Well that sucks. You already closed, right? If so, you're likely of SOL. You could probably take them to smalls claims court but your realtor or attorney would be able to tell you if that was a good idea based on the specifics. We had a similar issue post-closing and that was an option for us, but we decided it wasn't worth pursuing (it was a smaller $$ issue).
Did you just ask for cleaning of the chimney or did you also ask for it to be certified as well?
Each state has different requirements for the sellers disclosure, so take a look at the form and see if there is any language that asks about fire damage/history. They would not have been required to disclose it in my state.
Good luck - I hope you can work it out in your favor.
Meh, it's $100 to have a chimney cleaned. It sucks that they're lying SOBs, but it's par for the course with real estate (see: the sellers of our second house that neglected to mention the roof leak they concealed which later cost is $4k and the sellers of our third house who lied to us about having the septic pumped as our contract required which later left us days, yes literally days, from having raw sewage all inside our home and on the lawn.)
Chimney fires are NBD as far as inspections go. I wouldn't disclose one. They're scary, but it just means the owners were fire idiots for never cleaning, but we already figured that out about them Chimney fires just happen when too much soot builds up - it's strictly a user error thing.