To make firefox managed by update-alternatives:
sudo update-alternatives --install /usr/bin/firefox firefox /usr/bin/firefox-2 50 sudo update-alternatives --install /usr/bin/firefox firefox /usr/bin/firefox-3 100
To make firefox managed by update- alternatives:
sudo update-alternatives --install /usr/bin/firefox firefox /usr/bin/firefox-2 50
sudo update-alternatives --install /usr/bin/firefox firefox /usr/bin/firefox-3 100