Thunderbird usually returns the issue notification:

These troubleshooting steps apply to Thunderbird version 68.8.0 and earlier.

Close and reopen Thunderbird.

It’s always worth a make a run at shut the system and then reopen it.

SelectThunderbird>Quit Thunderbirdfrom the menu, and then reopen the program to see if this solves the problem.

Close Thunderbird on another computer.

Then access again on the computer you’re using.

Kill Thunderbird’s background processes.

Even if you closed Thunderbird, the tool might be running in the background.

Ending Thunderbird processes that are running in the background might fix the issue.

On a Windows system, do this from theTask Manager.

With macOS, force quit all Thunderbird processes from theActivity Monitor.

On a Unix system, use thekillall -9 thunderbirdcommandin a terminal.

Restarting is an easy fix that often solves many technical issues.

Start Thunderbird in Safe Mode.

This starts the software without certain extensions or add-ons that caused the issue notification.

Open in Safe Mode and see if this solves the problem.

Delete the parentlock file.

The parentlock file is created every time Thunderbird starts and should automatically clear after you close Thunderbird.

If Thunderbird fails to complete the closing process properly, the parentlock file isn’t deleted.

Manually delete the file to see if this solves the problem.

On a Mac, open a terminal window and typecdand a space.

PressEnterto fire off the command and then enterrm -f .parentlock.

On Unix, deleteparentlockandlockfrom the Thunderbird folder.

Use the LockHunter file-unlocking tool.

A folder may be corrupted.

Repair Thunderbird folders to see if this solves the problem.

Create a new Thunderbird profile.

There may be something wrong with your Thunderbird profile.

Profiles in Thunderbird and Firefox store information about your parameters, mail, accounts, and extensions you installed.

If something goes wrong,back up your profileand then create a fresh one.

Then, reinstall Thunderbird without a profile present.

Everything should start fresh.

The cause might be a stale profile lock that was left after Thunderbird crashed.

This means Thunderbird didn’t close properly or correctly clean up temporary files.

Additional processes are running in the background and Thunderbird is confused, frozen, and unable to open.

Another cause might be that Thunderbird is running on another computer.

Thunderbird can’t run on more than one computer at the same time with the same profile.