The Phoenix is a symbol of rebirth, such as rising out of the ashes of a cast off life, to live again, renewed, reborn. It is a symbol for the future of the resistance or the book people that Montag joins at the end of the story. It foreshadows a rebirth of the society, providing a sense of hope that the current society, with its fascination for burning books and eliminating diversity of opinion, will be cleansed through the use of fire.
"By ending the book in a fire storm of bombs, there is the sense that this old society of conformity will die and a new one will be born out of the ashes, like the mythical phoenix to which Granger refers. "A time to break down, and a time to build up. Yes. A time to keep silence, and a time to speak," Montag thinks as the book people move up the river at the end of the story."
And, as the Phoenix rises out of the ashes, symbolically representing a new life, cleansed of the past, to live once more, so too is the hope of the rebels who have secretly memorized books to keep them alive.
The Phoenix is a symbol of change and transformation and this is particularly true for Guy Montag, who experiences his own transformation in the book. So through Guy Montag, and the rebels, the Phoenix represents the cycles in nature, meaning that out of the current society, the events of the moment, a new cycle of life will emerge, that is history, all societies go through cycles.