Arson is a special crime. Only 2% of arsons are cleared by a conviction. This is because arson is a secretive, cowardly crime with few witnesses, and also that fire tends to destroy evidence. If a case can be made, it is usually circumstantial in nature.
This dude operated in DC from the 1980s to 2005. He set over 320 fires. His name was Thomas Sweat. The below article is listed because it goes into his incendiary device, a very simple and powerful trick.
Please note how he got caught. Because he was so active, just one of his fires had a wick with some DNA on it. By itself, that was nothing. But eventually they caught him in the act, making the connection.
Here is a short 20 minute docu on how he was caught. Essentially, he got too active and was following his pattern too much.