Firebricks having chipped edges or broken corners can be still used in construction of ovens, fireplaces, stoves or chimneys. There is no problem with firebrick seconds. Simply turn all bricks with their best sides inward into a visually accessible side.
Watch out for their details as the insulating fire bricks, the soft and lightweight refractory bricks, are NOT the hard dense and heavy firebricks. Due to those many empty air spaces in the brick body, insulating firebricks will not absorb nor conduct heat nearly at all, thus they have a very specific application purpose.