Using techniques that built on the slipstream research, a malicious webpage could have exposed both an internal network's hosts as well as services running on the user's local machine. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.
https://www.mozilla.org/security/advisories/mfsa2020-56/
https://www.mozilla.org/security/advisories/mfsa2020-55/