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

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Debian Security Advisory DSA-4600-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
January 09, 2020   https://www.debian.org/security/faq
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Package : firefox-esr
CVE ID : CVE-2019-17026 CVE-2019-17024 CVE-2019-17022
CVE-2019-17017 CVE-2019-17016

Multiple security issues have been found in the Mozilla Firefox
web browser, which could potentially result in the execution
of arbitrary code, data exfiltration or cross-site scripting.

For the oldstable distribution (stretch), this problem has been fixed
in version 68.4.1esr-1~deb9u1.

For the stable distribution (buster), this problem has been fixed in
version 68.4.1esr-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/