I hadn't noticed. Why has it gone, do you know?
Firefox
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officially the developer mentioned that the changes the youtube devs broke it completely in firefox and slightly broke it in chrome so he stopped it temporarily.
I hope @MaximeRF find the time/motivation to fix it. I just was reminded it'd gone when I went to use the boost volume feature.
I hope ad-blocking isn't the blocker, because uBlock Origin handles that (although I suppose uBO doesn't cover things like channel whitelisting).
but you can also get youtube channel whitelist for ublock origin to do that! https://addons.mozilla.org/en-US/firefox/addon/youtube-channel-whitelist/
I thought you were talking about the volume boost thing at first and was really surprised.
FYI: I keep using it, it kind-of-works for me if I open the tab in the background and let it load (< 1min) before focusing on it. It also works if I'm not logged in (e.g.: in incognito mode).
Better look at Freetube:
https://addons.mozilla.org/de/firefox/addon/freetube-redirect/
i have libredirect for somethings but i like youtube because i have certain creators i like and i like commenting and participating with them.
would recommend https://addons.mozilla.org/en-US/firefox/addon/youtube-addon/, idk if 100% of the features you mentioned are on there but worth a try