The names of Yosef’s two sons.
Menasheh received his name because, “God has caused me to forget (NaSHaNi) all my hardships and all that was in my father’s house” (41:51). This expressed how Yosef was pained by the fact that he found himself in a place which made him forget his father’s house. Efrayim was named, because “God has made me fruitful (hiFRani) in the land of my subjugation” (ibid. 52), expressing how Yosef had succeeded in Egypt.
Menasheh and Efrayim thus represent the two different reactions that a Jew has to being in exile. On the one hand he longs to leave the exile and return to “his father’s house.” But, on the other hand, since he finds himself—by Divine Providence—in exile, he realizes that there is a mission to be carried out there and he toils to succeed.
Menasheh was the firstborn because first of all a Jew needs to feel “out of place” in exile, to ensure that he does not assimilate. But the primary purpose of being in exile is to succeed in Divine service there—which is why Ya’akov wished to bless Efrayim first.
(Based on Likutei Sichos vol. 15, pp. 432-4)