Firestarter vs gufw after 2018?

It seems that firestarter is obsolete in Ubuntu.

It was shown in pkgs.org listings - Debian 7 (Wheezy) and Ubuntu 12.04 LTS (EOL now), currently repology does not show any Debian-based distros.

Download link on official FireStarter site does not open.
But you can try to build (from this source) on 16.04 LTS ESM or 18.04 LTS:

sudo apt-get install build-essential bzr libglade2-dev libgnomeui-dev checkinstall
bzr branch lp:ubuntu/trusty/firestarter
cd firestarter
./configure
make
sudo checkinstall make install # change version to 1.0.3, release to 1, name to firestarter
sudo dpkg -i firestarter_1.0.3-1_amd64.deb
sudo firestarter

Self-compiled firestarter launches but writes many errors and warning to the terminal.

I think there was a reason of removing it from official repositories, so you should use gufw.