May be, I was somewhat late in noticing this Firefox plugin but like me there would be many others who might have missed this one.Hence,thought of updating this on my blog 
Many people might have wanted to grab the videos on YouTube(or some other site where live streaming of video occurs) and Unplug is the solution to all your problems 
What is UnPlug?
UnPlug is an extension which lets you save video and audio which is embedded on a webpage – it’s a video download tool
What can UnPlug do?
UnPlug works for lots of sites including Youtube, IFilm, current.tv, MySpace(video) and 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.
My Experience with UnPlug:
After installing UnPlug for FireFox 2.0,I tried unplugging my favourite movie “Guru” and as per my expectation,it was excellent.It saved the file in a .flv format and you can download flv player from here
I saw the feedback for UnPlug and found that the user might encounter some problems with the Firefox versions which are older than 2.0
[My Experience with UnPlug]
You can download this excellent extension from here.For playing Unplugged files,you can download the flv player from here