Ultimate YouTube Downloader is a little Firefox add-on to download YouTube videos as FLV, MP4 and 3GP.
The best side of this add-on is its small placement below the video. It does not take larger area. Install this add-on from here. You need to check the Let me install this experimental add-on option.
After installing the add-on, restart the Firefox and you will notice the “Download” option below the video. You don’t need to use any extra tool, simply select the format from the drop-down menu and click on the “here” link.
:)
Link : Ultimate YouTube Downloader
Also Try : Youtubemp – Another Firefox Add-on to download Videos in MP4 Format
Thank you , but I suggest you to take a look at this site : http://www.youtubeto3gp.com/ it will help to download youtube videos in both FLV and 3GP formats.