Sorry for the dumb question and hopefully this is relevant enough to the sub. I have my own firewall and right now it connects to my ISPs provided home router over rj45, their router gets a fiber hookup to their network and it’s the only ISP device in my home. If I have a firewall with a fiber port, can I take the fiber to the modem and hook that straight to my firewall, or is there a reason I need their device?
You might be able to do mac address cloning for fiber ports, if you have openwrt or similar it usually offers it.
A firewall may not offer full routing and NAT though for ipv4 devices, or wifi if you need it, or ipv6, or many other features. I’ve also used a cable modem and stuck the router in the dmz which essentially makes it a passthrough device also in the past
Basically, play with it and good luck!