A seat includes a seatback including a top portion, a seat bottom coupled to the seatback, an airbag, a tether fixed to the seatback below the top portion and attached to the airbag, and an airbag extension. The airbag is inflatable from an undeployed position in the top portion to a deployed position attached to the top portion and extending in front of a front panel of the seatback. The airbag extension is inflatable from the airbag in the deployed position in front of the seat bottom.