Thank you for this extension! This extension helps me not to delete cookies and site data when the browser is closed. Though I think there are two points to improve:
1. When a new site is opened, the extension could recommend a container for this site. Will save some time instead of doing things on my own.
2. Be able to delete all the site data only from the "main" space (when no container is in place) when a browser is closed. At the moment, "Delete cookies and site data when Firefox Developer Edition is closed" flag is applied not only on the "main" space, but also drops the data from the containers (which doesn't seem correct to me).