That still doesn't explain why they breath fire though.
How about people who have mutated into rabid zombie looking things that store so much radiation that they can use it as a weapon? (Glowing One's)
Or how about people that turned into a race of moll people with florescent lights eluminating from their bodies? (Tunnelers)
Or how about lizards that became bipedal and runs like penguins, some of which can also breathe fire? (Fire Gecko's)
There isn't always an explanation for how things come to mutate the way it did.
It's bittersweet.
On one hand, why should we find out how they came to mutate the way they did? Some things in life remains a mystery, mysteries that can't be solved without the proper equipment, knowledge and area of origin. Maybe they were created by someone? But why should that person's lab be in New Vegas? It's too much of a comfortable storytelling when everything is explained as being right next to you.
On the other hand, it leaves questions which we want answers to, and not having the source of the mutation on record there is no way of knowing just how far this breed of ants could have spread.
But some things doesn't get explanations.
It's understandable, it kinda svcks.
But that's just the way the world works.
Fire Ants, along with Glowing One's, Fire Gecko's and Tunnelers could very well be mutants that mutated under special conditions with no additional science involved.
No one had to create them, some things just mutate 'naturally'.