Cant access firewall in system preferences; firewall tab is greyed out
My first assumption is that you have a app like Anti Virus that took over the control of your Firewall.

Here is a article on using the Terminal commands with Firewall.

It is for Lion, but I assume they still work with Mavericks.

For example to start it: 

/usr/libexec/ApplicationFirewall/socketfilterfw --setglobalstate on


Give it a try.