I've been using this add-on for a long time now, in Firefox, Brave and Chrome with not a single problem. I can't use YT without it now, it has so many quality-of-life improvements it's ridiculous.

Just to name a few I love that I can hold down right-click and scroll to change volume in full-screen, hold down ctrl and scroll to speed up/speed down. Not to mention that it's possible to scroll down even in full-screen, if you want to quickly see views, uploader, description, or how many viewers if it's a live stream.

Totally 5/5, with very few permissions to top it off.