Fire Giants are composed of fire tamed by magic. They can take on any shape or form for as long as they don't lose too much mass in the process. It usually takes days for them to regenerate much mass.
It is known that a few Fire Giants, such as Slim'Roth, are sapient and can talk, though others have been known to not interact with people at all. Because of this, it is unclear as to whether the species is classififed as sapient or not. Scholars remain undecided about their nature.