The ornamental design for a firearm, as shown and described.
FIG. 1 is a front view of a firearm showing our new design; and,
FIG. 2 is a partial enlarged front view showing the claimed area in FIG. 1.
The unclaimed broken lines immediately adjacent to the shaded area represent the boundaries of the design. The broken lines showing the remainder of the firearm are for environmental purposes only and form no part of the claimed design.