I would guess that there's some functionality provided via XUL which doesn't exist in the native OSX context menus (i.e. precisely the same functionality doesn'

I would guess that there's some functionality provided via XUL which doesn't exist in the native OSX context menus (i.e. precisely the same functionality doesn't exist, not that the native OSX context menus are somehow deficient). That's going to be particularly relevant for extensions.