QFF is effectively the pressure measured at a specific datum (airfield, meteorological station etc) adjusted to sea level using the actual conditions occurring at the time.
If you had the QFF set and landed on the sea your altimeter would read zero.

QNH is the pressure measured at a specific datum (airfield, meteorological station etc) adjusted to sea level using the lapse rates etc of the International Standard Atmosphere.
If you had the QNH set then at the airfield your altimeter would read the elevation of the airfield and if you landed on the sea it would only read zero if the actual conditions were the same as the ISA conditions ( that is if QNH=QFF).

QNH at an airfield located above sea level is 1009. Because the temperature is colder than ISA, the air is more dense and thus pressure will drop faster with altitude.
QNH takes into account the International Standard Atmosphere average drop of 1hPa with 27 feet.
QFF is the pressure measured at the airfield adjusted to mean sea level according to the actual weather conditions.
Because of the increased density the QNH has been lowered compared to QFF to let the airfield still read the correct elevation with QNH set.
That way we know that the QFF must be higher than 1009 hPa.

Remember:

ABOVE MSL:

• Temp warmer than ISA => QFF < QNH
• Temp colder than ISA => QFF > QNH

• Temp warmer than ISA => QFF > QNH
• Temp colder than ISA => QFF < QNH