I never ever had to switch profiles in Firefox in my 20 years of usage.
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.
It's not the only issue Firefox has, but it's the highest impact, lowest effort one they have. Performance improvements are hard, PWA is hard to implement. Putting a better UI on existing functionality is simple by comparison. They could score an easy win here, but they choose not to.