The box is held firmly in place by the Madison Bars. It should not be loose. Note how the upper and lower screw hole tabs on the box are set back into the wall slightly. Inside the rear of the box is the bare copper ground wire attached to the metal box using a 10/32 screw.
Home Electrical Receptacle Box Installation in an Existing Wall Using Madison Bars GemBoxWires-Copyright-IMG_1147-B-460x613