If an existing cookie is changed to be "HttpOnly" while a document is open, the original value remains accessible through script until that document is closed. Network requests correctly use the changed HttpOnly cookie. This vulnerability affects Firefox < 58.
https://www.mozilla.org/security/advisories/mfsa2018-02/
https://usn.ubuntu.com/3544-1/
https://bugzilla.mozilla.org/show_bug.cgi?id=1421324