title: JavaScript check if browser extension is installed for Chrome, Firefox and Opera
body: <p><br />
  I want to show custom bar ( notification like: Install our plugin. ) on our site <strong>if extension is not installed</strong> for Chrome, Firefox and Opera. None will be displayed if extension is installed.</p>
  
  <p><strong>The idea is</strong>:<br />
  1. User visits our site and a bar with message appears - "Hey, install our extensions".<br />
  2. User clicks on install and extension is installed. No need for the user to open it.<br />
  3. User visit our site again and the bar does not appear again as the extension is installed.<br />
  <strong>We talk only for installation check, not cookies</strong>.</p>
  
  <p>So far I was able to find solutions only for Chrome: <a href="http://blog.kotowicz.net/2012/02/intro-to-chrome-addons-hacking.html" rel="noreferrer">http://blog.kotowicz.net/2012/02/intro-to-chrome-addons-hacking.html</a><br />
  and Firefox: <a href="http://webdevwonders.com/detecting-firefox-add-ons/" rel="noreferrer">http://webdevwonders.com/detecting-firefox-add-ons/</a><br />
  Opera is never mentioned.</p>
  
  <p>Is there a way to do it for Opera, based on the idea of checking for a local resource which is part of the extension: <br />
  chrome-extension://--Extension ID--/blank.gif<br />
  Firefox chrome://firebug/content/blank.gif<br /></p>
  
  <p>Or else:
  <strong>Is there a simple way to check if extension is installed for Chrome, Firefox and Opera?</strong></p>