I don't see how it could be a firewall issue. I've been running Bitcoin-Qt for years with no problem from this same iMac.
How do I navigate to the "debug console"? I don't see it.
The firewall could be blocking connections to and from the program.
To get to the debug console, go to Help > Debug Console. It should tell you how many connections you have. If it has some connections, click on the Peers tab and see if the peers are actually connected. If they are connected the ping time should NOT be N/A. If it is N/A, then something is wrong.