This is covered in the addendum to my book:
p. 253: Correction: The F/f flags no longer appear in the output of fwaccel conns starting
in version R80.30; if a connection is being handled in the F2F path it does not appear at
all in the output of fwaccel conns, so commands such as fw ctl conntab and fw tab -u -t
connections -f must be used to verify the presence of an F2F-handled connection.
So it would appear that the F/f flags (which haven't appeared since R80.30) have been reborn in R81 as indicating a fast_accel'ed connection. The letter choice here is a bit unfortunate.
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com