Multifox is an extension that allows Firefox to connect to websites using different user names.
Once you have installed the Multifox extension there are three ways to access it :
Menu : File > New Identity Profile
Tabs Context Menu : rightclick any tab > Open in a New Identity Profile
Context Menu for your bookmarks : Bookmarks > rightclick any bookmark > Open in a New Identity Profile
Any of the three will cause a new and separate window to open. Counting starts from 2 : notice a blue square with a number at the end of the Address Bar.
Logins made in windows with different numbers are isolated.
Clicking on any of the blue numbered squares will open a drop-down window with basic information about that particular identity profile.