cannot login with multiple accounts from the same browser

when you are not logged in, when you press the login button you are *immediately* logged in, without actually stopping at a login screen that asks for credentials and allows you to select a different email address or username.

Can work around with a private window, but it should be possible for people with multiple (work/personal, etc) accounts to log out and back in under the other account without a private/incog window.

(huh, i also just learned that private/incog windows share the same cookies!! that seems crazy https://bugzilla.mozilla.org/show_bug.cgi?id=117222 )

(@wlach @hamilton)

Start by tracing the login button, logout flow, and the login screen that the issue describes, then identify where the current browser session is reused. Reproduce the problem with two accounts in one browser; done means logout permits entering or selecting different credentials without requiring a private window, with the existing login behavior still working.

Written by the indexing model from the issue text.