title: How to check Firefox engines version?
body: <p>In Chromium, I can browse <code>chrome://version</code> that will tell me the JavaScript engine version (V8 engine version 10.3.174.14 for example).</p>
  <p>In Firefox, <code>about:support</code> give information like <em>Firefox Features</em> version, some library Versions (NSS) and that's pretty all. But I want to know Firefox's JavaScript Engine (SpiderMonkey) version, layout engine (Gecko) version. Where can I find that?</p>
  <p>As a bonus I'm also interested in knowing versions of some API and libraries used by Firefox (zlib, libpng sqlite, x264, Opus, ...), is there a page listing them too?</p>