The ornamental design for a firearm, as shown and described.
FIG. 1 is a left side view of a firearm showing my new design;
FIG. 2 is an enlarged top, rear, left perspective view thereof, wherein some unclaimed subject matter has been omitted for clarity;
FIG. 3 is an enlarged left side view thereof;
FIG. 4 is an enlarged right side view thereof; and,
FIG. 5 is an enlarged top, rear, right perspective view thereof.
The even length broken lines immediately adjacent to the shaded area represent unclaimed boundaries of the design. The even length broken lines showing the remainder of the firearm are for environmental purposes only and form no part of the claimed design. The uneven length broken lines represent imaginary break lines.