Mozilla has fixed three high-severity flaws with the release of Firefox 81 and Firefox ESR 78.3. Two severe bugs are errors in the browser s memory-safety protections, which prevent memory access issues like buffer overflows. Another bug is a use-after-free issue, which is a type of vulnerability related to the incorrect use of dynamic memory. The remainder of flaws fixed in Firefox 81 are moderate in severity, including a download-origin spoofing flaw and a cross-site scripting flaw.
Source: https://threatpost.com/firefox-81-release-bugs/159435/