If I remember correctly, the big Youtube issue were with the "recent" redesign that involed a lot of web components code using the Polymer Material Design frame

If I remember correctly, the big Youtube issue were with the "recent" redesign that involed a lot of web components code using the Polymer Material Design framework. And Firefox just wasn't as performant with that (polyfills required?) I've always found the old design better (just like reddit), so I disabled polymer for YT and got both more performance and better UX out of that deal. To try it out, just append &disable_polymer=1 after any YT video url.