Green dragons have never breathed fire in D&D
Various fantasy artwork, both old and recent, has depicted a green dragon who breathes fire. This is generally because in some myths, including those which inspired D&D's conception of the creature, dragons are imagined as reptilian, who are often green; and in some myths and stories, including The Hobbit, dragons breathe fire. More recently, Tohru, the green dragon in Miss Kobayashi's Dragon Maid (and, it is implied, who is named after Tolkien), breathes fire.
However, in D&D, the green dragon has always breathed poison gas (or corrosive acid gas, in later editions), but never fire. This is been true since the original D&D (1974).
D&D creator Gary Gygax describes the story how he invented D&D's dragons in The Slayer's Guide to Dragons. First, there was what we now call the red dragon, which breathes fire as per The Hobbit. (Gygax does not specifically credit The Hobbit for this in D&D, though it is noted as such in D&D's precursor Chainmail, which is now known to have taken many of its fantasy elements from a 1970 fan-made Middle Earth wargame by Leonard Patt, and the fire-breathing dragon in that is inspired by the dragon in The Hobbit.)
Next, looking to the original Norse myth that inspired The Hobbit (perhaps to fend off claims of plagiarasing Tolkien), Gygax noticed the dragon Fafnir in the Saga of the Volsungs, who breathes or exudes poison, rather than fire. In the original Dungeons & Dragons, this inspired the green dragon, who breathes a 5" by 4" cloud of chlorine gas.
Gygax actually says he invented the new chromatic dragon types to create more variety for his players, who could otherwise reliably outfit themselves with fire-proofing before any dragon fight. In other words, not only were you surprised that a green dragon did not breathe fire, but so was the first gaming group to ever fight a green dragon in D&D.
It's entirely plausible that somewhere in the myriad of official and third-party D&D books there's a spell or variant that lets a dragon use another dragon's breath weapon. A potion of dragonbreath which lets a player character breathe fire could be used by a green dragon, for example.
I faintly recall an old Nodwick comic where a village asks the heroes to fight a green dragon, so they show up equipped with a Green Orb of Dragonkind. One unfortunate encounter later, they learn that it was actually a red dragon, and a rare genetic trait causes everyone in that village to be red-green colour-blind.