The ornamental design for a fabric tissue box cover, as shown and described.
FIG. 1 is a top, front, and right side perspective view of a fabric tissue box cover showing my new design, the broken-line disclosure of a portion of tissue is for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right side elevational view thereof, the left side being a mirror image of that shown; and,