B757-200
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Commercial
Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Wake Vortex Encounter; Inflight Event / Encounter Unstabilized Approach
General None Reported / Taken
Environment - Non Weather Related; Procedure; Human Factors
The First Officer flew a great approach. He was stable at 500 ft. Over threshold (MEM RWY 27); the aircraft encountered a slight wind shift which created a slight airspeed loss.  Sink rate increased a bit and I directed the First Officer to begin his flare. The touchdown was a bit firm; so we wrote up the firm landing on the AML upon reaching our gate. Target Vref was 119 + 5 = 124 kias. Winds calm.I believe a wind shift or wake turbulence from the preceding landing aircraft caused this event.Because the aircraft was light weight; I had instructed the First Officer to carry power into the flare. As I felt the sink rate increase on very short final; I instructed the First Officer to begin his flare.  The event occurred so close to landing that there was very little time for the First Officer to implement my recommendation.
Stable approach at 500 ft.; continued stable approach with small corrections until 100 ft. and over the threshold; when Pilot Flying and Pilot Monitoring's eyes transitioned to mostly outside at the aim point. Pilot Monitoring felt a sinking feeling about 50 ft.; and instructed Pilot Flying to 'Flare' approaching 30 ft. Pilot Flying was slow to put flight control inputs; resulting in a firm touchdown. Pilot Monitoring took controls of the aircraft once on the ground. Last recognized crosscheck of airspeed showed on VREF +5 (119 +5). Because aircraft was light weight; Pilot Flying held power through the flare. This resulted in a firm landing. [The cause was] most likely a last minute change in winds or turbulence from [preceding] aircraft.Suggestion is if Pilot Flying recognized sink rate sooner and applied an increase in power; or if the Pilot Flying timely inputted the correct flight control inputs instructed by the Pilot Monitoring.
B757-200 flight crew reported a firm landing resulted after encountering a wind shift or wake turbulence.