What have you noticed about mp4 video playback that is broken? I've used Firefox as my daily driver for many, many years (even before the Quantum updates) and have never noticed any playback errors.
[0]: https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&comp..., you can log in with a github account or create one
They should be able to do this fairly easily -- two distros here:
https://help.ubuntu.com/community/RestrictedFormats
https://docs.fedoraproject.org/en-US/quick-docs/assembly_ins...