Anyway, the balloon would be relatively stable. The atmosphere gets increasingly dense as you go towards the surface, while the balloon has a particular density which is more than the wispy far atmosphere and less than the dense low atmosphere. Therefore, if you were to drop it at the top, it would fall (while it's more heavy) then approach the altitude where it's equally dense, and start bobbing around there, until it settles at its equilibrium.

Picture a glass cylinder of water and oil. It's cleanly separated with the denser water on the bottom and oil on top. Then drop an ice cube in. It will sink through the oil and then float, in the middle of the cylinder, on the water.