Firefox 80 introduces a highly anticipated feature for Linux users, namely VA-API/FFmpeg hardware acceleration for video playback on systems using the traditional X11/X.Org Server display server. But the GPU acceleration feature is disabled by default, so you would have to open the about:config page and enable the media.ffmpeg.vaapi-drm-display.enabled and media.ffmpeg.vaapi.enabled options. With the upcoming Firefox 81 release, it appears that Mozilla has enabled the GPU acceleration by default.