Ok. I started playing with XULRunner again, but this time on my Mac. I downloaded a Mac build; I went through the getting started page and reviewed the tutorial; I had my XUL application ready, and then started it up:

/Library/Frameworks/XUL.framework/xulrunner-bin application.ini

IT FAILED, ARG!!! The xulrunner-bin process launched and promptly exited, and I couldn't find any debug information about why. I checked and double checked, but I didn't find any problems with my super simple XUL application. I even purposely broke my sample XUL application to try to get an error message-- but nothing.

SOLUTION: Out of a hunch, I followed the deployment instructions and packaged up a "MyApp.app" Mac application. I launched it from Finder-- `open MyApp.app` also would've worked from the Terminal-- and it worked like a charm. It's kinda hackish to develop in MyApp.app/ subdirectories, but it works...