I am a long-time user of the deprecated Multifox extension and I have switched to the Firefox containers ever since they have been introduced to the stable releases about a year ago. This feature is actually builtin in Firefox, you only need to change some config settings to enable the UI (probably this is also all what the linked extension does?).

As Multifox was one of the old XUL/XPCOM extensions, I am glad that this functionality was integrated natively before Firefox 57 will disable all extensions that are not WebExtensions.

It is a great way to login to multiple accounts on various sites such as Twitter, without going through the hassle of a full logout/login cycle. You can use the accounts side-by-side in different tabs, which will be color coded to indicate which container they belong to.

More details can be found on the Mozilla wiki: https://wiki.mozilla.org/Security/Contextual_Identity_Projec...