May be diet
Firm stools are often due to specific dietary tendencies.
Lots of dairy (esp. Milk & cheese), starchy foods (rice, bread, potatoes, bananas), but a diet
low in fiber
are a typical cause. Increasing fruits & veggies- while drinking lots of water- can help. Otc fiber supplements
(such as benefiber) may help. If not improving, call your doctor as some medical conditions can cause constipation