I love this feature... on one machine at a time.

Without a built-in sync feature, I continue to use the simplest form of cookie management: Private Windows.

I know there's been some discussion on sync: https://github.com/mozilla/multi-account-containers/issues/3....

As of Dec 2019, it looks like some traction will be coming to syncing just Containers and Site Assignments (which I believe would perfectly suffice most basic needs).

If anyone really wanted to keep an eye on sync, I'm guessing these features will be committed to their syncserver, when available: https://github.com/mozilla-services/syncserver.