Good day,
sorry to say, I was mistaken about the compatibility. As I just found out, full support to running .mp4 inside FF (and thus also the TBB) is limited to Windows, because of licensing, see here: Media type and format guide: image, audio, and video content - Web media technologies | MDN That’s one of the problems with “cross platform apps” what is possible on one OS may not be possible on another OS.
I’d recommend using VLC at this point anyway though: download - How to play a Vimeo video in VLC media player? - Super User
Have a nice day,
Ego