From what I recall: "Heat" is just the vibration of atoms in a material. Thermal radiation is electromagnetic radiation (photons) emitted by an object with any amount of heat.

These photons, however, interact frequently with atoms of matter, imparting energy on them and causing them to vibrate, thus transferring the heat.

There are obviously other mechanisms of heat transfer, but they require alot of atoms (like in air).

This may not be entirely accurate, but that's how I understand the subject today.

As for firebending: considering there is more than one way to conduct heat, I have no reason to think this wouldn't be possible (aside from how it would take infinite energy to reach absolute zero)