The Developer Tools feature suffers from a XUL injection vulnerability due to improper sanitization of the web page source code. In the worst case, this could allow arbitrary code execution when opening a malicious page with the style editor tool. This vulnerability affects Firefox ESR < 52.3 and Firefox < 55.
https://www.mozilla.org/security/advisories/mfsa2017-19/
https://www.mozilla.org/security/advisories/mfsa2017-18/
https://www.debian.org/security/2017/dsa-3928
https://bugzilla.mozilla.org/buglist.cgi?bug_id=1371586%2C1372112
https://access.redhat.com/errata/RHSA-2017:2456