After today's KDE 4.5.2 update (not sure which component is to blame), mailto links no longer worked in firefox or chromium-browser. I use kmail for mail.
firefox did nothing (not even an error message) when clicking a mailto link. I was able to fix it by installing gnome-control-center and setting the GNOME preferred mail client to kmail. I'm fairly certain that those links used to work just fine before this update, but not completely sure as I use firefox quite rarely these days on this box.
I haven't been able to get mailto links working again in chromium-browser, and I use it a lot more than firefox so this is quite annoying. It displays an error dialog when clicking such a link saying
| Could not start process Unable to create io-slave:
| klauncher said: Error loading 'kmailservice %u'.