This is a natural way to relieve built-up stress in refractories. It has no effect on operation, performance or useful life of the unit. The firebox is an arch structure, the most stable and permanent construction known. These cracks will develop over time and is nothing to worry about.
Due to it being cast as a one piece cylinder it goes through some expansion and contraction every time it is heat cycled. This is just the cylinder relieving its inert tension and results in a variety of different levels of cracking.
These cracks and blisters can slowly grow over time due to erosion through use. If you do not like the appearance of the cylinder when cracks appear, you can purchase veneering cement which can be mixed to a toothpaste like consistency and inserted into the cracked areas.
The story goes that the two original designers each had a Pyro and one touched up his cylinder every year and the other never touched his…25 years later both fires were still working albeit one looked in better looking condition internally than the other!
