The card will autonegotiate with the other endpoint. In case this is a hub it will go half-duplex. If you are running this at a switch you should get full duplex. If you want to override the autodetection check the hidden config.xml options ( http://doc.m0n0.ch/handbook-single/#FAQ.hiddenopts
has some info about how to set duplex mode. they are the same with pfSense). But beware, overriding these settings might cause problems if the other endpoint of that link doesn't join the setting which will end up in a lot of collisions.