The ornamental design for a firearm, as shown and described.
FIG. 1 is a right side view of a firearm showing my new design;
FIG. 2 is a top, front, right perspective view thereof with some unclaimed subject matter omitted; and,
FIG. 3 is an enlarged right side view thereof with all unclaimed subject matter omitted.
The uneven-length broken lines represent unclaimed boundaries of the claimed design. The even-length break lines in FIG. 2 are for illustrative purposes only and form no part of the claimed design or the article of manufacture. The even-length broken lines showing the remainder of the firearm are for environmental purposes only and form no part of the claimed design.