There is the external, physical fire which destroys and takes away--the books, the houses, the characters themselves are decimated and extinguished by the hungry fire which the firemen themselves unleash. It's power is mighty, and it is worthy of respect.
There is also the internal, symbolic fire that burns in people like Clarisse and Montag. They "burn" to know the truth about the past; "burn" with curiosity concerning the books that they have been told their whole lives are forbidden and off-limits; "burn" to find their purpose in life and happiness in their existence. People like Mildred and her friends never burn with discontent...they do what they are told and when they are told to do it. Those with higher intelligence ask the burning "why" and "how" questions which reach beyond the "who," "what," and "where" questions. This fire can also be destructive, but for some--Montag, Faber, the others who memorize the books--it proves to be a sort of salvation, cathartic and cleansing.