Installs Firefox addon.

        Returns identifier of installed addon. This identifier can later
        be used to uninstall addon.

        :param path: Absolute path to the addon that will be installed.

        :Usage:
            ::

                driver.install_addon('/path/to/firebug.xpi')

def install_addon(self, path, temporary=None):
        """
        Installs Firefox addon.

        Returns identifier of installed addon. This identifier can later
        be used to uninstall addon.

        :param path: Absolute path to the addon that will be installed.

        :Usage:
            ::

                driver.install_addon('/path/to/firebug.xpi')
        """
        payload = {"path": path}
        if temporary is not None:
            payload["temporary"] = temporary
        return self.execute("INSTALL_ADDON", payload)["value"]