docs: def update_ff_binary_env(self, variable):
          """
          If a ``FIREFOX_BINARY`` was specified, this method updates an
          environment variable used by the ``FirefoxBinary`` instance to
          the current value of the variable in the environment.
  
          This method is a no-op if ``FIREFOX_BINARY`` has not been
          specified or if the configured browser is not Firefox.
  
          A common use-case for this method is updating ``DISPLAY`` once
          an Xvfb or Xephyr instance has been launched. Typically, by
          the time these displays are launched, the configuration file
          has already been loaded and whatever ``FirefoxBinary``
          instance was created for ``FIREFOX_BINARY`` has a stale
          ``DISPLAY`` value.
  
          :param variable: The name of the variable to update.
          :type variable: :class:`str`
          """
          if self.config.browser != 'FIREFOX':
              return
  
          binary = self.local_conf.get('FIREFOX_BINARY')
          if binary is None:
              return
  
          # pylint: disable=protected-access
          binary._firefox_env[variable] = os.environ[variable]