Mysterious space added by mozilla browser

Hello,

I am testing my page using mozilla firefox 3.5.3 version. I have this annoying problem wherein a mysterious space is being added automatically on top(between the top side of the browser display window and the first element within the div) when I use a div
Ex:
html code:
   <div id="headerdiv">--%>
       <%-- <div id="homepicdiv" style="float:left;margin-left:20px">
        <asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="pictures/homelogo.gif" PostBackUrl="Default.aspx" style="margin-left:3%;"/>
        </div>
      <div id="menutabdiv">
    <div id="menutabinnerdiv"><ul id="navigation">
  &n ...