title: Firefox WebExtension APIs "browser is not defined"
body: <p>ReferenceError: browser is not defined</p>
  
  <p>Cannot find WebExtension APIs on page via console.
  In background script all APIs working fine.</p>
  
  <p><a href="https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Content_scripts#WebExtension_APIs" rel="nofollow noreferrer">https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Content_scripts#WebExtension_APIs</a></p>
  
  <p>How to send/receive message via WebExtension APIs on page (Firefox 51).</p>