var req = Components.classes["@mozilla.org/xmlextras/xmlhttprequest;1"].createInstance();
var path_file="D:\\test.txt";
req.open('GET', "file:///" + path_file.replace(/\\\\/g,'/').replace(/ /g,'%20'), false);
req.send();
var a=req.responseText;
alert(a);
Bạn thay đường dẫn vào path_file chạy...