In this chapter, Moses warns Pharaoh about the last plague, the death of the firstborn.
The LORD gave the people favor with the Egyptians? This is really peculiar. I already discussed this somewhat back in Exodus 3. I'm not sure why the Egyptians would be so happy to give wealth to the Israelites. It says the Hebrews had "favor" with the Egyptians, but how and why? Why give money to your former slaves? It says the "The LORD made the Egyptians favorably disposed" to the people, but I can only imagine why given that just a few chapters ago I was emphasizing the religious and cultural animosity between the Egyptians and the Israelites. That the Egyptians would then turn around and give away what little wealth they have left after the destruction of all their livestock (twice!), crops and water supply is unexpected to say the least.
The killing of the firstborn is the culmination of the plagues, and a parallel to Genesis 1 where it is the creation of man that culminates the events described. In that sense, the plagues are like the inversion of creation: where before it is the creation and population of realms (earth, sky, water, the heavens), here the creation is driven askew by the sinfulness and persecution of the Israelites, and as a result the realms are being successively overthrown and destroyed. It ends with the overthrow and destruction of man himself, with the death of the firstborn. In that sense, these plagues are a metaphor for sin and the death of man that is foretold in Genesis 3 with the curse of Adam. The curse of Adam was the warping of his rulership over the world: what should have been his rightful dominion was turned into a land of thorns and thistles, rebellious against his hand, and ending in his death when he should have lived forever.
What is truly novel, then, is not the destruction of Egypt in these cataclysms, because the destruction was foretold, but the redemption of Israel out of the midst of death. To the best of my knowledge, Genesis 3 did not predict a redemption of mankind out of bondage. Some people try to weasel it in, but I would have to disagree based on a plain reading of the text. However, beginning with the disappearance of Enoch (who thusly avoided death) and even stronger with the rescue of Noah from the flood in Genesis 7, we begin to see the first hints of a redemptive process at work, that not all men are doomed to die in a hostile world. This theme appears even more strongly in the redemption of Israel here. In this case, what we are seeing is the outworking of the Abrahamic Covenant which delivers the entire nation of the sons of Jacob from destruction, not because of their righteousness (Noah was saved because he was deemed a righteous man and the same with Enoch), but for the sake of Abraham, Isaac and Jacob, the patriarchs of Israel and holders at one time of the Abrahamic Covenant (among other sources, see Exodus 3:6 and 3:16). This is really important because the Israelites are being saved from destruction due to the righteousness of another which is protecting them, the righteousness of Abraham (Genesis 15:6): we are watching the genesis of the Christian ideology. I will continue on this subject in the next chapter.
This is the tenth plague (blood, frogs, gnats, flies, death of livestock, boils, hail, locusts, death of firstborn), and as before, 10 is the number of completion or fullness, and thus this is the fullness of plagues and curses upon those who enslave the people of God.
Note that the plague didn't actually happen in this chapter, it was simply Moses telling Pharaoh what was about to happen.