After installing the Murus front end for the built-in pf firewall in macOS, almost all my network services were unavailable. Print sharing, file sharing and other services simply didn’t work. The firewall has the mDNS port (UDP/5353) open by default, but that didn’t seem to matter. Disabling the firewall helped, so clearly there was a configuration problem somewhere.
After a little reading, I found the problem: mDNS uses a multicast address (18.104.22.168 specifically) for advertisements. These are not enabled in Murus by default. Adding a new group to handle the addresses fixed the problem!