Hi, it's a good question; the need for access rights to the API "privacy" is already explained on the presentation page of the addon:
Access and modify various privacy-related browser settings (the FirstPartyIsolation flag here).

Detecting the status of the FirstPartyIsolation option is absolutely necessary from Firefox 58 for people who want to use this extra protection.
This is a nightly feature that is currently not available other than through about:config for public versions of Firefox.

The API is here:
https://developer.mozilla.org/fr/docs/Mozilla/Add-ons/WebExtensions/API/privacy/websites

The way Firefox handles permissions is all or nothing: all permissions are granted or none of them are.
To my knowledge I can not ask the user to selectively accept a permission and not another.

If you have an example of this type of form, please create an issue on the GitHub so that I can explore that.
In any case if the access to the privacy API is refused, the addon will not be able to manage the cookies created with the option FirstPartyIsolation activated, even if this one is deactivated later.

Regards.