Neither did I for 10 years until I discovered the concept. Then I started making use of it and liked it. Same for everybody else around me, they either already knew of profiles and used them, or started using them once they learned about them.

Thing is, they used them in Chrome, because that's where it was easy and obvious to do. And since Firefox does not have an equivalently easy and obvious workflow, they keep using Chrome, and teach others as well to use Chrome. Reason why the drop in numbers happen.