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Firefox can occasionally complain that it's already running when you might not see it anywhere on your desktop. It's not lying, it's just hiding.

I installed Firefox because IE7 just freezes up and runs really slow. So far I like it. It runs really fast, but every now and then I try to start it and get a message saying Firefox is already running and that I need to stop the program or restart my computer. I was wondering what you think I should or could do, plus I was just wondering what you think of Firefox.

I'm a big fan of Firefox, and I use it every day. I still occasionally run into websites that don't work properly in Firefox, but for the most part I rarely have to open Internet Explorer for my normal surfing.

But yes, I have run into the scenario that you've described. I'm always not sure what causes it, but I can show you what I do.

What I've seen when Firefox complains that it's already running is simply that it is already running, but it doesn't actually have any windows open. (One way to force this on purpose is to set a scheduled task that runs Firefox, and then log out of your computer and stay logged out while that task runs. At least that's how it happens to me.)

If you run task manager, or my favorite: process explorer, after getting this message you'll probably find that firefox.exe is indeed running:

Process Explorer fragment showing Firefox running

(In Task Manager you may need to make sure that "Show process from all users" is selected.)

If you find a Firefox.exe in the process list but don't see one running on your desktop, then you can right click on it and click on "End Process" (task manager) or "Kill Process" (process explorer). That should halt the unexpected Firefox and allow you to start a new copy that's visible.

An alternative for the command-line fans out there is the SysInternals utility called "pskill" which can be downloaded for free from Microsoft.

"... it is already running, but it doesn't actually have any windows open."

One you have the tool, then in a command shell you can enter the following command:

pskill firefox

That should result in something like this:

PsKill v1.10 - Terminates processes on local or remote systems
Copyright (C) 1999-2004 Mark Russinovich
Sysinternals -

Process firefox killed.

If more than one copy of firefox was running, that would be terminated as well. In fact, any program with "firefox" in it's name would be killed. And of course if no such process exists it does nothing.

So rather than looking for and terminating firefox.exe in the task list, you can just use the command line tool to "kill 'em if you got 'em".

Article C3240 - December 17, 2007 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

December 19, 2007 11:24 AM

All you have to do is hit''control-alt-delete', when box comes up ,you'll see firefox in the box, just highlight and hit end process, the firefox browser will be shut down, now click on icon for firefox and it will load.....

Carl R. Goodwin
December 21, 2007 6:27 PM

I use FF all the time, and only run into this when I try to install a back-up copy of a profile into the "Profiles" folder. You cannot copy the entire folder - only the contents inside it (by pasting it). :)

Peter Woerner
December 22, 2007 5:04 AM

My problem is that after I've put my computer into Stand By mode (where it actually shuts off and comes back to the same spot once you restart your computer), my firefox won't restart properly no matter how many times I follow this procedure. I need to ensure that I don't close my firefox when I put the computer into Stand By mode.

Terry Hollett
December 22, 2007 6:51 AM

I've had this problem on and off,where programs would not open up, yet in the task manager, it indicated they where open in the background.

Not just with Firefox. I do have it on my system but my browser choice is Opera, and I've had it happen with it. There was no error message, it just wouldn't open, except in the background, according to task manager.

My sister called recently and had a similar problem with Windows Live Messenger. And then another guy called me with the same problem. I first opened task manager by right clicking on the task bar and chosing it from the menue. Closed all instances of Messenger (there would be about a half a dozen or so where they tried to get it to run). Then to the 'control panel', double-clicked on 'Add/Remove programs', clicked on the list item for 'Windows Live Messenger', there where two options, 'change or uninstall', clicking on the change button will give you two options, to 'repair' or 'uninstall', I choose 'repair'. It seemed to work temperarily. I eventually uninstalled and reinstalled. So far so good.

And to top it all off, I still cannot run the newest versions of Adaware 2007 or Spybot 1.5, I still have to use the old versions. I just keep checking and downloading the newest version occassionally, maybe one of them will work one day.

Terry Hollett

concerned student
March 13, 2008 11:54 AM

i use a school mac and i always get "firefox is already running" error im new to macs and the school admins are baffled any ideas on how to view processes on a mac running osx

August 4, 2008 11:31 AM

There's no reason to use IE even on sites that arent supported by Firefox. Just download IE tab extension. It emulates IE in Firefox. You can download it from here:

SFC doesn't run "on a drive", it runs on your system. That means it checks the Windows installation in use wherevere it might have been installed.


September 12, 2008 5:42 PM

I use FF all the time, and only run into this when I try to install a back-up copy of a profile into the "Profiles" folder. You cannot copy the entire folder - only the contents inside it (by pasting it). :)

Posted by: Carl R. Goodwin at December 21, 2007 06:27 PM

You sir, saved me about 30 minutes of headache. Old post, new info. Thanks.

October 21, 2008 12:57 PM

I am having the same problem, only firefox.exe is NOT in my task manager...what do I do ??

Use process explorer ( - it shows processes that Task Manager by default does not.
- Leo

December 4, 2008 1:18 AM

I get this error all the time when im in uni. It says it cant open cause its already open, BUT its not listed in the processes. We have to use firefox for all our projects. It never works until you restart (it sometimes takes 2 or 3 restarts) almost everytime I log on in uni i restart (and the systems in my uni take at least 30 mins to restart!!!) the computer and log in again. I have reported many times this and they keep saying they've fixed it but it still keeps friend gets it about once or twice a week and i get it almost evey time i log in. Rahhhhh!!! Its extreamly fustrating. I never get this error a home!

