Serial arsonists are a specific type of pyromaniacs that habitually and compulsively set fires. A serial arsonist, like a serial killer, has a cooling-off period between three or more fire-setting episodes.
We haven't seen many over the seasons, but the team did encounter a serial arson case in Season 4. They helped catch Tommy Wheeler, a disgruntled young man using death-by-fire to punish the people of a small town.
"House on Fire" (Episode 19, Season 4)