Very nice, still missing some features – that would make the Containers a solid alternative to multiple Firefox profiles/Chrome's „people/supervised users“. Especially that Firefox forces a single instance (I consider this a deep architectural bug by itself).
1. Synchronization via Firefox sync.
2. Setting a default container for a domain is clumsy arnd unclear.
3. A better integration with Password Manager (..., History, Bookmarks, Addon Manager, Preferences,...) would be very nice.
4. A possibility to reopen the current URL in a different container is a must!
5. I'd expect that container tabs inherit from the „default“. Currently, the „default“ is just an ordinary container without any added value, just a bit different menu/toolbars/uncolored tabs, which is misleading.
6. Protect containers with a password? Especially if the Password Manager would be containerized.

Also it is not clear, which Addon is „THE containers“. There are many extensions on AMO with very similar functionality.