Firefox won't start - find solutions

This article describes what to do if Firefox doesn't open when you click the Firefox icon.

  • If you get a message you can dismiss (e.g. "TypeError: Components.classes['@ ... ;1'] is undefined", "onload error TypeError: ...", "Exc in ev handl: ...") and can open Firefox, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.
  • If Firefox starts but won't load any websites, see Websites don't load - troubleshoot and fix error messages.

The Refresh Firefox feature can fix many issues by restoring Firefox to its default state while saving your essential information. Consider using it before going through a lengthy troubleshooting process. The Firefox Safe Mode window includes a Refresh Firefox option. You can start Firefox in Safe Mode by holding down the shift key while starting Firefox.  Mozilla Firefox Help

Specific error messages

Mozilla Crash Reporter

If you get the "Mozilla Crash Reporter" window when you start Firefox, see Troubleshoot Firefox crashes (closing or quitting unexpectedly) for solutions.


Firefox is already running but is not responding

See "Firefox is already running but is not responding" error - How to fix for solutions.

Your Firefox profile cannot be loaded. It may be missing or inaccessible

See How to run Firefox when your profile is missing or inaccessible for solutions.

Failed to read the configuration file

This error is caused by a problem with Firefox program files. The solution is to remove the Firefox program and then reinstall Firefox. (This will not remove your passwords, bookmarks or other user data and settings which are stored in a separate profiles folder.)

  1. Uninstall Firefox.
  2. Delete the Firefox installation directory, found in one of these locations by default:
    • C:\Program Files\Mozilla Firefox
    • C:\Program Files (x86)\Mozilla Firefox
  3. Use another browser to download the Firefox installer from the Firefox download page and install it. See How to download and install Firefox on Windows for details.

XULRunner - Error

If you see the message, "XULRunner - Error: Platform version <x.x.x.x> is not compatible with minVersion <x.x.x.x>"" ("<x.x.x.x>" depends on version number), after a software update, use another browser to download the Firefox installer from the Firefox download page, save it to the desktop or other location, delete the Firefox installation directory and then reinstall Firefox.

  • If Firefox still does not start, see the Firefox will not start after updating section below.

Error opening file for writing…

If you receive the above error when launching Firefox on a Windows system, you may be required by Firefox to log in as an administrator. You can either reboot the computer or, if you have an administrator account, you can execute the program as an administrator.

  1. Find the firefox.exe program file in the Firefox application folder on your system.
  2. Right-click the icon.
  3. Select Run As Administrator…. Select The following user:. From the User name drop-down, select Administrator or another user that has administrator privileges. Enter the password required to log in to this account. Click OK.
  4. The Firefox application is started using the administrator Windows account. The user that was used to log in to Windows at startup is still the active one.

Other error messages

If you get another error message (e.g. "XML Parsing Error: undefined entity Location", "Secure Connection Failed … uses an invalid security certificate"), see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

No error message

If you click the Firefox icon but no Firefox window opens and no error message is shown, a simple first step is to restart your computer and then try again. If Firefox still does not start, see the following sections.

Firefox will not start after updating

  • Outdated versions of Comodo Firewall or Comodo Antivirus security software (or a leftover file after Comodo was uninstalled) can cause Firefox not to start with no error message. Update Comodo to the latest version, if you have it installed. If Comodo software is not installed, check if there's a file called IseGuard64.dll or IseGuard32.dll in your C:\Windows\system32\ folder or your system drive and, if so, manually delete the file.
  • If you use certain versions of ZoneAlarm or other Internet security software that includes a "Virtual Browsing" feature, it can prevent Firefox from starting. Check virtualization settings and clear the virtual cache, if your Internet security software includes this feature, or visit the software provider's support site for more help.
  • Firefox may fail to start for some Windows users after updating, if AVG or AVAST antivirus is installed. For details, see this Techdows article.

Firefox will not start after installing add-ons

If Firefox previously worked but stopped after you installed extensions or themes, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Firefox previously worked

If Firefox previously worked but suddenly doesn't start, it may be due to some corrupt data in your settings. Uninstalling and reinstalling Firefox would not fix this problem because your settings are not removed on uninstall.

Specific error messages

Mozilla Crash Reporter

If you get the "Mozilla Crash Reporter" window when you start Firefox, see Troubleshoot Firefox crashes (closing or quitting unexpectedly) for solutions.

Firefox is already running but is not responding

See "Firefox is already running but is not responding" error - How to fix for solutions.

Your Firefox profile cannot be loaded. It may be missing or inaccessible

See How to run Firefox when your profile is missing or inaccessible for solutions.

Failed to read the configuration file

This error is caused by a problem with Firefox program files. The solution is to remove the Firefox program and then reinstall Firefox. (This will not remove your passwords, bookmarks or other user data and settings which are stored in a separate profiles folder.)

  1. Uninstall Firefox.
  2. Delete the Firefox installation directory, found in one of these locations by default:
    • C:\Program Files\Mozilla Firefox
    • C:\Program Files (x86)\Mozilla Firefox
  3. Use another browser to download the Firefox installer from the Firefox download page and install it. See How to download and install Firefox on Windows for details.

XULRunner - Error

If you see the message, "XULRunner - Error: Platform version <x.x.x.x> is not compatible with minVersion <x.x.x.x>"" ("<x.x.x.x>" depends on version number), after a software update, use another browser to download the Firefox installer from the Firefox download page, save it to the desktop or other location, delete the Firefox installation directory and then reinstall Firefox.

  • If Firefox still does not start, see the Firefox will not start after updating section below.

Error opening file for writing…

If you receive the above error when launching Firefox on a Windows system, you may be required by Firefox to log in as an administrator. You can either reboot the computer or, if you have an administrator account, you can execute the program as an administrator.

  1. Find the firefox.exe program file in the Firefox application folder on your system.
  2. Right-click the icon.
  3. Select Run As Administrator…. Select The following user:. From the User name drop-down, select Administrator or another user that has administrator privileges. Enter the password required to log in to this account. Click OK.
  4. The Firefox application is started using the administrator Windows account. The user that was used to log in to Windows at startup is still the active one.

Other error messages

If you get another error message (e.g. "XML Parsing Error: undefined entity Location", "Secure Connection Failed … uses an invalid security certificate"), see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

No error message

If you click the Firefox icon but no Firefox window opens and no error message is shown, a simple first step is to restart your computer and then try again. If Firefox still does not start, see the following sections.

Firefox will not start after updating

  • Outdated versions of Comodo Firewall or Comodo Antivirus security software (or a leftover file after Comodo was uninstalled) can cause Firefox not to start with no error message. Update Comodo to the latest version, if you have it installed. If Comodo software is not installed, check if there's a file called IseGuard64.dll or IseGuard32.dll in your C:\Windows\system32\ folder or your system drive and, if so, manually delete the file.
  • If you use certain versions of ZoneAlarm or other Internet security software that includes a "Virtual Browsing" feature, it can prevent Firefox from starting. Check virtualization settings and clear the virtual cache, if your Internet security software includes this feature, or visit the software provider's support site for more help.
  • Firefox may fail to start for some Windows users after updating, if AVG or AVAST antivirus is installed. For details, see this Techdows article.

Firefox will not start after installing add-ons

If Firefox previously worked but stopped after you installed extensions or themes, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Firefox previously worked

If Firefox previously worked but suddenly doesn't start, it may be due to some corrupt data in your settings. Uninstalling and reinstalling Firefox would not fix this problem because your settings are not removed on uninstall.



                   Call 1-856-514-8666 for help and support of Mozilla Firefox Help.

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