I need to block all outgoing connection attempts made by a particular app in order to prevent it from downloading updates automatically via ipfw?
You could try to edit the hosts file and redirect the application's requests into the void that is 127.0.0.1.
If you want a graphical front end to ipfw (it's still installed on 10.5 and 10.6, though not as prominently), I'd recommend Flying Buttress (Shareware). If cost is an issue, Wikipedia has a list of alternative user interfaces for ipfw, many of which run on Mac OS X. Alternatively, you could go all old school and configure ipfw by hand.
Ipfw is, however, only an IP packet filter - you'll only be able to block access for all applications or none and only for a specific host and port. If you want to block all connection attempts by a particular application, you need an application-level firewall, the best-known of which is probably Little Snitch.