Changing netfilter packet flow

Hi I want to have squid in My iptables firewall.As you know when we start squid in a Linux machine, it listens on input ports of machine like 3128, but i have firewalling rules with iptables netfilter firewalls to.So somehow i want to change packet flow to do web filtering with squid and also check iptables rules for client requests.Others firewalls and UTMs like opensense or pfsense and ... can do this but i can not! I don't know how they do this!! Are they change their packet flow or what??How do you use squid in ipfire?? or are there any other ways to solve it?

Tank you.