When are containers going to graduate to a built-in feature rather than an extension?
I am not sure if that'll be the case. The Multi Account Container (https://github.com/mozilla/multi-account-containers) seems like it's dead. No new commits since end of March and according to https://github.com/mozilla/multi-account-containers/issues/3..., it's only going to get bug fixes and maintenance patches. No new development.
I have a feeling that they are relying on extension developers to add features/functionality.