I live in an area where we have many victorian homes, a whole historical society to deal with then even, so I can picture your situation. Unfortunately I can not offer any ideas on what happened to the firebox but I will suggest something you NEED to have checked b4 you lite anything in that chimney. That is, you need to get a CSIA cert. sweep to make sure those chimneys are properly lined and constructed. An area sweep may even be able to help with your original question. Regardless though, the chimney and flue should be checked by a certified chimney technician. This is for your safety. Many old chimneys are completely unlined and using that chimney can open you to some serious potential problems. Burning that beautiful victorian home down is one. Don't want to scare you just get you informed. Good luck.