Multifox is an extension that allows Firefox to connect to websites using different user names. Simultaneously!
For example, if you have multiple Gmail accounts, you can open them all at the same time. Each Firefox window, managed by Multifox, accesses an account without interfering each other.
Install Multifox
Version 1.3.1 forFirefox 4.0.
You must click the Firefox Allow button to proceed with installation.
