On Intel, in some cases VA-API might not work with the Intel iHD driver intel-media-driver. This might be workaroundable by using the Intel i965 driver libva-intel-driver. This workaround does not work anymore with Intel Iris Xe graphics, which are only supported by intel-media-driver, only solution there is to wait until Firefox implements a GPU process for X11/Wayland (planned FF94) [14][15] [16].
From the wiki page you linked, it seems hardware acceleration for Firefox with Iris Xe graphics is dependent on waiting for Mozilla to fully implement it.
One way to get around this issue for the time being is to use an external video player, which is helpful for things like YouTube. I can confirm that hardware video acceleration is working with VLC, and also with MPV after some configuration (it’s not enabled by default on mpv), using the open source graphics drivers.
In VLC (and maybe mpv, haven’t tested) you can drag a YouTube URL into a VLC window and it will start streaming it. There’s also a menu option to open a video stream URL and have it play that way instead of click and drag.
There is also software you can use to download full YouTube videos first and then play them off your storage, such as youtube-dl - ArchWiki which is a command line interface. There are graphical layers that are separate that you can use if you don’t want to use the CLI (the wiki page has the links).
I know it’s not exactly what you’re asking for, but it’s what we have for now until Mozilla gets to fully enabling hardware acceleration on Firefox.