Due to the man vs. nature theme presented in "To Build A Fire," yes, it is an example of naturalism. While the story itself might not be as flowery as the naturalism exhibited by Emerson or Thoreau, it is naturalism nonetheless, as man is subjected to the forces of nature around him; in this case, extreme cold. Jack London, along with being a socialist, is considered one of the prime examples of naturalist literature writers.
Jack London is a poster boy for naturalism, and his short story, "To Build a Fire," is a good example of why.
Naturalism is an extreme form of realism, with a heightened attention to nature and its effects on humans. Humans are animals like any other when pitted against the forces of nature. Humans are subject to the forces of nature.
The main character in "To Build a Fire" is pitted against nature and he is vulnerable to it the same as any other creature: disrespect it or make a mistake, and you will suffer the consequences. In fact, in the climate of the story, man is inferior to other animals. The character takes the environment too lightly and does not prepare properly, and there are consequences for this.
Naturalism in literature refers to the idea that men are governed by an uncaring fate or an indifferent environment rather than by the will of a benevolent God or by their own moral agency. Jack London's "To Build a Fire" displays Naturalism through its deterministic themes, its representation of the environment, its emphasis on objective facts, and its subject matter.
Determinism is an outgrowth of Naturalism which posits that man has no free will. Applying the ideas of Darwinism to humans, determinism argues that a person is shaped by his environment to such a degree that he cannot truly choose how he will act in a given situation. Thus the negative events that transpire in the man's day in the story are described as "his mistake" or an accident; London notes that "it happened." The man is not held morally accountable for his actions in the story.
The environment in the story is cold, not just in the sense of temperature, but in the sense of its indifference toward the man. Although the sub-zero weather presents the man with profound challenges, it would be that cold whether he was outdoors or not. Mere survival against a hostile environment becomes the man's goal--a familiar goal in naturalistic writing.
Naturalism places a great emphasis on objective, scientific knowledge as the only way to truly understand the world. Thus London gives hard numbers throughout the story, including the fact that fifty below zero is the danger zone for traveling alone and the exact number of matches the man lights at once (seventy). The man is preoccupied with the distance to the camp and the time at which he will reach it. This numerical, factual approach is typical of naturalistic writing.
Finally, naturalist writing typically focuses on the often-ignored classes, and the main character in "To Build a Fire" is such an undistinguished person. He is just an everyday guy trying to strike it rich. He does not even warrant a name in the story. Often naturalistic stories focus more on narrative than on the character; in this story we only get a few glimpses into the mind and thoughts of the man.
Jack London has created a quintessential story of Naturalism in "To Build a Fire" by espousing determinism, representing nature as indifferent, focusing on facts, and elevating the narrative over the undistinguished main character of the story.