You might need to load the firmware for the chip.
Open up the shell and go to /boot/
If not created, create /boot/loader.conf.local
Input this into the file:
Reboot and you should have the firmware loaded at boot.
If you run pfsense on nanobsd, so you have a read only filesystem. You can run /etc/rc.conf_mount_rw to make the filesystem readwrite, then edit the file and then run /etc/rc.conf_mount_ro to make in read only again.