We claim the ornamental design for a firearm accessory, as shown and described.
FIG. 1 is a top, front, left side perspective view.
FIG. 2 is a top, rear, right side perspective view.
FIG. 3 is a front elevation view.
FIG. 4 is a rear elevation view.
FIG. 5 is a top plan view.
FIG. 6 is bottom plan view.
FIG. 7 is a left side elevation view; and,
FIG. 8 is a right side elevation view.
The firearm accessory described in this design patent is characterized by a protruded ellipse having a number of features on one end, which are seen best in the front elevation view.