Arioch,
Sorry to take so long getting back to you. While fire ships are not mentioned specifically, the article does state that if a ship attempts to grapple a burning ship the grappling vessel has a 50% chance per round of catching fire itself.

Fire-throwers are specifically designed to set ships on fire but you can also load catapults, mangonets and shot ballistas with incendiary shot. This kind of shot suffers a -2 penalty to hull damage rolls or a -1 penalty to its missle rating on the War Card.

That said, here's the excerpt dealing specifically with firing a ship:
"Whenever a ship suffers a hit from a fire thrower, incendiary shot, or cerain fiery spells such as fireball or produce fire, it may be set afire. The chance is 25%, plus 25% for each point of hull damage inflicted by the attack. For example, a ship that suffers no damage from the attack of a shot ballista firing a pitch pot has a 25% chance to be set afire, while another vessel that suffers 2 hull points of damage from a fire thrower has a 75% chance to be set afire. Once a ship is on fire, it loses 1 hull point per round until it sinks. There is a 10% chance per round that the crew can extinguish the flames before the ship suffers more damage, although some spells or magic items may be able to smother the flames automatically."

Pretty cool, huh?