Firmamento translates to "firmament" or "sky." It's a more poetic and less common word for the sky than cielo.
In the song, Buika uses firmamento to evoke a sense of vastness and timelessness, setting a melancholic mood. She sings, "Sólo las estrellas en el firmamento" (Only the stars in the firmament), painting a picture of a lonely night sky that mirrors her inner sorrow and the enduring pain of a lost love.
Buika's "Culpa Mía" is a beautiful, melancholic song about the heavy weight of regret after a breakup. The singer looks back at a relationship that ended, and she takes all the blame for its failure. She sings, "Mientras no haya sol / Que ilumine el cielo / Sólo las estrellas / En el firmamento" [As long as there's no sun / To light up the sky / Only the stars / In the firmament], setting a somber mood for her reflection.
She describes how this sadness is slowly killing her soul: "Corazoncillo loco / Que mata mi alma / Tan poquito a poco" [Crazy little heart / That kills my soul / So little by little]. The pain is so intense, she says, "Nunca me ha dolido tanto / Este sufrimiento que me está matando" [This suffering that is killing me / Has never hurt me so much]. She remembers the day her love left, "Como llovía / La tarde en que te fuiste" [How it rained / The afternoon you left], and even now, thinking of it makes her sad. She repeatedly says, "Considero que fue todo culpa mía" [I consider it was all my fault], listing her silences, unanswered calls, and even happy memories like dancing under the first moon as her fault. It's a powerful and emotional journey through the pain of self-blame.