Many times I have to either test security for various users, or make customizations and test them with different accounts. As you’ve probably seen by now, your authentication/authorization is cached with the first session you log in, and any subsequent browser sessions will use the same account to log you in automatically.
Now, the trick to logging in with various users at the same time is to use the new InPrivate Browsing (Ctrl+Shift+P):
This will create a new and completely independent browsing session, allowing you to use multiple credentials at the same time.
Once you start the InPrivate session, your screen will look like so:
Additional details are provided in the window description:
Do check out the help file for additional benefits and detailed description of this feature.
Image: two sessions running in parallel: