The ornamental design for an adjustable ice cream cone holder, as shown and described.
FIG. 1 is a perspective view of an adjustable ice cream cone holder showing my new design;
FIG. 2 is an enlarged side elevational view thereof, the undisclosed side being the same;
FIG. 3 is an enlarged front elevational view thereof, the undisclosed rear being the same with an ice cream cone shown in broken lines for illustrative purposes only and forming no part of the claimed design;