Granger says this to Montag at the end of the novel, after their civilization has been destroyed in a war and the survivors are camping by the river. Granger is speaking about civilization rising anew from the ashes, like the Phoenix, and he says that the survivors need to think about all the mistakes their civilization has made and how to improve it. He believes that remembering the past and the mistakes of the past is the key to improving the future. He says that they are going to "build the biggest goddam steam-shovel in history and dig the biggest grave of all time and shove war in and cover it up." He realizes that war must become a thing of the past if civilization is to have a rebirth.
When he says that they are going to build a mirror factory, he is speaking metaphorically. He means that in order to rebuild their society, the survivors must take a long, hard look at themselves and think about their faults and mistakes. They must first reflect on what society has been before they can attempt to rebuild it along better lines in the future.