Used to be a no-brainer add-on, but currently is a huge performance hog that will constantly freeze youtube tabs, so there's a trade-off to make between the great features it offers and having the speed of using youtube in Firefox roughly cut in half.

Update: Turns out the slowness was/is actually a bug with Firefox 86 and later on Windows 8.1 and only affects exactly one computer of mine. It's still a no-brainer add-on.