When did they start supporting VA-API? I didn't notice. Lacking that was one of the reasons I got a new computer.
https://mastransky.wordpress.com/2020/06/03/firefox-on-fedor...
I've also had to disable media.ffvpx.enabled in previous versions to enable VA-API for more than just VP9 (I haven't yet been able to verify if it's needed for this one).