In the book Fahrenheit 451, firemen knew which houses the books were in, because there was an alarm system, that people used to tell the firemen which houses the books were in and where within the house to look for hidden books.
The firemen in Fahrenheit 451 are responsible for the mass book-burning required by the governmental regime. It is ironic because in our world, firemen try to prevent, weaken, and eliminate fires rather than cause them.
Since they are in the future and that firemen in that time are sent to destroy any piece of literature, books, documents, etc. of the past believe it or not all she tells them is: That firemen use to put out fires and not start them. (in a basic summary.) dont you just love the irony of it?!?