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-52.6.0-i586-1_slack14.2.txz: Upgraded.
This release contains security fixes and improvements.
For more information, see:
https://www.mozilla.org/en-US/thunderbird/52.6.0/releasenotes/
(* 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-52.6.0-i586-1_slack14.2.txz

Updated package for Slackware x86_64 14.2:
ftp://ftp.slackware.com/pub/slackware/slackware64-14.2/patches/packages/mozilla-thunderbird-52.6.0-x86_64-1_slack14.2.txz

Updated package for Slackware -current:
ftp://ftp.slackware.com/pub/slackware/slackware-current/slackware/xap/mozilla-thunderbird-52.6.0-i586-1.txz

Updated package for Slackware x86_64 -current:
ftp://ftp.slackware.com/pub/slackware/slackware64-current/slackware64/xap/mozilla-thunderbird-52.6.0-x86_64-1.txz


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

Slackware 14.2 package:
56c00cce049a10b4085dbed3bd65d93c mozilla-thunderbird-52.6.0-i586-1_slack14.2.txz

Slackware x86_64 14.2 package:
d61a83b6a2fe47dc10c71c2ecf59586a mozilla-thunderbird-52.6.0-x86_64-1_slack14.2.txz

Slackware -current package:
9c9ac14d50ca93d7020f71978001662f xap/mozilla-thunderbird-52.6.0-i586-1.txz

Slackware x86_64 -current package:
d973836e05b5ca2653c3f2b3ab9f128c xap/mozilla-thunderbird-52.6.0-x86_64-1.txz


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

Upgrade the package as root:
# upgradepkg mozilla-thunderbird-52.6.0-i586-1_slack14.2.txz

  Mozilla-Thunderbird (SSA:2018-025-01) Update for Slackware