Can somebody explain to me what is the difference between FFSK (Fast Frequency Shift Keying) and FSK (Frequency Shift Keying) modulation schemes?
Fast FSK is just another term for minimum-shift keying (MSK). In the modern literature, the term fast FSK has been almost completely replaced by MSK.
There is a very good tutorial paper on MSK by S. Pasupathy:
Subbarayan Pasupathy, Minimum Shift Keying: A Spectrally Efficient Modulation, IEEE Communications Magazine, 1979.
The following is a quote from that paper:
Since the frequency spacing is only half as much as the conventional $1/T$ spacing used in noncoherent detection of FSK signals, MSK is also referred to as Fast FSK [...]
You should also be able to find a section on MSK in any good text on digital communications.