While trying to upgrade XULRunner and ChatZilla on my Mac OS X (10.5), I got into a state where running ChatZilla.app didn't have any effect.
When I tried to run /Applications/ChatZilla.app/Contents/MacOS/xulrunner in the terminal, I got "Couldn't load XRE functions."
Going into /Library/Frameworks/XUL.framework/Versions/ and deleting the old 1.8 version of XULRunner fixed this (I don't know why, though).
