Makes browsing even more more secure by separating your bank accounts infos, from your Facebook infos, from the rest. Much better than the "private" browsing concept because the cookies for each of these world remain after each browsing session, so you don't have to start over again each time. Much better also, because switching between each container is automatic (if desired), so you don't have to remember who goes where : in itself this is a great security feature. Of course, as the name implies, if you have many accounts for a given site, it also allows you to stay logged-in as these various accounts all at the same time, which is great. Loses a star because setup is not smooth, but once everything is set-up, Firefox Multi-Account Containers becomes totally transparent and a joy to use.