We claim the ornamental design of a firearm magazine, as shown and described.
FIG. 1 is a perspective view of a firearm magazine showing our design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top view thereof.
The broken lines in the drawings represent features, which are illustrated only for the purpose of understanding the design, and form no part of the claimed design.