Click the extension button to save current tab's cookies (containers only). They will then be restored upon startup.
I use this to keep logins in containers, but have the default container wipe cookies when I shut down Firefox. Because Firefox doesn't have an option to wipe only the default container.
On the addon's options page, you can list domains where cookies should be restored. Clicking the toolbar button makes the extension keep cookies for the current tab.
https://github.com/losnappas/cookie-save-webextension
