Is ipfw available in Mountain Lion?

Yes it is. I use it for my firewall needs. A great way to test if it still exists, is to just run it. Open Terminal.app and enter the following command:

sudo ipfw list

It should report the following (rules may vary):

65535 allow ip from any to any

Additionally, Apple will not remove it as it is actually the backend for the default Application Firewall found in System Preferences, at least not for some time.

You can read up more on ipfw in OS X using Apple's Manual Page (or simply type man ipfw in Terminal.app).

As for pf (the actual implementation of OS X is pfctl), it is a much more object orientated program that can control network access translation and packet filter (while ipfw requires external components to handle them). It is born from OpenBSD (as opposed to FreeBSD) and has been around for quite a while as well. It's power comes from being able to accomplish more internally, but also optimize long rulesets (that sometimes bogs ipfw down). They are both, however, excellent firewalls and can coexist quite happily. You can read more about the differences between pf and ipfw, here.