DropDownExtender cannot work correctly in IE6 and Firefox 2.0. Theme cannot change in Fire

Hi there,
A week later, I begin develop a new shop cart application with Ajax enabled.
My problem is:
1. My Page option feature (to change site's theme) is put in a panel with a DropDownExtender.
It cannot show/hide correctly:
- In Internet Explorer 6.0, italway show a "area" at top or bottom in the page.
Click on darkblue and pink icon still can change page skin.
- In Firefox browser, my application hide/show "area" correctly. But, when click on any icon
(an asp:ImageButton), after a postback,page's theme doesn't change.
- In Opera, and IE 7, everything is fine.

Please help me.
This is my demo site url:
http://ajaxshop-1.at.vwdhosting.net/
"Page option" in my language is: "Tùy ch?n trang" - under the search bar.

My source code:

1/*2* This is my user control aspx code3*/4"C#" AutoEventWireup="true" CodeFile="SelectTheme.ascx.cs" Inherits="UserControls_SelectTheme" %>5"AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>6
"right">7
class="PageOptionTextStyle">8 "HyperLinkPageOption" runat="server" Text="Tùy ch?n trang      "9 Width="100px">10 "DropDownExtender1" runat="server" DropDownControlID="PanelPageOption"11 TargetControlID="HyperLinkPageOption" HighlightBorderColor="transparent" >1213 "PanelPageOption" runat="server" Width="100px">14
"vertical-align:middle;text-align:center; background-color:LightYellow; height:20px; border-right: gray 1px ridge; border-top: gray 1px ridge; border-left: gray 1px ridge; border-bottom: gray 1px ridge;" align="left">15
16"middle" height="20px">17"middle">18class="innerlink"> Màu s?c: 19"ImageButtonDarkBlueSeclect" runat="server" SkinID="DarkblueSkinSelectButton" OnClick="ImageButtonDarkBlueSeclect_Click">20 21"ImageButtonDarkRedSelect" runat="server" SkinID="DarkredSkinSelectButton" OnClick="ImageButtonDarkRedSelect_Click">222324
2526
2728
29
303132/*33* This is code behind34*/35public partialclass UserControls_SelectTheme : System.Web.UI.UserControl36{37protected void Page_Load(object sender, EventArgs e)38 {3940 }41protected void ImageButtonDarkBlueSeclect_Click(object sender, ImageClickEventArgs e)42 {43 MySetTheme("Darkblue");44 }45protected void ImageButtonDarkRedSelect_Click(object sender, ImageClickEventArgs e)46 {47 MySetTheme("Darkred");48 }4950protected void MySetTheme(string themename)51 {52//Apply new theme53 Profile.CurrentTheme = themename;5455//Transfer to previous page56 Server.Transfer(Page.AppRelativeVirtualPath.ToString());57 }58}5960

Sorry, y writing "A week later, I begin developing a new shop cart application with Ajax enabled.
" i mean "A week before, I began develop a new shop cart application with Ajax enabled"

And, if you don't want to check my error in my web page, you can view this image