title: Cannot attach to an existing Selenium session via geckodriver
body: <p>After upgrading to geckodriver I'm unable to reuse my Selenium's sessions. Here's my setup:</p>
  
  <p>I have a <code>start_browser.py</code> script, which launches a Firefox instance and prints a port to connect to, like:</p>
  
  <pre><code>firefox_capabilities = DesiredCapabilities.FIREFOX
  firefox_capabilities['marionette'] = True
  browser = webdriver.Firefox(capabilities=firefox_capabilities)
  print browser.service.port
  wait_forever()
  </code></pre>
  
  <p>... and another script, which tries to connect to the existing instance via Remote driver:</p>
  
  <pre><code>caps = DesiredCapabilities.FIREFOX
  caps['marionette'] = True
  driver = webdriver.Remote(
          command_executor='http://localhost:{port}'.format(port=port),
          desired_capabilities=caps)
  </code></pre>
  
  <p>But it seems to be trying to launch a new session, and failing with a message:</p>
  
  <pre><code>selenium.common.exceptions.WebDriverException: Message: Session is already started
  </code></pre>
  
  <p>Is there an ability to just attach to the existing session, like in previous versions of Selenium? Or is this an intended behaviour of geckodriver (hope not)?</p>