Multiple security issues have been found in the Mozilla Firefox web browser, which could potentially result in the execution of arbitrary code, information disclosure or spoofing. Debian follows the extended support releases of Firefox. Support for the 78.x series has ended, so starting with this update we"re now following the 91.x releases. Between 78.x and 91.x, Firefox has seen a number of feature updates. For more information please refer to https://www.mozilla.org/en-US/firefox/91.0esr/releasenotes/