Fedora 28, Firefox 59.0.2 (64), installed ffmpeg still unable to stream mpeg, mpeg-4 videos. Solution: install compat-ffmpeg28 from rpmfusion-free-updates.
Installed Fedora 28. Installed rpmfusion-free-release-28 and rpmfusion-nonfree-release-28. Installed ffmpeg. But video streaming did not work, in Netfix or Twitter or any other site using mpeg, mpeg-4 videos. Solution: install compat-ffmpeg28 from rpmfusion. Streaming worked after that...
Every distro may be slightly different in packages and package name as FFmpeg is needed for Firefox (and SeaMonkey) to fully support the HTML5 players. Firefox was still running when installing the package then a restart of Firefox may be needed.
You can test at https://www.youtube.com/html5/ as all check marks should ideally be blue.