Wifi Router Plugged into pfSense ethernet port - are the router ports usable?
I already have a working pfsense box with 3 network ports, one of which is plugged into a wifi router and it is all working.
Someone has asked me to build them a wifi setup with pfsense as cheaply as possible.
Would the ethernet ports on a wifi router be usable in such a setup, in addition to the wifi?
Yes. The WiFi router will not be acting as a "router", strictly talking. The cable coming from the pfSense needs to be connected on one of the "LAN" ports of the WiFi router, and whatever device you want to connect here also needs to be connected to the LAN ports (you shouldn't use the WAN port on this setup). This is something that a lot of people fail to realize (if you plug the cable from pfSense to the WAN port, you will most likely be creating a double NAT and you might get some issues that will be a pain to troubleshoot).
Also, make sure the DHCP server on the Wi-Fi router is turned off.