you talk about a feature that I had to remove to fix a bug ( https://bugzilla.mozilla.org/show_bug.cgi?id=1465063 ) in Firefox that will be corrected on version 62 (Sept 2018). I could have kept it but cookies with the First-Party Isolation flag could not have been managed by my application. The addon is already ready for version 62 of Firefox on which the feature will reappear ;)
