Firebricks do need to be replaced periodically due to the constant high temperatures that are created within the firebox.
When the stove is cold, clean out the ash and debris from the firebox. If the baffle plate rests on the firebricks this should be removed first. The baffle plate, or throat plate as it is sometimes called, normally slides out quite easily, but sometimes it needs a bit of coaxing as it may have become choked up with soot and tar. You will now be able to lift each fire brick out of the stove. Check each one for cracks, or if broken remove the pieces. Replace with new, as necessary. Once you have replaced the firebricks (and baffle) the stove will be ready for use.