I believe there have been big floods in the past - there is evidence of them in the geology and seabeds and so on - but floods, even big ones, are things we know happen so it's easy to believe in them. Rain of fire? I'm willing to believe that someone saw a volcano somewhere and the story of it got passed around and so the idea of fiery rain could easily have been embellished and attributed to something more.
But I'm not one to use the bible(s) as historical documents. Not that I'm against religious texts being used as documents, but the bible-as-published-today has been through too many rewrites and translations to be of much use. Even when you get back to the source you ought to back it up the anecdotal evidence with other records and, hopefully, some physical evidence.