Firefox
Firefox and KDE
KDE integration
To bring the KDE look to GTK apps (including Firefox), install breeze-gtk and kde-gtk-config. Afterwards, go to System Settings -> Application Style -> GTK. Be sure to choose 'Breeze' in 'Select a GTK2/GTK3 Theme' and check 'Show icons in GTK buttons' and 'Show icons in GTK'.
To use KDE's KPart technology with Firefox, by embedding different KDE file viewers into the browser, you can install kpartsplugin.
For integration with KDE’s mime type system and file dialogs, one can use firefox-kde-opensuseAUR variant from AUR with OpenSUSE’s patches applied. This then is a "standalone web browser from mozilla.org with OpenSUSE patch, integrate better with KDE".
Add-ons may provide some integration, such as KWallet integration and Plasma notifications (i.e. extension "PlasmaNotify" of 2011).