Mac Python3.6 を使用しているものです。

seleniumをimportして、
browser=webdriver.Firefox()
を実行してみたのですが、
うまくいかず困っています。(エラー文は下記)

geckodriverのPATHについては問題ないと思うのですが。

初心者なのでわかりやすく教えて頂けるとありがたいです。

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/anaconda3/lib/python3.6/site-packages/selenium/webdriver/firefox/webdriver.py", line 167, in __init__
    keep_alive=True)
  File "/anaconda3/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 156, in __init__
    self.start_session(capabilities, browser_profile)
  File "/anaconda3/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 251, in start_session
    response = self.execute(Command.NEW_SESSION, parameters)
  File "/anaconda3/lib/python3.6/site-packages/selenium/webdriver/remote/webdriver.py", line 320, in execute
    self.error_handler.check_response(response)
  File "/anaconda3/lib/python3.6/site-packages/selenium/webdriver/remote/errorhandler.py", line 242, in check_response
    raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.SessionNotCreatedException: Message: Unable to find a matching set of capabilities