The first firmament is a creature created at the same time as the One-Above-All and One-Below-All, and at one point in time he was reality, time, space, and the universe all at once, dubbed the 'first cosmos,' but in his loneliness he created life, known as Celestials/Aspirants. The 'second cosmos' emerged from creation, but it paled in comparison to the first firmament because it possessed just a fraction of his might. The first firmament absorbed the second cosmoses existence after the 'second cosmos' produced the third cosmos, this deity was the essence of the first ever multiverse and at this time Oblivion, Death, Living Tribunal, Power Cosmic and The Brother Yin were created. The new Multiverse was given the name "Seventh Cosmos" after the embodiment of the Sixth Cosmos died(destroyed by Black Winter). Unlike the other Cosmoses, the Seventh Cosmos was not embodied by a single entity, but 2 Infinity and Eternity, they are the incarnations of the Seventh Cosmos. The majority of the Marvel universe was constructed at the creation of the seventh cosmos.
The first firmament is an omnipotent, omnipresent, and omniscient being. The 2nd cosmos was absorbed by the first firmament, which then he absorbed the rest of the cosmos until only the 7th remains.
The first firmament is the second most powerful marvel being, he is behind the One-Above-All and One Below All who have power that is comparable to or nearly equal to each other.
The first firmament attempted to absorb eternity but was unable to do so since Dormammu already took Eternitys power, so he "destroyed" him completely instead.
" Nothing can help you now. Your own systems have been corrupted. Turned against you. They will devour you from the inside out. And I will absorb what scraps are left of you, Eternity. And all will be as it was, long ago. Before your foul kind. Before the Multiverses. Eighth and seventh... and sixth... and fifth... and back and back... dying and being born, over and over. Like filthy little organisms. We will return to a saner time. When there was but one cosmos. One universe. One reality. One totality. When there was only I. I am the First Firmament. And soon... I will be the last. " -first firmament