This will only work if squid is on a different interface. At least many months ago I couldn't get it to work with a redirect rule with squid on the same interface. Mostly due to not being able to add a rule thats like below
rdr on {iface} inet proto tcp from !{squid ip} to any port = 80 -> {squid ip} port 3128
Three options:
Place squid box on different interface on the pfSense box and make the redirectrule.
Set squid box up with a bridge and redirect traffic going through the bridge to squid.
Change the pfsense dhcp lease default gateway to squid box and set the squid box gateway pointing to the pfsense box with traffic being redirected to squid.