message: Add GHE notice for Firefox
diff: <h2>Information</h2>
  <p>
  GitHub Enterprise user? While on your site, right-click on Refined GitHub's icon in the toolbar and select <strong>Enable Refined GitHub on this domain</strong>.
  - <a href="https://user-images.githubusercontent.com/1402241/32874388-e0c64150-cacc-11e7-9a50-eae3727fd3c2.png" target="_blank">Example</a>
  + <a href="https://user-images.githubusercontent.com/1402241/32874388-e0c64150-cacc-11e7-9a50-eae3727fd3c2.png" target="_blank">Example</a>. This is <a href="https://github.com/sindresorhus/refined-github/issues/840#issuecomment-349936195" target="_blank">not yet supported by Firefox</a>.
  </p>
  +
  <h2>News feed</h2>
  <p>
  <label>
  