If you have been suffering for years with no in-game music while you play on mozilla firefox firefox firefox here's something that works:
Enable Adobe Flash at the left side of the search bar (where the lock, slider, and lego block are).
That's literally it and I don't know how I've never noticed this before. Thank you and I hope this helped someone
In December 2020, Flash support will be completely removed from consumer versions of Firefox. Adobe will stop shipping security updates for Flash at the end of 2020.