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`

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]