title: Initial tab doesn't fire open, ready events
body: <p>I'm build an SDK Firefox Add-on that is supposed to read a tab's URL and parse it. To this end, I'm listening to the <code>'ready'</code> event in <code>lib/main.js</code>,</p>
  
  <pre><code>var tabs = require('sdk/tabs');
  tabs.on('open', function(tab){
    tab.on('ready', function(tab){
      console.log(tab.url);
    });
  });
  </code></pre>
  
  <p>as described in <a href="https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/tabs" rel="nofollow">Mozilla's documentation</a>.</p>
  
  <p>When debugging with <code>cfx run</code>, this appears to work well for new tabs. The tab that's already open on <code>cfx run</code>, however does <em>not</em> fire the <code>open</code> and <code>ready</code> events.</p>
  
  <p>What's the reason for this and how to fix it?</p>