Comment 14 for bug 197781

The problem is that apparently this prohibits pefpanes (which are loaded with this function) from being overlayed at all, which does not only reduce extensibility (which we probably want) but also would be something that SeaMonkey needs in our new toolkit-derived prefwindow: Once I overlay (with a conventional manifest-based overlay) the pane that's being loaded with loadOverlay(), the observer to that loadOverlay() call doesn't fire any more.