Answer
The balloon moves forward, as you can verify yourself (or look up a video).
Work Step by Step
In the reference frame of the car, which is accelerating, there appears to be a gravitational field that is pointing toward the back of the car. The balloon feels a buoyant force with a component in the opposite direction, that is, it floats forward.
Here’s another way to think about it. The inertia of the air inside the car causes it to stay still while the car accelerates forward. Unlike you, the air molecules don’t have the seatback to push them forward. The air rushes back, displacing the balloon forward.
