The FreeBSD pf is an (very) old version of what is available on OpenBSD.
No it's not, it's a fork.
The config file syntax matches that of the old pre-4.5 OpenBSD pf but the code under the hood has seen much evolution. FreeBSD's pf is SMP-capable, for one thing, unlike OpenBSD's that "will only use one processor": http://www.openbsd.org/faq/pf/perf.html