The ornamental design for a firearm, as shown and described.
FIG. 1 is a top, front, left perspective view of a firearm showing the new design.
FIG. 2 is a top, front, right perspective view thereof.
FIG. 3 is a left view thereof.
FIG. 4 is a front view thereof.
FIG. 5 is a right view thereof.
FIG. 6 is a back view thereof.
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines immediately adjacent to the solid line disclosure represent the unclaimed boundaries of the claimed design. The broken lines showing the remainder of the firearm are for illustrative purposes only and form no part of the claimed design.