It depends on where they are standing
The darkness spell is described in such a way as to give the impression that it is something tangible that spills out around corners, but can be blocked by objects. Think of darkness as not unlike a fog cloud.
The darkness spreads around corners....
Completely covering the source of the darkness with an opaque object, such as a bowl or a helm , blocks the darkness.
Likewise, darkness engulfs non-magical light or rather it cannot be illuminated by non-magical light. This means that magical light can illuminate it.
Light in 5e is described as having bounds. Faerie fire indicates that afflicted creatures shed dim light, which means that area imposes disadvantage for Wisdom(Perception) checks ONLY. Attacking into dim light behaves as normal.
This is where location comes into play. The bounds of the dim light must penetrate the bounds of the darkness in order to be seen. Put another way, there must be no darkness between you and the creature. You need to check the positions and radius of the darkness and creatures affected by the faerie fire. If the radius of the darkness completely engulfs the radius of the dim light, they cannot be seen by you from any angle. If they are 5 feet or more away from the center of the darkness, they can be seen from the direction the dim light is from the center. This is sort of mathy, but really no more difficult than normal line of sight. Darkness spills out around corners continuously, so just visualize or draw the circles and draw a line to the center of the dim light.
Who do I have advantage against?
Advantage and Disadvantage hinge on who can see whom. In general, you have disadvantage on targets you cannot see and advantage on targets who cant see you. However, when neither target can see each other (such as when a target is standing in darkness and you are not), you have neither disadvantage or disadvantage. From the basic rules on advantage
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa.
This means that Faerie Fire's advantage only has an effect when no disadvantage is imposed. It is important to note that, as explained in the question, faerie fire must be cast at a level greater than 2 or darkness will dispell it
You have advantage against
Targets you can see that can't see you. It shouldn't matter the circumstances in which this takes place (Devil's Sight, location, etc), if you can determine who can see whom, this will suffice for advantage.
Targets affected by Faerie Fire that you can see
Any situation in which you have at least 1 source of advantage and no sources of disadvantage
You have disadvantage against
You have neither against
Targets you can't see that also can't see you
Targets affected by Faerie Fire that you can't see
Targets standing in dim light that you have line of sight to (ie you can both see eachother)
Any situation in which you have any number of advantage AND disadvantage sources
This answer hinges on the fact that the darkness spell is dark in 3-dimensions, much like a 15 foot radius of fog or smoke. I believe this is true because of how the darkness spills around corners and can be blocked by objects. If the darkness only "darkens" objects and ground in its radius, then it makes sense to imply that the Faerie Fire penetrates through it, making line of sight meaningless. Is it a floating black orb or is it a radius of darkened objects? Again, I say the former, but I would personally leave this up to the DM.