In times where privacy both matters and is at risk, it's great to keep your logged in sites in something that looks like a different browser (the containers). Besides it also allows to have different accounts on a given website and be logged to both of them at the same time, just in different containers. This is the best thing Mozilla could have made for browsing. A fantastic enhancement on Private Mode (which is a bit like a container too, but it does not preserver your logins, history, etc., whereas containers do.)