My son has had several febrile seizures. The younger the child when they have the first one, the more likely they are to have another one. But I think it's only about a 25% chance that a child will have a repeat seizure. And yes, it's very scary but it has no long lasting effect on the child. My son's seizures have all happened during the night except for one that actually occurred in the car on the way home from the pediatrician's office (his temp had been 102 at the doctor's office but spiked really quickly, it was 104.8 when we took it as soon as we got home).
I've found the best way to try and avoid them is to keep my son carefully medicated when he gets a fever. I give him meds every 4 hours. He responds too strongly to motrin (it drops his temp too quickly) so I give him Tylenol which doesn't drop as temp too much but keeps it more steady, i.e., he's less likely to have a have a fever at a steady 102 than at a temp that goes down to 100 and then shoots up to 103.5 in a short period of time. Also I never give him baths to try and bring down a temp since it works too well for him, Good news is that despite being almost continuously ill all last winter (culminating in getting ear tubes and his adenoids out at the end of March) and having numerous bouts of croup this winter and one of pneumonia, knock wood he hasn't had a seizure since December of 2004, mostly I think because I monitor him so carefully whenever there's a hint of fever or illness.