The internal feed reader APIs that crossed the sandbox barrier allowed for a sandbox escape and escalation of privilege if combined with another vulnerability that resulted in remote code execution inside the sandboxed process. This vulnerability affects Firefox ESR < 52.1 and Firefox < 53.
https://www.mozilla.org/security/advisories/mfsa2017-12/
https://www.mozilla.org/security/advisories/mfsa2017-10/
https://bugzilla.mozilla.org/show_bug.cgi?id=1341191
https://access.redhat.com/errata/RHSA-2017:1106