The challenge here is that Gecko is the heavyweight bit here, not Firefox. If you wanted to shrink the size of Firefox dramatically, you could not do so without removing Web functionality (as opposed to Firefox functionality.)

I'd love to see someone give that a shot. It was really three or four people who "Firefoxed" the old Mozilla Suite and there's nothing stopping some similarly motivated group from giving it a go.

Still, I don't think they'd get nearly as far as we did back in 2002 and 2003, because Firefox is actually very trim and fit. There aren't a lot of extra features to cut and the code implementing most features is smart enough not to waste resources when not in use.