The Multi-account containers extension does sync via Firefox sync, but you have to go into the extension's settings in each browser and manually enable it. Anno

The Multi-account containers extension does sync via Firefox sync, but you have to go into the extension's settings in each browser and manually enable it. Annoying, and took me a while to find it, but it's there at least. Hopefully won't be necessary to manually switch on once containers are a part of Firefox proper.