packages/react-devtools-extensions/deploy.firefox.html

<ol>
  <li><a href="ReactDevTools.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>ReactDevTools</code>)</li>
</ol>