in the first instance
First, before any subsequent actions. In the first instance, tell your boss you're sick and then call and make an appointment with your doctor.
in the first place
1. From the outset; in or at the very beginning. You should have said you were feeling ill in the first place! I wouldn't have given you such a hard time if I'd known. I never would have bought this car in the first place if I'd known there weren't airbags.
2. As the first of a given set of points (e.g., in an argument). Why am I quitting? Well, in the first place, I haven't been paid for overtime in nearly a year, and I just don't feel valued as an employee in general.