Why are Safari and Chrome's Javascript engine's running 1.5?
<p>I was browsing the MDC Doc Center page on <a href="https://developer.mozilla.org/en/JavaScript/Guide/Closures" rel="nofollow">Closures</a>. And it mentions the introduction of the <strong>let</strong> keyword but only for use with browsers running at least the 1.7 release.</p>

<p>So I looked on <a href="http://en.wikipedia.org/wiki/Javascript_1.7#Versions" rel="nofollow">Wikipedia</a> only to find that Safari and Chrome are on 1.5, while it seems every new release of Firefox has been using a newer release of JS.</p>

<p>Why is that?</p>
