Because the developer tools on Mac OS X 10.6 default to building for 64-bit now, there are a few changes you have to make in order to build Mozilla on Mac OS X 10.6. Josh Aas wrote a very helpful blog post a few days ago about this.
I followed his instructions and successfully built Namoroka on my Mac at home, so I revised the Mac OS X build prerequisites article to explain how to configure things to successfully compile a 32-bit Intel build of Mozilla on Mac OS X 10.6. It also still explains how to do it on previous versions of Mac OS X.
Hopefully this added information will help you get going!
In related news, progress is being made on building Firefox as a 64-bit binary; the guys working on it are estimating it’ll be running within a month or so. I look forward to seeing it in action!