To install the latest Firefox 3.5  (final) or Firefox 3.6 in Ubuntu, open a terminal and:
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

Or  use the automatic Launchpad PPA script to add the GPG key.

And  finally, install Firefox 3.5 final:

sudo apt-get update && sudo apt-get install firefox-3.5

or Firefox 3.6 pre alpha:

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:

sudo apt-get update && sudo apt-get upgrade