There are plenty other video downloaders out there but none of them is able to directly convert the video.
This is a video downloader and video converter in one simple extension. So 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 a new icon will be added to your Firefox toolbar. It is located right next to the address bar and the google search box (see the screenshots).
As soon as you visit a website where Media Converter is able to download the video the icon will become blue otherwise it is 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.
Here is a small list of supported websites:
There are many other websites supported but then you have to sign up to be able to download and convert videos from them.
For further details please visit http://www.mediaconverter.org