Here's a very odd wrinkle which may be causing a few people some grief. I have an HP printer which has no interfacse other than wifi. It does not get automatically detected either by system-config-printer (which is what MCC->Hardware->Printing uses) nor by hp-setup (the hplip printer setup utility), despite the fact that there is no firewall on the machine. Not a big problem, since I happen to know the printer's IP address, which does the trick, but puzzling none the less.
I have now discovered, entirely by accident that both utilities do detect the printer if (a) the firewall is enabled and (b) the ethernet interface is checked off. I.e. the firewall is defined but not applied. Very odd. Probably affects all network printers and/or scanners, not just HP ones.