Firestone is a phosphine bearing rock found on Pern. Dragons and fire lizards can chew it to produce phosphorous gases which are ignited to burn Thread out of the sky. There are two types of firestone, one (also known as "Flamestone") is highly unstable and will often explode on contact with water, the other is more stable, but needs some time to be digested by dragons before they go to fight thread, this is the type that was originally found in Dragonsdawn and was used in all Pern novels following Dragon's Fire in Pern's chronology.
The chewed firestone must be expelled from the body after it is used up, for the dragons cannot digest it. This is done by regurgitation. The end product is a wet and smelly sand-like substance. Presumably, fire-lizards simply wait until they feel the need to expel it. Dragons usually learn that they need to do this after fighting Thread. A dragon that doesn't know of this will later find that the need to expel the remains becomes more like an involuntary bodily function.
All the dragons could chew firestone except for the golden Queens, who would simply belch up a steaming brown-grey puddle. Green dragons would be sterilized by chewing firestone - something thought to be a precaution added by Kitti Ping Yung to prevent Pern becoming overwhelmed with dragons - and by the Ninth Pass this was thought to apply to all female dragons; it was believed that chewing firestone would render a queen infertile.
If the gold queens could not use firestone, they would be less likely to fight Thread directly. They thus would likely not put in harm's way, being the only breeding females. It is also suspected that it may have been a result of her traditional beliefs, that the mother should not be allowed to fight.
It was hard to handle. Water would make it explode.
It had yellow and gray crystals in it. It also was harder for dragons to hold on to, many dragons were killed by the gas that build up leaving many dragonless men.