Sounds like I need to move some of my devtesting back to FF. The ability to open a new session with a fresh disconnected session (so no shared session cookies) is one of the few things I miss from bad-old-IE (as simple as "File|NewSession"). Very useful, as you say, for testing workflows between different users.

Of course you've always been able to setup multiple profiles for FF, but that is setup that I was usually too lazy for given it could be done ad-hoc in IE.