Which main-series Final Fantasy game was the franchise's first MMORPG?

The first main-series Final Fantasy MMORPG was Final Fantasy XI.

Also called Final Fantasy XI Online, it was developed by Square and released in Japan for the PlayStation 2 and Windows PCs in 2002. It moved the numbered series from primarily single-player adventures into a persistent online world, Vana’diel, where players created avatars, chose jobs, completed quests, fought monsters, and cooperated with other subscribers.

Final Fantasy XI was especially notable for cross-platform play between PlayStation 2 and Windows, a feature unusual for its era. Its subscription-based structure and expansions supported a long-running shared world rather than a self-contained campaign. Final Fantasy XIV is also a main-series MMORPG, but it launched later, in 2010, and its original version was replaced by the rebuilt A Realm Reborn in 2013. Final Fantasy XII and Final Fantasy X are single-player numbered entries, so neither is a competing answer.