Firefox

Hardware Accelerated Video

Install Intel driver and optionally Intel GPU tools to watch GPU activity.

pacman -S intel-media-driver intel-gpu-tools
sudo intel_gpu_top

Enable hardware accelerated video in Firefox in about:config

media.ffmpeg.vaapi.enabled = true

I did not have to do this. It just worked out of the box with the settings above.

These are settings in Firefox about:config.

Disable software decoders (they supposedly overrule hardware accelerated video)

media.ffvpx.enabled = false

Needed due to Firefox trying to put some video decoding in a remote video process. This currently breaks accelerated VP8/VP9 video decoding:

media.rdd-vpx.enabled to false

Enable hardware decoding for WebRTC

media.navigator.mediadatadecoder_vpx_enabled to true