diff -up firefox-115.0/dom/media/mp4/MP4Demuxer.cpp.1670333 firefox-115.0/dom/media/mp4/MP4Demuxer.cpp
--- firefox-115.0/dom/media/mp4/MP4Demuxer.cpp.1670333	2023-06-06 23:14:43.000000000 +0200
+++ firefox-115.0/dom/media/mp4/MP4Demuxer.cpp	2023-06-08 08:15:48.214109403 +0200
@@ -32,6 +32,8 @@ mozilla::LogModule* GetDemuxerLog() { re
   DDMOZ_LOG(gMediaDemuxerLog, mozilla::LogLevel::Debug, "::%s: " arg, \
             __func__, ##__VA_ARGS__)
 
+extern bool gUseKeyframeFromContainer;
+
 namespace mozilla {
 
 using TimeUnit = media::TimeUnit;
@@ -404,6 +406,12 @@ 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-115.0/dom/media/platforms/PDMFactory.cpp.1670333 firefox-115.0/dom/media/platforms/PDMFactory.cpp
--- firefox-115.0/dom/media/platforms/PDMFactory.cpp.1670333	2023-06-06 23:14:44.000000000 +0200
+++ firefox-115.0/dom/media/platforms/PDMFactory.cpp	2023-06-08 08:09:33.145289602 +0200
@@ -67,6 +67,8 @@
 
 #include <functional>
 
+bool gUseKeyframeFromContainer = false;
+
 using DecodeSupport = mozilla::media::DecodeSupport;
 using DecodeSupportSet = mozilla::media::DecodeSupportSet;
 using MediaCodec = mozilla::media::MediaCodec;
@@ -562,7 +564,7 @@ void PDMFactory::CreateRddPDMs() {
 #ifdef MOZ_FFMPEG
   if (StaticPrefs::media_ffmpeg_enabled() &&
       StaticPrefs::media_rdd_ffmpeg_enabled() &&
-      !CreateAndStartupPDM<FFmpegRuntimeLinker>()) {
+      !(mFFmpegUsed = CreateAndStartupPDM<FFmpegRuntimeLinker>())) {
     mFailureFlags += GetFailureFlagBasedOnFFmpegStatus(
         FFmpegRuntimeLinker::LinkStatusCode());
   }
@@ -738,9 +740,10 @@ void PDMFactory::CreateDefaultPDMs() {
 
   CreateAndStartupPDM<AgnosticDecoderModule>();
 
-  if (StaticPrefs::media_gmp_decoder_enabled() &&
+  if (StaticPrefs::media_gmp_decoder_enabled() && !mFFmpegUsed &&
       !StartupPDM(GMPDecoderModule::Create(),
                   StaticPrefs::media_gmp_decoder_preferred())) {
+    gUseKeyframeFromContainer = true;
     mFailureFlags += DecoderDoctorDiagnostics::Flags::GMPPDMFailedToStartup;
   }
 }
diff -up firefox-115.0/dom/media/platforms/PDMFactory.h.1670333 firefox-115.0/dom/media/platforms/PDMFactory.h
--- firefox-115.0/dom/media/platforms/PDMFactory.h.1670333	2023-06-06 23:14:42.000000000 +0200
+++ firefox-115.0/dom/media/platforms/PDMFactory.h	2023-06-08 08:09:33.145289602 +0200
@@ -103,6 +103,7 @@ class PDMFactory final {
   RefPtr<PlatformDecoderModule> mNullPDM;
 
   DecoderDoctorDiagnostics::FlagsSet mFailureFlags;
+  bool mFFmpegUsed = false;
 
   friend class RemoteVideoDecoderParent;
   static void EnsureInit();