Here are the details from the Slackware 13.1 ChangeLog:
+--------------------------+
patches/packages/mozilla-firefox-3.6.16-i686-1.txz: Upgraded.
This release fixes a security vulnerability by blacklisting several
invalid HTTPS certificates.
For more information, see:
http://www.mozilla.org/security/announce/2011/mfsa2011-11.html
(* Security fix *)
+--------------------------+


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

HINT: Getting slow download speeds from ftp.slackware.com?
Give slackware.osuosl.org a try. This is another primary FTP site
for Slackware that can be considerably faster than downloading
directly from ftp.slackware.com.

Thanks to the friendly folks at the OSU Open Source Lab
(http://osuosl.org) for donating additional 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 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware-13.0/patches/packages/mozilla-firefox-3.6.16-i686-1.txz

Updated package for Slackware x86_64 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.0/patches/packages/mozilla-firefox-3.6.16-x86_64-1_slack13.0.txz

Updated package for Slackware 13.1:
ftp://ftp.slackware.com/pub/slackware/slackware-13.1/patches/packages/mozilla-firefox-3.6.16-i686-1.txz

Updated package for Slackware x86_64 13.1:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.1/patches/packages/mozilla-firefox-3.6.16-x86_64-1_slack13.1.txz


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

Slackware 10.2 package:
36068a4243b05cc791620ddbbf88edbb mozilla-firefox-2.0.0.20-i686-1.tgz

Slackware 11.0 package:
36068a4243b05cc791620ddbbf88edbb mozilla-firefox-2.0.0.20-i686-1.tgz

Slackware 12.0 package:
36068a4243b05cc791620ddbbf88edbb mozilla-firefox-2.0.0.20-i686-1.tgz

Slackware 12.1 package:
36068a4243b05cc791620ddbbf88edbb mozilla-firefox-2.0.0.20-i686-1.tgz

Slackware 12.2 package:
44a3d85cb39de9b698fc266ea54ce1fa mozilla-firefox-3.0.19-i686-1.tgz

Slackware 13.0 package:
bcbfe4cc2fff86976fdf73cc17570c5e mozilla-firefox-3.6.16-i686-1.txz

Slackware x86_64 13.0 package:
cb4171670047e641ebfe2f21efa1c2d2 mozilla-firefox-3.6.16-x86_64-1_slack13.0.txz

Slackware 13.1 package:
53695a9e7b230bfe590149e63c02d67b mozilla-firefox-3.6.16-i686-1.txz

Slackware x86_64 13.1 package:
4141f75f0deadf914d3238ea31d5f554 mozilla-firefox-3.6.16-x86_64-1_slack13.1.txz


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

Upgrade the package as root:
# upgradepkg mozilla-firefox-3.6.16-i686-1.txz


+-----+

Slackware Linux Security Team
http://slackware.com/gpg-key