"I have already told thee, Sancho," said Don Quixote, "not to give thyself any uneasiness on that score; for if an island should fail, there is the kingdom of Denmark, or of Sobradisa, which will fit thee as a ring fits the finger, and all the more that, being on terra firma, thou wilt all the better enjoy thyself.
There are no more uses of "terra firma" in the book.
Show samples from other sources
When the elevator thumped to a stop, Langdon was relieved to feel terra firma beneath his feet.
Dan Brown -- Angels & Demons
God Almighty, I’m glad I’m right here on old terra firma.