First, download firefox. Then:
$ # Untar the download
$ tar xjf firefox-<version-num>.tar.bz2
$ # Remove current firefox
$ sudo rm -r /opt/firefox
$ # Move new firefox to old firefox location
$ sudo mv ~/Downloads/firefox /opt/firefox
$ # Make the symbolic link between the app and your PATH bin folder
$ sudo ln -s /opt/firefox/firefox /usr/bin/firefox