They completed the dance, and waited for judgement. The dragons looked at them. They were already fairly sure, but they had to be completely so.
The shorter one was the Avatar. He sought peace. He had locked his inner fire away to avoid hurting the ones he cared about, but wanted to use it to protect the world. His heart was pure, and he passed.
The other one was trickier. His heart had struggled before. His inner fire had grown lost, used to being fed on hatred and anger and having lost its fuel. Before, he would not have been worthy, but would he stay so?
They made their decision, and showed them Firebending's true self.
Note: Sorry if you saw the first, really screwed up version. FFdotnet is being weird.