usage of java script in asp.net

ok please give solution for my specific issue

I used Checkboxlist with textbox popupcontols for multi selection dropdownlist and selected items text displayed in the text box.which is work in google chrome...but does not work in firefox....selected item does not displayed in text box in the firefox browser

Here is my coding

Multicheckcombo.ascx

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<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="MultiCheckCombo.ascx.cs" Inherits="Mobius.D2K.Web.Administration.Modules.MultiCheckCombo" %>


<script type = "text/javascript">
//Script para incluir en el ComboBox1 cada item chekeado del chkListMateriales
function CheckItem(checkBoxList) {


var options = checkBoxList.getElementsByTagName('input');
var arrayOfCheckBoxLabels = checkBoxList.getElementsByTagName('label');
var s = "";

for (i = 0; i < options.length; i++) {
var opt = options[i];

if (opt.checked) {

s = s + ", '" + arrayOfCheckBoxLabels[i].innerText.replace("'", "''") + "'";

}
}
if (s.length > 0) {
s = s.substring(2, s.length); //sacar la primer 'coma'
}

var TxtBox = document.getElementById("<%=txtCombo.ClientID%>");
TxtBox.value = s;
document.getElementById('<%=hidVal.ClientID %>').value = s;
}
</script>



<asp:TextBox ID="txtCombo" runat="server" ReadOnly="True" Width="200"
Font-Size="Large" ForeColor="#660033">
<ajaxToolkit:PopupControlExtender ID="PopupControlExtender111" runat="server"
TargetControlID="txtCombo" PopupControlID="Panel111" Position="Bottom" >


<input type="hidden" name="hidVal" id="hidVal" runat="server" />

<asp:Panel ID="Panel111" runat="server" style="vertical-align:top;" ScrollBars="Vertical" Width="250" Height="150" BackColor="AliceBlue" BorderColor="Gray" BorderWidth="1">

<asp:CheckBoxList ID="chkList" AutoPostBack ="true"
runat="server"
Height="150" onclick="CheckItem(this)">









Multicheckcombo.ascx.cs



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using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.Odbc;
using Mobius.D2K.Common.Utils;
using Mobius.D2K.Common.Utils.Html;
using System.Data.SqlClient;

namespace Mobius.D2K.Web.Administration.Modules
{
public partial class MultiCheckCombo : BaseMobiusAdministrationUserControl
{

protected void Page_Load(object sender, EventArgs e)
{

}

///

///Set the Width of the CheckBoxList

///

public int WidthCheckListBox
{
set
{
chkList.Width = value;
Panel111.Width = value + 20;
}
}
///

/// Set the Width of the Combo
///

public int Width
{
set { txtCombo.Width = value; }
get { return (Int32)txtCombo.Width.Value; }
}
public bool Enabled
{
set { txtCombo.Enabled = value; }
}
///

/// Set the CheckBoxList font Size
///

public FontUnit fontSizeCheckBoxList
{
set { chkList.Font.Size = value; }
get { return chkList.Font.Size; }
}
///

/// Set the ComboBox font Size
///

public FontUnit fontSizeTextBox
{
set { txtCombo.Font.Size = value; }
}



///

/// Add Items to the CheckBoxList.
///