coastal plain vs terra firma

coastal plain and terra firma both are nouns.

Word	Noun	Adjective	Verb	Adverb
coastal plain	Yes	No	No	No
terra firma	Yes	No	No	No

As nouns, terra firma is a hypernym of coastal plain; that is, terra firma is a word with a broader meaning than coastal plain:

Other hypernyms of coastal plain include dry land, earth, ground, land, solid ground.

coastal plain (noun)	terra firma (noun)
a plain adjacent to a coast	the solid part of the earth's surface