(In reply to Darkspirit from comment #52)
I assume this bug will be fixed once video decoding has been moved to a utility process (bug 1722051).
Until then, MOZ_DISABLE_RDD_SANDBOX=1 environment variable is required to try out experimental VAAPI.
It doesn't work reliably regardless, as also some other reports here indicate: intel_gpu_top shows that hardware decoding initially is used for a brief moment, but then Firefox falls back to CPU decoding with intel_gpu_top reporting 0% hardware video decoding load. For whatever weird reason, it very rarely can continue to show hardware decoding load, but then breaks upon next Firefox launch too. VAAPI is completely unusable since Firefox 99 with at least Intel, also with MOZ_DISABLE_RDD_SANDBOX=1. Reports that claim it to be working without making sure it really does so via intel_gpu_top over an extended period of usage (i.e. days) are unfortunately not reliable.
I think I'll just go back to 98 and perhaps set up some 3rd party sandboxing in case there are open CVEs. Hardware decoding is not a bonus on slow CPUs, it's a must. Really annoying that, despite of still being experimental, it has been broken so hard.