test.js

    var pbService = Components.classes["@mozilla.org/privatebrowsing;1"].
                  getService(Ci.nsIPrivateBrowsingService);
    
    var listener = {
      QueryInterface: function(iid) {
        if (Ci.nsIPrivacyTransitionObserver.equals(iid) ||
            Ci.nsISupportsWeakReference.equals(iid) ||
            Ci.nsISupports.equals(iid))
          return this;
        throw Cr.NS_ERROR_NO_INTERFACE;
      },
      privateModeChanged: function() {
        alert('change!');
      }
    }
    
    window.gBrowser.docShell.addWeakPrivacyTransitionObserver(listener);
    
    pbService.privateBrowsingEnabled = true;
    window.gBrowser.docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing = false;
    alert(window.gBrowser.docShell.QueryInterface(Ci.nsILoadContext).usePrivateBrowsing);