FiveD is the name of the original firmware used on the Darwin and early Mendels.
While it featured acceleration in principle, it was unusable for the lack of a movement buffer. It's speciality: acceleration was calculated on the host side, so it would have been able do lookahead long before other firmwares came up with this feature. When more modern firmwares arrived, FiveD was pretty much abandoned.
One modern replacement is Teacup Firmware, which is currently the only firmware supporting old electronics like Generation 3 Electronics and featuring this special kind of acceleration (ACCELERATION_REPRAP).