While Firefox is a GTK+ based application, it’s high customizability together with the new possibilities of CSS 3 gradients and shadows gives us the opportunity to recreate KDE’s default theme “Oxygen” pixel perfect (or at least near pixel perfect) using these styling methods.
Here is an example page showing the capabilities of this approach.
In this way, by avoiding the use of images, a pure css clone of this theme can be easily generated out of the theme configuration files. (While “easily” means “easily from that moment on, when we have a finished copy of the theme using the default color scheme”)
Supporting KDE should be the second highest priority after ambiance/radiance, because KDE is considered one of the two “big” DEs alongside Gnome, which can be interpreted as “It’s standard theme is on most linux desktops alongside GNOME’s”.