Hastings' Dictionary of the Bible (1898)
FIR'MAMENT. The word "expansion" would more perfectly convey the meaning of the original word. Gen 1:17. A similar idea is suggested Ps 104:2; Isa 40:22, and the same word is used to denote a "covering," Num 16:38-39, or a "spreading over," Isa 40:19, or "spread forth." Isa 42:5. The Jews probably understood the word "firmament" to denote an immense arch or canopy sprung from one side of the horizon to the other, studded with stars and forming a
sort of separating wall between the upper and lower waters. See Ps 19:1; Dan 12:3. The stars are represented as dropping from their settings in it. Isa 34:4; Matt 24:29.