solid ground vs terra firma

solid ground and terra firma both are nouns.

Word	Noun	Adjective	Verb	Adverb
solid ground	Yes	No	No	No
terra firma	Yes	No	No	No

As nouns, solid ground and terra firma are synonyms defined as:

Other synonyms of solid ground include dry land, earth, ground, land.

solid ground (noun)	terra firma (noun)
the solid part of the earth's surface	the solid part of the earth's surface