Here is a copy of the rule
3.7: Fire-Retardant Clothing: Full SFI 3.2/A- or FIA 8856-2000-certified fire-retardant driving suits must be worn by all drivers at all times while inside the car. If using a single-layer SFI 3.2/A1 or 3.2/A3 suit, driver must also wear fire-retardant SFI- or FIA-certified longjohns. Multilayer suits rated SFI 3.2/A5 or higher are highly recommended and may be worn without longjohns. Fire-retardant FIA- or SFI-rated racing gloves and shoes are required. And yeah, while they may very well be superior, military-spec or firefighter suits are not FIA- or SFI-rated, so we can't accept them.
