The ornamental design for a toothbrush handle, as shown and described.
FIG. 1 is a perspective view of the toothbrush handle embodying the new design;
FIG. 2 is a right side view of the toothbrush handle, the left side view being a minor image thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
FIGS. 1-6 show a replacement toothbrush head, and a portion of the toothbrush handle in phantom lines. The portions shown in phantom lines are shown for environmental purposes only and form no part of the claimed design.