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.
"Mein Herz brennt" opens like a children's bedtime show. Its first lines echo the real intro to a beloved German TV program that used to send kids off to sleep: "Now dear children, pay close attention." Rammstein twists that gentle voice into something unsettling.
The singer becomes "the voice from the pillow," a figure who visits children at night, promising a story but really coming to take something from them: their tears, their fear, maybe their trust. Demons and ghosts crawl out of the cellar while the storyteller sings until dawn, his heart burning like a light in the sky.
Till Lindemann has described the song as a comment on the strange power of being a public voice that reaches into people's homes, much like the old TV puppet did every night. It plays on how comforting a familiar voice can be, and how easily that comfort can turn into something darker.