running firefox selenium webdriver from python on mac OS fails to start

when trying to launch firefox from Pycharm IDE or from Terminal with the next simple script:

from selenium import webdriver
a = webdriver.Firefox(firefox_binary='/usr/local/bin/geckodriver')

no browser session is opened and i'm getting this issue:

Traceback (most recent call last):
  File "/Users/avihaiezaguy/Desktop/a.py", line 2, in <module>
    a = webdriver.Firefox(firefox_binary='/usr/local/bin/geckodriver')
  File "/Library/Python/2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 150, in __init__
    keep_alive=True)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 92, in __init__
    self.start_session(desired_capabilities, browser_profile)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 179, in start_session
    response = self.execute(Command.NEW_SESSION, capabilities)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 236, in execute
    self.error_handler.check_response(response)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 104, in check_response
    status = value["status"]
KeyError: 'status'

when webdriver is called without binary from Terminal, the browser opens and i get:

Traceback (most recent call last):
  File "/Users/avihaiezaguy/Desktop/a.py", line 2, in <module>
    a = webdriver.Firefox()
  File "/Library/Python/2.7/site-packages/selenium/webdriver/firefox/webdriver.py", line 150, in __init__
    keep_alive=True)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 92, in __init__
    self.start_session(desired_capabilities, browser_profile)
  File "/Library/Python/2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 180, in start_session
    self.session_id = response['sessionId']
KeyError: 'sessionId'

on the other hand Chrome works fine.

can anyone help solving this issue?