[em-bee]

the Multi-Account Containers extension always only added additional UI functions to manage containers. it was never a different container implementation. so they are migrating UI features from the extensions back into the core.

"Not all of the features of the add-on are available in the first-party version of containers just yet, we’re still building them out."

the migration is not complete yet. i expect that eventually the extension will be made redundant.

[Izkata]

Except this announcement only lists features already available natively (also enable privacy.userContext.ui.enabled for them), that's why it's confusing.