One surely can edit containers in Nightly. I did this for myself long time ago, when I've first learned about container tabs, as the default set was almost usel

One surely can edit containers in Nightly. I did this for myself long time ago, when I've first learned about container tabs, as the default set was almost useless to me. There's a setting, privacy.userContext.ui.enabled than enables management UI in the "privacy" section of "options" system (about:preferences#containers)