Incidentally, Firefox on my M1 MacBook Air suddenly got very slow. Scrolling got super laggy. I've tried refreshing it and disabling all extensions, nothing helps. It's on the latest version and the rest of the system is very smooth as always.
It's even laggy after I completely wiped it from my system and used a fresh Firefox installation.
Late update: I feel extremely stupid. I'm using an app called "App Tamer"[0] and had it configured to run Firefox on the efficiency cores only, even when it's in the foreground. Maybe I thought originally that the e-cores would be more than fast enough to handle a browser, but I guess that's not the case. This also explains why I might not have noticed this issue before: I only use Firefox for work-related tasks and I rarely work while on battery.
Changing that option doesn't change anything in my subjective feeling. It's not only scrolling that is laggy, I think. Also I didn't change any settings between a week ago and now, still suddenly it got laggy. It works fine on my iMac (albeit an older Intel iMac) with the exact configuration. (I use Firefox as my work browser and since I usually work at the iMac, it's been a week or two since I've last used Firefox on my M1 MBA.)
It's even laggy after I completely wiped it from my system and used a fresh Firefox installation.
Any ideas?