Right now, you can only login with one username or email address, in one browser. When you open a new browser tab and login with a second username/email address, it cancels out the previous username/email address's login. This implies you can be logged in to one username (or email address) per Usermin instance.
It'd be nice to support multiple login sessions (one session per username or email address) in one browser. This way, you could watch multiple email inboxes in one browser.