they just made up the word to be different.

It's just the ancient Jewish cosmology. The "firmament" was a dome which held the sky, and rested on "the foundations of the heavens".

Funny how the Bible reflects the mis-beliefs of its day rather than reality, eh?

You can do a google search whenever you have questions.

Link: [en.wikipedia.org]

Definition of firmament
1 : the vault or arch of the sky : heavens
Stars twinkled in the firmament.

2 obsolete : basis

3 : the field or sphere of an interest or activity
the international fashion firmament
She's a rising star in the city's artistic firmament.