This add-on is a bit different form the rest of add-ons, because you won't find any new toolbar or button whihle you surf the Internet, but when you access any Youtube video, you'll seea "Download as MP4" link below the 'Embed' option at every YouTube video page.
By that way, you can download videos directly from YouTube in high quality MP4 format. Just click that link and the video will be downloaded to the folder you want.
The downloaded video will be formatted as mp4, so you'll enjoy high quality and good compression, ready to play on any portable media player: mobile phone, iPod, PSP...
If you are a Firefox user and a fan of Youtube videos, Download Youtube As MP4 is a must-have.