Assuming the photo is fairly representative of the entire firebox, what's it going to take to repair all of the mortar joints in terms of time/cost? The width at the back is 24, the front is 30, and the height of the area needing repair is 30.
When dry skin is the cause of an itching back, it is easily remedied by applying a lotion or cream to moisturize the area. When chronic dryness and itchiness is an issue, a person may need to increase hydration efforts.
Dryness of the skin is a common cause of itch. Itching of skin can lead to tears in the skin (excoriations) from scratching. Less commonly, generalized itching can be a sign of chronic medical conditions such as liver disease.