I left a preliminary review before, and this has turned out to be as good as predicted. However I would still like to see
1. Syncing with firefox account or save option
2. Ability to reload current tab in container or to convert current tab to a container tab.
3. More flexibility with moving items on the list of containers
4. Integration with tabs + button so you can choose which tab to open from there.
5. More colors/Flexible color choices
Great job. Such a good idea!