old_contents: from marionette_driver import By
  from marionette_driver.errors import MarionetteException
  
  from marionette import MarionetteTestCase
  
  class Test(MarionetteTestCase):
      def test_useragent(self):
          with self.marionette.using_context('content'):
              self.marionette.navigate('about:robots')
              js = self.marionette.execute_script
              # Check that useragent string is as expected
              # We better know the ESR version we're testing
              self.assertEqual("Mozilla/5.0 (Windows NT 6.1; rv:38.0) Gecko/20100101 Firefox/38.0",
                                js("return navigator.userAgent"))
  
new_contents: from marionette_driver import By
  from marionette_driver.errors import MarionetteException
  
  from marionette import MarionetteTestCase
  
  class Test(MarionetteTestCase):
      def test_useragent(self):
          with self.marionette.using_context('content'):
              self.marionette.navigate('about:robots')
              js = self.marionette.execute_script
              # Check that useragent string is as expected
              # We better know the ESR version we're testing
              self.assertEqual("Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0",
                                js("return navigator.userAgent"))
  
subject: update useragent