Great extension! only there is not enough control for several tabs at the same time and so that the video is captured from all existing sites. there are similar extensions, maybe something can be used based from them: https://addons.mozilla.org/ru/firefox/addon/sloth_music_controller/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search
https://addons.mozilla.org/en-US/firefox/addon/pin-video-controller/
https://addons.mozilla.org/ru/firefox/addon/sound-control/?src=search