The last serious performance issue I saw with FF+YT was when YT added a glowing border around videos when the tab was in dark mode. FF just wasn't able to efficiently run that effect. I don't think it's very reasonable to say that YT or any other site should be limited by the performance problems of FF. Maybe they should have detected the issue before release but the explosion of browser x platform x dark/light theme x graphics driver is a large space to sweep.
"Should have detected the issue" implies it was an accident. YouTube has intentionally tanked performance in non-Chrome browsers before and been caught. Even when they weren't actively sabotaging other browsers they had big Chrome banners and "Works Better in Chrome" signs for several years in the past. If they did it _then_, it's hard to give them the benefit of the doubt that they just "missed it" now, and easier to assume they do things that are broken in other browsers intentionally.
Edge (Spartan or Legacy) had a big fight with YouTube that YouTube was doing a hidden (not visible to users) DOM animation underneath the video player in a way that Chrome ignored but tanked Edge performance. There were several release cycles where Edge would specifically target that hidden animation performance, only for YouTube to make the animation worse and ratchet it back.
I lived through that as a user. (I was one of the like 5 Edge Spartan users, I know, lol.) I could see the dumb animation in dev tools and manually delete it for better performance. It was a nice thing I had some technical skills. I know for mainstream users the solution was "watch YouTube in Chrome".
It's hard to find to other sources because Microsoft intentionally broke the SEO on the Edge brand and you know Google is the only other major search engine. Not that they'd intentionally down-pagerank bad news about a Google property, I'm sure.
