I thought you might be interested in something I did (or you might want to improve it further). As iAqua currently doesn’t draw the little curved indentations on the buttons or nestle into a top curvature at the end of the thumb, I tinkered around until I figured out how to do that:

http://hostilefork.com/2007/11/06/workaround-firefox-scrollbar-bug-on-mac/

Someone handier with bitmap editing than I am might be able to get the pixels all perfect, but I just wanted to do “about as good a job as MacFox II does” (it handles the case where the scroll buttons are at the top and the bottom, and I used those graphics for that case.)

Any feedback you can offer would be helpful. Thanks!