browser compatibility problem

i have ajax script, its work in IE but not in mozilla. Actually there is no problem in ajax but the value has not posted.
Example :

if i select the state, the correspondent cities will display but the value of the city could not get another page.


<form method=post action=next.php>
<select name="state" class="selectfield" id="state" onChange="javascript:selectcity()">
<option value="">..Select State..</option>
<?
$sql_state = "select * from {$prefix}state order by statename";
$res_state = $DB->doSQL($sql_state);
while($row_state = @mysql_fetch_assoc($res_state))
{
?>
<option value="<?=$row_state[stateid]?>">
<?=$row_state[statename]?>
</option>
<? }?>
</select>




javascirpt
function selectcity()
{
val=document.sea_agency.state.value;
xhReq.open("GET", "ajaxval.php?id="+val, true);
xhReq.onreadystatechange = onSumResponse1;
xhReq.send(null);
}
function onSumResponse1()
{
if (xhReq.readyState != 4) { return; }
var serverResponse = xhReq.responseText;
var cell = document.getElementById("fac_c");
cell.innerHTML = serverResponse;
}

ajaxval.php
-------------
<select name="city" class="selectfield" id="city" >
<option value="">..Select City..</option>
<?php
include_once("includes/checksession.php");
$getRes = mysql_query("select * from cities where stateid = '$_GET[id]' order by cityname");
if($getRes)
{
if(mysql_num_rows($getRes) > 0)
{
while($getRow = mysql_fetch_assoc($getRes))
{
?><option value="<?=$getRow['cityid']?>"><?=$getRow['cityname']?></option>
<?php
}
}
}
?></select>

if i posted this value


next.php
---------
$cityname=$_REQUEST['city'];

here i could not get the value of the city.

But its work in IE, anybody know the problem?