Hey I am an avid use of Mozilla Firefox..cant use IE anymore..and i am totally a fan of the addons in Mozilla..
I think they are quite innovatively created..greasemonkey for example..and mouse gestures is also a good instance of this fact..
by grease monkey u can actually download videos from youtube..check out this link for the same:
you will find it very useful..
ppl are welcome to tell about other useful scripts related to greasemonkey :)