Rated 5 out of 5 stars

Please update in consideration of Youtube modifications !
NetVideoHunter works very well everywhere except Youtube since Google decided (one of their regular crisis) to modify Youtube code (mama mia, I'm starting to move off towards Dailymotion by the way).
Otherwise, the best on-line video downloader for Firefox, IMO.

This review is for a previous version of the add-on (0.4.3).  This user has other reviews of this add-on.