XUL/Firefox: create new browser window on addon load

Question

I'm developing bootstrapped Firefox addon. In my startup function I am constructing new instance of object, that creates new Firefox window with the following code:

function createWindow (cb) {
    const appHiddenWindow = Services.appShell.applicationProvidedHiddenWindow
        , hidden = Services.appShell.hiddenDOMWindow;

    // For definition of OpenBrowserWindow() see
    //   http://mxr.mozilla.org/mozilla-central/source/browser/base/content/browser.js
    // Prior to Firefox 20 "private" option is silently ignored
    let parent = appHiddenWindow ? hidden : RecentWindow.getMostRecentBrowserWindow()
      , window = parent.OpenBrowserWindow({ private: true });
    window.addEventListener("load", onWindowLoad);
    return window;

    function onWindowLoad (e) {
        window.removeEventListener("load", onWindowLoad);
        window.gBrowser.addEventListener("pageshow", onPageShow);
    }

    function onPageShow (e) {
        dump("onPageShow\n");
        try {
            cb(window);
        } catch (e) {}
    }
}

For some unknown for me reasons this code behaves differently in different circumstances: on Firefox prior to 20 if it is being executed on app startup, onPageShow callback never fires. I assume this is because gBrowser's selectedBrowser just doesn't load homepage and stops after about:blank has loaded. Why does it load homepage in new window when I reactivate my extension manually (when Firefox is loaded already)?

Trouble appears when it yet loads homepage into selectedBrowser (in newer Fx versions): in this case it doesn't start loading URL, specified by me and instead continues to load my homepage.

So how can I determine if gBrowser is gonna load homepage or not? And why does it behave differently on App startup and on addon reactivation?

PS: For the above code to work one must do these imports and use following fallback:

Cu.import("resource://gre/modules/Services.jsm");
try {
    Cu.import("resource:///modules/RecentWindow.jsm");
} catch (e) { // Firefox prior to v20
    var RecentWindow = {
        getMostRecentBrowserWindow: function ()
            Services.wm.getMostRecentWindow("navigator:browser")
    };
}

Answer 1

It seems I was wrong in some of my assumptions. I don't have to wait until pageshow event. After a long research I found this bug. According to it there is an observer notification browser-delayed-startup-finished that is enough to start doing things I had to do with window.