1-Click YouTube Download is a lite, easy to use, open source Firefox add-on that allows you to download videos directly from YouTube website with a single click and save it to your local computer.
The add-on adds small links in YouTube video webpage to download videos and music in 3GP, MP4 HD (High Definition), MP4 High-Quality(iPod compatible) and FLV. This is one of the lightest and simple tools to download videos directly from YouTube, while browsing with FireFox.
Using 1-Click YouTube Download is very simple and intuitive. Once the add-on is installed, downloading videos is only a click away.
If you like to download a video from YouTube, all you have to do is to visit the video page and click on the MP4/FLV/3GP download links located at the bottom of the video.
After downloading the video, you can then watch it offline or transfer it to Apple’s iPod and to most modern mobile phones. To watch videos on your computer you’ll need a video player capable for playing MP4,3GP or FLV files. [source]