The first Founders' homeworld was an M-class rogue planet located within the Omarion Nebula in the Gamma Quadrant. The Changelings who later became the Founders of the Dominion withdrew to this planet, thousands of years ago, to escape persecution by the solids.
In 2371, Odo experienced a mental compulsion that led him and Major Kira Nerys to the planet, after the USS Defiant was captured by the Jem'Hadar. The crew of the Defiant was also brought to the planet and subjected to an experiment by the Vorta to gauge their level of resistance to a Dominion invasion. (DS9: "The Search, Part I", "The Search, Part II")
Thirty percent of the planet's crust was destroyed by weapons fire from warships operated by the Cardassian Union's Obsidian Order and the Romulan Star Empire's Tal Shiar later in 2371, during the Battle of the Omarion Nebula. The Founders had previously evacuated the planet, and deliberately enabled the attack so as to ambush the two intelligence agencies and render them incapable of resisting the planned conquest of their respective empires. (DS9: "The Die is Cast")
During the first two seasons of Star Trek: Deep Space Nine, viewers were left to wonder where the Changeling character of Odo had come from. The actor who played him, Rene Auberjonois, wanted this question to remain unanswered and mysterious, though he was happy with how the answer was revealed. (Cinefantastique, Vol. 32, Nos. 4/5, p. 40)
Upon devising the first Founders' homeworld, Ira Steven Behr and his writing staff on Star Trek: Deep Space Nine wanted to create a planet which was extremely strange and mysterious. (Star Trek: Deep Space Nine Companion, p. 168)
Much effort went into depicting the first Founders' homeworld. "The optical shots that were involved in our first view of the Founders' planet were very complicated," admitted Producer Steve Oster. "There was a lot of interfacing between what the director was planning and what the Art Department was doing. The planet needed to be fairly dark in order for us to see the glowing, gelatinous sea that makes up the Great Link." (Star Trek: Deep Space Nine Companion, p. 168) Illustrator Jim Martin, who did some concept art of the locale, stated, "It was all supposed to be this mysterious surface of a planet at night, without any sort of building or structure at all; just kind of a bizarre, alien landscape. I tried to throw everything I thought looked alien at them." ("Jim Martin Sketchbook", DS9 Season 4 DVD special features)
A miniature created by Model Maker Gregory Jein included part of the planet, portraying the area surrounding the Great Link. First, his team constructed a perspective landscape, which was ten feet long and twelve feet deep. "Greg had to build the miniature three weeks before the actual set was built," laughed Special Effects Supervisor Glenn Neufeld. The physical configuration of the landscape model was influenced by a visit to the cave set – which was later to be utilized for representing the planet – and multiple sketches provided by the art department. The illustrations gave Jein an impression of "a lot of weird colors and rocks." Next, he started carving urethane foam into artificial rocks and painting "funky florist flora with wild colors so it looked like an alien glade." (Star Trek: Deep Space Nine Companion, pp. 168-169)
There were subsequent problems with the set used for the first Founders' homeworld, such as with its extremely bright colors. "We went to the set one day, expecting to see it looking one way, and it didn't," remembered Glenn Neufeld. "So we ran to the phone and told Greg, 'You know those purple and yellow trees? Well, paint them orange or rip them out. We don't care which!'" The DS9 producers were ultimately disappointed with the planet, Behr thinking "the dark planet with the weird things on it" turned out to be a "set [that] never worked" and actually "sucked!" (Star Trek: Deep Space Nine Companion, pp. 169 & 168)
However, according to Startrek.com and the Star Trek Encyclopedia (4th ed., vol. 2, p. 103) (in entry "Omarion Nebula"), the first Founders' homeworld was an M-class planet. The planet supported native flora, which thrived on processes other than photosynthesis.