::Other,
     NewRunnableMethod("ChannelMediaResource::UpdatePrincipal",