I can confirm that this doesn't work out of the box.
When running the firefox snap with MOZ_LOG="PlatformDecoderModule:5", I can see that libva tries to load drivers from $SNAP/gnome-platform/usr/lib/x86_64-linux-gnu/dri/, where it can't find any such driver. This is bug #1947180, for which I've proposed a fix in snapcraft.
Once that is fixed, the next step is to ship said VA-API drivers, either in the firefox snap itself, or in the platform snap (gnome-3-38-2004).
I can confirm that this doesn't work out of the box.
When running the firefox snap with MOZ_LOG= "PlatformDecode rModule: 5", I can see that libva tries to load drivers from $SNAP/gnome- platform/ usr/lib/ x86_64- linux-gnu/ dri/, where it can't find any such driver. This is bug #1947180, for which I've proposed a fix in snapcraft.
Once that is fixed, the next step is to ship said VA-API drivers, either in the firefox snap itself, or in the platform snap (gnome-3-38-2004).