When setting up a new pfSense firewall via the web interface, if you change the ip address and/or device name while connected you may receive the following error:

Error: 501 Description: An HTTP_REFERER was detected other than what is defined in System -> Advanced (https://192.168.1.1/). If not needed, this check can be disabled in System -> Advanced -> Admin.

The fix, is to connect to the console of the device, and use option 2 to set the interface IP addresses and enable DHCP.