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Missing mozglue.dll when launching Firefox

  • 20 replies
  • 367 have this problem
  • Last reply by ffw62

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Hi, I'm getting an error "The program can't start because mozglue.dll is missing from your computer. Try reinstalling the program to fix the problem." when launching Firefox. Firefox was working last night. I have Windows 7.

I have uninstalled Firefox and reinstalled it from latest version from Mozilla website but got same error when I launched Firefox again. I have tried reinstalling Firefox about 4 times but this did not help to fix the issue.

Would it help fix the issue if someone provides me a copy of mozglue.dll to place in C:\Program Files\Mozilla Firefox\ on my computer?

Please help. Thanks.

Regards, Andrew

Request for information added here where it will be seen

Hi, there seem to be a couple of other users affected by this reading this thread (judging by 30 users clicking "I have this problem too").

It would be helpful if you could chime in as well & report what antivirus software (& sandboxing software, if any?) you are using on your system. Also if you take a look into your Firefox program folder: Is the mozglue.dll file actually missing there?

Firefox program folder is in one of these locations:
Windows 32-bit users: C:\Program Files\Mozilla Firefox
Windows 64-bit users: C:\Program Files (x86)\Mozilla Firefox

Modified by NoahSUMO

Chosen solution

thanks, please try to update to a current version since avg might not maintain older versions of their program anymore:

Read this answer in context 👍 10

All Replies (20)

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Hi Andrew, if you haven't already, could you try it this way:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

It only takes a few minutes.

(A) Download a fresh installer for Firefox 49.0 from to a convenient location. (Scroll down to your preferred language.) Choose the "Windows" version (32-bit) rather than the 64-bit version for maximum plugin compatibility (and of course for 32-bit Windows systems).

(B) Exit out of Firefox (if applicable).

(C) Using Windows Explorer/My Computer, rename the program folder as follows:

C:\Program Files\Mozilla Firefox


C:\Program Files\OldFirefox

64-bit Windows users: Rename

C:\Program Files (x86)\Mozilla Firefox


C:\Program Files (x86)\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

  • \OldFirefox\Plugins
  • \OldFirefox\browser\plugins

Any improvement?

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Hi jscher, I tried your suggestion but unfortunately this did not fix the problem. I still get the same error when launching Firefox even after renaming C:\Program Files\Mozilla Firefox\ to C:\Program Files\OldFirefox\ and installing Firefox. One thing to note is there's no mozglue.dll and no PlugIns folder under C:\Program Files\Mozilla Firefox\ and C:\Program Files\OldFirefox\. I have already tried reinstalling Firefox many times this morning and it's the same problem everytime I launch Firefox.

Any other ideas? Thanks.

Regards, Andrew

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In case this is a new problem in Firefox 49.0, could you try Firefox 48.0.2. Its installer is available here:

That leads to a list of folder by language code (locale), for example, en-US is U.S. English. If you're not sure of the language you want, check the address bar while you are viewing a thread on this site. It should be in there.

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Also, do you have any security software that could be removing files from the Firefox program folder? Usually the software would at least light an icon to let you know it detected something, or possibly pop up an alert, and then it may have a log of deleted or quarantined files.

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Hi Jscher, Thank you for your prompt reply.

Installing Firefox 48.0.2 from the link helped to fix the issue with missing mozglue.dll. I could see the dll in the C:\Program Files\Mozilla Firefox\ folder after the install.

I could then launch Firefox browser but soon after, it automatically started to download and install some updates. When I restarted Firefox after these updates, I got a different error "The application was unable to start correctly (0xc0000022). Click OK to close the application."

I think this was the first error I got this morning when I first launched Firefox (it was working last night) and I suppose this is due to the updates it automatically installed.

Could there be an issue with the updates ? I don't know which version it automatically updated it with (after the 48.0.2 install).

Regards, Andrew

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If Firefox did a program update, that would have to have been Firefox 49.0. Those are not complete installers; I don't know exactly how they're constructed but obviously the one your Firefox is downloading isn't working for your system. Not that the full installer works any better for you.

