Is this sheathing fire rated?

Polystyrene is quite flamable and gives off toxic fumes when burned. Under most conditions you will not want to leave it exposed.

No. But being sheathing it does not have to be fire rated as it's on the exterior of the house. Gypsum wallboard on the interior of the stud walls along with fire-blocking at the top and bottom plate of all walls will meet fire rating codes. Fire rating is to slow down the spread of a fire inside a house to allow emergency personnel to arrive. The outside of your house could burn away like a torch, we don't want it to spread from room to room instide...