An assembly for a vehicle includes a seat having a seat bottom and a seatback supported by the seat bottom. The seat bottom and the seatback define an occupant seating area. The seat includes an airbag supported by an upper end of the seatback. The airbag is inflatable downwardly from the upper end of the seatback enclosing the occupant seating area between the airbag and the seatback. The seat includes a pyrotechnic device fixed to the seat below the airbag. The pyrotechnic device is connected to the airbag.