This happens when security software somehow interrupts an update. My suggestion: - backup profile (just for safety, not usually affected). - delete the program in c:\program files\mozilla thunderbird or c:\program files(x86)\mozilla thunderbird - do NOT uninstall - now, download current version of Thunderbird and install. - if your profile is not immediately accessed, then click tools>moretroubleshootinginformation, scroll down to 'profiles', click 'about:profiles' and select your profile.