shells/browser/shared/deploy.firefox.html

<ol>
  <li><a href="RelayDevTools.zip">download extension</a></li>
  <li>Extract/unzip</li>
  <li>Visit <code>about:debugging</code></li>
  <li>Click "Load Temporary Add-on"</li>
  <li>
    Select the <code>manifest.json</code> file inside of the extracted extension
    folder (<code>RelayDevTools</code>)
  </li>
</ol>