I sure you have but have you tried checking the out put of vainfo Arch wiki since Firefox only supports Hardware acceleration with VA-API.

Firefox Hardware video acceleration

If hardware video acceleration is blocked with error code FEATURE_HARDWARE_VIDEO_DECODING_DISABLE or FEATURE_FAILURE_VIDEO_DECODING_TEST_FAILED in about:support, you can override it with media.hardware-video-decoding.force-enabled=true. See [10] for more information. Alternatively, you can install firefox-vaapiAUR.

Also make sure that in about:support Compositing in the Features table under Graphics is set to WebRender if it is not than set gfx.webrender.all in about:config to true.

I had the same issue but I have a NVIDIA gpu. but I just had to install libva-nvidia-driver and edit Kernel parameters and Environment Variables . I got this to work with both Firefox and Librewolf but only some video codecs.

note i have 1080ti so i don’t have hardware acceleration for av1. also the only time a problem when I turn off hardware decdoing on firefox is when I watch a HEVC stream on twitch for some reason i get drop frames every second.

Firefox V137.0 Linux

Librewolf V136.0.4 (HEVC decoding is only add support on Firefox 137.0 on linux)