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.

In the 'profile name' folder (that have a similar name to this: 2154edr5.default) there should be a 'parent.lock' file You would need to Exit Thunderbird first and then access profile name folder and delete the 'parent.lock' file.

Where did you get the original Thunderbird program download? Where did you get the latest download after uninstalling original Thunderbird?

Did you install Thunderbird from the official download website: https://www.thunderbird.net/en-US/thunderbird/all/

Or did you get Thunderbird from the 'Windows App Store' ?

This info is important as it determines where your profile named folders s are stored.

If you had actually deleted the profile name folders using the Profile Manager or manually directly then you would have lost everything including address books, so did you really remove the profile name folders or did you just remove the pop or imap mail accounts via Account Settings?

I have the same problem, and installed it via downloaded file from website. I've uninstalled, installed new version, old version... All solutions I've seen online. Nothing works. I've even deleted EVERYTHING Thunderbird related on my PC I could find. Still no luck.

I have the same problem, and installed it via downloaded file from website. I've uninstalled, installed new version, old version... All solutions I've seen online. Nothing works. I've even deleted EVERYTHING Thunderbird related on my PC I could find. Still no luck.

I usually find these Youtube tech "help" videos to be pretty useless if not completely wrong, but this one has some valid suggestions for you to try. I will assume you're using Windows since you don't mention otherwise. You should begin by rebooting the computer, then using Task Manager, confirm that there are no invisible instances of Thunderbird running. Then follow the suggestions in this video:

https://www.youtube.com/watch?v=jV31ztP3Tvk

to start Thunderbird in Safe/Troubleshoot mode, which will temporarily disable all add-ons. If it works after that, that would suggest that one of them is misbehaving. You you should also take this opportunity to disable Hardware Acceleration, which can cause these kind of symptoms on some systems.

Please let us know your results.

By the way, it's best form to start a new thread for your own issue rather than piggy-back on someone else's. But since you're already here, please continue.

I've already tried to start in Troubleshoot mode (i.e. holding in shift and clicking Thunderbird), but the same error message popped up.

Not sure what you mean by "Hardware Acceleration"? What type? How?

I thought adding to the same thread would be helpful if others arrives here by googling, like me.

The Hardware Acceleration setting can be found in <Settings | General | Indexing | Use hardware acceleration when available>. The setting is designed to improve performance, but in my experience it doesn't make much difference and it can clash with some display drivers.

If thunderbird says Another version is running, then it's because the 'parent.lock' file in a profile name folder is saying to Thunderbird program that another instance is already running. You would need to Exit Thunderbird if you see it running, access each 'profile name' folder and delete: the 'parent.lock file' and I would also delete the 'session.json' file as well. It's important to check all the 'profile name' folders and do the same deletions.

Thunderbird Profiles are auto stored in this location if you downloaded/installed Thunderbird from the official website: https://www.thunderbird.net/en-US/thunderbird/all/

• C://Users/username/Appdata/Roaming/Thunderbird/Profiles/profile name folder

Deleting that 'Thunderbird' folder would delete absolutely everything and I mean everything. So if you need to backup that profile to reinstall any address books, then get a copy of that Thunderbird folder or the actual profile name folder and put it in a safe easy acessible location like the desktop as a temp location. Then when you start Thunderbird, it would auto create a new profile folder name and ask you to create a mail account. after which you can import all your old address books.

I'm asking this because some people think just removing the mail account is deleting a profile and it is not.

If by any chance, in the past, you installed Thunderbird from the 'Windows App Store' then there may be a profile in this location causing the problem - you could just remove it.

• C://Users/username/AppData/Local/Packages/MozillaThunderbird.MZLA_h5892qc0xkpca/LocalCache/Roaming/Thunderbird/Profiles/profile name folder

If you installed a previous version then it would not work on the current profile as version installations use a dedicated profile. So if you then installed an older version - did you do this:

• Exit Thunderbird.
• Uninstall Thunderbird program.
• Access profile name folder and delete the 'compatibility.ini' file.
• Then get a download for an older version from here:
• https://ftp.mozilla.org/pub/thunderbird/releases/
• Select version number, win64, language and click on the Thunderbird exe file to get download.

I have the same problem, and installed it via downloaded file from website. I've uninstalled, installed new version, old version... All solutions I've seen online. Nothing works. I've even deleted EVERYTHING Thunderbird related on my PC I could find. Still no luck.

Are you saying you completely deleted your entire 'Thunderbird' profile folder? If you did that then you would have lost all mail accounts, address books, calendar data etc. If you used Imap accounts then all emails would still be on the server. So recreating an imap account would be able to access emails again.

Please tell me how did you delete everything...Did you do this?

• Exit Thunderbird
• In Windows search type: %Appdata%
• Select: %Appdata% file folder
• Select 'Roaming'
• Delete the 'Thunderbird' folder

Or did you start the 'Thunderbird Profile Manager' when Thunderbird closed

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles#w_start-the-profile-manager-when-thunderbird-is-closed

Then select the profile name and click on 'Delete Profile' You said: "I have deleted the existing profiles" Did you remove all of them via the 'Thunderbird Profile Manager' ? Did you create a new profile via the 'Thunderbird Profile Manager'?