After the bombing of the city, Granger compares mankind to a phoenix that burns itself up and then rises out of its ashes over and over again. This means that man can reconstruct itself from the ground up by recognizing its mistakes and learning. The phoenix also represents the change of life for Montag.
At the end of the novel, the men talk about starting a mirror factory so that they can "take a long look at themselves." The mirror represents how they (including Montag) will reflect on their lives. For Montag, this remembered him of Clarisse and how he remembered how empty and fake the life she was living. The mirror gave clarity to Monag to understand himself better.
In Fahrenheit 451, fire symbolises destruction because the firemen, instead of putting out fire, start fires by burning books. They believe books are problems so they burn them. In this society, this shows that people don't have to face their responsibilities and avoid their consequences. Also, the "451" represents the temperature where papers and books burn at.