Many people have more than one account on different online platforms, using Gmail one for personal and other one for business or Facebook's two different accounts for anonymous identity, well you understand but regular browsers do not allow you to run multiple instances together, so if you wanna sign in to another account you need to logout and than sign in or you have to open another browser for that account, well Multifox removes that hassle by enabling you to open multiple accounts instances all together right on one browser.
With Multifox extension installed in your firefox browser you just need to open that same website from bookmarks as "Open in New Identity Profile" and that will enable you to login to your another account and side by side working on the earlier account.
So its simple if you wanna open out 3 different Gmail accounts than you need to open three different tabs in Open New Identity Profile mode, and than you can sign in to your different accounts.
Well Multifox is currently available only for the firefox and you can install extension from their official website. More information can be gained from their default site.