Issue while displaying image as preview in Mozilla Firefox

Hi,
I am selecting an image file from file upload control and after selecting image file I want to display that image as preview.
I am using the following code.
It is working fine in IE but it is not working in Mozilla Firefox.
    <asp:FileUpload ID="FileUpload1" runat="server" onchange="TestImage();"
    style="background-color:White" />
    <img id="Image1" runat="server" alt="" src="" width="100" height="60" />
My javascript function is
    function TestImage()
    {
      if (document.getElementById(FileUpload1).getAttribute("value") != "") 
      {             
           document.getElementById(Image1).setAttribute("src", "HandlerImage.ashx?ima ...