Just tried Firefox 4 (Minefield) on my Kubuntu box and I must say, it is THE fastest browser I have ever used. Plus the interface is quite impressive as well.
The integration with KDE was easy. Just download this theme (it is compatible with FF 4.0).
You might also want to use a KDE-ish file picker. To do that, type about:config in your address bar, and search for file_picker. Set ui.allow_platform_file_picker to false.
Viola! You’re done.