The ornamental design for a firedetector, as shown and described.
FIG. 1 is a top perspective view of an inventive fire detector;
FIG. 2 is a bottom perspective view of the fire detector depicted in FIG. 1;
FIG. 3 is a bottom view of the fire detector depicted in FIG. 1;
FIG. 4 is a top view of the fire detector depicted in FIG. 1; and,
FIG. 5 is a side view of the fire detector depicted in FIG. 1, where orthogonal side views and an opposing side view to that shown in FIG. 5 are identical that depicted in FIG. 5.