Can febrile seizures be the cause of permanent brain damage or be fatal?

Benign by definition: It is good to point out a difference here between a seizure occurring with a fever & a "febrile seizure". Anyone with a variety of different types of seizures can have a major seizure during a fever.The outcome of this is related to the type of seizure & complications common to that type. Febrile seizures are by definition, brief, don't impair oxygenation or result in long term injury.

Not likely: simple febrile seizures are not associated with brain injury. What is known as complex febrile seizure is actually a form of epilepsy that is triggered by fever. What is often thought of as brain damage from a prolonged seizure is more likely to be due to the cause of the fever, for instance encephalitis, meningitis.