1.modify apt-get repository
add two lines as below
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
2.import PPA GPG key
#sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE
3.run command to install firefox35 or firefox36
sudo apt-get update && sudo apt-get install firefox-3.5