How to make spreading fire

You'd want to clone your particle system every preset amount of time. The only tricky part would be making sur ehte fire does not appear in a place that there already is fire.

To just clone the fire, you'd use a script like this attached to your fire particle system:

 private var startTime : float;
 var pollTime : float = 5.0; // Seconds between fire spreading
 var spaceToSpread : Vector3 = new Vector3(1,0,1);
 function Start()
 {
 startTime = Time.time;
 }
 
 function Update()
 {
     if(Time.time > startTime + pollTime)
     {
         startTime = Time.time;
         CloneFire();
     }
 }
 
 function CloneFire()
 {
     var newFirePos : Vector3 = transform.position + Vector3(Random.Range(- spaceToSpread.x, spaceToSpread.x), Random.Range( -spaceToSpread.y, spaceToSpread.y), Random.Range(- spaceToSpread.z, spaceToSpread.z)); 
     var newFire = GameObject.Instantiate(gameObject, newFirePos, Quaternion.identity);
 }