the problem is in using spherical basis functions together with the 6-31G basis set. As a general rule, older basis sets are usually designed to use Cartesian d functions. In particular, the following sets in Firefly should be used with Cartesian d functions:
STO-NG, N-21G, N-31G, SBKJC, HW, DH, DZV, TZV, MINI, and MIDI.
If you disable the d5 option ($CONTRL d5=.f.), it will work.