The ornamental design for coffin furniture, as shown and described.
FIG. 1 is a perspective view of the coffin furniture showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a right side view thereof; the left side being a mirror image thereof.
The broken lines shown in the drawings illustrate portions of the coffin furniture that form no part of the claimed design.
The coffin furniture is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.