title: Not able to update firefox prefrenecs in geckodriver
body: <p>I am launching firefox version 51 using geckodriver with following code to setpreferences but when i see in action:config in firefox . Preferences are not getting updated</p>
  
  <pre><code>nodeAddress = 20.20.20.20 #Node addres where browser is getting launched
  d = DesiredCapabilities.FIREFOX
  d['loggingPrefs'] = {'browser': 'ALL'}
  d['marionette'] = True
  d['binary'] = 'C:\\Program Files (x86)\\Mozilla Firefox\\firefox.exe'
  f = webdriver.FirefoxProfile()
  f.set_preference('media.navigator.permission.disabled', True)
  d['firefox_profile']=f.encoded
  driver = webdriver.Remote(command_executor=nodeAddress,desired_capabilities=f)
  </code></pre>
  
  <p>Also tried this too</p>
  
  <pre><code>nodeAddress = 20.20.20.20 #Node addres where browser is getting launched
  d = DesiredCapabilities.FIREFOX
  d['loggingPrefs'] = {'browser': 'ALL'}
  d['marionette'] = True
  d['binary'] = 'C:\\Program Files (x86)\\Mozilla Firefox\\firefox.exe'
  f = webdriver.FirefoxProfile()
  f.set_preference('media.navigator.permission.disabled', True)
  driver = webdriver.Remote(command_executor=nodeAddress,desired_capabilities=f,browser_profile=f.encoded)
  </code></pre>