Firewall Switch

Firewall Switch is a simple to use and free AppleScript based application that will help you turn OS X Lion's built-in firewall on or off without having to open the System Preferences.

The Firewall Switch app will automatically  detect whether or not the system firewall is turned on.

If it is already running, it will ask you to confirm that you want to turn the firewall off. However, If the firewall is off, you will be asked for confirmation to switch the firewall back on (in "Block all incoming connections" mode).

If your system is configured to ask for administrator credentials to make changes in System Preferences, you'll also be prompted to enter those.