A: firefox-esr security update has been released for Debian GNU/Linux 10 and 11 to address multiple security issues.

DSA 5026-1: firefox-esr security update

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Debian Security Advisory DSA-5026-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
December 19, 2021   https://www.debian.org/security/faq
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Package : firefox-esr
CVE ID : CVE-2021-43546 CVE-2021-43545 CVE-2021-43543 CVE-2021-43542
CVE-2021-43541 CVE-2021-43539 CVE-2021-43538 CVE-2021-43537
CVE-2021-43536 CVE-2021-43535 CVE-2021-43534 CVE-2021-38509
CVE-2021-38508 CVE-2021-38507 CVE-2021-38506 CVE-2021-38504
CVE-2021-38503

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 (ESR) 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/

For the oldstable distribution (buster) one more final toolchain update
is needed, updated packages will shortly be available as 91.4.1esr-1~deb10u1

For the stable distribution (bullseye), these problem have been fixed in
version 91.4.1esr-1~deb11u1.

We recommend that you upgrade your firefox-esr packages.

For the detailed security status of firefox-esr please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/firefox-esr

Further information about Debian Security Advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://www.debian.org/security/