The rules for Faerie Fire state:
Any attack roll against an affected creature or object has advantage if the attacker can see it, and the affected creature or object can't benefit from being invisible.
As I read it, there are two ways to interpret this. The first is that being held unseen is a benefit of being invisible, and therefore the spell removes that benefit. Since the invisible creature is then visible, you have advantage against it.
The other interpretation is that the order of the sentence matters; first, check if you can see them, and you have advantage if you can. Then, strip them of the benefits of invisibility. In this case, you would have a regular attack roll against the creature, without disadvantage from being invisible nor advantage from Faerie Fire.
What interpretation of the rule aligns with the intention of the spell?