Yes, it's possible.
If you're just using a firewall with public addresses inside (NOT 192.168.x.x, 172.16-32.x.x or 10.x.x.x), just set up the firewall to allow requests to port 80 through.
If you're using a NAT firewall, you have to set it up to "port-forward" requests to port 80. See your Firewall's manual for this. You'll want to set up a rule that any request for port 80 coming to the firewall's public address (e.g. 188.8.131.52) is forwarded to your web server's internal address (e.g. 192.168.1.100). That'll do it.
Another issue is that some ISPs, particularly cable companies, block port 80. This was done to stop the spread of some virus that affects Windows web servers (blech!). Since you are using a Mac, you're an innocent victim. The work-around for this is to tell people to direct their browsers to a different port with a command like: http://www.mydomain.com:8080. Then set up port-forwarding to direct requests for port 8080 to port 80 on your internal mac. How to figure out what ports are allowed through by the ISP is beyond the scope of this discussion.