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