I just found a bug in the latest Thunderbird release. It leaves behind a folder called "Thunderbird" in %APPDATA% with the usual Mozilla Crash Reports folder in it including a file with the install date.
Firefox and Seamonkey handle the folder correctly (=backup and restore), it seems that this functionality has been forgotten in the Thunderbird Launcher.
Tested on Win XP SP3 limited account.
Can anyone confirm this?
EDIT: This has been fixed.