Mozilla adds "Containers" feature to Nightly build.

Firefox has added an experimental feature to its latest Firefox Nightly build called “Containers” that allows users to be perceived as different identities depending on what they’re doing. I know that sounds a bit odd or even suspicious but it could be quite helpful in certain scenarios.
Say, for example, you work from home and manage social media accounts for your employer. It wouldn’t be uncommon for you to be logged into those accounts but also want to share something on your own personal Facebook account that’s not work-related.

With Containers, you could do just that – assume two personas simultaneously without having to log out of your work accounts or open different browsers. It could also be handy if you want to do some online shopping and not have ads for products you viewed follow you around the web afterwards.
As Mozilla security engineer Tanvi Vyas explains, the new feature provides four default identities – personal, work, banking and shopping – which are color-coded to make it easier to know which identity you’re using at any given time. Each identity has its own set of cookies, IndexedDB data, local storage and caches.
The idea isn’t new but as the Firefox team notes, nobody has really figured out how to best implement it. Although Mozilla is giving it a shot, it concedes that there are still a lot of unknowns in which it doesn’t yet have answers to.

It’s worth noting that different identities share the same bookmarks, browsing history and saved passwords as well as search and form data. Ad trackers could still technically “figure it out” but on the surface, there’s no easy way to tie it all together without some serious digging.
Vyas stresses that this is an experimental feature in Nightly and that they want to collect feedback and iterate on the design before advancing it to a more stable channel. A full FAQ on the new feature can be found over on Mozilla’s blog.