Thunderbird only gives Close Thunderbird dialog. Why?
Thunderbird only shows "Close Thunderbird" dialog box -Thunderbird is already running, but is not responding. To open a new window, you must first close the existing Thunderbird process, or restart your system. - Task Manager shows no running Thunderbird process and rebooting system does not clear the error. How do I fix this?
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Duplicate thread - https://support.mozilla.org/en-US/questions/1003994