Thunderbird includes a Software Update feature that automatically checks for updates and prompts you to install them when necessary. For more information about the Software Update feature, see Updating Thunderbird.
Sometimes, the updater fails and displays the following error message:
This article presents possible solutions to this problem and workarounds to get Thunderbird updated.
If you share your computer with other users, and another user's account is still logged in with Thunderbird running, this error can occur. Make sure you're the only user logged in to the computer when updating Thunderbird. The easiest way to do this is to restart your computer. Once the computer starts up again, start Thunderbird, click the Help menu, and select Check for Updates....
End any Thunderbird processes that may still be active, either manually or by restarting your computer.
Once all instances of Thunderbird have been ended or after the computer starts up again, restart Thunderbird to see if the updater is now working.
You can manually end the Thunderbird process using Windows Task Manager.
If you are using Windows 7 or Vista:
If the issue is not resolved, end Thunderbird again, as explained above, but this time, open the C:\Program Files\Mozilla Thunderbird folder, right-click on the thunderbird or thunderbird.exe file icon and select Run as Administrator.
If the issue is not resolved, you may have stopped the service "Application Experience" or disabled "Application Experience" in msconfig.
If you have tried every solution above and you still see the Software Update Failed message, you can update Thunderbird by installing the new version over top of your current version.
Based on information from Software Update (mozillaZine KB)