Earlier quoted context omitted.
Wouldn't opening several "Private Windows" achieve the same situation? What are the differences?
I don't think so: I'm currently logged in on HN. When I open a new private window, I'm logged in as well. This is really annoying when you always use your web browser in private mode, but don't close it regularly. It means that e.g. youtube already builds a profile about me from my previous searches even though I'm not logged in. If I were that concerned I would close Firefox, but the usability issue is just too big…
Even Internet Explorer has File > New Session.