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.