What's the "built-in" firewall in OS X 10.9?
The firewall in OS X 10.9 is now "pf".

It is controlled by the tool pfctl which can also give you a lot of status about the firewall itself. Try sudo pfctl -s all and you will get a huge dump of info. If you want a lot of information you can add one or two -v to the front of the options for even more info - sudo pfctl -v -v -s all