It's not really written down anywhere but you can see the gory history of how we got it together in bugzilla:
https://bugzilla.mozilla.org/show_bug.cgi?id=921040We used the MacOSX SDK from Xcode, packaged up into our internal file store for build machines to use and passed a bunch of compiler options that clang sets by default when you're compiling on macOS:
https://searchfox.org/mozilla-central/rev/72e3388f74458d369a...
Getting DMG creation working was a bit of a hassle but we managed to get it done with some dmg/hfsplus tools from the iPhone jailbreak scene.