Q:

method.toString() exclude/include 'use strict'

method.toString() add 'use strict' in Firefox, not in IE or Chrome.
Javascript code:
'use strict';
var myMethod = function foo(){return 1};
result = myMethod.toString();
console.log(result);

Plunker to reproduce it
Results: 

Chrome 39.0: "function foo(){return 1}"
Internet 11: "function foo(){return 1}"
Firefox 33.1: "function foo(){ "use strict"; return 1}"

Is it possible to define somehow that the directive 'use strict' should be include/exclude in toString()?

A:

You can try using this to determine strict mode and 'insert' it into the toString result:
var strictMode = (function() { return !this; })() ? '"use strict"; ' : '';
var result = myMethod.toString()
                 .replace(/"use strict";/, '') // FF prevent doubling
                 .replace(/{/, '{'+strictMode);

See updated Plunker or check the code snippet

'use strict';

var strictMode = (function() { return !this; }()) ? '"use strict"; ' : '';
var myMethod = function foo(){return 1};

$(function(){
  $("#function").append(
      myMethod.toString()
       .replace(/"use strict";/, '') // to prevent a double 'use strict' in firefox
       .replace(/{/, '{'+strictMode) );
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<h1>Hello stackOverflow!</h1>
<h2>My Function to String</h2>
<p id="function"></p>

