I think Firefox is included with the latest testing version of Debian GNU/Linux...
if you aren't using Debian, you can download the official binary or source code at
http://www.getfirefox.com
here is the direct download link of the latest version for GNU/Linux:
http://ftp.mozilla.org/pub/mozilla.o...ux-i686.tar.gz
after installing it, create a symlink or launcher on the desktop to <install_dir>/firefox and double click this icon you'll start the firefox.
Anyways I've seen no reason to use Firefox if you already have Mozilla, as Mozilla contains all the functions in Firefox.