Just quickly tested youtube playback on two different machines:
Kabylake (9th gen): VA-API is working and 1440p60 is playing smoothly with hardware acceleration.
Tigerlake (12th gen): VA-API seems to initialize just fine with the iHD_drv_video driver but no hardware acceleration. This laptop does also have an RTX3050 with the nvidia driver loaded but that isn't used for Firefox.
BZ#2094953 VA-API does not work even though it is supposed to according to update description
On my Skylake system you might see a blip in the Video engine in intel_gpu_top when the youtube video page is loading but for for video decoding VAAPI is still non-functional unless I disable RDD sandbox.