There are a number of symbols in Bradbury's classic book. The title of the second portion of the book is "The Sieve and the Sand." Montag remembers trying to fill a sieve with sand. That's an impossible task: there are always holes in the sieve, and the sand will always pass through, like the memories of the books they read (and try to memorize).
The phoenix was a figure from mythology. In the myths, the phoenix never died, but instead, went into flames and was reborn all young and healthy again. Late in the book, the phoenix is used as a symbol of mankind, with war the fire that will force its rebirth.
And the firemen use the salamander consciously, as a symbol of their activity. Salamanders were also mythological. They were at ease with fire, and at home in it. They were even nourished by it, as the firemen claim to be.