The ornamental design for a dental flossholder, as shown and described.
FIG. 1 is a front view of a dental floss holder, showing our new design;
FIG. 2 is a right side view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a sectional view thereof, taken along line 4-4 of FIG. 1.
In the drawings, the broken lines shown represent portions of the dental floss holder that form no part of the claimed design. The dot-dash lines represent only the boundary between the claimed portion and unclaimed portions. The rear view is omitted, because it is a mirror image of the front view. All sides not shown except for the rear view form no part of the claimed design.