Let's say you have a an internal /24 network and you want to add a secondary (local) IP address (this could be useful in case you remove an old machine from the network and your new firewall is on a different address - but your new firewall should offer on of it's services on the old address).
In the ipfire GUI this is not possible. But you can use the file "rc.local" (which is executed while the firewall boots).