I sometimes have difficulty with YouTube 4K on Firefox on my Mac Studio (M2 Max with 64GB). I don't really care though, because 1440p is always well and truly good enough. (And really 1080p is more than good enough except that the default bitrate is absolute dogshit. Most of the time I'm choosing 1440p is because it's bitrate is not dogshit.)

4k? That might have something to do with it. Though given a big enough box that too shouldn't be an issue. But I'm using a decade+ old thinkpad driving a fairly high res dell and it works well enough.

"Works on My Machine" reply.