Both ESR (52.8) and unstable (60.0.1) have broken playback for H.264 files: MP4 videos cannot be played: “Video format or MIME-type is not supported”. Opening Youtube HTML5 page shows, that H.264 and H.264 MSE are not available any more. Fedora has a package, that solves the issue: https://forums.fedoraforum.org/showthread.php?317721-fedora-28-and-firefox-video(h264-youtube-gstreamer1) The package itself: https://fedora.pkgs.org/28/rpmfusion-free-x86_64/compat-ffmpeg28-2.8.14-1.fc28.x86_64.rpm.html Gentoo probably needs something like that, too.
works fine for me with ffmpeg-3.4.2-r1 and ff-60.0.1 which version of ffmpeg you have installed? Can you please provide emerge --info firefox output? bottom part (that says Package Settings) of emerge --info ffmpeg can be useful as well.
I used ffmpeg-9999. Yes, as I could see with ┎ /var/tmp/portage/www-client/firefox-52.8.0/work ┖ at home # grep -F 'sLibs[]' -A12 firefox-52.8.0esr/dom/media/platforms/ffmpeg/FFmpegRuntimeLinker.cpp static const char* sLibs[] = { #if defined(XP_DARWIN) "libavcodec.57.dylib", "libavcodec.56.dylib", "libavcodec.55.dylib", "libavcodec.54.dylib", "libavcodec.53.dylib", #else "libavcodec-ffmpeg.so.57", "libavcodec-ffmpeg.so.56", "libavcodec.so.57", "libavcodec.so.56", "libavcodec.so.55", it links at runtime against specific libavcodec version(s?), and as upstream ffmpeg is in subslot 56.58.58, there were no libraries to link against. I decided to downgrade for a time being.
