Is the coolant level OK?????
I once had a hose burst (on Christmas Eve) in the garage on my MGB and it filled the garage with steam, as the water hit the exhaust manifolds. Like you I thought it was on fire. So a possibility is a jet of water from a loose coolant hose squirting on the exhaust, or something like the washer bottle pipe coming off and dumping the contents on the exhaust. Check those levels.
