Fi says: I should draw my ethical principles from my own internal sense of what is ethical in a vacuum. Ethics would still exist as a fundamental property of the universe even if we were not here to think about them.
Fe says: I should draw my ethical principles from what my community surroundings hold as important values. Ethics exist only as a means of fostering emotional connection between people so that they can work toward external goals together.
The key is that to Fe, nothing is actually ethical until some external goal has been defined. If the goal is integrating into a given culture, then the methodology would clearly involve integration into that culture's moral and social norms. Fe's ethics change and adapt based on the emotional texture of its environment.
Fi, on the other hand, sees ethics as private, personal and non-negotiable. It neither seeks nor requires any external validation, because it knows what is ethical and what is not, end of story.