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?
No, you are likely looking at an ONT (optical network terminal), and it is not a router. Even with a port that accepts the fiber (sfp or sfp+ for 10g) on your equipment, the OLT (optical line terminal) likely will not provide you with service.
If you were to match the wavelengths the ISP is using you are likely to become a “rogue” on their PON that can knock out service for other customers that share the same passive network as you.
I make assumptions about you being on PON since you say AT&T, generally all I have ever seen from them are passive networks (one fiber with splitters for 1 port to many customers) unless you are paying extra for “dedicated” ($$$$$) internet.
If they are using a ONT with an “RG” (residential gateway) which is the typical “all in one” you can request the gateway service can be removed and replaced with a layer 2 bridge, where you’re router/firewall gets the “external” addressing and there is nothing being done by the ISP equipment other than sending you traffic and OAM (operations, administration, and maintenance; usually check or alert for light levels, software status, if a part of the ONT fails etc).