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.
It simply doesn't work for some non-trivial amount of videos, e.g. some instagram stories or gfycat mp4s. If you load the link directly in the browser it will say the file is corrupt. Loading the same link in chrome plays fine.
Would you mind filing a bug at [0] with the content of about:support and maybe a link or two where it doesn't work? Happy to have an initial look into this (I work on the media team at Mozilla).