Pfsense one card network SOLO PROXY
Can I use pfsense only as a proxy with a single network card? ONLY PROXY
It is possible as a stand-alone proxy, though not ideal. It can't act as a gateway, so whatever the actual gateway on the network is, it would have to forward traffic to the proxy. That or you'd have to add the proxy settings to user's PCs/Browsers/Devices directly and maybe setup WPAD.
Same as any other stand-alone proxy. Basically you'd only be using it as a GUI for squid at that point.