Cannot share my internet connection via Wi-Fi from my Mac

I was desperate to get internet sharing going on my mac as, at the moment, it is my only router. It was going fine and suddenly stopped, and I hadn't updated recently. I ran the WirelessDiagnostic tool after doing a bit of searching, and that made the internet shareable again. It's an app--not a setting; I mention this because I wasted a lot of time trying to find it in System Preferences.

Turn Off firewall, so far it's the only way that you can share the connection. I don't like this idea of turning off the firewall but sharing just doesn't work when firewall is set to On. Apple should give a fix on this without turning Off the firewall and still able to share the connection.