code: async function installFirefoxPreferences(executablePath) {
    const firefoxFolder = path.dirname(executablePath);
    const {helper} = require('../lib/helper');
    const mkdirAsync = helper.promisify(fs.mkdir.bind(fs));
  
    let prefPath = '';
    let configPath = '';
    if (os.platform() === 'darwin') {
      prefPath = path.join(firefoxFolder, '..', 'Resources', 'defaults', 'pref');
      configPath = path.join(firefoxFolder, '..', 'Resources');
    } else if (os.platform() === 'linux') {
      if (!fs.existsSync(path.join(firefoxFolder, 'browser', 'defaults')))
        await mkdirAsync(path.join(firefoxFolder, 'browser', 'defaults'));
      if (!fs.existsSync(path.join(firefoxFolder, 'browser', 'defaults', 'preferences')))
        await mkdirAsync(path.join(firefoxFolder, 'browser', 'defaults', 'preferences'));
      prefPath = path.join(firefoxFolder, 'browser', 'defaults', 'preferences');
      configPath = firefoxFolder;
    } else if (os.platform() === 'win32') {
      prefPath = path.join(firefoxFolder, 'defaults', 'pref');
      configPath = firefoxFolder;
    } else {
      throw new Error('Unsupported platform: ' + os.platform());
    }
  
    await Promise.all([
      copyFile({
        from: path.join(__dirname, '00-puppeteer-prefs.js'),
        to: path.join(prefPath, '00-puppeteer-prefs.js'),
      }),
      copyFile({
        from: path.join(__dirname, 'puppeteer.cfg'),
        to: path.join(configPath, 'puppeteer.cfg'),
      }),
    ]);
  }
docs: Install browser preferences after downloading and unpacking firefox instances. Based on:   https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Enterprise_deployment_before_60#Configuration