Fireships advantages:
1. cheaper than birems
2. build way faster than birems
3. are guaranteed to take out an attacking ls
Now, you don't want them to have some sort of disadvantage? If you remove the disadvantages, then we have the perfect weapon? Not a real choice then...
Fireships are slower than birems, so they hit a bit later than birems. The commanding admiral of defending ships decided to not let the incoming armada get to close to the city, he fears the mighty weapons of the light ships will torch the city from the water. No, he decided to hit fast the enemy... moving from one side to another, knowing his birems are faster than those light ships. In this time, his fireships, that could have been spoted easily because their slow, set sail to the remaining of the enemy armada while the enemy is busy with the fast birems....
Did you liked my story? I did. And see, reasons why the fire ships go last.
Yes, they have disadvantages.
But think on how the attacker feels when the defender only builds fire ships. THe attacker knows that he can't match the build speed of those coffins, a ls lost will be recovered in 2:40:55 in a lvl 20 harbor, while the fireship that suicided to sink that ls will be recovered in only 0:44:42, 4 times faster.