try this code it uses jquery ajax simply it send post request to the login screen. it works well for all browsers but it doesn't work for firefox
Code:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
$.post('http://forum-URL/login.php?do=login',
{"vb_login_username": "admin",
"vb_login_password" :"123456",
"s":"",
"securitytoken":"guest",
"do":"login",
"vb_login_md5password":"e10adc3949ba59abbe56e057f20f883e",
"vb_login_md5password_utf":"e10adc3949ba59abbe56e057f20f883e" },
function(data,textStatus) {
alert(textStatus);
});