How to deal with this error(I use python+selenium+chrome).Only chrome happens,Firefox didn't happened

Traceback (most recent call last): File "/Users/wuyong/PycharmProjects/Run/Firefox.py", line 5, in driver = webdriver.Chrome() File "/Library/Python/2.7/site-packages/selenium/webdriver/chrome/webdriver.py", line 67, in init desired_capabilities=desired_capabilities) File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 87, in init self.start_session(desired_capabilities, browser_profile) File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 141, in start_session 'desiredCapabilities': desired_capabilities, File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 201, in execute self.error_handler.check_response(response) File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 181, in check_response raise exception_class(message, screen, stacktrace) selenium.common.exceptions.WebDriverException: Message: session not created exception from disconnected: unable to connect to renderer (Session info: chrome=55.0.2883.95) (Driver info: chromedriver=2.27.440174 (e97a722caafc2d3a8b807ee115bfb307f7d2cfd9),platform=Mac OS X 10.12.0 x86_64)