I used the Kde4 + Firefox3 theme before I upgraded to Firefox 3.5. When I upgraded, I noticed how incredibly ugly Firefox looked because the theme was not compatible with this new version. After a few days, I set out to search for a slightly more compatible theme.
As it turns out, the author of the original theme also has a Firefox 3.5 compatible one called KFirefox. I think it looks a little better than the original, even. Here it is.