title: WebExtension: browser.tabs is undefined in FireFox (elementary OS)
body: <p>I'm trying to get access to the browser.tabs using an extension. My manifest.json currently includes these permissions:</p>
  
  <pre><code>"permissions": [
    "&lt;all_urls&gt;",
    "activeTab",
    "tabs",
    "storage"
  ]
  </code></pre>
  
  <p>When I try to access <code>browser.tabs</code> I still get <code>undefined</code>.</p>
  
  <p>I'm running Firefox 66.0.1 (64 bit) on the latest elementary os 0.5 - Juno.</p>
  
  <p>Maybe someone could point me in the right direction.</p>
  
  <p>Cheers
  Peter</p>