Getting such error while using selenium with Python means that you most probably updated Firefox and didn’t selenium.

[dande@j ~]$ python zs.py
Traceback (most recent call last):
  File "zs.py", line 6, in <module>
    browser = webdriver.Firefox()
  File "/usr/lib/python2.6/site-packages/selenium/webdriver/firefox/webdriver.py", line 58, in __init__
    self.binary, timeout),
  File "/usr/lib/python2.6/site-packages/selenium/webdriver/firefox/extension_connection.py", line 47, in __init__
    self.binary.launch_browser(self.profile)
  File "/usr/lib/python2.6/site-packages/selenium/webdriver/firefox/firefox_binary.py", line 48, in launch_browser
    self._wait_until_connectable()
  File "/usr/lib/python2.6/site-packages/selenium/webdriver/firefox/firefox_binary.py", line 95, in _wait_until_connectable
    self.profile.path, self._get_firefox_output()))
selenium.common.exceptions.WebDriverException: Message: 'Can\'t load the profile. Profile Dir: /tmp/tmpOGd4Im Firefox output: Xlib:  extension "RANDR" missing on displa
y ":1".\n*** LOG addons.xpi: startup\n*** LOG addons.xpi: checkForChanges\n*** LOG addons.xpi: No changes found\n'
[dande@j ~]$ 

Fixing:

[dante@j ~]# pip-python install -U selenium
Downloading/unpacking selenium
  Downloading selenium-2.32.0.tar.gz (2.5Mb): 2.5Mb downloaded
  Running setup.py egg_info for package selenium
Installing collected packages: selenium
  Found existing installation: selenium 2.30.0
    Uninstalling selenium:
      Successfully uninstalled selenium
  Running setup.py install for selenium
Successfully installed selenium
Cleaning up...
[dante@j ~]#

All set.