You don't want to revert to the older version, ie 56, because of security. You can download the
ESR version which currently is at v52.5.0 and work with legacy add-ons. ESR will be at v52 until Firefox 60 when it will be 52.8.0. This will be the last version before upgrading to Quantum on ESR 59.1.0 which will be June 2018