B757-200
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Confusion; Time Pressure
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Weather; Environment - Non Weather Related
At normal 500 feet mark; Pilot Monitoring called 'Stable; Cleared to Land Runway XXL' to which Pilot Flying replied; 'Landing'.  Pilot Flying was observed to be within parameters of stable approach to land during all phases. Winds were variable out of the east. Pilot Monitoring remarked a CRJ holding short for departure on [Runway] XXL as the flight approached runway to land. As aircraft crossed threshold; aural cadence stated '50....'then; startling us both; it sounded '40; 30; 20; 10' rapidly as the cadence sped up. Pilot Flying held pitch and applied power to attempt to slow descent rate but it seemed as though the bottom dropped out immediately.  Aircraft landed firmly.It was a startling event and one which took us by complete surprise. I wonder if there could be a connection between the CRJ holding short; perpendicular to the numbers and the easterly; variable winds--possibly generating a unique mechanical turbulence which caused the rapid decay in descent so close to the runway.Everything leading up to the '50' aural call was stable and within normal parameters.  I'm not certain this event could have been 'prevented'; per se; as it happened so very quickly; and close to the runway.  Pilot Flying absolutely attempted to mitigate the rate by adding power and holding pitch.
B757-200 First Officer reported that the approach to landing was stable up until 50 feet AGL; then the descent rate rapidly increased; resulting in a firm landing.