title: How to know if a new tab is opened by user click on new tab button?
body: <p>In Firefox WebExtensions, how to know in which way is the new tab opened?</p>
  
  <ul>
  <li>By user click on the new tab button (+)?</li>
  <li>By User click on a link such as <code>&lt;a href="http://www.google.com/"&gt;</code>?
  
  <blockquote>
    <p>Note: I don't care if a new tab is opened by <code>window.open()</code></p>
  </blockquote></li>
  </ul>
  
  <p>I found that, in callback of <code>chrome.tabs.Tab.onCreated</code>, there is a parameter passed in, assume it is named as <code>firefoxTab</code>:</p>
  
  <ul>
  <li>For tabs opened by click on +, its URL is <code>about:newtab</code></li>
  <li>For tabs opened by click on <code>&lt;a href="" target="_blank"&gt;</code>, its URL is <code>about:blank</code></li>
  </ul>
  
  <p><strong>But</strong>, there is an exception, if the second tab after Firefox started, is opened by click on "+", its URL will be <code>about:blank</code>, not <code>about:newtab</code>. I think it is a Firefox defect, have posted a bug on Bugzilla.</p>
  
  <p>In the meantime, is there any other way to do this?</p>