Here are the details from the Slackware 13.37 ChangeLog:
+--------------------------+
patches/packages/mozilla-firefox-5.0-i486-1_slack13.37.txz: Upgraded.
This release contains security fixes and improvements.
For more information, see:
http://www.mozilla.org/security/known-vulnerabilities/firefox.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 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware-13.0/patches/packages/mozilla-firefox-3.6.18-i686-1.txz

Updated package for Slackware x86_64 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.0/patches/packages/mozilla-firefox-3.6.18-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-3.6.18-i686-1.txz

Updated package for Slackware x86_64 13.1:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.1/patches/packages/mozilla-firefox-3.6.18-x86_64-1_slack13.1.txz

Updated package for Slackware 13.37:
ftp://ftp.slackware.com/pub/slackware/slackware-13.37/patches/packages/mozilla-firefox-5.0-i486-1_slack13.37.txz

Updated package for Slackware x86_64 13.37:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.37/patches/packages/mozilla-firefox-5.0-x86_64-1_slack13.37.txz

Updated package for Slackware -current:
ftp://ftp.slackware.com/pub/slackware/slackware-current/slackware/xap/mozilla-firefox-5.0-i486-1.txz

Updated package for Slackware x86_64 -current:
ftp://ftp.slackware.com/pub/slackware/slackware64-current/slackware64/xap/mozilla-firefox-5.0-x86_64-1.txz


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

Slackware 13.0 package:
6bae11a48ff0dbd131535523da47dfa3 mozilla-firefox-3.6.18-i686-1.txz

Slackware x86_64 13.0 package:
9c6b5b65cb74692f8e51e92a9af1b174 mozilla-firefox-3.6.18-x86_64-1_slack13.0.txz

Slackware 13.1 package:
5de7a771024cf248144cc84bf22e6c0b mozilla-firefox-3.6.18-i686-1.txz

Slackware x86_64 13.1 package:
9868ece365ad9c8fb88492e98124e989 mozilla-firefox-3.6.18-x86_64-1_slack13.1.txz

Slackware 13.37 package:
0f44d12d4cab622f99c20025090cc344 mozilla-firefox-5.0-i486-1_slack13.37.txz

Slackware x86_64 13.37 package:
cd7eb8f9e363e10026b1ecfa88a927b1 mozilla-firefox-5.0-x86_64-1_slack13.37.txz

Slackware -current package:
082655c72f19e52b4d6b97881d3b4388 xap/mozilla-firefox-5.0-i486-1.txz

Slackware x86_64 -current package:
a9dd701bf34e6141b386530ad38d4292 xap/mozilla-firefox-5.0-x86_64-1.txz


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

Upgrade the package as root:
# upgradepkg mozilla-firefox-5.0-i486-1_slack13.37.txz


+-----+

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