M-A-C dev here ...

We're actually pretty deep into the work of adding sync to M-A-C. https://github.com/mozilla/multi-account-containers/pull/161...

Predictably, synchronizing data is complicated, so it's taking a while and we're trying to do it in a way that doesn't destroy any existing data. So we'll be doing some heavy internal testing on it before we release it.

But it's definitely coming.