You didn't allow every other Add-on to access the API, but explicitly only the Add-ons with the "contextualIdentities" permission ("Container feature"). Which i

You didn't allow every other Add-on to access the API, but explicitly only the Add-ons with the "contextualIdentities" permission ("Container feature"). Which is the reason the "management" permission ("Monitor extension usage") was needed in the first place - to check if the Add-ons accessing the API have that permission. Also, the API doesn't give external Add-ons access to the permissions you gave the Multi-Account Containers Add-on, but instead only a really specific amount of information; which is whether a specific domain is assigned to a container (you can read about it here https://github.com/mozilla/multi-account-containers/wiki/API https://github.com/mozilla/multi-account-containers/wiki/API). Now, why "automatically" for Add-ons that have the "contextualIdentities" permission? It's simple, if you grant an Add-on that permission, you already gave your consent that the Add-on can access the Container API; and the Multi-Account Containers Add-on itself being from Mozilla, is just additional Container functionality, but as an Add-on. So if you grant the Container permission, you also get API access to some additional informations from the Add-on; and with that increase interoperability between Container Add-ons.