mainland vs terra firma

mainland and terra firma both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainland	Yes	No	No	No
terra firma	Yes	No	No	No

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

mainland: the main land mass of a country or continent; as distinguished from an island or peninsula
terra firma: the solid part of the earth's surface

Other hypernyms of mainland include dry land, earth, ground, land, solid ground.

mainland (noun)	terra firma (noun)
the main land mass of a country or continent; as distinguished from an island or peninsula	the solid part of the earth's surface