I have issues with FireFox running YouTube on windows 10 - it gets super laggy - the issue is nonexistent if I used the Piped frontend. I think it depends a lot on what website you are using - some don’t play well with FireFox.
That being said, I did not have issues with FireFox on Mac when I used that, or on Linux, though I don’t use my Linux laptop a lot for web heavy stuff
It’s believed to be that Google will serve different websites to non-Chrome (maybe non-Chromium) browsers, or they specifically use features that only they implement to ensure that it performs worse on other browsers. And I don’t mean they add a new feature and it’s only them, but that . Honestly, fuck Google.