Let's say you have multiple Twitter accounts, or several Google accounts, or even a few different bank accounts. It sucks to have to switch between those accounts in your browser. Firefox is testing out a new feature called Containers in its beta channel that hopes to fix this. The idea here is simple: You have a lot of different web identities and you might have multiple accounts on the same service. Containers let you create separate browsing sessions with separate rules all in the same Firefox window. So in one tab you can log into your work gmail account, and on another, your personal one. These containers also won't share data like cookies with each other, so it could be helpful to set up a container for all those weird searches you do after a few drinks. This is all still in beta on the Firefox Nightly build, but it's working well enough to play around with right now.
Contextual Identities on the Web [Firefox]