Containers are nice, but they are, as you said, not as powerful as the new profiles system (which is, incidentally, almost as easy to use as containers now that we can switch between them right in the UI).
At this time there are essentially three levels of indirection: "legacy" profiles -> new profiles -> containers, that is we can have multiple legacy profiles with each having multiple new profiles, each with an independent group of containers in them.
To choose a legacy profile, use the -P CLI option (with or without a profile name). Once firefox starts, you can switch between the new profiles stored inside the current legacy one using the new UI.
At this time there are essentially three levels of indirection: "legacy" profiles -> new profiles -> containers, that is we can have multiple legacy profiles with each having multiple new profiles, each with an independent group of containers in them.
To choose a legacy profile, use the -P CLI option (with or without a profile name). Once firefox starts, you can switch between the new profiles stored inside the current legacy one using the new UI.