Add import for FirefoxOptions (#5120)

Author: greyli

py/selenium/webdriver/__init__.py

@@ -17,6 +17,7 @@
 
 from .firefox.webdriver import WebDriver as Firefox  # noqa
 from .firefox.firefox_profile import FirefoxProfile  # noqa
+from .firefox.options import Options as FirefoxOptions  # noqa
 from .chrome.webdriver import WebDriver as Chrome  # noqa
 from .chrome.options import Options as ChromeOptions  # noqa
 from .ie.webdriver import WebDriver as Ie  # noqa