Excellent workaround for Google's stranglehold on YouTube video downloading. Free version allows for decent to mediocre downloads, with the possibility of higher quality downloads provided a pro user has downloaded the video recently. Pro version (price dictated by personal honesty) allows for downloads of excellent quality at exceptional speeds. This is a MUST HAVE app if you want to download YouTube videos without paying ridiculous subscription rates and/or don't want YouTube (aka, Google) watching your every move. The best part? With this add-on, you don't even need to log in to YouTube in order to download.
FOR MOBILE USERS: to use the add-on in the mobile Firefox browser, you need to request YouTube's (NOT FIREFOX'S) desktop site. It's located amongst the options found above the video window