Issues with Thunderbird 1.5
As posted on Slashdot, there are some issues with the new Thunderbird 1.5.
The most important: Uninstall your old versions of Thunderbird before running the installer for 1.5. Don't let the installer for 1.5 just overwrite the older version, as it will result in a BROKEN installation! Backup your profiles, uninstall old version, then install 1.5.
This is officially listed under Known Issues: