How to open new window with gecko browser?

You can use the event CreateWindow2 to handle a new popup window:
GeckoWebBrowser wb1 = new GeckoWebBrowser();
wb1.CreateWindow2 += new EventHandler<GeckoCreateWindow2EventArgs>(wb1_CreateWindow2);


Here event CreateWindow2:
void wb1_CreateWindow2(object sender, GeckoCreateWindow2EventArgs e)
    {
        //Keep popup new window here!
        e.Cancel = true;

        //e.WebBrowser.Navigate(e.Uri);

        // OR

        //GeckoWebBrowser wb1 = new GeckoWebBrowser();
        //wb1.Navigating += new EventHandler<GeckoNavigatingEventArgs>(wb1_Navigating);
        //wb1.Dock = DockStyle.Fill;
        //wb1.CreateControl();
        //TabPage tab1 = new TabPage("New WebBrowser");
        //tabBrowser.TabPages.Add(tab1);
        //tab1.Controls.Add(wb1);
        //wb1.Navigate(e.Uri);
    }