I think parent comment wanted something more like First-Party Isolation (privacy.firstparty.isolate and privacy.firstparty.isolate.restrict_opener_access in about:config, use with caution - it will break things, including breaking Cookie Auto-Delete extension)
https://www.reddit.com/r/firefox/comments/6y7lpw/what_is_fir... (sorry, don't know any mozilla.org link for FPI that has any good description what it does and how it works)