Yangchen's predecessor

The unnamed fire Avatar was an Avatar who hailed from the Fire Nation centuries prior to the War.

Background
The unnamed fire Avatar lived before Avatar Yangchen's era. His attire bore striking similarities to the garments worn by the Fire Sages, suggesting that he was affiliated with the religious faction during his time. As aforementioned, his life remains a mystery, though his appearances are strong indications of his skill and bending prowess, as at one point of his life he erupted multiple volcanoes at once. The unnamed fire Avatar eventually passed away and Yangchen was born, bearing the Avatar Spirit.

Appearances in Aang's life
During a retreat to General Fong's fortress, Aang inadvertently entered the Avatar State as a result of his anger towards General Fong, who endangered Katara in an attempt to force Aang into the state.

Amidst the situation, Aang was taken by Avatar Roku on a journey to the Spirit World, where he was taught about the Avatar State and the Avatar Spirit in general. While explaining the extent of the Avatar State's capabilities, the unnamed fire Avatar was briefly shown erupting several volcanoes with the use of enhanced firebending and earthbending.

The unnamed fire Avatar made another appearance later on during the Battle at Wulong Forest, where he was seen amongst a line of Avatars directly after Aang unlocked the state as a result of impacting a jutting rock.

Abilities
Albeit being shown only briefly, it is evident that the unnamed fire Avatar had complete control of the Avatar State, able to enter and exit it at will. He was seen controlling a number of small volcanoes with the use of his abilities, though the exact extent of his power remains unexplored.