The ornamental design for a double doorunit for a firebox, as shown and described.
FIG. 1 shows a front view of a double door unit for a firebox with both
doors closed;
FIG. 2 shows a side view of the double door unit for a firebox, the
opposite side being substantially identical except for the chain lock and
two closure members;
FIG. 3 shows a front view of a double door unit for a firebox with both
doors open; and,
FIG. 4 shows a front view of a double door unit for a firebox showing a
second embodiment of my new design, the sole difference from the first
embodiment being the window in the upper door.
The broken line showing of a firebox in FIGS. 1-4, is for illustrative
purposes only and forms no part of the claimed design.