Firefox is already running, but is not responding

Martin Brinkmann
Oct 27, 2008
Updated • Feb 11, 2015
Firefox, Firefox add-ons

The message "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system" popped up quite frequently on a test system running the latest official version of Firefox 3.

The issue was noticed regularly after closing Firefox and opening the browser soon thereafter again.

There did not seem to be a pattern to it though. The only indicator was that a firefox.exe process was still shown as running in the Windows Task Manager even though all windows of the browser had been closed already and non was visible anymore on the desktop.

First time I noticed the issue I decided to wait to see what would happen. In particular, I was interested if the Firefox process would terminate automatically. Turns out it did not and after waiting 15 or so minutes I decided to troubleshoot the issue to find a solution for it.

The only option that helped was to kill the Firefox process which would free the resources and make it possible to restart Firefox. The problem with killing the process though is that it could damage important files that are needed to run Firefox. It could damage files in the profile folder like the bookmarks.

firefox not responding

The Mozilla website lists a few possible causes that could be the root of the problem. The lists includes problematic extensions, Java applets or Internet security software. There is also an error description in the Firefox Knowledge Base about the Firefox is already running error message which gives a few additional pointers.

These pages list many different causes for the issue. A common one is that Firefox is still processing data in the background which can be the case if you have configured it to delete browsing data on exit for example.

This process should not take longer than a couple of seconds tops though.

You can terminate the process but that may cause data loss and is generally not advised unless there is no other option to deal with the issue.

One solution may be to delete the parent.lock file in the Firefox profile folder if it exists as it may be responsible for the issue.

The problem is that non of the possible causes listed on the two pages were the cause for the error message on the test system. It is easy to test if extensions are the cause of the error by starting up Firefox in safe mode which will load Firefox without extensions. If the problem persists it is clear that extensions are not the source.

Anyone else ever got that problem and found a solution for it?

Firefox is already running, but is not responding
Article Name
Firefox is already running, but is not responding
The guide provides you with troubleshooting tips for the Firefox is already running error message when you try to run the browser.

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  1. Anonymous said on September 24, 2009 at 9:28 pm

    Dear users!
    I had the same problem on FF v3.5. But when I unchecked all 3 “Automatically check for updates to” check boxes (Tools->Options->Update) it started to go right. But I don’t know he reason…

  2. Happowal said on September 8, 2009 at 7:14 am

    I fixed the problem on my computer, i don’t know if it would work for anyone else but they might as well try it. If you’re on vista, click start and then type into the search box:


    click the folder that comes up (that is your profile), and delete “parent.lock” (make sure you don’t have firefox open when you do this otherwise it wont work)

    hope it helps. and thankyou to Damian, as he pointed me in the right direction.

    [on a sidenote, i think its to do with the profiles and the lock.. i’m not sure i understand this correctly, but when firefox starts up, its supposed to create a lock and then that lock disappears when you close it, my firefox had a lock before it opened, and somehow i think that affects it? i hope this helps if anybody can understand this)

  3. scube said on March 30, 2009 at 5:29 am

    I have experienced this problem after I installed the latest Java.

    I could restart Firefox every time by doing nothing else but shutting java down in the processes window of Task Manager.

    It may be a Firefox issue or it may be a Java issue.

    I have now installed an earlier Java and the issue is still there.

    Windows 2000
    Firefox 3.0.8
    Java – current


  4. linux said on December 16, 2008 at 12:00 pm

    Right, because AVG is the problem, when I’m having this error message in Linux…

  5. HellBringer said on December 16, 2008 at 1:44 am

    Good News people!!!
    I’ve finally found the problem (at least in my conputer)

    the piece of junk that has been anoying our try for a succesful execution of firefox iiiiis: “the AVG add-ons”

    After iniciating FF3 in Safe-Mode, taking all the add-ons and everything else out, I realized the problem had stoped. I exited FF over and over again to make sure it worked.
    Then, I started activating the add-ons 1 by 1 and exiting FF to try if it still worked. Until… guess what! After adding the AVG add-ons, FF didn’t work anymore. So then I deactivated the AVG add-ons. But it still didnt work. So I had to start FF in Safe-Mode once again.
    I activated the add-ons again (except the AVG add-ons) and now I have my FF working perfectly.

    Thanks for the help. Hope that this info. can also help you guys.

  6. HellBringer said on December 16, 2008 at 12:40 am

    thank Martin

    I’ll do the Safe-Mode thing

    but how do I create a new profile?
    and can someone explain how to do the batch file
    and what to put in it?
    i know what it is but not how to create it

  7. Martin said on December 15, 2008 at 11:54 pm

    Hellbringer you could try the following:

    a) create a new profile and see if it happens there as well
    b) run Firefox in Safe-Mode which disables all add-ons so that you know if an add-on might be the cause of the problem.

