From the offical docs:
With the Firefox Multi-Account Containers extension, you can…
Sign in to two different accounts on the same site (for example, you could sign in to work email and home email in two different Container tabs.
Keep different kinds of browsing far away from each other (for example, you might use one Container tab for managing your Checking Account and a different Container tab for searching for new songs by your favorite band)
Avoid leaving social-network footprints all over the web (for example, you could use a Container tab for signing in to a social network, and use a different tab for visiting online news sites, keeping your social identity separate from tracking scripts on news sites)
Protect your browsing activity in individual Containers using Mozilla VPN, so you can shop while travelling abroad but check your bank account from a server in your home country.