I upgraded to the latest version of Firefox in Fedora using yum and it stopped working with the error Firefox doesn't know how to communicate with the server.
I started firefox with firefox -profilemanager and created a new profile.
I then copied cp ~/.mozilla/firefox/profilename/compreg.dat ~/.mozilla/firefox/default.rtt/ where profilename is the name of the new profile. Thats it.

updating nss was the only method that worked in my case
If you are using YUM,
# yum update nss (if you don't have NSS installed at all #yum install NSS)
# yum remove firefox
# yum install firefox
If you are not using YUM,
Download latest rpm package,
# rpm -Uvh nss*
# rpm -ivh --force firefox-1.5.0.6-2.fc5.i386.rpm
best,
s
-Gabe
- run "firefox -profilemanager"
- delete the profile and create another one
That's it!