--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1555,16 +1555,18 @@ MediaDecoder::RequestDebugInfo()
     [str] () {
       return DebugInfoPromise::CreateAndResolve(str, __func__);
     });
 }
 
 void
 MediaDecoder::GetMozDebugReaderData(nsACString& aString)
 {
+  aString += nsPrintfCString("Container Type: %s\n",
+                             ContainerType().Type().AsString().get());
   if (mReader) {
     mReader->GetMozDebugReaderData(aString);
   }
 }
 
 void
 MediaDecoder::NotifyAudibleStateChanged()
 {

--- a/dom/media/mediasource/MediaSourceDecoder.cpp
+++ b/dom/media/mediasource/MediaSourceDecoder.cpp
@@ -250,16 +250,17 @@ MediaSourceDecoder::SetMediaSourceDurati
   } else {
     SetExplicitDuration(PositiveInfinity<double>());
   }
 }
 
 void
 MediaSourceDecoder::GetMozDebugReaderData(nsACString& aString)
 {
+  aString += NS_LITERAL_CSTRING("Container Type: MediaSource\n");
   if (mReader && mDemuxer) {
     mReader->GetMozDebugReaderData(aString);
     mDemuxer->GetMozDebugReaderData(aString);
   }
 }
 
 double
 MediaSourceDecoder::GetDuration()