Using techniques that built on the slipstream research, a malicious webpage could have scanned both an internal network's hosts as well as services running on the user's local machine utilizing WebRTC connections. This vulnerability affects Firefox ESR < 78.9, Firefox < 87, and Thunderbird < 78.9.
https://www.mozilla.org/security/advisories/mfsa2021-12/
https://www.mozilla.org/security/advisories/mfsa2021-11/