The ornamental design for a suctiontoothbrushholder, as shown, and described.
FIG. 1 is a perspective view of a suction toothbrush holder;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines shown in the figures represent portions of the suction toothbrush holder that form no part of the claimed design. The broken line showing immediately adjacent the shaded areas represents boundaries to the claimed design and forms no part of the claimed design.