I claim the ornamental design for the firearm lower receiver, as shown and described.
FIG. 1 is a top perspective view of a firearm lower receiver, showing my new design.
FIG. 2 is a bottom perspective view of the firearm lower receiver.
FIG. 3 is a left side view of the firearm lower receiver.
FIG. 4 is a right side view of the firearm lower receiver.
FIG. 5 is a rear view of the firearm lower receiver.
FIG. 6 is a front view of the firearm lower receiver.
FIG. 7 is a top view of the firearm lower receiver; and,
FIG. 8 is a bottom view of the firearm lower receiver.
The dashed broken lines represent a possible environment of a complete lower receiver and form no part of the claimed design. The long-dash-short-dash broken lines define the bounds of the claimed design and form no part thereof.