Special "about:" pages used by web content, such as RSS feeds, can load privileged "about:" pages in an iframe. If a content-injection bug were found in one of those pages this could allow for potential privilege escalation. This vulnerability affects Firefox < 51.
https://www.mozilla.org/security/advisories/mfsa2017-01/
https://bugzilla.mozilla.org/show_bug.cgi?id=1309310