yes, there are many grades and types of firebrick; i'm not sure of the size of your brick, but if you are lucky, locate an old powerplant, they will usually have some old high-duty insulative firebrick out back somewhere and may let you take some. generally the size is roughly 3x5x9". this brick will not crack, disintigrate etc. and will increase combustion efficiency compared to the junk brick used in most homeowner applications. however if you have an epa wood stove, many of the secondary burn features require a brick that insulates even more than the type i mentioned above. firebrick is much more complicated than that, but suffice it to say that a "high-duty" brick like that from a powerplant will last longer than your stove, vs. the junk that came in it.
on edit, i realize that i didn't read very carefully at all; you mentioned the size of your firebrick