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
Bewerkt door iamjayakumars op
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Check this article.
Problem solved. I had to delete parent.lock file from Profile