The the system firewall of OS X is a packet filter that determines for each program if a data packet should be passed or blocked. If the firewall is enabled every program must be entered on a list to respond to requests from outside. Moodle4Mac (or MAMP) uses the httpd program to deliver the sites. You will find httpd in the MAMP folder ... the path is /Application/MAMP/Library/bin/httpd. To enter httpd into the whitelist of the firewall you must click forward on this path ... no idea if there would be a configuration file which you can edit.