Some time ago I released a simple Firefox extension, Open With Konqueror. In the time since I released it, a new version of KDE and the Crystal icon set have been released, as well as a slew of new “Open With X”-type extensions. Open With Konqueror is simply obsolete.
I recommend you install Open With, a generic extension capable of opening the current page or selected link with any application. KDE users can simply open the extension preferences, select the “Manual Entries” tab, select “Add”, and enter “/usr/bin/konqueror”.