firefoxDriver.js

FirefoxDriver.prototype.dismissAlert = function(respond) {
  webdriver.modals.dismissAlert(this);
  respond.send();
};
FirefoxDriver.prototype.dismissAlert.preconditions = [ webdriver.preconditions.alertPresent ];

preconditions.js

/**
 * Guard to ensure that no modal dialog is open.
 *
 * @param {!Object} driver A WebDriver instance.
 */
webdriver.preconditions.alertPresent = function(driver) {
  if (driver.modalOpen !== false) {
    return new WebDriverError(ErrorCode.NO_SUCH_WINDOW,
        'A modal dialog, such as an alert, is not open.');
  }
};