Multifox is an extension that allows Firefox to connect to websites using different user names.
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Latest commit f2f943c Aug 24, 2016 @hultmann committed on GitHub Merge pull request #256 from DaAwesomeP/classic
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README.md

Multifox

Development has stopped due to deprecation of XPCOM extensions.

However, Mozilla has implemented a very similar feature in the testing versions of Firefox (a.k.a. Nightly). Container Tabs are "contextual identities." There is a pre-set and pre-named list of the containers, but they have the same effect as MultiFox, and they officially will work with Firefox (even e10s). Read more details on the Mozilla blog post or Mozilla Wiki page.