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