January 14, 2009 3:11 AM

the following happens when I use the pskill command in order to delete the blocking firefox.exe (Windows Vista business, running with administration rights):
C:\Users\Rolf>pskill -t firefox

PsKill v1.12 - Terminates processes on local or remote systems
Copyright (C) 1999-2005 Mark Russinovich
Sysinternals -

2 processes named firefox killed.

Ok, 2 processes? There is only 1 in the task manager list!
And this process remains in the list, it is not killed.
Also using pskill with the PID of this process has no success:

C:\Users\Rolf>pskill -t 5560

PsKill v1.12 - Terminates processes on local or remote systems
Copyright (C) 1999-2005 Mark Russinovich
Sysinternals -

Process 5560 killed.

But the process 5560 firefox.exe is still there! I have to restart to get rid of it.

February 6, 2009 8:42 AM

Yes, I've been troubled by the same FireFox (FF) problem on a Vista Home Premium system with SP1. In addition to the other problems I've seen mentioned, I'd like to add a little regarding my own situation. In my case, when FF locks up, although I can easily close the FF window, the process does not terminate. Task Manager, Process Explorer, and even "PsKill" are all ineffective in killing the FF process. I hoped that PsKill might work, but I tried referring to the process by its name as well as by the process number, but neither method worked.

Experience already tells me that whenever FF is left hanging like that, although it's not consuming any CPU cycles, I will not even be able to perform a normal restart/shutdown. The shutdown sequence gets hung up for an extraordinarily long period of time and I usually have to force it by using my system's power button, i.e., a hard, ungraceful power off. But quite by accident, I figured out a way to at least get around the shutdown difficulties I have when FF hangs like this. I kill "lsass.exe", the Local Security Authority Process. I was experimenting once and was trying to eliminate unnecessary processes to improve performance and I thought killing lsass might help. When I killed "lsass.exe" -- you can try it yourself -- I immediately learned that this is apparently an essential/mandatory Windows process. Killing "lsass.exe" DOES NOT "crash" Windows nor does it cause a Blue Screen of Death, but once the OS senses that lsass is no longer running, after a one minute delay, a graceful restart is forced.

So, although I regained control of my ability to restart when FF hangs, this hasn't done anything to remedy the question as to why FF hangs in the first place.

Good luck to all.

February 12, 2009 5:48 AM

I had this FF-problem under Vista business at least 2 times a day (see my post Jan 14, 2009).
2 weeks ago I replaced the ZoneAlarm (free) firewall by the free COMODO firewall - the problem is gone!

March 26, 2009 1:17 AM

I experience exactly the same problems as described in the post by Tony at February 6, 2009 8:42 AM. I am running Vista Home Premium SP1. I can add to what he said that after waiting for something like 15 minutes, Windows finally shuts by itself, without necessity to kill lsass.exe first. At least this is what happened on my machine when I decided once not force 'power off'.

I also want to say that the problem with Firefox hanging and being unterminable by Task Manager, taskkill or pskill, started to regularly happen after I installed Zone Alarm firewall a few weeks ago. I will uninstall it and check whether the problem goes away. To Zone Alarm users I want to say that this problem happens even though I turned off Prosess Control in Zone Alarm.

I want to say that I am bitterly disappointed with Zone Alarm and consider it to be another piece of malware pretending to protect your computer: it sneaks toolbars in both Internet Explorer and Firefox, and it is very difficult to get rid of them. I consider this practice to be abominable and thus strongly advise against installing Zone Alarm.

April 10, 2009 3:24 AM

It's unfortunate that you don't answer the question you posed. I presume you don't know the answer?

I get this issue constatntly, running 3.0.8 and, since FireFox is such poor software, I'll always have to rely on proper browsers.

It's a shame... :(

August 16, 2009 5:25 PM

You, sir, are a genius and scholar. This article provided invaluable help to restoring my Firefox browser. I am indebted to you forever. Thank you!!!

August 30, 2009 12:30 PM

Thanks, this pskill really helps

March 25, 2010 8:12 PM

Like so many of you, I also have the same problem with Firefox running on Windows XP (and ZoneAlarm is not installed). When I close Firefox, I cannot use my computer because Firefox continues to run in the background; and when I open Task Manager it shows that Firefox is using 99% of CPU resources. Sometimes, it shuts itself off after 5-10mins and other times it just runs forever. Each time is different. Regardless, I can always kill it in Task Manager. [Itís interesting though, because on my laptop, this has never happened, even though ZoneAlarm is installed.] Iím getting extremely frustrated by having to go through these steps each time to be able to use the computer. Iím not an expert, but I find it hard to believe that someone out there hasnít been able to find the real problem; and a proper solution. If anyone can help out, I know many of us would be very appreciative.
Thanks very much, George.

October 23, 2010 5:46 PM

I use Vista; everytime I click on Firefox to start, it will tell me Firefox is running but not responding. It tells me to close the existing firefox process or restart the system. I checked in Task Manager but I can not see the process, I used pskill to stop it but it says firefox does not exist and do nothing. Yet I still can not start up firefox!

March 7, 2011 4:06 PM

Hey Everybody,
Yes he did answer right, but sometimes there is no firefox.exe process in the Task Manager.
Here are some links which may help you with this problem.

Hope this helps.

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