Before Firefox v115 I had to go into about:config and enable vaapi. For some reason it was disabled by default even on Intel graphics PCs. With v115 that step i

Before Firefox v115 I had to go into about:config and enable vaapi. For some reason it was disabled by default even on Intel graphics PCs. With v115 that step is not needed. One can confirm with intel_gpu_top and check the video decode bar.