"I woke up with the urge to defaecate, only to be interrupted with the knowledge that I have Firehole. It was more painful than ramming a CD drive into your nose."
Is slang for when you go to take a dump after eating spicy food and you get that hot and sometimes unbearable burning sensation at your butthole. You know you have had firehole before if you have ever felt the need to sit in ice water after or during a hot dump. Firehole can be bad and will force you to take small breaks between turds so your butthole can cool off.