If you specify the eventCallback parameter, it should be a JavaScript function that gets called when interesting things happen related to the notification box. This function receives as its only parameter a string indicating what event occurred. At this time, there's just one event type: "removed". This indicates that the notification box has been removed from its window.

Example:

function testNotificationBoxWithButtons() {
    //Create some common variables if they do not exist.
    //  This should work from any Firefox context.
    //  Depending on the context in which the function is being run,
    //  this could be simplified.
    if (typeof window === "undefined") {
        //If there is no window defined, get the most recent.
        var window=Components.classes["@mozilla.org/appshell/window-mediator;1"]
                             .getService(Components.interfaces.nsIWindowMediator)
                             .getMostRecentWindow("navigator:browser");
    }
    if (typeof gBrowser === "undefined") {
        //If there is no gBrowser defined, get it
        var gBrowser = window.gBrowser;
    }

    function testNotificationButton1Callback(theNotification, buttonInfo, eventTarget) {
        window.alert("Button 1 pressed");
        //Prevent notification from closing:
        return true;
    };

    function testNotificationButton2Callback(theNotification, buttonInfo, eventTarget) {
        window.alert("Button 2 pressed");
        //Do not prevent notification from closing:
    };

    function testNotificationCallback(reason) {
        window.alert("Reason is: " + reason);
    };

    let notifyBox = gBrowser.getNotificationBox();
    let buttons = [];

    let button1 = {
        isDefault: false,
        accessKey: "1",
        label: "Button 1",
        callback: testNotificationButton1Callback,
        type: "", // If a popup, then must be: "menu-button" or "menu".
        popup: null
    };

    buttons.push(button1);

    let button2 = {
        isDefault: true,
        accessKey: "2",
        label: "Button 2",
        callback: testNotificationButton2Callback,
        type: "", // If a popup, then must be: "menu-button" or "menu".
        popup: null
    };

    buttons.push(button2);

    //appendNotification( label , value , image (URL) , priority , buttons, eventCallback )
    notifyBox.appendNotification("My Notification text", "Test notification unique ID",
                                 "chrome://browser/content/aboutRobots-icon.png",
                                 notifyBox.PRIORITY_INFO_HIGH, buttons,
                                 testNotificationCallback);
}