docs: function FirefoxDriver(
    desiredCapabilities,
    requiredCapabilities
    ) {
    var instance;
    var first = desiredCapabilities;
    var len = arguments.length;
  
    if (!len) {
      instance = new Class();
    } else if (len === 1 || len === 2) {
      assert(first)
        .extends(Capabilities)
        .or(FirefoxProfile)
        .throws(
          'The first argument wasn\'t an instanceof Capabilities.'
          );
      if (len === 1) {
        instance = new Class(first._instance);
      } else if (len === 2) {
        assert(requiredCapabilities)
          .extends(Capabilities)
          .throws(
            'The second argument must be an instance of Capabilities.'
            );
        instance = new Class(first._instance, requiredCapabilities._instance);
      }
    } else {
      throw new Error('The wrong number of arguments was given.');
    }
  
    addFinalProp(this, '_instance', instance);
  }