Create a VLAN on the switch with two ports: WAN and pfSense (WAN) (both untagged).
And even better…add an additional (and different) VLAN for your internal network with as many ports as required and available: pfSense (LAN) and LANx (all untagged).
To get to know VLAN a bit more:
http://forum.mikrotik.com/t/using-routeros-to-vlan-your-network/126489/1