Installing firefox will cause a lot of dependencies to be installed, .e.g X and a desktop environment or at least a light weight WM.
In fact, 12.04.4 LTS already has firefox 28.0 in its official repository, you don't have to add a PPA to get the latest stable, just do sudo apt-get -y install firefox
and it'll do.
BTW: You are getting the error because it's not the correct line(s) to be added in sources.list or split files under /etc/sources.list.d/*.list
it should be like below:
deb http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu precise main
deb-src http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu precise main
To use add-apt-repository
command, you'll have to install python-software-properties
beforehand.