We get calls frequently about cracks in a firebox. Cracks depending on where they are and what type of cracking can be potentially hazardous situations or insignificant. There are about 3 different construction types of fireplaces employed in the last 70-80 years. A virtual assessment can often determine the severity of the situation and type of construction.
This picture shows a fireplace built probably prior to 1980 or so, where there were cracks inside the firebox top front opening. One can see staining where heat was traveling toward the wood combustibles. Someone removed the lintel and bricks of the the façade (incidentally a terrible idea as someone was looking to remodel). So, some cracks are potentially dangerous, while others not so much. But a virtual assessment can often determine the necessity for an in person inspection.