I want to know how to adjust audio (treble/bass balancer) for videos playing in Firefox. Your page at https://support.mozilla.org/en-US/kb/control-audio-or-video-playback-your-keyboard#w_what-is-the-media-control-feature refers me to https://specifications.freedesktop.org/mpris-spec/2.2/ but this URL is a 404. Is there another page that might help? Thanks.