Thunderbird gives "Close Thunderbird" dialog box. Why?
Thunderbird only shows "Close Thunderbird" dialog box with - 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. - Why? Task Manager shows no Thunderbird application and rebooting system (warm or cold) does not clear the error. What do I do to fix this?
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As you mentioned, Task Manager not showing any background running Thunderbird.
Check it for All user, instead of current user.
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Create a new profile as a test to check if your current profile is causing the problems.
See Creating a profile:
If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem
Profile Backup and Restore
Τροποποιήθηκε στις από το χρήστη iamjayakumars
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Check this article.
Problem solved. I had to delete parent.lock file from Profile