Just giving it a wild shot, but it looks to me like the difference is in the reduction method. QNH uses ISA temp and pressure gradient, while QFF uses actual station temperature?
Or in other words, the QFF value tells you what the pressure at sea level would be (useful for plotting isobar charts), and the QNH value tells you what to dial on the altimeter so that it reads airfield elevation at the threshold when you land (useful for, obviously, landing a plane)
At least that is my understanding from reading the article on
Atmospheric Pressure at the Wikipedia and this
list of Q codes. Other than that, meteorology is not my strong point at all.