Download 
-rw-r--r-- 1 toby toby 18078847 Jun 16 17:55 firefox-13.0.1.tar.bz2
to /Downloads

cd /usr/local/bin
sudo rm -rf firefox
sudo rm /usr/bin/firefox
cd /home/toby/Downloads/

GUI - Places -Downloads - firefox-13.0.1.tar.bz2
  archive manager launched
    extract
  extracted to /home/toby/Downloads/firefox


sudo mv /home/toby/Downloads/firefox /usr/local/bin