Really, you're out of luck due to the fact that Firefox is a GTK based application, while all the native KDE applications are based on
One way to get around this is to install a GTK theme that blends with QT as much as possible
gtk2-engines-gtk-qt - theme engine using Qt for GTK+ 2.x
thats the package on debian based distributions, allows you to set GTK styles from within the KDE control center, its not perfect.
here is what i've been able to do with a firefox theme that emulates crystal...
however, I would suggest just using Konqueror. It is a great browser/file manager.