Usually, all that is needed to defeat "fire proofing" is an accelerant. For instance, the material most seats in modern cars is made of is "fire proof." You can drop a cigarette on it or hold a match to it and it won't catch fire. However if you introduce an accelerant, for instance gasoline, poof, you have a roaring fire that will burn even the "fire proof" materials to ashes.
Excuse me, but, they never said they 'burned down' any houses, just the books inside them. they set fire to the houses, burning everything inside that did not have the fireproof plastic layer, but not the structures themself...
It's been a while since I read this book, so I can't verify if either of these are mentioned in the text, but as I remember it the firemen either had control of fire-proofing systems throughout the city, and Beatty would shut it off for a target before they went out to burn.
The flamethrowers they use aren't the type that we have today, but futuristic ones designed to circumvent the fire-proofing. Remember, fire-proof materials today can still be burnt- it just has to get *very* hot for it to happen. The flamethrower Montag used destroyed the first robot, after all.
When they talked of burning one woman's house (and she was burnt with it) they talked about taking the fire-proofing off and that the house was so old it was the only thing holding it together. So therefore the firemen had ways of taking it off.
I inferred that the people who read books(and also kept them in their houses) were not very modern people, and did not follow the times. Therefore, their houses were not fireproof. It could be said that they did not like what society was turning into, and were rebelling any way they could.