In case you don’t know this, Amber Grove was the cheerleader who burst into flames and was rescued from a fiery death by Buffy in Episode 3 (Episode 4 if you count the unaired pilot).
The actor is not credited for her role, and even the Buffy Wiki states that she was portrayed by an unknown actress.
There were two female stunt doubles who worked on that episode. Neither was credited, but it is known that Diana Lee Inosanto doubled for Sarah Michelle Gellar. The other female stunt person was Lisa Dempsey. It is reasonable to conclude that, since the character did basically one thing, that being to catch on fire, a stunt person would be employed in the role. I believe it was Dempsey.
Dempsey is 1.6m tall, and weighs 54.5kg. She has brown hair and blue eyes. She lists one of her specialties as “partial burns”.