Firebrick is referred to as "common brick" because it is the most commonly used

type of brick.

Firebrick is referred to as "common brick" because it is the most commonly used  type of brick --- False

Explanation:

 Firebrick is not the most commonly used,   it is  a  refractory ceramic brick used in lining of  furnaces and  kilns, which is built basically  to withstand or resist high temperature.The most commonly used type of brick, is the Building Brick called "common brick"   because it is versatile and used for  applications where appearance is not an  important factor.