--- a/dom/media/MediaMIMETypes.cpp
+++ b/dom/media/MediaMIMETypes.cpp
@@ -47,18 +47,16 @@ MediaMIMEType::SizeOfExcludingThis(Mallo
 MediaMIMEType::MediaMIMEType(const nsACString& aType)
   : mMIMEType(aType)
 {
 }
 
 Maybe<MediaMIMEType>
 MakeMediaMIMEType(const nsAString& aType)
 {
-  MOZ_ASSERT(NS_IsMainThread());
-
   nsContentTypeParser parser(aType);
   nsAutoString mime;
   nsresult rv = parser.GetType(mime);
   if (!NS_SUCCEEDED(rv) || mime.IsEmpty()) {
     return Nothing();
   }
 
   NS_ConvertUTF16toUTF8 mime8{mime};
@@ -158,18 +156,16 @@ MediaExtendedMIMEType::MediaExtendedMIME
   : mOriginalString(aType.AsString())
   , mMIMEType(Move(aType))
 {
 }
 
 Maybe<MediaExtendedMIMEType>
 MakeMediaExtendedMIMEType(const nsAString& aType)
 {
-  MOZ_ASSERT(NS_IsMainThread());
-
   nsContentTypeParser parser(aType);
   nsAutoString mime;
   nsresult rv = parser.GetType(mime);
   if (!NS_SUCCEEDED(rv) || mime.IsEmpty()) {
     return Nothing();
   }
 
   NS_ConvertUTF16toUTF8 mime8{mime};