I'm not often interested in downloading YouTube videos, but when I am, this add-on is invaluable. An online search turns up many results that purport to tell you how to download such videos, but they are out of date and don't work now. This one does, thanks to the ongoing maintenance of the add-on by the developer. He puts in the time and effort to stay ahead of YouTube's attempts to thwart downloading, even for benign purposes.