I claim the ornamental design for a toothbrush sterilizer, as shown and described.
FIG. 1 is a Perspective view of a toothbrush sterilizer showing my new design;
FIG. 2 is a Back Perspective view thereof;
FIG. 3 is a Front view thereof;
FIG. 4 is a Back view thereof;
FIG. 5 is a Left-side view thereof;
FIG. 6 is a Right-side view thereof;
FIG. 7 is a Top view thereof;
FIG. 8 is a Bottom view thereof; and,
FIG. 9 is a Perspective view of a toothbrush sterilizer showing in use view.
The broken lines depicting the toothbrushes in FIG. 9 represent environmental subject matter and form no part of the claimed design; and the other broken lines illustrate portions of the toothbrush sterilizer that form no part of the claimed design.