Selenium "Unable to find a matching set of capabilities" despite driver being in /usr/local/bin

I'm trying to follow a tutorial about Selenium, http://selenium-python.readthedocs.io/getting-started.html. I've downloaded the latest version of `geckodriver` and copied it to `/usr/local/bin`. However, when I try ``` from selenium import webdriver driver = webdriver.Firefox() ``` I get the following error message: ``` Traceback (most recent call last): File "/Users/kurtpeek/Documents/Scratch/selenium_getting_started.py", line 4, in <module> driver = webdriver.Firefox() File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 152, in __init__ keep_alive=True) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 98, in __init__ self.start_session(desired_capabilities, browser_profile) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 188, in start_session response = self.execute(Command.NEW_SESSION, parameters) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 252, in execute self.error_handler.check_response(response) File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 194, in check_response raise exception_class(message, screen, stacktrace) selenium.common.exceptions.WebDriverException: Message: Unable to find a matching set of capabilities [Finished in 1.2s with exit code 1] ``` From https://github.com/SeleniumHQ/selenium/issues/3884, it seems like other users are experiencing similar issues, but the Selenium team is unable to reproduce it. How can I get Selenium working with Firefox? (It does work with `chromedriver` and a `webdriver.Chrome()` instance, so I suspect this might be a bug in Selenium).