A simple Firefox plugin that can automatically copy cookies from the 'main' Firefox container to all your other containers for the domains that you can configure in the settings. This can be useful for instance to copy your existing login sessions to other containers.
Note that this will have some privacy impact, because containers are not completely isolated anymore. This is the first version, in the future a config option to filter on fine-grained cookie-level (instead of domain level) or a config option to only copy to some containers might be added.