I believe you're thinking of the "Facebook Container" addon - it's "official", in that it's made by people at Mozilla & has their corporate blessing, but it's not installed by default.
AIUI, the only "container" functionality built-in to the browser is... infrastructure, I guess you'd call it: Firefox provides all the tools for addons to do stuff with containers, but doesn't actually do anything with them itself.
AIUI, the only "container" functionality built-in to the browser is... infrastructure, I guess you'd call it: Firefox provides all the tools for addons to do stuff with containers, but doesn't actually do anything with them itself.