Since you mention Eset, have you tried temporarily disabling it to see if it's tying up a Thunderbird data file?

This doesn't quite fit your scenario but it's worth checking... Windows 10 and 11 have a feature called "Fast Startup" which does, indeed, make boot-up a little faster, but at the cost of not thoroughly cleaning up processes from the previous session. The consequence is that if something crashes and even if you reboot, the crashed program's process may still be running in the background! This could cause an "already running" error like what you see. IMO the feature creates more problems than it solves and I always turn it off. To do that:

To disable Fast Startup on Windows 11, use these steps:

- Open Control Panel. - Click on Hardware and Sound. - Click on Power Options. - Click the "Choose what the power button does" option. - Click the "Change settings that are currently unavailable" option. - Under the "Shutdown settings" section, UNcheck the "Turn on fast startup" option. - Click the Save changes button.

Lin, Thank you for your response, I tried both suggestions, and neither worked. I am hoping someone has a solution. I really don't want to give up on Thunderbird, but if I can't find out where/how/why my PC thinks it is running I don't seem to have a choice.