What you can do with Multi-Account Containers:

• Log in to multiple accounts on a single website (e.g. personal and work email accounts).
• Prevent tracking and security attacks (e.g. harmful clicks stay within a container).
• Assign websites to containers (e.g. a specific website opens in a specific container).
• Avoid social network footprints (e.g. use a container for social networks).

Never mind, finally found the answer.

See also:

• https://support.mozilla.org/en-US/kb/containers