The ornamental design for a light mount block for a firearm, as shown and described.
FIG. 1 is a left, rear, top perspective view of a light mount block for a firearm attached to a firearm showing my new design;
FIG. 2 is a left, rear, top perspective view of the light mount block for a firearm of FIG. 1 with the firearm omitted;
FIG. 3 is a front elevation view of the light mount block for a firearm of FIG. 1;
FIG. 4 is a rear elevation view of the light mount block for a firearm of FIG. 1;
FIG. 5 is a left side elevation view of the light mount block for a firearm of FIG. 1;
FIG. 6 is a right side elevation view of the light mount block for a firearm of FIG. 1;
FIG. 7 is a top plan view of the light mount block for a firearm of FIG. 1;
FIG. 8 is a bottom plan view of the light mount block for a firearm of FIG. 1;
FIG. 9 is a left, front top view of the light mount block for a firearm of FIG. 1; and,
FIG. 10 is a right, front, bottom view of the light mount block for a firearm of FIG. 1.
The broken lines showing portions of the design in all figures are included for the purpose of illustrating environment and unclaimed features and forms not part of the claimed design.