I have a mac and don't have a firewall.
Another useful troubleshooting step is to look at the Messages tab in MultiBit. If you don't already have a Messages tab alongside your Send, Request, and Transactions tabs, go to View->Messages to open it. In that Messages tab, when things are working normally, you should see an "Opening wallet" message, followed by a growing list of "Synchronising with network" messages. Hopefully, if yours isn't connecting, that tab will say something helpful about it.
If MultiBit simply refuses to cooperate, or if you'd rather chuck it and try something else, you could:
1. Install another Bitcoin client.
2. Export your keys from MultiBit.
3. Import them into the other client.
Moving those keys effectively moves your funds to the new wallet. You should then completely uninstall MultiBit to avoid future conflicts with your new wallet. If you'd like more detail on any of those steps, let me know.
