We claim the ornamental design for the disposable toothbrush holder, as shown and described.
FIG. 1 is a front view of the disposable toothbrush holder.
FIG. 2 is a left side view of the claimed design.
FIG. 3 is a right side view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design. The toothbrush of FIG. 6 is of broken lines, and is environmental matter forming no part of the claimed design.