The following javascript function demonstrates how to add new items to Combobox in HTML. The solution utilizes appendChild (select) and createElement (document) functions accordingly. Works both in Internet Explorer and Firefox.
<html>
<title>
<head>
<script type="text/javascript">
function add() {
var newOpt = mySel.appendChild(document.createElement('option'));
newOpt.text = "New Item";
}
</script>
</head>
<body>
<elect id="mySel">
<option>item 1
</select>
<input type="button" value="Add" onclick="add();">
</body>
</html>