The FreeBSD Handbook has a summary of the firewalling options available under FreeBSD. It includes examples as well as configuration guidance and can be accessed at http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/firewalls.html.
PF: The OpenBSD Packet Filter
This handbook is a great resource for detailed, up to date information on the PF firewall. There are several mirrors on the Net, including http://openbsd.mirrors.pair.com/doc/pf-faq.pdf.
FreeBSD Packet Filter (pf)
This site, http://pf4freebsd.love2party.net, is maintained by the team that has ported PF to FreeBSD. There are mailing lists, a TODO file, and the source code for the port.