The ornamental design for a toothbrushholder, as shown and described.
FIG. 1 is a perspective view taken from a front, left side of a toothbrush holder according to our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a rear elevation view thereof; and,
FIG. 6 is a top plan view thereof.
Throughout the Figures, the broken lines are for the purpose of illustrating environment only and form no part of the claimed design. Portions of the toothbrush holder not shown in the Figures and not otherwise described in this specification do not form part of the claimed design. All surfaces not shown form no part of the claimed design.