const {Cu,Ci,Cc} = require("chrome");
const {OS} = Cu.import("resource://gre/modules/osfile.jsm", {});

/*
var t; // тут объявлял t
*/
var file = "c:\\c.txt";
var buttons = require('sdk/ui/button/action');
var tabs = require("sdk/tabs");


var button = buttons.ActionButton({
  id: "mozilla-link",
  label: "Visit Mozilla",
  icon: {
    "16": "./icon-16.png",
    "32": "./icon-32.png",
    "64": "./icon-64.png"
  },
  onClick: handleClick
});

function handleClick(state) {

let promise = OS.File.read(file , { encoding: "utf-8" });
promise = promise.then(
  function onSuccess(text) {

/*
t=text; // пытался присвоить
*/

	var url = "http://www.example.com/";

	var cookieString = "your_key_name="+text+";domain=.example.com;";

	var cookieUri = Cc["@mozilla.org/network/io-service;1"]
    		.getService(Ci.nsIIOService)
    		.newURI(url, null, null);

	Cc["@mozilla.org/cookieService;1"]
    		.getService(Ci.nsICookieService)
    		.setCookieString(cookieUri, null, cookieString, null);
	

  }
);

/*
console.log(t); // ну и в консоль, а там undifined
*/

promise = OS.File.remove(file);

}