diff -up firefox-81.0.2/dom/media/mp4/MP4Demuxer.cpp.old firefox-81.0.2/dom/media/mp4/MP4Demuxer.cpp
--- firefox-81.0.2/dom/media/mp4/MP4Demuxer.cpp.old	2020-10-14 17:06:31.424309071 +0200
+++ firefox-81.0.2/dom/media/mp4/MP4Demuxer.cpp	2020-10-14 17:07:31.029533759 +0200
@@ -394,7 +394,7 @@ already_AddRefed<MediaRawData> MP4TrackD
           [[fallthrough]];
         case H264::FrameType::OTHER: {
           bool keyframe = type == H264::FrameType::I_FRAME;
-          if (sample->mKeyframe != keyframe) {
+          if (sample->mKeyframe && sample->mKeyframe != keyframe) {
             NS_WARNING(nsPrintfCString("Frame incorrectly marked as %skeyframe "
                                        "@ pts:%" PRId64 " dur:%" PRId64
                                        " dts:%" PRId64,