diff --git a/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp b/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp
--- a/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp
+++ b/dom/media/platforms/agnostic/gmp/GMPDecoderModule.cpp
@@ -86,7 +86,7 @@ bool GMPDecoderModule::SupportsMimeType(
 
 bool GMPDecoderModule::SupportsMimeType(
     const nsACString& aMimeType, DecoderDoctorDiagnostics* aDiagnostics) const {
-  return false;
+  return MP4Decoder::IsH264(aMimeType);
 }
 
 }  // namespace mozilla
diff --git a/dom/media/platforms/agnostic/gmp/GMPVideoDecoder.cpp b/dom/media/platforms/agnostic/gmp/GMPVideoDecoder.cpp
--- a/dom/media/platforms/agnostic/gmp/GMPVideoDecoder.cpp
+++ b/dom/media/platforms/agnostic/gmp/GMPVideoDecoder.cpp
@@ -67,6 +67,8 @@ void GMPVideoDecoder::Decoded(GMPVideoi4
   RefPtr<GMPVideoDecoder> self = this;
   if (v) {
     mDecodedData.AppendElement(std::move(v));
+    mDecodePromise.ResolveIfExists(std::move(mDecodedData), __func__);
+    mDecodedData = DecodedData();
   } else {
     mDecodedData.Clear();
     mDecodePromise.RejectIfExists(