Cookiebro can delete unwanted cookies from other containers as well. However, it requires that you have a tab open for that specific container. This is an unfortunate limitation that Firefox devs didn't want to fix. See https://bugzilla.mozilla.org/show_bug.cgi?id=1578096

With Cookiebro you can also inspect and manage cookies in a particular container. Just open a container tab and then access e.g. Cookie Editor with Cookiebro.

EDIT: there's an upcoming workaround in the following versions for this.