If you want to connect to your IPFire-3.x you have to make sure that the network is configured correctly and that the IPFire machine is able to access the internet.
We need to install an additional package:
pakfire install openssh-server
After that we are able to enable it so that it is started right after booting up the IPFire:
systemctl enable sshd
And we need to start the service:
systemctl start sshd
The first start may take some time because the ssh keys are generated at first start.
If you want to log in to IPFire from another machine you have to make sure that the password of the root user is set. therefor you just have to create a password on the IPFire machine like this:
passwd
Enter the Password twice and reboot IPFire. Now you should be able to access the SSH from another machine.
All of the above steps can now be done via a ssh connection. It is more comfortable than working in a kvm or virtualbox window.