The ornamental design for a christmasbell, as shown and described.
FIG. 1 is a perspective view of a Christmas bell showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is bottom plan view thereof; and,
FIG. 8 is a perspective view thereof, with the top ribbon in broken lines shown for illustrative purposes only, and forming no part of the claimed design.