In a common Jewish or Christian view of the world, the sky is a support for something. I don't recall much of anything about this, but I know that explains the English term firmament. However, did firmamentum ever appear before to mean the sky? Some dictionaries online suggest that the term can mean sky without context, but is this true? I'm guessing they were simply providing that definition to reduce confusion for translations of religious texts, but I just want to make sure. If so, could the word be used generally? I've seen the heavens called the firmamentum in miscellaneous non-religious, more modern texts, but when was this first assumed? Was it used by Romans? Perhaps it was popularized in scientific texts from the renaissance to 1800s? I need to know a time period of its usage other than a support.
Firmamentum first shows up in the Vulgate to translate the Hebrew רָקִיעַ raqiyaʻ "firmament" (Strong's Hebrew #7549), which means expanse or support, but also the mythological arch of the heavens, separating "the waters above and the waters below" (Gn 1:7). It's a concept that finds parallel in other Ancient Near East mythologies (Wikipedia, for instance, reports that the Babylonians thought the "vault of heaven" to be fashioned from the ribs of the slain primeval goddess Tiamat).
That word רָקִיעַ is translated in the LXX by the Greek words στερέωμα steréōma "support", "that which furnishes a foundation" (Strong's Greek #4733) and οὐρανός ouranós "sky" (Strong's Greek #3772). The first word is translated in turn as firmamentum and the second as caelum in the Vulgate.
Therefore, while firmamentum has been used in pre-Christian writings, it was never in the context of the arch of the heavens, which is an Ancient Near Eastern concept anyway, but generally meaning "foundation" (either for building, etc. or an argument).