The ornamental design for a switch for lightbulbholder, as shown and described.
FIG. 1 is a perspective view of a switch for light bulb holder showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line portions of the figure drawings is included to show portions of the switch for light bulb holder that form no part of the claimed design.