UnPlug Firefox Extension

From the site:

UnPlug is an extension which lets you save video and audio which is embedded on a webpage - it's a video download tool.

UnPlug scans web pages and tells you where media players are getting their data from, and displays it as a simple hyperlink: in most cases, you can then follow the simple download link to save the media file.

UnPlug works for lots of sites including youtube.com, ifilm.com, current.tv, revver.com, peekvid.com, video.google.com, mobuzztv.com and lots, lots more.

Unlike many other extensions, UnPlug scans the page, and if it doesn't know there the data comes from it can guess - correctly most of the time.