I you are using ubuntu 9.10 karmic/ 10.04 Lucid.
just run these commands on your terminal:
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get install firefox-3.6
Now for those who are using ubuntu 9.04/8.10/8.04.
You will need to edit your source list and add the appropriate ppa.
Open sources.list
sudo gedit /etc/apt/sources.list
and add the below lines to it.
PPA for 9.04 (Jaunty): deb http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu jaunty maindeb-src http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu jaunty mainPPA for 8.10 (Intrepid): deb http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu intrepid maindeb-src http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu intrepid mainPPA for 8.04 (Hardy): deb http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu hardy maindeb-src http://ppa.launchpad.net/mozillateam/firefox-stable/ubuntu hardy main
save and exit
Now get the updates
sudo apt-get update
and install firefox 3.6
sudo apt-get install firefox-3.6