Email, Messaging, & Video Calls Email How to Fix Mozilla Thunderbird Not Starting Solve the 'Thunderbird Is Already Running, but Not Responding' error by Heinz Tschabitscher Writer A former freelance contributor who has reviewed hundreds of email programs and services since 1997. our editorial process Heinz Tschabitscher Updated on May 11, 2020 Email Yahoo! Mail Gmail Tweet Share Email Mozilla Thunderbird is a full-featured, secure, and reliable email client. Users, however, have noticed an issue where Thunderbird appears to freeze, not starting up even though it seems to be running. Thunderbird usually returns the error message, "Thunderbird is already running, but not responding. To open a new window, you must close the existing Thunderbird process or restart your system." Often, closing Thunderbird returns this additional error: "Your Thunderbird profile cannot be loaded. It may be missing or inaccessible." If you're having these problems, here's what might be happening and what you can do to fix it and get Thunderbird back up and running again. These troubleshooting steps apply to Thunderbird version 68.8.0 and earlier. Causes of Thunderbird Not Starting If Thunderbird refuses to start and returns an error about an existing Thunderbird process, it's because Thunderbird thinks your profile is already in use. The cause might be a stale profile lock 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 you have Thunderbird running on another computer somewhere. Thunderbird can't run on more than one computer at the same time with the same profile. Fortunately, there are some simple troubleshooting actions users can take to fix this problem. CC BY 2.0 / Flickr How to Make Thunderbird Start Again If Thunderbird is "already running, but not responding," or says that your profile is in use, here are the steps you should try. Close and reopen Thunderbird. It's always worth a try to close out of the application and then reopen it. Select Thunderbird > Quit Thunderbird from the menu, and then try reopening the application to see if this solves the problem. Close Thunderbird on another computer. If Thunderbird is running with your profile on another computer, close out of Thunderbird on that machine by selecting Thunderbird > Quit Thunderbird from the menu. Then try logging in again on the computer you're using. Kill Thunderbird's background processes. Even if you've closed out of Thunderbird, the application might still be running in the background. Ending Thunderbird processes still running in the background might fix the issue. On a Windows system, you do this via the Task Manager. With macOS, force quit all Thunderbird processes via the Activity Monitor, and on a Unix system, use the killall -9 thunderbird command in a terminal. Restart your computer. Restarting is an easy fix that often solves myriad technical issues. Start Thunderbird in Safe Mode. Start up Thunderbird in Safe Mode. This starts the application without certain extensions or add-ons that might be causing the error message. Try opening 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 clear automatically after you close Thunderbird. If Thunderbird fails to complete the closing process properly, this file isn't deleted. Try manually deleting the file to see if this solves the problem. On a Mac, open a terminal window and type cd and a space. From the Thunderbird folder in Finder, drag the icon into the terminal window so that the path to the folder will immediately follow the "cd" command. Hit Enter to run the command and then enter rm -f .parentlock. Unix users should delete both parentlock and lock from the Thunderbird folder. Use the LockHunter file-unlocking tool. Use LockHunter to see what's restricting Thunderbird from opening, and then shut down any holds on the program so that you can use it normally. Repair Thunderbird folders. A folder may be corrupted. Try repairing 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 all the information about your settings, mail, accounts, and any extensions you've installed. If something goes wrong, first back up your profile and then create a fresh one. Reinstall Thunderbird. If none of these troubleshooting steps solves the problem, try moving your profile folder to a different location to back it up. Then, reinstall Thunderbird without a profile present. Everything should start fresh.