The best one I have found, after tryin all the Mozilla Firefox Plugins which didnt work in my machine ---

You simply download this small program: VideoCacheView :

http://videocacheview.en.softonic.com/

Then go to your YouTube clip (preferably using Mozilla) and let it to stream to the end.

Then you simply double click on the "VideoCacheView" executable and you right click and save the clip on preferred hardisk location.

w/r
Stephen N.