https://addons.mozilla.org/firefox/2390/
Download videos from Youtube, Google, Metacafe, iFilm, Dailymotion... and other 60+ video sites ! And all embedded objects on a webpage (movies, mp3s, flash, quicktime, etc) ! Directly !
VideoDownloader add a small icon on the status bar at the bottom of your firefox window, and a toolbar button. Just click that and download the video you are watching !