Read youtube videos with another player in firefox

i’m trying to revive some old PCs (2Go DDR2) with linux lubuntu 18 (the last 32bits). Firefox works well but youtube is too slow while video players as mpv read films (.mkv and others) smoothly
with some modifications (fr) in bash, it is possible to open youtube videos in mpv and it is smooth.
is there a way to introduce a new parameter in firefox 32bits allowing the choice of the video player to use it in youtube pages?

Sadly, this is probably not as easy as adding a new checkbox to tick in the settings. I never tried it myself but you may have some luck using youtube-viewer as an alternative. It offers both a command line interface and a GUI one.