NB// If like me you ignore the part about un-installing Iceweasel you'll end up with an apt-get error message of 'half-installed' for FF.
Go back, remove Iceweasel and then :
apt-get install --reinstall firefox-mozilla-buildI also needed to manually edit the sources.list as the cut and past didn't work. You entry should look like :
deb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main