I believe I had to pay something for this one, but you might want to take a look at
http://dhtml-menu.com/ which I found to be an easy-to-use, very, very powerful (if you are DHTML/JS familiar with hand coding to tweak it) and cool little menu that did not annoy me by hiding behind images or even form fields (a common killer of older DHTML/JS I built/bought in the first half of this decade).
Whatever the price I paid might have been, for the client, who was fantastically excited by how sleek this was, it was a no-brainer. And for me, I think I spent about two days making what would have been a suicide call if I'd tried to do it on my own. I used the "tree" if I recall, but messed with the others like the dropdown banners and such just because it was a blast.
I checked the site out in Firefox just now to be sure it was compatible, and sure enough, check out examples like this one,
http://dhtml-menu.com/tree-examples/tree-menuxp.html
and they work similarly in FF or IE. Not a lot of third-party tools or snippets I have used or been satisfied with, all in all, but this is one of the precious few I'd use again.