Some ECMAScript 6 element are available in Firefox

es6.js

// To test this code in Firefox, I encourage you to use Firefox Aurora
// and to include your script this way:
// <script type="application/javascript;version=1.8">
// (necessary to make `let` work)

/* let ********************************************* */

  // let is the new var :)
  // Declares a block scope local variable

  var a = 5;
  if (1 == 1) { let a = 6; alert(a); } // 6
  alert(a); // 5

/* Default Arguments ****************************** */

  function myLog(str, prefix="> ", suffix="") {
    console.log(prefix + str + suffix);
  }

  myLog("foobar!");

  // output:
  // > foobar!

/* For ... of ************************************** */

  // Iterates over iterable objects

  let arr = [ 3, 5, 7 ];
  arr.foo = "hello";

  for (let i of arr) {  // OF
    console.log(i); // logs "3", "5", "7"
  }

  for (let i in arr) {  // IN
    console.log(i); // logs "0", "1", "2", "foo"
  }

  let articleParagraphs = document.querySelectorAll("article > p");
  for (let paragraph of articleParagraphs) {
    paragraph.classList.add("read");
  }

/* Rest arguments ********************************* */

  function myLog(str, ...args) {
    console.log(str + ":");
    for (let arg of args) {
      console.log(" " + arg);
    }
  }
  myLog("foo", "foobar", "mopmop");
  // output:
  // foo:
  //  foobar
  //  mopmop

/* Map ******************************************** */
// Not iterable yet. Will be in a week. bug 725909.

  // key/value maps.
  // let you associate an object to another one.
  // Better than using an Object because you don't
  // have to use a string.

  let m = new Map();

  m.set(obj_1, obj_a);
  m.set(obj_2, obj_b);

  m.get(obj_1); // return obj_a;

/* Set ******************************************** */
// Not iterable yet. Will be in a week. bug 725909.

  // Unordered collections of unique elements
  // Better than using an array + indexOf as it ensures
  // you the object is stored only once.

  let trackedElement = new Set();

  trackedElement.add(div1);
  trackedElement.add(div2);
  trackedElement.add(div1);

  trackedElement.has(div1);    // true
  trackedElement.delete(div1);
  trackedElement.has(div1);    // false

A detail: the if (1 == 1) in if (1 == 1) { let a = 6; alert(a); } // 6 at line 11 is not necessary. { let a = 6; alert(a); } works just fine.