New mozilla-thunderbird packages are available for Slackware 14.2 and -current to fix security issues.

Here are the details from the Slackware 14.2 ChangeLog:
+--------------------------+
patches/packages/mozilla-thunderbird-60.6.1-i686-1_slack14.2.txz: Upgraded.
This release contains security fixes and improvements.
For more information, see:
https://www.mozilla.org/en-US/thunderbird/60.6.1/releasenotes/
https://www.mozilla.org/security/known-vulnerabilities/thunderbird.html
(* Security fix *)
+--------------------------+


Where to find the new packages:
+-----------------------------+

Thanks to the friendly folks at the OSU Open Source Lab
(http://osuosl.org) for donating FTP and rsync hosting
to the Slackware project!

Also see the "Get Slack" section on http://slackware.com for
additional mirror sites near you.

Updated package for Slackware 14.2:
ftp://ftp.slackware.com/pub/slackware/slackware-14.2/patches/packages/mozilla-thunderbird-60.6.1-i686-1_slack14.2.txz

Updated package for Slackware x86_64 14.2:
ftp://ftp.slackware.com/pub/slackware/slackware64-14.2/patches/packages/mozilla-thunderbird-60.6.1-x86_64-1_slack14.2.txz

Updated package for Slackware -current:
ftp://ftp.slackware.com/pub/slackware/slackware-current/slackware/xap/mozilla-thunderbird-60.6.1-i686-1.txz

Updated package for Slackware x86_64 -current:
ftp://ftp.slackware.com/pub/slackware/slackware64-current/slackware64/xap/mozilla-thunderbird-60.6.1-x86_64-1.txz


MD5 signatures:
+-------------+

Slackware 14.2 package:
8819ba1c1bb0b82acedde947fe831567 mozilla-thunderbird-60.6.1-i686-1_slack14.2.txz

Slackware x86_64 14.2 package:
9999b34e9acd3100d592e825c1c912b2 mozilla-thunderbird-60.6.1-x86_64-1_slack14.2.txz

Slackware -current package:
6280813fd16a6563e12330ffd92ee5ff xap/mozilla-thunderbird-60.6.1-i686-1.txz

Slackware x86_64 -current package:
97a9367a19d5eab63ba37e75302a9d78 xap/mozilla-thunderbird-60.6.1-x86_64-1.txz


Installation instructions:
+------------------------+

Upgrade the package as root:
# upgradepkg mozilla-thunderbird-60.6.1-i686-1_slack14.2.txz

  Mozilla-Thunderbird Update for Slackware