And all the firstborn in the land of Egypt shall die, from the firstborn of the Pharaoh who sits on his throne, even to the firstborn of the slave girl who is behind the millstones; all the firstborn of the cattle as well. (Exod. 11:5)As these verses describe it, the distinction is not between master and slave, but between the Hebrews, all of whose firstborn are spared, and the non-Hebrews, all of whose firstborn are slain.
Now it came about at midnight that the LORD struck all the firstborn in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the captive who was in the dungeon, and all the firstborn of cattle. (Ibid. 12:29)
However, the sages drastically minimize this plague, arguing that many non-Hebrews participated in the Exodus, while many Hebrews perished during the plagues -- specifically, more than 20 million of the former and one trillion of the latter.
***WARNING: BORING MATH PART***603,550 adult, able-bodied Israelite males make it to the end of the Book of Exodus (38:26). Add back in 3,000 golden calf fatalities (32:28) and 8,580 adult Levites counted separately (Num. 4:48). That's 615,130.
But what about all the children (below 20)? The elderly (over 60, Talmud Bava Batra 121b) and infirm? The women? After all, Pharaoh spared the girls.
It is not unreasonable to assume that for every male 20-59, there was, on average, one younger and one older. Double that to account for (probably more than) half of the population which is double-X, and one arrives at 3,690,780. (Indeed Targum Pseudo-Jonathan mentions five dependents for each adult male explicitly, Exod. 13:18).
***END OF BORING MATH PART. FOR NOW.***So we have a nation of about 3.7 million, all told. Well, according to Rabbi (not-Pseudo) Jonathan in Yalkut Shimoni 209, the non-Hebrew "mixed multitude" accompanying the Israelites outnumbered them six-to-one. Hence, 22 million non-Jewish Exoduers.
What about the Jews who perished during the plagues? Rabbi Simai states (Talmud Sanhedrin 111a)
Just as at their entry into the Land there were but two out of 600,000 [Joshua and Caleb], so at their exodus from Egypt there were but two out of 600,000.This passage posits a 99.99967% fatality rate for the Jews during the plagues. Which means that they had to start out with 300,000 X 3,690,780. 1.1 trillion who were not righteous enough to leave.
I don't mean to take these numbers literally. If you're unfortunate enough to be seated next to someone who does, at the seder or on a flight, you should probably move. But what they do tell us is that the sages adopted a perspective that made the numbers of the slaying of the firstborn a below-the-fold story. Food for thought, along with your matzo ball soup and brisket.