I just wanted to follow up after having had some time to test and tinker.
@johnpoz : Thanks for your help and patience! Your insight was invaluable.
RECAP : IssueMy original issue was identified by the supposed failure of pings to traverse through pfSense between two devices on different networks (ex. 192.168.11.xx and 192.168.12.xx).
RECAP : Issue No.1 : Windows Firewall BehaviorImportant issue no.1 didn't have anything to do with pfSense or, for that matter, with the network in general. Windows firewall blocks ICMP Echo requests and this behavior seems to continue even with the firewall turned off in the Control Panel.
The weird part with this issue is that both pfSense AND the managed switch could ping both computers. The issue was revealed when the computers could not ping each other (pings timed out).
The simplest way to fix this behavior is to add an Allow Rule to Windows Firewall for ICMP behavior. Just... make sure to turn it off before using those test machines elsewhere.
RECAP : Issue No.2 : pfSense DHCPImportant issue no.2 had to do with weird behavior from the DHCP service on my pfSense machine. I cannot say if this is the result of a bug. I would have to do further testing (which I may follow up on later).
This was described by johnpoz as : "if your device... doesn't have a gateway, then you would never be able to talk to it from some other network."
Or even simpler : No door (gateway), no exit.
This issue was revealed by the ping attempt on one of the computers throwing a "General Failure" error when trying to ping the other computer. Investigation of ipconfig results confirmed the issue (missing network gateway).
The proposed solution that fixed the issue was simply to enter a value in the DHCP configuration screen : Other DHCP Options/Gateway. Adding a value here propagated to the two testing machines.
The value I used was the IP address of the associated firewall interface (... the default value...).
FinThat's it.
Pings between the two computers works as expected, even when they are in different networks. The ping works in both directions.
Thanks again!