我正在使用Python 2.7，因为我想编写一个简单的自动登录机器人，我已经安装了Selenium和Geckodriver。在浏览互联网（这里也是）之后，我将Geckodriver可执行文件放在/usr/local/bin中。现在，当我尝试做的时候：
from selenium import webdriver
driver = webdriver.Firefox()
我收到以下错误：
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
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
为什么会这样，我该如何解决这个问题？