I assume you are refering to the fire mentioned in Chapter Four of this text. The fire is clearly used as a symbol of the group as they meet in their early stages of utopian fervour, representing their enthusiasm and the spark of idealism that can bring warmth and comfort to so many. Consider how the narrator describes the fire for us and then relate that description to the people and the stage that they are at:
But it was fortunate for us, on that wintry eve of our untried life, to enjoy the warm and radiant luxury of a somewhat too abundant fire. If it served no other purpose, it made the men look so full of youth, warm blood, and hope, and the women--such of them, at least, as were anywise convertible by its magic--so very beautiful, that I would cheerfully have spent my last dollar to prolong the blaze.
The fire seems to therefore be a symbol of the fervour of the group at this stage in the novel, but also the way that their dreams and hopes are somewhat naive when faced with the harsh realities of world. Note that their lives are "untried," and that the narrator bestows upon the fire a somewhat negative description: "a somewhat too abundant fire." It makes things look more beautiful and hopeful than they actually are, and can be related to the way that the utopian ideals of the group are blazing high at this stage, only to dwindle away somewhat in the following chapters.