"Isn't the word "firmament" a synonym for the skies/heavens?"
Only if you don't know Biblical Hebrew. Then anything can be synonyms with anything else. Funny how that works. Where are those waters above the firmament (1:7)? And notice, 1:17, how the firmament is merely a structure _in_ the heavens.
Granted that God calls to the firmament "Heaven" in 1:8, but that'sjust as meaningful as God calling light "day" and darkness "night."These titles seem to tell us when and where we can find these things. We'll find light during the day, darkness at night, and the firmament in Heaven.
Anyway, the prophets of old knew Biblical Hebrew than either of us and one, Yechezkel, uses the same word "Rakia" in his book. It is said to be the surface on which God's throne sits (1:26). I think that's meaningful.