The ornamental design for a toothbrush holder, as shown and described.
FIG. 1 is a perspective view of a first form of a toothbrush holder showing my new design, the broken-line disclosure of a toothbrush being for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevational view thereof, the side opposite being identical in appearance thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a rear elevational veiw thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view of a second embodiment thereof; and,
FIG. 8 is a perspective view of a third embodiment thereof.