I'm going to fire my coworker today because of something I brought up
Why are you firing them for something you brought up?
I've been through that over and over again with them over the last two years
If I were you I would have fired the person as well, especially if I have said something multiple times and they were not able to follow the instructions. I would recommend that you would warn them first before firing them, but if they still were not able to follow the instructions, I would go through with the termination of their employment.
I think that is what I am going to do.
It is unfortunate that sometimes people cannot follow instruction, but the job must get done. If they cannot perform, they should be let go.