Firefox using far too much resources in Amber / plasma

15v4 amber plasma, true with firefox-esr, firefox (76), firefox 76 flatpak, safe mode / not safe mode…

I see FF using > 100% of CPU on at least two cores when playing video in the browser. The frame rate stutters, the fans are going full force. I know that the VA-API video acceleration isn’t working yet, at least in my setup, but with a 15v4 it shouldn’t be that bad even doing the decoding via CPU.

Is acceleration working in Wayland session + Wayland Firefox? Does gnome (amber) do this by default yet?

Any ideas as to why resource usage is so high? Mozilla’s troubleshooting isn’t super helpful. I know I can use mpv or vlc, but I’d prefer to get the browser working correctly. I think it’s laggy even not doing video. I’d also like to avoid switching back to Gnome, too many simplifying design choices I don’t agree with.