Not float, but you can keep aloft while spending minimal energy using https://en.wikipedia.org/wiki/Dynamic_soaring https://en.wikipedia.org/wiki/Dynamic_soarin

Not float, but you can keep aloft while spending minimal energy using https://en.wikipedia.org/wiki/Dynamic_soaring https://en.wikipedia.org/wiki/Dynamic_soaring Jupiter seems to have sufficiently dynamic atmosphere. Not sure about cold brown dwarfs...