I agree that it would be nice to have a way to show that permission prompt only if an external extension wants to use the API - however, that's not possible because such prompts can only be triggered after an explicit user interaction, not from the Add-on itself or external Add-ons.

Also, you didn't effectively "agreed telemetry". The same way you didn't "agreed to sent all website data you visit to the Add-on developers" when you accepted the "Access your data for all websites" permission that the Add-on also requires. If you don't trust the Add-on or its developers, then that permission should scare you more than the "Monitoring extensions" permission. But as you probably know; it's just a wording you have to take with a grain of salt and a lot of Add-ons/Extensions require permissions which would in theory allow them to inject any arbitrary content in websites you visit, or read data from them for that matter. It's how permissions work.

You didn't explain WHY should I'd ever want to allow that every other "external extension" automatically "uses the API" of the extension to which as you properly note I anyway gave a lot of major permissions. And why the extension forces me to allow something to the "other extensions" automatically.

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).

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.