I was having trouble with unread mail messages being shown on the welcome screen of Windows XP after using Thunderbird Portable.
I found that the launcher correctly backs up the UnreadMail key (HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail) and any subkeys it has unless the UnreadMail key itself does not exist. In that case Thunderbird Portable will leave data behind.
Please fix this. (This still happens with the latest version - 22.214.171.124).
[Subject modified by mod JTH]