The Freddy Fazbear mask makes all of the animatronics not reconize you and thinks you are Freddy Fazbear. It only doesn't work on the Marionette ( The puppet ) which technically kind of made the animatronics, as in put the souls into them.

Who does the Freddy mask work on?

The Freddy Fazbear Mask is one of the core mechanic in the FNaF 2 challenge. When any Toy Animatronic is in the vents, or any Withered Animatronic is entering the office, the player must immediately wear on the Freddy Fazbear Mask. It can ward off the animatronics.

Why does the Freddy mask not work on Foxy?

The working theory is that Foxy's endoskeleton is very twitchy and doesn't have proper recognition software installed. Therefore, he views everything as the same entity, but he is susceptible to lights due to his malfuntioning photoreceptors being too sensitive to it.