Alright, I think I know what to do. Since the pfSense device isn't the DHCP server it needs an internal IP from my modem/router.
Not exactly, on pfSense you setup the WAN interface to use DHCP to automatically get a WAN IP address from the modem.
Because your modem is also a router, the address pfSense gets will be an "internal" RFC1918 address that cannot be routed on the Internet (192.168.x.x for eg.)
You need to make sure the pfSense WAN interface is setup for DHCP on IPv4, None on IPv6 and uncheck the box that says "Block private networks and loopback addresses".
Enabling DHCP on the LAN interface makes it so the rest of my devices can get their IP's from the modem/router and through the firewall (pfSense)? I do have a question about the upstream gateway… is that asking for the local IP of my modem/router? I entered that figuring that's what it meant but it didn't really seem to do much. Is that something that only gets configured for the WAN interface?
pfSense does provide DHCP on its LAN interface (make sure to set it up that way).
DHCP on the pfSense LAN interface is provided to all your attached devices so that they get an "internal" (RFC1918) address that matches the subnet defined for the LAN interface.
They will ask pfSense to tell them how to get "out to the internet" (or anywhere other than their LAN subnet).
The neat thing is they have no idea (nor do they need any) how pfSense does that, they don't know about the modem/router or the WAN IP
They know the address/subnet pfSense gave them and that the pfSense LAN address is where they can go to get "outside".
This why you DO NOT WANT to enter a gateway address anywhere, leave it at default and pfSense can make things work.
Another subtle gotcha in this setup is that the pfSense LAN subnet CANNOT be the same as the subnet handed out by the modem/router.
This goes back to my earlier advice to move off of the "default" RFC1918 addresses (192.168.0.x,192.168.1.x, etc)
As always, the description of these setups is always WAAAAAY longer than actually doing them.