Versions 1.5.0.4 of both Thunderbird and Firefox were released by the Mozilla Corporation today.  The release notes state that each contained "several security fixes", but the known vulnerabilities page hasn't been updated yet, so we don't know exactly what they are. 

Update 2: (2006-06-02-15:09 UTC): the known vulnerabilities page linked to above is now up to date with the 1.5.0.4 updates.

Update: Thanx to fellow handler Koon-Yaw Tan for bringing this to our attention and pointing out the advisories.

http://www.mozilla.org/security/announce/2006/mfsa2006-31.html
http://www.mozilla.org/security/announce/2006/mfsa2006-32.html
http://www.mozilla.org/security/announce/2006/mfsa2006-33.html
http://www.mozilla.org/security/announce/2006/mfsa2006-34.html
http://www.mozilla.org/security/announce/2006/mfsa2006-35.html
http://www.mozilla.org/security/announce/2006/mfsa2006-36.html
http://www.mozilla.org/security/announce/2006/mfsa2006-37.html
http://www.mozilla.org/security/announce/2006/mfsa2006-38.html
http://www.mozilla.org/security/announce/2006/mfsa2006-39.html
http://www.mozilla.org/security/announce/2006/mfsa2006-40.html
http://www.mozilla.org/security/announce/2006/mfsa2006-41.html
http://www.mozilla.org/security/announce/2006/mfsa2006-42.html
http://www.mozilla.org/security/announce/2006/mfsa2006-43.html

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Jim Clausing, jac --at-- isc dot sans dot org