At key points throughout the novel, mirrors are mentioned, showing how the culture of the future has no idea what it itself looks like from the outside. Montag sees himself in the mirror of Clarisse, and thinks that all the firemen are mirror-images of himself; he is not an individual, but a societal clone created to fill one function. At the end, when the city is destroyed, Granger says:
"Come on now, we're going to go build a mirror-factory first and put out nothing but mirrors for the next year and take a long look in them."
(Bradbury, Fahrenheit 451, Google Books)
The idea is that culture has become so insular and self-absorbed that it has no perspective, no objective view of itself. Without this, culture must continue on its path of self-destruction, because no one is able to see the invertible result. A mirror reflects what is really there; Granger wants society to take a long time examining itself, to decide if there is something worth saving, and if not, to see what needs to be cut out, destroyed, or fixed.