admin@smartproteam$ sudo gedit /etc/apt/sources.list

add below lines to /etc/apt/sources/list depends on Ubuntu version

Ubuntu Hardy :

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

Ubuntu Jaunty :

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 then run below command

admin@smartproteam$ sudo apt-get update && install firefox-3.5