sfr {fail-open | fail-close [monitor-only]} <- There's a couple different options here. The first one is fail-open which means that if the Firepower software module is unavailable, the ASA will continue to forward traffic. fail-close means that if the Firepower module fails, the traffic will stop flowing. While this doesn't seem ideal, there might be a use case for it when securing highly regulated environments. The monitor-only switch can be used with both and basically puts the Firepower services into IDS-mode only. This might be useful for initial testing or setup.