The ornamental design for a firearm hammer, as shown and described.
FIG. 1 is a rear, top, left side perspective view of a firearm hammer in accordance with my new design;
FIG. 2 is a left side view thereof, the right side view of the firearm hammer being a mirror image of the left side view thereof; and,
FIG. 3 is another left side view thereof, the firearm hammer shown in combination with a trigger mechanism, wherein the firearm hammer may form a part thereof.
The broken lines having repeated short dashes in the Figures are for illustrative purposes only and form no part of the claimed design. The shade lines having a combination of long and short lines in the Figures show contour and not surface ornamentation.