Javascript to close IE6, IE7, IE8 and Firefox without confirmation box

if ((userBrowser.browser == "Explorer" && (userBrowser.version == "8" || userBrowser.version == "7"))) {
        window.open('', '_self', '');
        window.close();
    } else if ((userBrowser.browser == "Explorer" && userBrowser.version == "6")) {
        window.opener = null;
        window.close();
    } else {
        window.opener = '';
        window.close(); // attempt to close window first, show user warning message if fails
        alert("To avoid data corruption/loss. Please close this window immedietly.");
    }