To install the latest Firefox 3.5 (final) or Firefox 3.6 in Ubuntu, open a terminal and type:
sudo sh -c “echo ‘deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu jaunty main’ >> /etc/apt/sources.list”
sudo sh -c "echo 'deb-src http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu jaunty main' >> /etc/apt/sources.list"
Then add the Launchpad PPA GPG key:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE
And finally, install Firefox 3.5 or 3.6
sudo apt-get update && sudo apt-get install firefox-3.5
or
sudo apt-get update && sudo apt-get install firefox-3.6
If you already have a version of Firefox 3.5 installed from a repo then upgrade it:
sudo apt-get update && sudo apt-get upgrade
courtesy: http://webupd8.blogspot.com/2009/06/firefox-35-rc-1-ubuntu-repository-deb.html