Comment by WorldMaker

To my understanding: the built-in "Junk" container auto-clears history, as well as "In Private" boundaries apply per usual for any container you open "In Private". Containers that I truly need to regularly clear history on, divorced from the rest of my Profile go into their own Firefox Profile. MAC is on top of/in addition to Firefox Profiles.

Multi-Account Container nicely reduces the need to juggle different Firefox Profiles, but it doesn't eliminate it. (I certainly still use a mixture of both for a myriad of reasons.)