Without a doubt my favorite Firefox extension. Available in both Windows and Linux. The Video download helper. It lets you download videos from Youtube and dozens of other websites.
DownloadHelper is a tool for web content extraction. Its purpose is to capture video and image files from many sites.
Just surf the Web as you are used to, when DownloadHelper detects it can do something for you, the icon gets animated and a menu allows you to download files by simply clicking an item
For instance, if you go to a YouTube page, you’ll be able to download the video directly on your file system. It also works with MySpace, Google videos, DailyMotion, Porkolt, iFilm, DreamHost and others.
Go have a nosey, its a simple plug in for Firefox so a quick and easy setup.