A fastener driver includes a housing defining a head portion and a handle portion, a drive mechanism positioned within the housing, and a firing mechanism. The firing mechanism includes a primary guide member supported within the head portion of the housing, a secondary guide member spaced from the primary guide member and supported within the head portion of the housing, a piston slidable along the primary guide member and the secondary guide member, a driver blade attached to the piston and configured to be movable along a drive axis, and a biasing member configured to move the piston and the driver blade from a top dead center (TDC) position toward a bottom dead center (BDC) position. A lifter assembly is operated by the drive mechanism to return the piston and the driver blade towards the TDC position, against the bias of the biasing member.