How is Facebook Container different from Multi-account Containers?
Facebook Containers specifically isolates Facebook and works automatically. The Firefox Multi-Account Containers is a more general extension that allows you to create containers and determine which sites open in each container.
You can use Multi-Account Containers to create a container for Facebook and assign facebook.com to it. Multi-Account Containers will then make sure to only open facebook.com in the Facebook Container. However, unlike Facebook Container, Multi-Account Containers doesn’t prevent you from opening non-Facebook sites in your Facebook Container. So users of Multi-Account Containers need to take a bit extra care to make sure they leave the Facebook Container when navigating to other sites. In addition, Facebook Container assigns some Facebook-owned sites like Instagram and Messenger to the Facebook Container. With Multi-Account Containers, you will have to assign these in addition to facebook.com.
Facebook Container also deletes Facebook cookies from your regular container on install and when you restart the browser, to clean up any potential Facebook trackers. Multi-Account Containers does not do that for you.