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:

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