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.

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.