Firmamento is a beautiful, poetic word for "the heavens" or "the sky". While you might know cielo for "sky," firmamento is a more literary and elevated term, often used to evoke a sense of grandeur and wonder.
In the song, Pablo Alboran uses it to create a stunning romantic image. He sings that the stars in the firmamento "die of jealousy" when they hear his lover's name, suggesting their beauty is so great it outshines the heavens themselves.
This is one of the most romantic songs you'll ever hear. Pablo Alborán is completely devoted to one person who has changed his entire world. He feels like they've brought color and light back into his life, singing, "Haces que mi cielo vuelva a tener ese azul" (You make my sky turn that blue again) and "Pintas de colores mis mañanas sólo tú" (Only you paint my mornings with colors).
He sees this person as almost magical. He says their voice is like an ocean he can sail on and that their eyes are like flashes of light. He even gets a little dramatic, telling them, "No menciones tu nombre que en el firmamento / Se mueren de celos" (Don't say your name because in the sky / They'll die of jealousy). The whole song repeats the central idea over and over: "Tú, y tú, y tú, y solamente tú" (You, and you, and you, and only you) are the one who makes his soul come alive.