The owner of this ship also says next rank (5) will give Fury. I believe it.
She says this upgrade is "Firestorm Upgrade Level Four," and that next upgrade is "Skull and Bones Fury."
Now, like I previously said, the thing only occasionally fires a Firebrand shot.
Edit:
There's the Fury shots. According to Obsidion, the owner of THIS Sloop, there's a 15% chance that a Broadside shot will be a Fury shot. A broadside of a Sloop consists of 5 shots. You do the math from there.
(Imagine that from a War Galleon or War Brig, which both have 12 Broadsides)