Multifox
Development has stopped due to deprecation of XPCOM extensions.
However, Mozilla has implemented a very similar feature in the Firefox Multi-Account Containers add-on. Container Tabs are "contextual identities. There is a manipulatable list of the containers (name, icon, and color), but they have the same effect as MultiFox, and they officially will work with Firefox (even e10s). Simply add the toolbar button and go to Firefox Preferences > Privacy > Container Tabs to configure them. Read more details on the Mozilla blog post or Mozilla Wiki page.