Mozilla Thunderbird is a mail client that uses the same code base as Mozilla Firefox. Just like Firefox, Thunderbird can suffer profile locks that block you from accessing the service. The error message reads: "Thunderbird is already running, but not responding. To open a new window you must close the existing Thunderbird process, or restart your system." You will need to remove the profile lock by deleting all Mozilla processes from your Windows taskbar.
Instructions
Unlock Mozilla Thunderbird
Access your Windows Task Manager by pressing the "Ctrl," "Alt" and "Delete" keys simultaneously.
Left-click the "Processes" tab.
Highlight each Mozilla process and then press the "End Task" button located at the bottom right corner of the window.
Close the Windows Task Manager.
Restart your computer to finish repairing Mozilla Thunderbird.
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