This script works great - for the newbies out there, you just have to copy and paste the listing above into a file (for total newbies, you could try K Menu->run command... "kedit /usr/local/firefox/firefox-agent.sh")
Paste the listing, check for obvious problems (when I pasted it, I ended up with some garbage characters at the end, presumably from the formatting on the page), save it.
Then drop to a terminal and chmod a+x /usr/local/firefox/firefox-agent.sh
Then K Menu -> Preferences -> Preferred Applications -> Web Browser
It should work! (at least, it does for me)