Evolving to sapience billions of years ago, The First Born and Lorien, the first among them, were humanoids born naturally immortal, never aging though still susceptible to death by injury or illness. At first they were kept in balance by the race's relatively low birthrates with less than a handful each year. Over time this changed as the younger generations began to grow old, infirm and died, until eventually only those that came first remained. They developed a peaceful advanced civilisation and eventually left their homeworld to explore the galaxy. In their travels they visited countless worlds and found many infant races. Thye acted as teachers and guardians before eventually stepping aside and allowing them to make their own paths, creating great empires, exploring and founding even younger races to whom they became teachers and guardians.
Gradually, over the course of a million years, the First Born who did not perish through injury or illness and most of their remaining pupils left the galaxy. Of those few who were left behind, Lorien was the only First Born to remain while among the handful of First Ones that did not leave, the Vorlons and Shadows took up the responsibility of the sole guardians and protectors of the younger races. Lorien eventually withdrew and dwelt deep beneath the planet that would become known as Z'ha'dum. He would remain there for a million years, but never forgotten by those he had taught. The Shadows, in their endless series of conflicts would always eventually return to the planet after being driven off, in respect for Lorien.
In 2261, Lorien left Z'ha'dum and helped bring an end to the Shadow and Vorlon conflict and left the Galaxy with his remaining pupils, mainly the Shadows and Vorlons.
The First Born are nowadays able to appear in their original humanoid form and in an evolved non-corporeal form which can travel through space at FTL speeds. They possess tremendous psychic powers.
Source: Babylon 5 universe