I don't think you could get them to be consistently warm on Earth. But the atmosphere on Venus is so thick you could have a bunch of these floating above the clouds.

Maybe you could replace some of the nitrogen with helium to achieve positive buoyancy without a temperature difference.

Wouldn't that leak out from .. basically anything solid?

We're not talking superfluidic Helium here... Glass or BPA should be fine... In that case the envelop is rigid and we might at well just suck out enough air to get buoyancy.

When I was reading about Sterling engines, people talked about H2 leaking straight through the steel walls of the container. I can't find a similar reference for helium, but it's conceivable, to a non-expert anyway. No superfluidity required.

Big difference between H2 and He. Leakage is a problem, but not so nasty a problem. (Hydrogen leaks through solid metal. Yes you read that right.)

Helium doesn't need to be superfluid to leak through a lot of materials. If you are designing a ultra high vacuum system you better make sure the type of glass you use doesn't leak He. Or if you get the chance to play with a mass spectrometer you can inhale helium and observe how it leaks through your chest.

It really depends how big you are. A 5C difference in temperature in a sphere 5m in radius will easily hold a weight of 10kg.

Or like mikeash replied, you can replace most of the nitrogen with helium.

Whether it would float would depend on the equilibrium that the ecosystem gets in. For example, carbon dioxide is heavier than air, so if the ecosystem has lots of it, a balloon would have to expand quite a bit to compensate for that before it floats. And of course, any balloon must overcome its own weight before getting airborne.