  8. HellBringer said on December 15, 2008 at 11:32 pm

    i spoke too soon
    it doesnt work
    i have no idea what to do

  9. HellBringer said on December 15, 2008 at 11:25 pm

    hey dude.head
    at last someone has solved the problem
    i did it too and it seems that it works
    you have no idea of how good i feel now (or probably you do)
    thaks to all
    woo hoo my FF works

  10. dude.head said on November 15, 2008 at 12:58 am

    hey thanks for this,, i went to the “root” link and i had trial and error with the different causes and it turns out that just disabling the AVG Safesearch extension fixes my problem. now runs perfectly and i can start fireox after closing it without having to end process :P

  11. R said on November 6, 2008 at 4:56 pm

    I run zone alarm , and have found that if I turn down the internet zone to medium the problem of this recuring notification seems to disappear, however I am not expert enough to really understand why , except for the fact that I am visible to hackers while online . sometimes after I have opened ff3 I will go back to zone alarm and turn the settings back up.
    works for me

  12. Damian said on October 29, 2008 at 1:11 am

    I used to have this problem fairly regularly at uni running Linux (Fedora to be precise) and the problem seems to have been cause by a lock not being released.
    In linux, these locks are found in ~/.mozilla/firefox/random string for profile/

    The 2 files which need to be deleted are lock and .parentlock.

    A simple shell script which removes these files and launches firefox is very easy to make and doesn’t seem to cause any problems either, as the locks are recreated as needed

  13. roggaro said on October 28, 2008 at 10:11 pm

    happened for the first time to day and I´m using FF2…
    to me it seems to be a an issue within windows.
    was running JetAudio with ‘time critical’ file playback at the same time… so I just lowered and will see if it happens again

  14. Jojo said on October 28, 2008 at 11:05 am

    @madforit – try rebuilding your user profile.

  15. AussieRodney said on October 28, 2008 at 9:38 am

    Occasionally happens when I close the FF Preloader & attempt to start FF itself. Usually goes away on the second attempt.

  16. madforit said on October 28, 2008 at 8:11 am

    Had this problem from the very first copy of firefox,and i agree its nothing to do with extensions/addons,glad to know its not just me.
    Had to reformat my pc yesterday and decided to
    start afresh with everything,even firefox 3
    which after testing the first version hated and
    was surprised how fast it now is,but still getting the firefox still running problem.

  17. venkat said on October 28, 2008 at 6:43 am

    Yes I found this problem once I solved it by killing the Firefox process that’s as the solution I mentioned to this in my blog post also .Unless Firefox comes with solution to this in their knowledge base ,this is the temporary solution seems to be the solution

  18. george said on October 28, 2008 at 5:51 am

    I have this problem all the time with firefox. I’ve always believed the cause was the privacy setting “always clear my private data when i close firefox”. You may have closed your firefox window but there may be a process in the background busy erasing your browser data. You can turn this privacy feature off and let a third party privacy utility take care of it. You might also try minimizing firefoxs cache.

  19. paulo said on October 28, 2008 at 4:14 am

    Yeah, I hit this issue about 2 or 3 times a day. I use update checker too but there should be a real less janky solution.

  20. clownboat said on October 28, 2008 at 3:37 am

    I have it with FF 3.x on an irregular basis. Alas, I force a shutdown through task manager. My problem is I love my FF, I just want it clean and lean. Too much resource use. Unless I get a new laptop every year I can’t keep ahead of the resource curve.

  21. linuxeventually said on October 28, 2008 at 3:09 am

    Happens occasionally in Linux as well, usually waiting a bit before attempting to reopening does the trick (and it’s one of those dealios, where if you try to open the instance before the process has completed and exited it will “add” time to it’s completion) If you are in a hurry create a new firefox profile: navigate to directory containing firefox.exe on the command line and append the parameters –no-remote -P
    or you could post on the bug report for this long-with-standing bug (but don’t get count on it getting fixed anytime soon, anything considered “minor” by the devs, is ignored): &

    BTW you might have a corrupt profile (although this is has not been common since FF2)

  22. B said on October 27, 2008 at 11:55 pm

    I have had this problem for many versions now, starting somewhere in the 2.x series (not earlier) if I recall correctly. I also haven’t been able to pinpoint the cause of the problem so far.

  23. Jojo said on October 27, 2008 at 11:50 pm

    I had run into this problem occasionally when running FF 2.x but not with FF 3.x

    You’ll never find the root problem without running sophisticated debugging tools. Mozilla support will blame the problem (as you found) on something/anything else. While they may be correct, my complaint with Mozilla is that they don’t provide built-in debugging tools to trace problems over time.

    When you have 40+ extensions like I do, trying to guess which one might be the problem is futile.

  24. Lee Mathews said on October 27, 2008 at 10:04 pm

    Not really a fix, but I added a shortcut to a batch file on my QuickLaunch that does quickly kills the process:

    taskkill /IM firefox.exe

    It does the job for now!

  25. rruben said on October 27, 2008 at 10:48 pm

    Indeed, experienced that also some times, but not often.

    But Firefox crashes everytime I close it though, not much fun in comparison with FF2 then.

    Speaking about comparing. Opening Chrome costs me 1 second, but opening Firefox cost me 40 seconds.
    So at the moment I use more Chrome then Firefox because I don’t have the patience to wait so long and don’t like it to crash everytime I close it.

    My conclusion is that Firefox 3 behaves like a beta with still a lot of bugs,while Chrome behaves like a finished product (although it is a beta) because there are simply no errors to spot while using it.
    So keeping FF2 or moving to Chrome is both better then using the crappy FF3 imho.

    @Commodity Trading
    That’s no excuse. Firefox runs on windows, not windows on Firefox. Btw Firefox 2 runs good so windows is not the one to blame.

  26. Commodity Trading Account said on October 27, 2008 at 10:41 pm

    the real problem is Windows…the browser is ok!

  27. -=Ben=- said on October 27, 2008 at 9:35 pm

    Use the extension “Update Checker”. It has a nice Reset Firefox option that will force a clean and nice reset (no data loss)

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