We claim the ornamental design for a football helmet window cling, as shown and described.
FIG. 1 is a perspective view of a football helmet window cling, showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof; and,
FIG. 5 is a top plan view thereof, the bottom plan view being a mirror image thereof.
The broken lines showing a window in FIG. 1 and text and a stripe design on the helmet in FIGS. 1-3 depict environmental matter and form no part of the claimed design.