Comment 18 for bug 67586

This isn't a Kubuntu docs issue either I don't believe, but this should fix your problem:

At the command line (terminal, console) type:

sudo update-alternatives --config firefox-homepage

enter your password

Select the option that looks like:

/usr/share/ubuntu-artwork/home/firefox-index.html

In my case it is option 1, so I would type 1 and press enter. If it is option 2 for you, then select 2 and press enter. You want the firefox-index.html selected. I think this is due to borkage from upgrading from Breezy to Dapper to Edgy. Possibly locations and/or filenames changed during these times and didn't upgraded correctly.