A year late but whilst i was looking for extensions i found this on the firefox site.
Media Pirate - The video downloader 2.2 by Pascal Beyeler
More and more websites publish their videos in those neat flash players. But if you want to download a video there is no download link. This doesn't mean that you can't download it. Media Pirate creates a download link for you!
Media Pirate helps you to download videos from the following websites:
blip.tv break.com dailymotion.com ifilm.com media.putfile.com metacafe.com myvideo.de video.google.com vids.myspace.com vidilife.com youtube.com zippyvideos.com
To see in a live demo how the extension works, visit my website:
http://www.mediapirate.org/livedemo.php
To convert the video files to other formats go to:
Media Converter -
http://www.mediaconverter.org
This is very useful if you don't have a FLV player.
Media Pirate - The video downloader is the service that 100% works!!
Hint: It opens a new tab with the mediapirate.org website where you can find the download link.