At present, from what you say, INCOMING traffic on Port 80 is basically being forwarded by your modem/router to Pfsense.
So all you should need to do is simply add a port forwarding rule for port 80 to the IP of your server.
so in Firewall menu select NAT, then create a new rule under the Port Forwarding tab
Set these settings:
interface -> WAN
Protocol -> TCP
Destination -> WAN Address
Destination Port Range -> HTTP (port 80)
Redirect Target IP -> 192.168.1.100
Filter Rule Association -> Create Associated Filter rule
other stuff can I think be left as default.
And that should be it…. It's just a simple port forward operation really, unless I'm misreading something.