title: Unable to Hide Page Action Button in Firefox 57
body: <p>I was wanting to remove the Page Action button in Firefox's url bar, so I added the following line to my <code>userChrome.css</code> file:</p>
  
  <pre><code>#urlbar-page-action-button {display:none !important;}
  </code></pre>
  
  <p>However, nothing happened. How can I accomplish what I'm attempting?</p>
  
  <p>I'm using Firefox Nightly <code>version 57.0a1</code> on a <code>Macbook</code>.</p>