The Firefox 50 Nightly test version introduced an interesting new functionality. The experimental "container tabs" feature allows you to separate different contexts while browsing the web.
At present the browser offers containers for "personal", "work", "banking" and "shopping" tabs and they all have separate cookies and data caches. Each container acts independently from the other and the default container, and is easily distinguishable by their color in the Firefox tab bar. Sites opened in a container have access to that container's browser storage, but no access to any other browser storage.
While it's unclear if it's going to end up in a stable version of Firefox, the container tabs definitely seem like a productivity enhancing solution. For example, you could use this to open two Twitter accounts in the same browser window; your personal account and one for work, or to open multiple Gmail accounts or Google accounts.
Containers improve privacy for users when used correctly. You could separate social media sites from your regular browsing session for instance, or avoid ad retargeting by opening shopping sites in their own container.
Another handy effect of using containers is that you can open different accounts on the same site at the same time. Open three Gmail accounts in the same Firefox window, or a work and personal Google account.
A more detailed explanation and how this differs from profiles can be read at Ghacks.