It may happen that the Thunderbird mail client opens two times or more. To resolve this problem, simply delete the file session.json so the file is regenerated correctly at the next opening of the program.

To locate the file you can use the search file on your operating system or systems to XP the path is:

C:\\Documents and Settings\\USER\\Application Data\\Thunderbird\\Profiles\\PROFILE

while other windows systems you can also use this shortcut: %APPDATA%\\Thunderbird\\Profiles\\PROFILE

USER: is the user name with which you access the operating system
PROFILE: it is generally a sequence of letters and numbers that usually ends with .default