Firmament translates to "firmament" or "sky". It's a more poetic and archaic word for the sky, evoking a sense of grandeur and vastness that isn't captured by the more common "Himmel" (heaven/sky).
In Rammstein's "Mein Herz Brennt," the word appears in the line "Ein heller schein am firmament" (A bright glow in the firmament). This choice of word adds a dramatic and almost mythical quality to the imagery, suggesting a powerful, almost supernatural event unfolding in the heavens as the heart burns.
Welcome to one of Rammstein's darkest fairytales. The song starts like a creepy bedtime story: "Nun, liebe kinder, gebt fein acht" (Now, dear children, pay close attention). The narrator is a terrifying figure, the "stimme aus dem kissen" (voice from the pillow), who brings children a special gift torn directly from its own chest: a burning heart.
This isn't a story about love. This heart gives the narrator the power "Die augenlider zu erpressen" (to blackmail the eyelids), controlling whether the children can sleep or not. It brings an army of nightmares with it, including "Dämonen, geister, schwarze feen" (demons, ghosts, black fairies) that crawl from the cellar to hide under the bed. These creatures then steal the children's tears and press them into the narrator's "kalten venen" (cold veins). The whole song builds around that one, intense declaration: "Mein herz brennt!" (My heart burns!).