Well I upgraded the KDE installation in my Sabayon system to KDE4 (by default, Sabayon installs KDE 3.5.10) and after logging in, and opening Firefox, the first thing I noticed was the absolutely FUGLY, and yes I MEAN FUGLY look of Firefox. Don’t believe me? Have a look
Thankfully, Firefox’s support for theme and the immense community out there meant I was able to get a fantastic theme, which makes Firefox look like a native KDE4 application (well, almost!).
You can download KDE4 + Firefox 3 theme from here, though do note that it is an experimental add-on and hence you will need a Mozilla account. Its well worth registering though, have a look at Firefox after applying the theme.