With OS X 10.7, Apple deprecated use of FreeBSD's ipfw and switched to OpenBSD's pf.
The control command for pf is pfctl(8).
You will find a brief discussion as of 10.7 here. This is useful for highlighting a couple ways OS X's PF differs from the stock BSD version.
Documentation on PF is widely available, including Hansteen's The Book of PF. The author also wrote a freely available tutorial, and the OpenBSD documentation is also freely available.
Note: If your machine is also running OS X Server under 10.8, you might need to fix a config error that Apple made before PF starts working.