How to install Firefox 3.1b2 in Ubuntu (from repo)
Go to System > Administration > Software Sources, to the Third-Party Software tab and add this repository:
deb http://ppa.launchpad.net/fta/ubuntu intrepid main
click on Close, open a terminal and type: sudo apt-get update
Then, go to System > Administration > Synaptic Package Manager and search for Firefox, select version 3.1 to be installed (it will also install additional files). After install is complete, open your file browser (because Firefox 3.1 does not create a shortcut) and navigate to /usr/lib/firefox-3.1b2. To open FIrefox 3.1, double click firefox-3.1.
To make a shortcut, simply drag firefox-3.1 to one of your panels.