Firefox won't start - find solutions

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

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.

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 mozilla.org/firefox, save it to the desktop or other location, delete the Firefox installation directory and then reinstall Firefox.

There was an error launching the application

This error indicates that either Firefox is not installed or your shortcut to it is incorrect. Make sure that Firefox is installed at the location specified by the shortcut. If it isn't, install Firefox if necessary, then modify the shortcut to point to the correct location.

The program must close to allow a previous installation attempt to complete

If you get a message saying "The program must close to allow a previous installation attempt to complete. Please restart." when you start Firefox, see The program must close to allow a previous installation attempt to complete.

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.
    • (Windows 2000/XP) Select Run As.... 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.
    • (Windows 7 and Vista) Select Run As Administrator.... Complete a similar set of steps, as those listed above.
  3. 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

  • If Firefox fails to start after an update, first check the Latest Firefox issues article to see if it contains a solution.
  • 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 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 will not start after cleaning the registry

Some registry cleaners clear more registry keys than they should, preventing some programs from starting, including Firefox. If Firefox previously worked but stopped after you've cleaned the registry, you should restore it using the registry backup you made before the cleanup.

  • If you didn't make a registry backup, use the Windows System Restore feature to restore your computer to a point before you cleaned the registry. These Microsoft articles have more information:
Windows 7 features - System Restore
What types of files does System Restore change?
How to restore Windows XP to a previous state
Note: If Firefox does not start after a System Restore, remove the Firefox installation folder and then reinstall Firefox using these instructions.

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.

To test to see if this is the problem, use the Profile Manager to create a new profile. If the problem goes away, you can then move your bookmarks and other important settings into the new profile.

Firefox will not start on OS X 10.4 or earlier or with a PowerPC processor

Mozilla has stopped supporting OS X 10.4 and earlier, and also OS X 10.5 with a PowerPC processor. None of these operating systems or architecture are supported by Apple (and have not been for quite some time) and have multiple vulnerabilities that will remain unpatched. As such, you have the following options listed from the most to the least secure:

  • If you have an Intel processor you can update to OS X 10.6 or 10.7. This is the most secure option. If you don't want to pay for a new operating system, or if you have a PowerPC processor you can install a Linux distribution. This ensures you have a completely secure system and access to all the great new features in Firefox.
  • On OS X 10.4 or 10.5 with a PowerPC processor, you can download a clone of the latest Firefox version designed only for PowerPC from the TenForFour home page.
    Note: TenForFour is not affiliated with nor supported by Mozilla in any way, and is not an official build.
    • You can also continue to use Firefox 3.6 which you can download from this page.
      Warning: Firefox 3.6 will only be maintained with security and stability updates for a short period of time.
  • On OS X 10.3 and earlier, you can continue to use Firefox 2.0.0.20. Just because the browser will no longer receive security updates doesn't mean it will simply stop working.
    Warning: You will be using unsupported software on an unsupported operating system which poses a significant security risk.

Firefox will not start on Windows 98/ME or earlier

Mozilla has stopped supporting Windows 98/ME and earlier. None of these operating systems are supported by Microsoft (and have not been for quite some time) and have multiple vulnerabilities that will remain unpatched. As such, you have the following option:

  • You can update to Windows XP, Windows Vista or Windows 7. If you don't want to pay for a new operating system or if your hardware is too old to be updated, you can install a Linux distribution which can run on older machines. This ensures you have a completely secure system and access to all the great new features in Firefox.

Firefox will not start because of missing libraries

If you installed Firefox outside of your package manager, make sure that your system has the libraries listed in the system requirements.




Based on information from Browser will not start up (mozillaZine KB)