I finally tracked down this prequel to The Gammage Cup, and it was worth the hunt. The Firelings have lived for generations on the slopes of Belcher, an active volcano. They survive by following strict rules on how to live and where not to go, along with an occasional human sacrifice down the Long Slip when Belcher gets hungry. But Belcher gets really sick, doom hangs over the Firelings, and their only chance of survival is in breaking all the rules.
It is full of memorable characters and plenty of action. It is also an exploration of the danger of unquestioning belief and conformity.