We claim the ornamental design for the firearm receiver, as shown and described.
FIG. 1 is a front left isometric view of a firearm receiver, showing our new design.
FIG. 2 is a front right isometric view of the firearm receiver.
FIG. 3 is a front right top isometric view of the firearm receiver.
FIG. 4 is a rear left isometric view of the firearm receiver.
FIG. 5 is a bottom front right isometric view of the firearm receiver.
FIG. 6 is a front elevational view of the firearm receiver.
FIG. 7 is a rear elevational view of the firearm receiver.
FIG. 8 is a left side elevational view of the firearm receiver.
FIG. 9 is a right side elevational view of the firearm receiver.
FIG. 10 is a bottom plan view of the firearm receiver; and,
FIG. 11 is a top plan view of the firearm receiver.
The broken lines show a boundary of the claimed design. The portions of the firearm receiver depicted in deemphasized half tone in each of the figures illustrate environmental structure which forms no part of the claimed design.