stoically

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.

I've added an "Multi-Account Containers Isolation" feature to the Temporary Containers Add-on. Enabling that will only allow Websites that are assigned to "Always open in" with Multi-Account Containers to load in their container - every other request will automatically opened in a new Temporary Container.

There's also a discussion about implementing such an Isolation feature directly in Multi-Account Containers.

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.

> How about not being required? Older versions really didn't require.

Unfortunately there's no other way for us to check whether an Add-on that tries to access the "API" has the needed "contextualIdentities" permission. I can assure you that it's in no way about telemetry and never will be. If the Multi-Account Features would be a Firefox platform feature then such an API would have the same requirements.

> I had some tabs (mostly YouTube) opened thrice when clicking on a YT link.

Some of the multiple tabs issues have been fixed in version 6.0.0. And the upcoming version will fix them completely.

> They don't automatically switch back to the default container. That's a big problem

I've added an "Isolation" feature to the just published version 0.67 of the Temporary Containers Add-on that gives you several ways to avoid accidentally staying in the same container - including an option to only allow "Always open in" assigned domains to load in their container.