Email, Messaging, & Video Calls Email How to Fix Mozilla Thunderbird Not Starting What to Do When Thunderbird Is Already Running, but Not Responding Share Pin Email Print Hero/Images Email Yahoo! Mail Gmail 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 June 24, 2019 If Mozilla Thunderbird refuses to start and complains about another instance or a profile in use, the cause may be a stale profile lock left from a crashing instance of Thunderbird. This is usually the error that's seen: Thunderbird is already running, but not responding. To open a new window you must close the existing thunderbird process, or restart your system. Of course, you've probably already tried restarting your computer and found that it doesn't work. One thing you can try is to remove the file that locks your profile so that Thunderbird will (hopefully) start up and run like normal again. How to Make Thunderbird Start Again If Thunderbird is "already running, but not responding," or opens the profile manager and says that your profile is in use, try this: Shut down all Thunderbird processes: In Windows, kill any instances of thunderbird in Task Manager.With macOS, force quit all thunderbird processes in Activity Monitor.With Unix, use the killall -9 thunderbird command in a terminal. Open your Mozilla Thunderbird profile folder. If you're on Windows, delete the parent.lock file. macOS users should open a terminal window and type cd followed by 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 on the keyboard to run the command (which will change the working directory to the Thunderbird folder), and then enter in another command: rm -f .parentlock.Unix users should delete both parentlock and lock from the Thunderbird folder. Try starting Thunderbird again. If the above steps don't work to open Thunderbird, one thing you can try is to use LockHunter to see what is restricting Thunderbird from opening and then shut down any holds on the program so that you can use it normally.