Hello Werner @whole,
I have pihole and IPFire working together. I'm not sure I understand the issues you see (beside it doesn't work). But connecting them together works fine for me.
On the IPFire Community there are others that have pihole working also. (FYI - the IPFire Community is a Discourse system just like this one).
To get things working I recommend turning off some of the services temporarily. Turn off IPS for now. Depending on your firewall rules you may want to turn off those also. Once you get everything working you can turn services back on one at a time.
Set the IPFire DNS (menu Network > Domain Name System) to your DNS of choice. I use Quad9 (9.9.9.9) also.
Set the PiHole DNS (menu Settings > DNS) to Quad9. Once everything works you can point it (upstream it) to the IPFire box.
Up until today that was my settings and all is working well!
Hope this helps!