It's slightly more complex as there are some hardware/software combinations that don't work. This article gives some good steps to make sure it's enabled:

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).