Kerosene. The firemen need an accelerant to spread the fire, so they hose kerosene all over the house, then light a match. The second sentence of the book states of the firehose that it is a "great python spitting its venomous kerosene". And, kerosene really smells; Clarisse knew Montag was coming down the street even before she saw him because the smell on Montag is so strong. Montag says, "you never wash it off completely," in acknowledgment that that smell of kerosene always stays on him, no matter what he does.
I hope that helps!