Mozilla issued me a new MacBook, and I've been trying to get it set up for development. In particular I wanted to get it set up for Firebug development since that's my primary focus right now. This should be easy -- Firebug is just an extension written in JavaScript. However, one thing I wanted to do is set up a build script that will automatically launch Firefox using a specific profile just for testing. It's not at all hard to launch Firefox from the command-line in OS X, except that I was getting really bizarre behavior: A Firefox window would appear, but there would be no menu bar, and indeed, Firefox didn't show up in the applications list when you hit command-tab. Even worse, there was no way to get keyboard focus into the window. You could close the window by clicking on it, but then you'd have a zombie Firefox process that you couldn't quit, and you couldn't Force-Quit it, because it didn't show up in that list either. It was necessary to kill it from the command-line.
I tried everything I could think of, and a handful of times, everything worked, and then it went back to being broken. Eventually I discovered that the path to the executable that I typed out differed from the path in the filesystem by the case of a single letter. I had /Applications/Firefox.app/Contents/MacOs/firefox-bin, when what I really wanted was /Applications/Firefox.app/Contents/MacOS/firefox-bin (MacOs ==> MacOS). That was it.Addendum: This is now bug #453913 in Bugzilla.