Using Media Converter Addon for Mozilla Firefox, you can download and convert online videos directly from your Firefox browser without the need for special software.
This is a video downloader and video converter in one simple Firefox browser extension. This addon will help you to download your favourite web videos and provide it in the necessary file format for your mobile device.
After installing this addon on your browser, a new icon will be added to your Firefox toolbar. It is located right next to the address bar, just before the Google search box (see the screenshot). As soon as you visit a website such as YouTube.com, where Media Converter is able to download the video, the icon will become blue, which is otherwise greyed out. Just click on the icon and the conversion wizard will open in a small popup. The steps are quite self-explanatory and it is definitely not a big deal to download your favourite stuff.
In order to use the tool, make sure that you use the latest compatible and safe browser – Download Firefox
The free program supports download of videos from websites such as: youtube.com, blip.tv, break.com, dailymotion.com, livevideo.com, metacafe.com, myvideo.de and veoh.com
Using this Firefox plugin, you can download the videos from the above websites into various formats of your choice such as 3g2, 3gp, asf, avi, flv, m4v, mkv, mov, mp3, mp4, mpeg, ogg, ogg, rm, wav, wma and wmv.
For YouTube, it works only for the direct watch link such as http://www.youtube.com/watch?v=. For playlists, the icon will not be shown blue.
There are many other websites supported but then you have to sign up to be able to download and convert videos from them. [source]