A absolutely must have extension. Unfortunately it is not updated and cause malfunction on some pages like this one https://www.ellinikahoaxes.gr/ (thumbnails of videos do not load) IMO that should be a build in firefox feature not an extension. That alone, that this extension is no longer updated, is reason enough for me to consider switching to chrome.
Must install for students and anyone who watches video essays, tutorials, animatic story times o&r any content where someone is talking. Side effect: may skew your perception of time. 11/10, Should be part of vanilla YouTube/Browsers.