The ornamental design for a firearmreceiver, as shown and described.
FIG. 1 is a front right isometric view of a firearm receiver according to the present invention.
FIG. 2 is another front right isometric view thereof.
FIG. 3 is a rear right isometric view thereof.
FIG. 4 is a front right bottom isometric view thereof.
FIG. 5 is a front view thereof.
FIG. 6 is a rear view thereof.
FIG. 7 is a left side view thereof.
FIG. 8 is a right side view thereof.
FIG. 9 is a top view thereof; and,
FIG. 10 is a bottom view thereof.
The broken lines show a boundary of the claimed design and form no part thereof. The portions of the firearm receiver depicted in de-emphasized half tone in each of the figures depict environmental structure and form no part of the claimed design.