Can adults get febrile seizures? and if so, how severe can these seizures get? do they only happen at really high temperatures, or any temperature?

Poor term: Febrile seizure is a term used to describe a process in childhood where their immature nervous system can develop benign seizures with febrile stress. It would never be applied to an adult with a mature nervous system. Childhood seizures are often benign, while adult seizures are not. Any seizure patient is more likely to have a seizure if they have a fever.