Frequently, I notice on Youtube the UI just freezes entirely for ~1-5 seconds, usually periodically. I've measured it most recently around every 10 seconds or so (manual stopwatch in another tab, so a little faster or slower possible). When in this state, I can often notice it frequently: on hovering over buttons the UI does not highlight them, the video controls will not appear, or the page will not fill the screen on resize, can't click into search, etc. Basically just full page freeze except for media playback. It's disappeared and come back a few times, but tends to be more often there than not. This occurs on every page of Youtube. I've tried disabling basically all my extensions and refreshing the page without any change. I've also captured a performance profile while I was moving around between buttons and observing the issue, as I assume it might help diagnose: https://share.firefox.dev/3Xqz7co