Add the following lines to /etc/apt/sources.list (skip this if these lines are already there

Add the following lines to /etc/apt/sources.list (skip this if these lines are already there)

deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu jaunty main
deb-src http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu jaunty main

and now run the following apt-get command

sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 247510BE
sudo apt-get update && sudo apt-get install firefox-3.5
this will install firefox 3.5 in your ubuntu machine. enjoy and have fun!