A firefox-esr security update has been released for both Debian GNU/Linux 9 and 10.

DSA 4678-1: firefox-esr security update

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Debian Security Advisory DSA-4678-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
May 06, 2020   https://www.debian.org/security/faq
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Package : firefox-esr
CVE ID : CVE-2020-6831 CVE-2020-12387 CVE-2020-12392 CVE-2020-12395

Multiple security issues have been found in the Mozilla Firefox web
browser, which could potentially result in the execution of arbitrary
code or information disclosure.

For the oldstable distribution (stretch), these problems have been fixed
in version 68.8.0esr-1~deb9u1.

For the stable distribution (buster), these problems have been fixed in
version 68.8.0esr-1~deb10u1.

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/