raqia in Hebrew derives from the root word to beat or stamp out, and is used in several places in the OT for stamping out gold or other metals into sheets. Greek stereoma, used by the Septuagint translators, has the exact same connotation of solidity, also referring to stretched or beaten out metal. St. Jerome, who knew both Hebrew and Greek, also adopted that sense, using the Latin firmamentum. Between the translators of the Septuagint and St. Jerome, they knew Hebrew and Greek, and St. Jerome Latin also, better than any living human being in modern times.
More importantly than etymological considerations, Sacred Scripture describes the firmament and its properties. It holds back waters from the earth, and it has windows that could be opened to allow these waters to inundate the earth during the great flood, and these were obviously literal H2O waters. Finally, and definitively, the Church Fathers all unanimously agreed that there were liquid H2O waters (not metaphorical waters) held up by this firmament.