The “Firefox is already running, but is not responding” error has haunted Firefox users for years. You don’t have to restart your computer when you see this error – you can usually fix it with a quick trip to the Task Manager.
This error occurs when Firefox is closed but is still running in the background. Firefox is either in the process of closing or is frozen and hasn’t quit properly. In rare situations, there may be a problem with your profile.
Wait a Few Seconds
If you’ve just closed all your Firefox windows and quickly tried to open a new one, you may see this message. This is because Firefox is still running in the background, doing housekeeping tasks and saving your data before quitting completely. If you wait a few seconds and try launching Firefox again, you may find that it opens properly.
End Firefox.exe in the Task Manager
If waiting a few seconds didn’t help, then Firefox has failed to quit properly. While all Firefox windows may be closed, Firefox itself is still running in the background. It may be frozen and using no system resources or it may be chewing up your available CPU time.
Luckily, ending Firefox in the Task Manager is simple. First, open the Task Manager by pressing Ctrl+Shift+Escape. You can also right-click your taskbar and select Task Manager.
Firefox won’t appear in the list of running applications because this list only shows applications with open windows. No problem – just click the Processes tab.
(On Windows 8, you should find Firefox under Background processes. You can also click the Details tab to see a full list of processes.)
Type the letter f and you should instantly jump to the firefox.exe process. You can also click the Image Name header to sort the process list alphabetically and locate firefox.exe.
Once you’ve located firefox.exe, select it and click the End Process button.
This is what the message means when it asks you to “close the existing Firefox process.”
Confirm the operation and Firefox will be unceremoniously quit. You can now restart Firefox.
Restart Your Computer
Restarting your computer is the nuclear option and, quite frankly, isn’t a very good idea. If you encounter this error, you can always fix it without restarting your computer.
The “restart your computer” suggestion is for less-experienced users that may not want to deal with the Task Manager – restarting your computer works because it ends all the processes on your computer, including firefox.exe.
Remove the Profile Lock
If you’ve tried everything above – including rebooting your computer – and you still see this message, it’s possible that your Firefox profile is still “locked.” Firefox is designed to ensure that only one copy of Firefox is using a profile at a single time. To ensure this, it places special lock files in the profile directory. In rare cases, Firefox could have left behind the lock file when it crashed or quit abnormally.
You can delete these lock files yourself. Ensure Firefox is not running (not even in the task manager) before you do this or you could potentially corrupt your profile.
First, you’ll need to open your Firefox profile folder. To do so on Windows, press Windows Key+R, copy-paste the following line into the box, and press Enter:
Double-click the profile folder ending in .default. The profile has a random prefix name on each computer, but it always ends in .default.
Locate the parent.lock file and delete it. Firefox should now launch properly.
If you’re using Mac OS X, you’ll need to delete the .parentlock file in your profile directory instead. Linux users will need to delete the .parentlock and lock files.
The methods above should fix the vast majority of “Firefox is already running” errors. If you’re still experiencing this problem, consult the Profile in use wiki page on mozillaZine for more technical solutions to rare problems.
We’ve also covered troubleshooting crashes and hangs with Firefox – while the article itself is for an older version of Firefox, most of the tips are still applicable today.