Masonry fireboxes, meaning that the fireplace is not a prefabricated or metal chimney, can deteriorate over time. Repairing your firebox is a job that is generally doable by the average homeowner, but professional consultation is always advised.
Older masonry fireboxes were often constructed with regular old Portland cement. This cement “is not” specially formulated to withstand the heating and cooling process that the firebox undergoes with every lit fire. As the firebox heats up, the cement stays pretty solid, but over time the cement will deteriorate such that only fire clay and sand remain in the joints. These loose joints will fall out easily and have occurred if your chimney professional states that you have ‘open or loose mortar joints’ in your masonry firebox.
Different localities have different regulations on exactly how the joints between firebricks in the firebox should be tuck-pointed. NFPA 2:11 has recently stated that refractory mortar is what is appropriate for filling the mortar joints between firebricks.
Local building codes have slowly started to adopt this specification as well. Portland cement, even with fire clay, is not appropriate due to the face that no matter what it is mixed with the Portland cement will deteriorate over time, leaving you with open mortar joints that become a fire hazard. 
joints in the firebox should never exceed a fourth of an inch. Larger mortar joints are prone to coming loose and falling out. The products of combustion from the fire in your fireplace are extremely hot and will seep through any cracks in the firebox. This leads to the possibility of combustibles in the frame of your chimney catching fire.
There is premixed refractory mortar and hydraulic refractory mortar. Premixed refractory mortar dries as opposed to hydraulic refractory mortar, which cures. Hydraulic refractory mortar has been approved for outdoor fireplaces and premixed refractory mortar has not. Similarly, only use the hydraulic refractory mortar when setting clay flue tiles. This is due to the fact that premixed mortar will dissolve with water and the acidic fumes that are the products of combustion. If you have questions as to which kind of mortar a particular refractory mortar is, consult the manufacturer or a professional because using the right type of mortar is key to making a lasting repair or strong first construction.
Protecting Your Firebox
When your chimney professional comes out to inspect your chimney, make sure that he takes a good look 
at the firebox as well as the rest of interior of your chimney.
Any missing mortar joints, whether in the firebox or elsewhere in the chimney, are dangerous and not according to code.
You can protect the back wall of your firebox as well as increase the efficiency of your by investing in a cast iron or stainless steel fireback. These firebacks cover nearly the entire back wall of the firebox and will deflect the heat from damaging the back wall and radiate it out into the room. Stainless Steel firebacks are less expensive than Cast Iron firebacks , so I would encourage you to look into both kinds before making a final purchase. What the stainless steel fireback lacks with its lower price is the intricate designs present in cast iron firebacks.