nsIShutdownListener::OnShutdown is never called

[Marius Mikucionis]

I wrote a nsIShutdownListener with C++ and registered it with this line:

nsServiceManager::GetService(
    "@mozilla.org/docloaderservice;1",
    NS_GET_IID(nsIDocumentLoader),
    (nsISupports**) &docl,
    shutdownListener
);

But the OnShutdown method is never called even if the browser is successfully closed.
What can be wrong?