The ornamental design of a footbath, as shown and described.
FIG. 1 is a perspective view of a foot bath with a lid being in a closed position;
FIG. 2 is a perspective view of the foot bath with the lid being in an open position;
FIG. 3 is a top view of the foot bath;
FIG. 4 is a front view of the foot bath;
FIG. 5 is a side view of a first side of the foot bath;
FIG. 6 is a side view of a second side of the foot bath; and,
FIG. 7 is a bottom view of the foot bath.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.