With all of Leopard's new highly-publicised features came a few subtle changes. One of those changes is the new Firewall built-in to OSX 10.5. Previously, the Firewall came turned on by default. With Leopard, it is turned off and works completely different than it did in Tiger.

In Leopard you set your Firewall on an app-by-app basis, much like the shareware application Little Snitch 2, rather than by which port you wish to control, as it was in Tiger.

If you're concerned about security on your Mac and you're running OSX 10.5, you should take a look at this brief Apple Support Document titled About the Application Firewall.