SRCDS is now working with pfSense. I ended up deleting all my SRCDS-related NAT port forwardings and firewall rules. I then added:
Port forwards:
27015 TCP/UDP
27011 TCP/UDP
NAT outbound:
27015 (Static port yes)
27011 (Static port yes)
I’m happy that it’s working now. That leaves the only remaining issue as the xfire file transfer. It’s weird because the UPnP service is running. Maybe I have to change the max upload/download rates from their defaults of blank? That’s for another day, though.