I've closed that bug as the problem as the originally reported problem is fixed by hardware accelerating filters as part of WebRender. Do you have more details on the performance problem that you're seeing?
Looks like WebRender has been enabled by default as of Firefox 92 (September 7, 2021) so that's great news! I think most of my performance problems were due to filters so I'm eager give it a try. I saw your comment on Bugzilla and will do that if I'm still experiencing issues after testing it. Thank you!