Yes, it's definitely possible to do that. But switching between Firefox profiles is nowhere near as convenient as logging into two different sites with two different identities in two tabs next to each other, which is really easy to do if you don't share identities across sites. As I said, juggling multiple incognito windows is not fun. If anything, juggling multiple profiles is even less fun.
But I guess I'm just ranting here, since it is well known that convenience and privacy don't always go together.
"But switching between Firefox profiles is nowhere near as convenient..." - don't switch, run them in parallel. I use 6 different profiles and often have most of them open at the same time.
But I guess I'm just ranting here, since it is well known that convenience and privacy don't always go together.