"let" has worked in Firefox for years; support for it first shipped in Firefox 2 back in 2006. You do have to opt in to it by putting type="application/javascript;version=1.7" on your script tag, since the feature is not backwards-compatible and hence couldn't be added to existing scripts without possibly breaking them.
JS 1.7 in general added a bunch of neat features (array comprehensions, destructuring assignment, etc) that are only now slowly making their way to specifications after some foot-dragging by certain parties. See https://developer.mozilla.org/en/JavaScript/New_in_JavaScrip...
