While opening the Mozilla Thunderbird application, you might have encountered an error causing the application to hang. The application might be shown as running but will not be responding. The error message may appear like “Thunderbird is already running, but not responding. To open a new window, you must close the existing Thunderbird process or restart your system.” There are several reasons for this issue. In this post, we’ll be discussing the solutions to fix this error. But before moving on to the solutions, let’s take a look at the possible causes of this error.

Causes of the Mozilla Thunderbird not Starting Issue

Mozilla Thunderbird might be running at another location on the same profile. So, when it detects multiple users, it fails to respond. Also, if Thunderbird is not closed properly, it would have trouble starting. There might be a lock file that is preventing Thunderbird from running properly. Some temporary files, which are present on the server after the crash, are not letting you access the application. Moreover, a process might be running in the background hampering the working of Thunderbird. 

Ways to Fix the Error

• Try Restarting the Application

You can try restarting the Thunderbird application. When you do that, the app resets its settings and launches in its default state. However, the app would fail to respond if any files are missing. You will also get to know if the files are corrupted.

• Close your Profile on Other Computers

If you have logged into your Thunderbird profile from more than one device, this is also a reason why your Mozilla profile has failed to respond. You can close your profile on any other platform and see if it resolves the issue.

• Close Background Processes

Sometimes, even when you close the Thunderbird application, it continues to run in the background. If the application is running in the background, you will not be able to launch it. In such a case, you need to close all the Thunderbird processes running in the background by accessing the Task Manager on your device. Then, try launching the application again.

• Start Windows in Safe Mode

Starting Windows in Safe Mode can help you resolve the issue. In the Safe Mode, Windows uses limited files and drivers. If Thunderbird works, it will help you narrow down the cause of the problem.

• Use the LockHunter tool

With the help of LockHunter, you will be able to track down the files which are not letting the Thunderbird start. After tracking, you can close all the comments and then start Thunderbird.

• Try Using the App in Safe Mode

When the application detects a threat, it automatically ceases to function. This is done to protect your data and application. It can also happen when you log in to an unknown network. Under such circumstances, try running the app in safe mode. The app restricts its functioning to a bare minimum and stops all ancillary functions in safe mode. It performs only the basic functions and allows you to access your files.

• Remove the ParentLock File

Every time you open the Thunderbird application, it creates a parent lock file. This file is automatically deleted when the application is closed. If due for some reason, the app fails to close properly, then this file will not get deleted. In such a case, you need to manually delete the file.

• Close Other Programs

There can also be programs that are holding back Thunderbird from starting. You need to look for such programs and close them. Similarly, try closing any web extensions that are running in the background. These apps can hinder the running of Mozilla Thunderbird.

• Repair the Application Files

If the app is still not responding, it means the files of the application are corrupted. You can repair the damaged files and re-launch the application.

• Create New Profile

The Thunderbird application may not work or respond if your profile is corrupt or damaged. You can create a new one to resolve the issue. 

• Reinstall the Thunderbird Application

If nothing works, you can reinstall the application. But before uninstalling the application, make sure to move the profile folder to another location. After reinstalling the application, you can copy your profile folder back into the application.

To Conclude

With the help of the above-given methods, you can resolve the “Mozilla Thunderbird not Starting” issue. Depending on the cause of the issue, you can apply the solution to resolve it.

However, in case you’re planning to migrate from Thunderbird to MS Outlook, you can use a professional MBOX to PST converter tool, such as Stellar Converter for MBOX. The software converts all the MBOX data into PST, which you can import into Outlook. It also saves MBOX data in various other formats, such as .pdf, .txt, .html, etc.