Mozilla developers reported memory safety and script safety bugs present in Firefox 73. Some of these bugs showed evidence of memory corruption or escalation of privilege and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 74.
https://www.mozilla.org/security/advisories/mfsa2020-08/
https://bugzilla.mozilla.org/buglist.cgi?bug_id=1181957%2C1557732%2C1557739%2C1611457%2C1612431