File | New Session
... and then you have a fresh session context that doesn't share cookies with the others so you can log into services as different users. As a developer this is very handy for testing multi-user workflows, and had numerous uses as a more general user too, and it was easy to do ad-hoc without needing to setup multiple profiles first.It didn't work for apps that are sensitive to permanent cookies (and other client-side storage) instead if session level options of course, where multiple profiles does (as presumably does Firefoxes containers? - I don't know as I've not yet used them).