As the poem opens, it seems as if "fire" might be referring to a natural disaster which is powerful enough to destroy humanity—perhaps a volcanic disaster or an incredible meteor which blazes into earth's atmosphere. Yet in lines 3 and 4, it is clear that fire is symbolic of something else entirely: humanity's "desire."
The poem was published in 1920, just as the world was rebounding from the conclusion of World War I. In this context, it's easier to see how the desires of humanity have inflicted much destruction on the world. In fact, the connotation of fire conjures gunfire within this context. In a more modern context, it could be associated with nuclear warfare.
Fire, therefore, is associated with those emotions and desires of humanity which are destructive, such has violence, bloodthirstiness, and revenge. The speaker believes that he has seen enough of these powerfully destructive capacities within humanity that it is quite likely that humans will one day bring an end to their own existence.
Humanity's capacity for fiery emotions presents a bleak portrait for the earth's future. The poem offers no hope that humanity will survive its own tendencies for violence and destruction. Instead, the speaker is convinced that humanity will eventually destroy itself, most likely because of its own propensity for violence.