ALLYouTubeDownloader extension (FLV/MP4/WebM; HD/HQ). Windows only. Works only with ALL YouTube Downloader 2.3 application or higher (versions 2.2 and lower doesn’t work anymore).
Here are the details from the developer:
Just install ‘ALL YouTube Downloader’ and immediately two buttons, both entitled “ALLYouTubeDownloader”, will appear in your browser on Youtube.com. Click on either of them, and the program will ask which format to use to write the selected film on disk.
Want to pick up this add-on? You can find it at the Firefox Add-ons website.