Description of the problem:
On Fedora 28 beta, with ffmpeg installed from the OS repository, Firefox can not play H-264-encoded MP4 videos.
Version-Release number of selected component (if applicable):
Visit https://www.quirksmode.org/html5/tests/video.html and try to play the video labeled "H.264/MP4".
Another relevant test: https://developer.jwplayer.com/tools/stream-tester/?playerversion=8.
HTML5 videos can not be played.
HTML5 videos can be played (as they are on Fedora 27).
Correction: ffmpeg comes from RPMfusion (http://download1.rpmfusion.org/free/fedora/development/28/Everything/x86_64/os/Packages/f/)
Works for me on Fedora 28 / Firefox 59.0.2. I have those ffmpeg packages installed:
rpm -qa \*ffmpeg\*
"compat-ffmpeg28" must be the key here since Firefox does not (to my knowledge) use Gstreamer anymore.
According to the referenced upstream bug, compatibility with FFmpeg 3.5+ will be part of the Firefox 61 release.
Fixed in Firefox 60 (see external bug).