How-To Geek

How to Fix the “Firefox Is Already Running” Error


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.

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.

  • Published 12/14/12

Comments (26)

  1. Cambo

    This is one of those things Mozilla should have fixed, YEARS ago. As well as the need to restart Firefox after adding extensions.

  2. Josh B.

    Ditto on that.

  3. CoughingMonkey

    Is that all that it was/is? It was very considerate of you to make this known to us FF users. It only took you a couple of years to get the memo out? Are you trying to get your house in order before the 21st of Dec?

  4. Paul

    Firefox uses FILE LOCKS when in use? OMG! How juvenile. No wonder a crash can stop it restarting again. Haven’t the devs heard of mutexes or semaphores to prevent multiple instances?

  5. EB

    I use C-Cleaner to close FF as well as clean (history, cache, temps, etc.) and start fresh. But FF should have feature for providing multi apps.

  6. DK

    It’s probably not “Ctrl+Shift+Esc” but “Ctrl+Alt+Del”. Or I am wrong?

  7. surgec411

    Works both ways, but “Ctrl+Alt+Del” is for more options such as “switch user”. I prefer “Ctrl+Shift+Esc” because it instantly opens task manager and is faster.

  8. surgec411

    I’ve had this problem for a few years, and then solved it by using task manager. Though it happened too much and I switched to chrome. If I find a bug in chrome, I think ill switch back. :P

  9. Kent Hinrichsen

    I too have had the occasional failure of firefox not closing properly, and would use task manager to get it closed, but recently discovered through experimentation that if I right clicked on my FF icon I could open a new page, then just close it again and all would be well

  10. houndless


  11. Bruce W Fowler

    Really appreciate the courtesy of telling up front that this was a Winders-only article.

  12. Raven

    Bruce, the article is for *nix and Mac OS also as it states ” 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. ” :)

    Btw, I went to school with a Bruce Fowler, class of ’68, N.W. IN. Might you be the same?

  13. Fort Worth

    Thanks for the tip. I actually quit FF and switched to Chrome because of this glitch (as did surgec411, apparently).

  14. Vinh

    The fastest way to terminate Firefox is to use the taskkill option.
    Press Start+R
    Type “taskkill /IM firefox.exe /F” and press Enter.

  15. Art€

    Yep, a good tip!. I’ve been doing this now for some time, i.e. using Task Manager, but I’ve just learnt one more trick from you: Alt+Ctrl+Esc, thank you brilliant! Always ‘FUN’ to learn!! (Don’t always like being taught though :/ )

  16. Steven

    I had thought this was only a WINDOWS issue. However, that’s untrue as it happens to me (quite often) in Linux.

  17. Richard Steven Hack

    Yes, it happens in Linux frequently, but in Linux I’ve never had to do anything but wait a few seconds and then restart Firefox. Apparently only in Windows does Firefox screw up the lock file clearing – which doesn’t surprise me since nothing works in Windows… :-)

  18. Richard Steven Hack

    By the way, the trivial fix to this is for Mozilla to simply write code to declare “Firefox is closing…” until it actually IS closed. But that’s way too obvious, simple and user-friendly for the morons at Mozilla…

  19. B Pratt

    I prefer using Process Explorer rather than the Task Manager which only allows you to ‘end’ a process as opposed to killing it like you can do in Linux/Unix land.

  20. Arun K

    I simply close the error message, right click on the desktop and “refresh” my desktop screen. If I then double click on the Firefox icon, then it opens without any further issues. Dont know why this trick works. I tried it one day as I was too lazy to restart the PC or open task manager! Hope this helps someone.

  21. Claude LaFrenière

    Just put this command line in a shortcut for any not responding process

    C:\Windows\System32\taskkill.exe /f /fi “status eq not responding”


    add this command in context menu with this .reg file from Mr. Vishal Gupta:

    Windows Registry Editor Version 5.00

    ;Created by Vishal Gupta for

    [HKEY_CLASSES_ROOT\DesktopBackground\Shell\Kill Not Responding Tasks]

    [HKEY_CLASSES_ROOT\DesktopBackground\Shell\Kill Not Responding Tasks\command]
    @=”taskkill /F /FI \”STATUS eq NOT RESPONDING\””


  22. mhenriday

    On rare occasions I have seen this notice, but it’s never happened that clicking OK and then waiting a couple of seconds has failed to resolve the problem. But then again, I generally run Ubuntu, which I prefer to Windows….


  23. Thomas

    Thanks, I didn’t know about Ctrl+Shft+Esc.

  24. Fern Kline

    After fooling with this problem. I finally decided that the best solution was to dump Foxfire. I have never regretted my decision

  25. Paul

    Arun, your “trick” works because by the time you’ve closed the message, right-clicked, refreshed, and then double-clicked the Firefox again, it has finished closing and is ready to re-open. That’s all. :)

  26. Navigator Numbskull

    Wouldn’t some/all this headache be avoidable if you just used the PORTABLE version?

    Check it out:

    …And just who decided that upgrading to a new Firefox version about every other month was a good thing? It stinks! About all they change are a couple of config settings, patch a bug or two that you didn’t even know was there and call it version 17. Wala! The new and “improved” Firefox. Yet, this memory bug lives on. It’s enough to make me want to go back to IE if it weren’t for the fact that IE stinks even worse!

    And don’t even get me started on those other crumby choices like Opera, Chrome or that biggest joke of all browsers, Safari. Pee-yew!

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