The ornamental design for a frontbumper for a motor vehicle, as shown and described.
FIG. 1 is a left side perspective view of a front bumper for a motor vehicle;
FIG. 2 is a right side perspective view thereof;
FIG. 3 is an elevation view from the left side thereof, the right side being a substantial mirror image thereof;
FIG. 4 is a front elevation view thereof; and,
FIG. 5 is a top plan view thereof.
The broken line showing of the front bumper for a motor vehicle is for the purpose of illustrating portions of the front bumper for a motor vehicle and forms no part of the claimed design.