Your child might like to build fires and watch a fire burning. At the same time, he understands the danger and is careful. Also, fire is not something he thinks about much.
Your child who has a problem with fire has a much different set of thoughts and feelings about fire. He thinks a lot about it and looks for chances to set fires, to watch fires, and to do things with fire.
Before ten or so, your child's playing with matches or lighters may cause a fire or get you concerned in other ways; but typically, this is a behavior problem and not a sign of mental illness. By ten to twelve, though, it is more than a behavior problem, although a simple accident can still happen.
If your child has a problem with fire, you can invest some effort in keeping matches and lighters away from him and to take time to teach him safe use of matches and fire. Also, talking with a fireman might have some educational value. Nonetheless, these activities should not be used to frighten or scare your child. It is likely he has other problems that may be less easy to see.
Getting angry with him or severely punishing him likely only make his problems worse. For example, he may become more secretive, more clever at hiding the behavior, and even more fascinated. Certainly, talking is important as is a warm, safe family environment. Even so, specialized help is necessary.