first you need to uninstall existing firefox !

impiza@impiza-desktop ~ $ sudo autoremove firefox                          

then type this command in a terminal to add repository to system

impiza@impiza-desktop ~ $  sudo add-apt-repository ppa:ubuntu-mozilla-daily/firefox-aurora        

then update your system

impiza@impiza-desktop ~ $ sudo apt-get update                     

then install firefox

this will install firefox Developer Edition (Aurora)

impiza@impiza-desktop ~ $ sudo apt-get install firefox