Window Location In An IFRAME

i have the following script:

function subCat(){
var d = document.all;
var i = d.sid.value;
if(i != "-1"){
window.location = "subcat.php?sid="+i;
}
}

That is called on the following drop down menu:
<select name="sid" class="dropBox" id="sid" onChange="javascript:subCat()">
<option value="-1">Please Select</option>
<option value="1">category 1</option>
<option value="2">category 2</option>
<option value="3">category 3</option>
<option value="4">category 4</option>
</select>

It works fine in IE but not in Mozilla Firefox, what do i need to chnage to
make it work correctly?