Will Firefox start in its Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

  • "3-bar" menu button > "?" button > Restart with Add-ons Disabled
  • Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

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Hi jscher, I tried that link for the new error message. I launched Process Monitor and found Access Denied messages after replicating the error when launching Firefox. The Access Denied was for the mozglue.dll in the C:\Program Files\Mozilla Firefox\ folder.

The article says that I should grant Full Control if I get any message with my account, however I'm already an Administrator on my computer and I have Full Control to the mozglue.dll or even to the Mozilla Firefox folder.

I don't know where to go from here.

Any idea?

Regards, Andrew

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Hi Andrew, I'm out of my depth on this. There probably are other things to check but I'm hoping someone else is more familiar with the problem.

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Hi Andrew, What security related software are you using ?

Other reports of this may have been due to security software quarantining or sandboxing mozglue. Are you able to find the mozglue.dll in the Firerfox program folder ?

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Hi John, I'm using AVG Antivirus and I tried disabling it when reinstalling Firefox but that did not help either.


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Did you try to check whether AVG had any reports of problems or lists of quarantined files ?

Did you look in the program folder. mozglue.dll should be found in

C:\Program Files\Mozilla Firefox\  

I presume it is a 32 bit tablet, if it is a 64 bit System it would instead be in

C:\Program Files (x86)\Mozilla Firefox

Those are the same folders as the main Firefox program file firefox.exe

If AVG or something else has already removed or quarantined mozglue.dll then just trying to launch Firefox with the AV disabled will probably not work. The file would need to be reinstalled from the AV, or Firefox itself would have to be reinstalled again and started with the AV off.

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hi, there seem to be a couple of other users affected by this reading this thread (judging by 30 user clicking "i have this problem too"). it would be helpful if you could chime in as well & report what security software (&sandboxing software, if any?) are you using on your system? also if you take a look into your firefox program folder: is the mozglue.dll file actually missing there?

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Hi John99, Philipp, There was no mozglue.dll in the C:\Program Files\Mozilla Firefox\ folder.

I found the cause of the problem. It was due to AVG Antivirus not allowing Firefox to launch. See the other issue I raised:

This is what I've done: (1) Go to AVG Antivirus and instead of selecting the option "Temporarily disable AVG protection", I opened AVG and went to Computer Protection-->Disabled. (3) Uninstall Mozilla Firefox from Control Panel-->Add/Remove Programs (4) Download Firefox 49.0 and install it on my computer. (5) I can now launch Firefox without any error. The mozglue.dll is created in C:\Program Files\Mozilla Firefox\ folder. (6) However, if I enable AVG Antivirus again, I'll get the other error ( ).

Does anybody know how I can get Firefox to run with AVG Antivirus enabled?


Modified by NoahSUMO

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thanks for following up, could you also give us the exact product information/version numbers that could be found like described at

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I've been trying all of these suggestions over the past few days. I was able to get Firefox to work following Andrewli7's directions to disable AVG. Just clicking "Temporarily Disable AVG" did not work. I had to open AVG and fully disable each type of protection, even though it looked at first like they were already disabled.

The AVG version on my computer is 2014.0.4158

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Hi, My AVG version is 2014.0.4570.

So, how do we get Firefox to run without disabling AVG Antivirus?

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Chosen Solution

thanks, please try to update to a current version since avg might not maintain older versions of their program anymore:

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Hi philipp, I removed AVG 2014 and installed AVG 2016 and it fixed the problem. I can now use Firefox with AVG enabled. Thanks.

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Was facing the same issue, Firefox would not start. It hanged at each 'Firstrun'. Installed Firefox over and over .. Disabled A/V products. It got a very long story. See: ( topic title: "Firefox v50.1.0 serious problem - unable to use it..."

By coincidence I discovered the solution

Right click Firefox shortcut->Select Properties-> click [Advanced-->tag Run ad administrator-> click [OK]->click on [Continue] at the warning "You will need to provide administrator permission to change these settings"->click on [Apply] and [OK]

In my case A/V tools had nothing to do with it. Neither mozglue whatever.

Admin rights made Firefox to launch again.

Nonetheless, still a strange issue.

I have the very same Firefox running as VM, with the same A/V products, all the same versions, and the same Windows 10 x64 of the same build. No issues there, no need to provide admin rights...