function click_me(element) {
    try {//pour ie
        document.getElementById(element).click();
    } catch (e) {//pour ff
        var evt = document.createEvent("MouseEvents");
        evt.initMouseEvent("click", true, true, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
        var cb = document.getElementById(element);
        cb.dispatchEvent(evt);
    }
}

document.observe('dom:loaded', function () {
    new Showcase.Horizontal($$('#horizontal ul li'), $$('#horizontal a.controls'), { duration: 0.3, size: 3 });

    window.setInterval(function () {
        click_me('anext');
    }, 3000);
});