Memory safety bugs were reported in Firefox 54. Some of these bugs showed evidence of memory corruption and we presume that with enough effort that some of these could be exploited to run arbitrary code. This vulnerability affects Firefox < 55.
www.securityfocus.com/bid/100199
www.securitytracker.com/id/1039124
bugzilla.mozilla.org/buglist.cgi?bug_id=1353763%2C1353356%2C1370070%2C1375435%2C1373663%2C1363150%2C1370817%2C1273678%2C1367850%2C1347968%2C1361749%2C1349138%2C1371982%2C1344666%2C1369836%2C1330739%2C1371511%2C1371484
www.mozilla.org/security/advisories/mfsa2017-18/