title: "Unable to find a matching set of capabilities" -- Selenium with Python 2.7
body: <p>Using Selenium 3.8.1 with Python 2.7 with Firefox Portable 54, 64 bit, I get the following error message when running this script:</p>
  
  <pre><code>from selenium import webdriver
  from selenium.webdriver.firefox.firefox_binary import FirefoxBinary
  
  binary_argument = FirefoxBinary(r'C:\Users\[removed]\FirefoxPortable.exe')
  driver = webdriver.Firefox(firefox_binary=binary_argument)
  
  driver.get("http://icanhazip.com")
  </code></pre>
  
  <p>.</p>
  
  <pre><code>Traceback (most recent call last):
    File "F:/cp/python-selenium3/ToyScripts/launch_portable_browser.py", line 5, in &lt;module&gt;
      driver = webdriver.Firefox(firefox_binary=binary_argument)
    File "C:\ProgramData\Anaconda2\envs\automation2\lib\site-packages\selenium\webdriver\firefox\webdriver.py", line 162, in __init__
      keep_alive=True)
    File "C:\ProgramData\Anaconda2\envs\automation2\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 154, in __init__
      self.start_session(desired_capabilities, browser_profile)
    File "C:\ProgramData\Anaconda2\envs\automation2\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 243, in start_session
      response = self.execute(Command.NEW_SESSION, parameters)
    File "C:\ProgramData\Anaconda2\envs\automation2\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 312, in execute
      self.error_handler.check_response(response)
    File "C:\ProgramData\Anaconda2\envs\automation2\lib\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
  </code></pre>
  
  <p>I do see the splash screen for a moment before it disappears. Also worth noting is that the browser works fine when used manually. </p>
  
  <p>I suspect that the portable browser needs some settings switched to allow Selenium to take over, but that is the extent of what i know/suspect.</p>
  
  <p>Thank you for any help with this.</p>