Any idea why my page load is working twice

All my requests are happening twice in FireFox..  The Page_Load and all the code is executing twice.. I have asked help on this issue before also.. but couldnt solve this yet..
At first I thought its some issue with my code or configuration.. so I just created a new website and added following code
protected void Page_Load(object sender, EventArgs e)
{
        Session["Counter"] = Session["Counter"] == null ? 1 : (int)Session["Counter"]+1;
        Response.Write(Session["Counter"]);
}
Now when the page loads in FireFox it will print 1. But for each subseqent refresh the number will be added by two..
I can see the code is executing twice if i put breakpoint..
IE is working fine..
And here is my whole web.config
<?xml version="1.0"?>

<configuration& ...