If you create or declare 
img and 
script element with empty 
src attribute or 
link element with empty 
href attribute you will receive unwanted GET request with latest FireFox. The problem with 
img elements exist even in 
IE 6/ IE 7 however the request will be to the 
application root. You can check all these using this simple example: 
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
    protected void Page_Load(object sender, EventArgs e)
    { 
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Untitled Page</title>
    <script src=""></script>
    <link href="" />
</head>
<body>
    <form id="form1" runat="server">
        <img src="" />
        <div>
        </div>
    </form>
</body>
</html> 
You should be extra careful especially when creating programatically such elements! 
The weird thing is that it will be very hard to determine what is causing such GET request - even the mighty 
FireBug will fail it in this case. Yesterday, after spending almost half an hour, trying to find what exactly is going on, finally 
Deshev pointed that we can try 
Fiddler as FireFox proxy and .... voila :-) 
Here is a screenshot from my FireFox network connection settings:  
 
      
You can identify what is causing the request verifying the request headers:  
