If you have a recent (>=10.7) version, pf [0] is installed. pf is a more capable firewall than ipfw, and can shape upstream connections in a number of ways. As an example of what it can do, the Network Link Conditioner [1] is built using pf. The version in OS X is not the most recent version in OpenBSD, but that FAQ is a good start, and the man page [2] can fill in the details.
0: http://www.openbsd.org/faq/pf/
1: http://mattgemmell.com/2011/07/25/network-link-conditioner-i...
2: http://developer.apple.com/library/mac/#documentation/Darwin...