Absolutely essential for watching YouTube nowadays, when ads are spread randomly even in the shortest videos! It's tiny (under 100 kB) and updated frequently, keeping YT always clean. Note: I've noticed if fails some times, and an ad is played, but it's obviously a Firefox's memory fault, as it starts chocking and crumbling down after playing too many videos (or running through too many badly coded Pages), what can only be fixed by restarting the browser.