The First Firmament was the first ever universe, the First Cosmos to ever exist before the Multiverse, and absolutely anything else.
The Celestials then started making their own creations to impress the First Firmament, hoping to get its approval. However, when the Celestials wanted their creations to have the capacity to mutate and evolve, the First Firmament saw this has heresy against it, and made the Astrals and fought against it.
During the finale hours of the Celestial War, the Celestial known as Aion, using the Hammer of Aion, shattered the First Firmament and what remained of the First Firmament fled to the edges of its existence along with the remaining Astrals. Eventually, the First Firmament shattered pieces formed other realities, forming the Second Cosmos, the First Multiverse.
Whenever the Multiverse renews, and reborn, the grows more hatred to everything within it. The First Firmament wants to be whole again, and bring forth the Apocalypse on every sentient being for daring to exist. But the time for that is not yet there, and so the First Firmament awaits in the Dark Void outside of the Multiverse.