Excellent add-on. Does what it says, but its a bit fiddly to set-up. The only sensible option seems to be to delete all cookies and create the containers in the order you want them to appear in drop down menu. Feature suggestions:

1. Sort containers in drop down list without editing JSON file or deleting all cookies and creating containers from scratch in required order;
2. Organize container contents without looking at each individual cookie (I hear the latter ability will soon be removed by Firefox changes);
3. Allow sync across devices (with option to auto-delete cookies which become container-less as a result);
4. Allow containers to be sync'd with bookmark folders to simplify management and make it easier to remember which websites belong to which containers (websites used in two containers will appear in two bookmark folders);
5. Option to auto-delete cookies in default (no) container;
6. Option to auto-delete cookies not in container specified for that website.