In life, we must employ two paradoxical approaches with regard to the world at large: On the one hand, we must be constantly vigilant against alien influences; on the other hand, we must engage the mundaneness of the world in order to influence it positively.
Influencing our environment is obviously more important than merely maintaining our values. Temporally, however, the latter must precede the former, since if we forget our roots we will no longer have anything to contribute.
The two sons of Joseph, born and raised in Egypt, personified these two aspects of exilic life. Manasseh, so named by Joseph "in order not to forget his family and heritage," personifies our need to resist assimilation. Ephraim, so named "because G-d has made me fruitful in the land of my suffering," demonstrates our purpose in the "land of suffering": to be fruitful there and influence it positively.
Chronologically, therefore, Manasseh preceded Ephraim—he was the firstborn.
According to Rabbi Dovber of Mezeritch, Ephraim represents consistently saintly individuals while Manasseh represents penitents. Each group is inspired by their pasts, but in different ways:
Ephraim…G-d has made me fruitful: The consistently saintly are inspired by the fact that G=d has made them fruitful in the past because of their good deeds. Their past experience spurs them on to continued good. This is like a person, who, after traveling some distance to reach a certain city, is advised by others to give up the trip. The traveler will reply, "I have come so far; how can I give up in the middle?"
Manasseh… G-d has made me forget: Penitents recall the fact that they have in the past forgotten G-d, and are thereby fired with a greater yearning for closeness with G-d.
—From the Kehot Chumash