If it's possible, you should give your built-in Media Player in Firefox because I use it more often than my VLC media player installed (the UI when you're trying to play a media file via Firefox) a revamp!
For MIME/Audio: You should give a shuffle, playlist, Visualization, etc. (and If playing an audio, you should add an option to:
* auto-add all audio files in the same directory/folder as the one that's playing to a playlist, or
* Play the single file only.
For MIME/Video: You should add support in Firefox to play .m3u8 files natively (in Desktop) (I needed an extension just to play it) (because If I'm not wrong, Firefox Mobile is only the one who supports it)
Samsung browser and banana browsers have their own media ui and controls..firefox should also provide similar experience to mobile devices
I believe Firefox should be at least half as good as VLC