What browsers currently support JavaScript's 'let' keyword?

I'm developing an app and don't have to ever worry about Internet Explorer and was looking into some of the features present in A+ grade browsers that aren't in Internet Explorer1.

One of these features I wanted to play around with is JavaScript's let keyword

I can't seem to get any of their 'let' examples to work in Firefox 3.6 (User-Agent string: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6 (.NET CLR 3.5.30729)). I get SyntaxError: missing ; before statement SyntaxError: missing ; before statement when executing let foo = "bar" .

So, what browsers support the let keyword? (Or am I doing something wrong?)

Those extensions are not ECMA-Standard, they are supported only by the Mozilla implementation.

On browser environments you should include the JavaScript version number in your script tag to use it:

<script type="application/javascript;version=1.7">  
  var x = 5;
  var y = 0;

  let (x = x+10, y = 12) {
    alert(x+y + "n");
  }

  alert((x + y) + "n");
</script>

Internet Explorer≥11（仅for范围内缺失）和所有当前浏览器（ECMAScript 6兼容性表：let）均提供完全支持。

As of April 2017:

 for (let i = 0; i < 3; ++i) {
      setTimeout(function() {
          console.log(i);
       }, i * 100);
  }