--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -2191,18 +2191,16 @@ MediaCacheStream::Seek(int32_t aWhence, 
 
   gMediaCache->QueueUpdate();
   return NS_OK;
 }
 
 int64_t
 MediaCacheStream::Tell()
 {
-  NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
-
   ReentrantMonitorAutoEnter mon(gMediaCache->GetReentrantMonitor());
   return mStreamOffset;
 }
 
 nsresult
 MediaCacheStream::Read(char* aBuffer, uint32_t aCount, uint32_t* aBytes)
 {
   NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");

--- a/dom/media/MediaResource.cpp
+++ b/dom/media/MediaResource.cpp
@@ -702,18 +702,16 @@ ChannelMediaResource::MediaReadAt(int64_
     curr += bytesRead;
   }
   bytes->SetLength(curr - start);
   return bytes.forget();
 }
 
 int64_t ChannelMediaResource::Tell()
 {
-  NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
-
   return mCacheStream.Tell();
 }
 
 nsresult ChannelMediaResource::GetCachedRanges(MediaByteRangeSet& aRanges)
 {
   return mCacheStream.GetCachedRanges(aRanges);
 }
 
@@ -1471,18 +1469,16 @@ nsresult FileMediaResource::UnsafeSeek(i
   if (!mSeekable)
     return NS_ERROR_FAILURE;
   EnsureSizeInitialized();
   return mSeekable->Seek(aWhence, aOffset);
 }
 
 int64_t FileMediaResource::Tell()
 {
-  NS_ASSERTION(!NS_IsMainThread(), "Don't call on main thread");
-
   MutexAutoLock lock(mLock);
   EnsureSizeInitialized();
 
   int64_t offset = 0;
   // Return mSize as offset (end of stream) in case of error
   if (!mSeekable || NS_FAILED(mSeekable->Tell(&offset)))
     return mSize;
   return offset;