The First Firmament was the origin of all universes. It was the only and the last, the sole totality of all existences.
It was shattered during an ancient war between Oa and the Celestials. Weapons were fired by a faction of rebellious Celestials called Aspirants, weapons invented by Oa to damage those otherwise invulnerable titans. The Godkiller attacked reality at the quantum level. It was deployed by one Oan scientist against the Source itself. This resulted in the First Firmament collapsing and shuddering apart into infinite uncounted universes, each segregated from one another by unique P-brane frequencies and forming the larger multiverse.
The end of the First Firmament may have marked the end of all reality in all universes. Some have expressed concern that the Oan superweapon has made it impossible for the multiverse to ever sustain a 'big crunch' that would lead to another explosion of life from the Source. All reality may, in time, wither and flicker into nothingness instead.