The phrase the more firma, the less terra is used to express distrust at air travel or sea travel.

With a pun on terror, this phrase jocularly decouples from each other the components of the Latin expression terra firma (literally firm land), meaning dry land, i.e., the land as distinct from the sea, and later also, the land as distinct from the air.
—Cf. also, on the same pattern, the tooter the sweeter, meaning the sooner the better, which jocularly detaches from each other the words constituting toot sweet, meaning straight away, immediately.

The phrase the more firma, the less terra was originally used of air travel.

The earliest occurrence that I have found is from one of the Googiegrams, in the column Side Lights, by ‘Googie’, published in the Ithaca Journal-News (Ithaca, New York) of Monday 16^th August 1926:

When asked why he didn’t go up in the airplane, a friend explained: “I believe in terra firma. The more firma the less terra.”

The second-earliest occurrence of the phrase that I have found is from the review by Rowland Field of The Bachelor Father, a comedy by Edward Childs Carpenter ¹, staged at the Belasco Theatre, Broadway, New York City—review published in the Brooklyn Daily Times (Brooklyn, New York) of Wednesday 29^th February 1928:

“Tony” soon decides to venture forth in search of honors in the air at a nearby flying meet at Morton Field. In spite of her father’s rather pathetic pleadings she hurries away to cut aerial capers among the clouds. All this takes place in the play’s final act and near its end “Tony” returns to earth in an airplane crash to realize that she belongs securely on terra firma in the welcome arms of Ashley, the handsome English solicitor. As some one has put it, the more firma the less terra.

¹ Edward Childs Carpenter (1872-1950) was a U.S. playwright.

The phrase then occurs in Lindy Misses Exhibit So Mayor Won’t Fly, published in the Detroit Times (Detroit, Michigan) of Sunday 8^th April 1928:

Col. Charles A. Lindbergh ² cannot come to Detroit for the National Aircraft Show, so—
Mayor John C. Lodge cannot take his first ride in an airplane.
Promoters of the show approached Mayor Lodge with a request that he go up for his first airplane ride to advertise the opening of the event.
“Well,” objected the Mayor, “it’s this way: I sort of promised to take my first ride with my grand-nephew, Colonel Lindbergh at the controls.”
Whereupon the promoters prevailed upon the Mayor to wire Lindy in Santa Barbara, Calif., inviting him to Detroit. The answer came Saturday, as follows:
“Would be glad to pilot plane for you; regret inability to be in Detroit on that date.”
So the Mayor will remain on terra firma—“the more firma the less terra.”

² Charles Augustus Lindbergh (1902-1974) was a U.S. aviator. In 1927, he made the first solo transatlantic flight in a single-engined monoplane, Spirit of St Louis.

The phrase is also used in British and Irish English. The earliest occurrence that I have found is from the column Alleged Humour, published in The Midland Counties Advertiser (Roscrea, Tipperary, Ireland) of Thursday 24^th May 1928:

Aviator: “How would you like a trip?”
Sambo: “No, sah. I stays on terra firma, and the more firma, the less terra.”

The phrase the more firma, the less terra soon came to be also used of sea travel—as in the following, from The Falkirk Herald and Scottish Midlands Journal (Falkirk, Stirlingshire, Scotland) of Saturday 4^th August 1928:

GRANGEMOUTH
“As we sweep through the deep—”
“A night on the river” is the attraction offered by the brethren of Lodge Zetland, No. 391, the date being Wednesday first. The Merry Masons have chartered that luxurious, commodious, up-to-date saloon steamer, “Fair Maid,” and hope to have their usual “bon voyage.”
Like other “Bairns,” our Falkirk neighbours dearly love an hour or two on the briny, and our local liner will return to port in time for a convenient bus to Falkirk.
If you happen to be one of those to whom a life on the ocean wave has no appeal, even per the Towing Company’s ocean greyhound, but who every time prefers terra firma (and, as the darkie said, the more firma the less terra) you can be delightfully suited.