@beau_tox :-
As xenial says, it's the same old story.
Google specifically optimise YouTube to work best with their own browser. I admit, I've been a Chrome/Chromium user ever since it "hit the shelves"; 13 years ago, Firefox was plagued with memory-leak issues, and was - to put it kindly! - "crash-happy" even on a GOOD day. YouTube has never been anything BUT smooth with Chromium derivatives; 'zilla-based browsers have always been something of a challenge.
The legacy of those days continues, right up to the present..... I don't ever see it changing. I mean, you can hardly blame Google; you purchase the most popular video-sharing site in history, AND you just happen to have the most widely-used browser on the planet. Put yourself in their shoes; in a capitalist world, what would you do?
Remember recently, when Mozilla outright accused Google of deliberately shutting their users out of YT, due to certain unpopular code changes?
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@amethyst :-
I don't deny it is possible to get a decent YouTube experience with a 'zilla-based browser. I guess you must have hit on a lucky combination of software where - rarely - everything just "plays nice" together. Although I confess, I'm surprised that particular PM release is still working OK; it's not "up-to-date", and Google are well-known for changing the YT site code on a whim, several times a year.
Still, while it keeps functioning, use what works for you, I say. Go for it. 
Mike. 