All the heavy lifting for containers is built-in. It's just the mapping of tabs to containers, the creation/disposal of containers and UI parts that's managed through the extensions because users do have different preferences and use-cases in that area. E.g. they may want automatic rule-driven assignment of domains to containers except when either blah blah holds or when manually overridden.

The built-in minimal and completely manual UI can be enabled with privacy.userContext.enabled = true