Firefox 3.6 can easily be installed in Karmic from the Mozilla Daily Builds PPA. Just setup the PPA as described here and run
sudo aptitude install firefox-3.6
Then to start the new browser: Alt-F2, and enter "firefox-3.6". All your settings from the regular Firefox will be copied and unchanged in the new installation.
At the time of this writing (1/13/2010), Firefox 3.6 is just a release candidate (RC1). The final version is due to be released this month. The Register has the best review I found on the new features.
The list of Firefox versions available per Ubuntu release is available on the UbuntuUpdates firefox search page. As a reminder, the officially supported version of Firefox in Karmic will stay at 3.5. You install other versions at your own risks. Firefox-3.6 is branded as "Namoroka" in Ubuntu 9.10.
