AJAX XMLHTTP Request

I have created a mozilla extension which is a button located on the browser. This button has a javascript which when clicked should send a XMLHTTLP request. I want to use a local HTML file which I have created in the URL field of it. When I use it I still can't view that HTML page. Why is that so? The code goes like this:

CustomButton = { 

1: function () 
  {

     var xmlhttp;
     if (window.XMLHttpRequest)
     {// code for IE7+, Firefox, Chrome, Opera, Safari
         xmlhttp=new XMLHttpRequest();
     }
     else
     {// code for IE6, IE5
         xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
     }
     xmlhttp.onreadystatechange=function()
     {
         if (xmlhttp.readyState==4 && 开发者_C百科xmlhttp.status==200)
         {
             document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
         }
     }
     xmlhttp.open("GET","http://localhost/sample.html",true); 
     xmlhttp.send();
  }
} 

The sample.html file is located in the htdocs folder of xampp.

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Accessing local files with XMLHttpRequest is not allowed for security reasons.