public class GeckoTestWebLogin
{
    private readonly string _user;
    private readonly string _pass;
    public GeckoWebBrowser Gweb;
    public Uri LoginUri { get; } = new Uri("https://website.com/login/");

    public bool LoginCompleted { get; private set; } = false;
    public bool Loaded { get; private set; } = false;

    public GeckoTestWebLogin(string user, string pass)
    {
        _user = user;
        _pass = pass;
        Xpcom.EnableProfileMonitoring = false;
        Xpcom.Initialize("Firefox");

        //this code is for testing purposes, it will be removed upon project completion
        CookieManager.RemoveAll();

        Gweb = new GeckoWebBrowser();
        Gweb.DocumentCompleted += DocLoaded;

        //right about here is where I get lost, where can I set a callback method for the observer to report back to? Is this even how it works?
        MutationObserver mutationObserver = new MutationObserver(Gweb.Window.DomWindow, (nsISupports)Gweb.Document.DomObject);
    }

    private void TestObservedEvent(string parms, object[] objs)
    {
        MessageBox.Show("The page was changed @ " + DateTime.Now);
    }

    public void DocLoaded(object obj, GeckoDocumentCompletedEventArgs e)
    {
        Loaded = true;
        if (Gweb.Url != LoginUri) return;
        AttemptLogin();
    }

    private void AttemptLogin()
    {
        GeckoElementCollection elements = Gweb.Document.GetElementsByTagName("input");
        foreach (GeckoHtmlElement element in elements)
        {
            switch (element.Id)
            {
                case "username":
                    element.SetAttribute("value", _user);
                    break;
                case "password":
                    element.SetAttribute("value", _pass);
                    break;
                case "importantchangedinfo":
                    GeckoHtmlElement authcodeModal =
                        (GeckoHtmlElement)
                            Gweb.Document.GetElementsByClassName("login_modal").First();
                    if (authcodeModal.Attributes["style"].NodeValue != "display: none")
                    {
                        InputForm form = new InputForm { InputDescription = "Captcha Required!" };
                        form.ShowDialog();
                        elements.FirstOrDefault(x => x.Id == "captchabox")?.SetAttribute("value", form.Input);
                    }
                    break;
            }
        }
        elements.FirstOrDefault(x => x.Id == "Login")?.Click();
    }

    public void Login()
    {
        //this will cause the DocLoaded event to fire after completion
        Gweb.Navigate(LoginUri.ToString());
    }
}

MutationObserver mutationObserver = new MutationObserver(Gweb.Window.DomWindow, (nsISupports)Gweb.Document.DomObject);