It looks like the auto-update feature in Mozilla's Firefox browser and Thunderbird email client is not working properly when installed in Microsoft Windows Vista. The download appears to start and complete as expected. However, the upgrade process itself fails. Firefox or Thunderbird will then download another upgrade copy and re-attempt to upgrade with the same failure. I have not seen this problem under Mac OS X or Linux.

The solution is to download a fresh copy of Firefox or Thunderbird and run the installer manually.