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.
Spanish singer Pablo Alborán sounds totally head over heels here, and he goes all out with poetic drama. The whole track revolves around one person having complete power over his mood. He sings that this special someone restores the blue to his sky (haces que mi cielo vuelva a tener ese azul) and colors his mornings (pintas de colores mis mañanas). In Spanish, when someone says pintas de colores mis mañanas, they mean you take away the gray, dull routine and make life bright again. He even falls into the ocean of romance with lines like me pierdo en este mar [I lose myself in this sea] and says he navigates the waves of their voice (navego entre las olas de tu voz).
Then he takes the compliments to a slightly ridiculous, cosmic level. He whispers, no menciones tu nombre que en el firmamento se mueren de celos [don't mention your name, because in the sky they are dying of jealousy]. The stars literally cannot handle the competition. Amid all this celestial praise, there is an open call for honesty too: enseña tus heridas y así las curarás [show your wounds and that way you will heal them]. For Alborán, love means laying out your flaws in plain sight while feeling like only one person (solamente tú) holds the key to waking up your soul.