firstborn of all cattle , does not mean the Egyptians owned them.
Then exactly who owned them? We've already established that the egyptian cattle was all dead.
Its clear that none of the cattle of the Egytpians remained where as all the cattle of the Ishraelites lived.
And that the Israelite firstborns were skipped---it they had the blood on the post.
The Israelites were to sprinkle the blood on the doorposts of the houses they observed the act in, which means neither their cattle or other livestock were excluded from the destruction that was to come.
So what was left to kill?
For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD