Some testing later, and I'm now certain that it's specifically Youtube (and *only* Youtube) that slows down Firefox to a crawl.
@joepie91 yeah I have been pretty certain of this for a while. Performance has just been consistently worsening for years and to the extent that things even have workarounds or solutions they involve features that have no reason to be performance intensive at all, like theater mode dimming the page. I haven't had access to my PC for a month now, but then it had gotten to the point where it would regularly take 6-7 seconds to register when I pressed play/pause or the skip keys. I haven't heard anyone else say that, so maybe it was an extension interaction, but I don't know. And the idle CPU use was also very high.
@someonetellmetosleep The "takes several seconds to register play/pause" is something I consistently experience when I've had a bunch of Youtube tabs open for a while, even if none of them have actually played a video yet.
