If these are 24-bit ICAO aircraft addresses, then they belong to an unallocated part of the address space. Several places on the net (1, 2) document that addresses starting with F are reserved for future use by ICAO.
The referenced documents, however, pre-date the widespread development and deployment of UAVs, and other sources on the net (e.g. here, here) indicate that "high-order" 24-bit addresses (which apparently means ones starting with F) are to be used for UAVs equipped with "ACAS Xu", a collision avoidance system (currently under development? testing?) which uses the same ADS-B compatible radio protocol as ordinary ACAS/TCAS systems, but cannot send interrogations.
The point of using addresses starting with F (rather than allocating addresses out of the nationally assigned range for the drone's country of origin) seems to be that the ACAS protocol uses "which aircraft has the higher address" as one of the inputs in coordinating which resolution advisory to issue. If a drone and an ordinary ACAS II equipped aircraft are converging, there's a risk that both ACAS systems would issue "climb" advisories (or both would issue "descend") and be technically unable to change it unless the drone has a higher address. At least, that's what I get from the sources linked above.
So your FFxxxx traffic, if you're sure they are 24-bit addresses, are probably drones exchanging ACAS reports with nearby aircraft. (DeltaLima's answer looks more convincing.)