diff -up firefox-84.0/dom/media/mp4/MP4Demuxer.cpp.1670333 firefox-84.0/dom/media/mp4/MP4Demuxer.cpp
--- firefox-84.0/dom/media/mp4/MP4Demuxer.cpp.1670333	2020-12-07 23:32:59.000000000 +0100
+++ firefox-84.0/dom/media/mp4/MP4Demuxer.cpp	2020-12-10 14:07:49.095266466 +0100
@@ -31,6 +31,8 @@ mozilla::LogModule* GetDemuxerLog() { re
   DDMOZ_LOG(gMediaDemuxerLog, mozilla::LogLevel::Debug, "::%s: " arg, \
             __func__, ##__VA_ARGS__)
 
+extern bool gUseKeyframeFromContainer;
+
 namespace mozilla {
 
 DDLoggedTypeDeclNameAndBase(MP4TrackDemuxer, MediaTrackDemuxer);
@@ -394,6 +396,12 @@ already_AddRefed<MediaRawData> MP4TrackD
           [[fallthrough]];
         case H264::FrameType::OTHER: {
           bool keyframe = type == H264::FrameType::I_FRAME;
+          if (gUseKeyframeFromContainer) {
+            if (sample->mKeyframe && sample->mKeyframe != keyframe) {
+              sample->mKeyframe = keyframe;
+            }
+            break;
+          }
           if (sample->mKeyframe != keyframe) {
             NS_WARNING(nsPrintfCString("Frame incorrectly marked as %skeyframe "
                                        "@ pts:%" PRId64 " dur:%" PRId64
diff -up firefox-84.0/dom/media/platforms/PDMFactory.cpp.1670333 firefox-84.0/dom/media/platforms/PDMFactory.cpp
--- firefox-84.0/dom/media/platforms/PDMFactory.cpp.1670333	2020-12-10 14:07:49.095266466 +0100
+++ firefox-84.0/dom/media/platforms/PDMFactory.cpp	2020-12-10 15:29:28.969006031 +0100
@@ -52,6 +52,8 @@
 
 #include <functional>
 
+bool gUseKeyframeFromContainer = false;
+
 namespace mozilla {
 
 // Set on the main thread, in content processes before any PDMFactory will ever
@@ -522,9 +524,11 @@ void PDMFactory::CreateContentPDMs() {
   }
 #endif
 #ifdef MOZ_FFMPEG
-  if (StaticPrefs::media_ffmpeg_enabled() &&
-      !CreateAndStartupPDM<FFmpegRuntimeLinker>()) {
-    mFailureFlags += DecoderDoctorDiagnostics::Flags::FFmpegFailedToLoad;
+  if (StaticPrefs::media_ffmpeg_enabled()) {
+    mFFmpegUsed = CreateAndStartupPDM<FFmpegRuntimeLinker>();
+    if (!mFFmpegUsed) {
+      mFailureFlags += DecoderDoctorDiagnostics::Flags::FFmpegFailedToLoad;
+    }
   }
 #endif
 #ifdef MOZ_WIDGET_ANDROID
@@ -536,8 +540,9 @@ void PDMFactory::CreateContentPDMs() {
 
   CreateAndStartupPDM<AgnosticDecoderModule>();
 
-  if (StaticPrefs::media_gmp_decoder_enabled() &&
+  if (StaticPrefs::media_gmp_decoder_enabled() && !mFFmpegUsed &&
       !CreateAndStartupPDM<GMPDecoderModule>()) {
+    gUseKeyframeFromContainer = true;
     mFailureFlags += DecoderDoctorDiagnostics::Flags::GMPPDMFailedToStartup;
   }
 }
diff -up firefox-84.0/dom/media/platforms/PDMFactory.h.1670333 firefox-84.0/dom/media/platforms/PDMFactory.h
--- firefox-84.0/dom/media/platforms/PDMFactory.h.1670333	2020-12-10 14:07:49.096266493 +0100
+++ firefox-84.0/dom/media/platforms/PDMFactory.h	2020-12-10 15:27:42.363034881 +0100
@@ -116,6 +116,8 @@ class PDMFactory final {
 
   DecoderDoctorDiagnostics::FlagsSet mFailureFlags;
 
+  bool mFFmpegUsed = false;
+
   friend class RemoteVideoDecoderParent;
   static void EnsureInit();
   template <class T>