XMLHttpRequest post hangs on Firefox / Mozilla

Hello, the following bit of (stripped down) code works fine in IE, but on Gecko
based browsers -
Firefox 1.0.7
Mozilla 1.7.12

it just hangs on the xmlhttpOpen

// IE object - works fine
// var xmlhttp=new ActiveXObject("Microsoft.XMLHTTP")

     var xmlhttp =new XMLHttpRequest();
        xmlhttp.Open("POST","http://www.anythin.com",false);

 xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");

        xmlhttp.send("data");

I'd be grateful for any ideas.

Cheers

--
Jim
a Yorkshire polymoth