Serviceable, but could use enhanced features to help manage and prevent unwanted cookie-cross pollination into containers. An automatic default container would be nice for any website not in the whitelist.

EDIT: Adding an extra star to this as I realized toggling "limit to designated sites" for each container forces non-whitelisted sites into the default container, preventing unwanted sites from contaminating my custom ones.

EDIT 2: Container logic seems to be confused by URLs crafted to redirect. Doing things like checking my eBay watch list or seeing my previous orders on Ali Express force containers into an infinite authentication loop whereby I have to disable designated site limiting in order for it to work.

EDIT 3: June 14, 2022. This extension is dead now that Mozilla has enabled site partitioning by default in Firefox 101. Great while it lasted but it was klugy with third party cookies and single sign-on, etc. I mean, it'll still be useful to allow multiple sign-ons to the same site, but that's about it.