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This article explains what to do if the Firefox window does not open or you get an error message when you try to start Firefox.

This article describes what to do if Firefox does not open when you click the Firefox icon. * If Firefox hangs or stops responding as soon as you open it, see [[Firefox hangs or is not responding - How to fix]]. * 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]]. {note}'''Note:''' [[Template:reset-fx]] The Firefox [[Diagnose Firefox issues using Troubleshoot Mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf|Troubleshoot Mode]] window includes a Refresh Firefox option. You can start Firefox in Troubleshoot Mode by {for win}holding down the {key shift} key while starting Firefox.{/for}{for mac}holding down the {key option} key while starting Firefox.{/for}{for linux}quitting Firefox and then going to your '''Terminal''' and running: <code>firefox -safe-mode</code><br>You may need to specify the Firefox installation path (e.g. {filepath /usr/lib/firefox}){/for}{/note} __TOC__ = Specific error messages = == Mozilla Crash Reporter == If you get the "''Mozilla Crash Reporter''" window when you start Firefox, see [[Firefox crashes - Troubleshoot, prevent and get help fixing crashes]] for solutions. == Firefox is already running but is not responding == See [["Firefox is already running but is not responding" error message - How to fix it]] 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. {for win} == 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 - Where Firefox stores your bookmarks, passwords and other user data|profiles]] folder.) #[[Uninstall Firefox from your computer|Uninstall Firefox]]. # Delete the Firefox installation directory, normally found in one of these locations by default: #*C:\Program Files\Mozilla Firefox #*C:\Program Files (x86)\Mozilla Firefox # Use another browser to download the Firefox installer from the [https://www.mozilla.org/firefox/windows/?utm_medium=referral&utm_source=support.mozilla.org Firefox download page] and install it. See [[How to install Firefox on Windows]] for details. {/for} == 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 [https://www.mozilla.org/firefox/new/?utm_medium=referral&utm_source=support.mozilla.org Firefox download page], save it to the desktop or other location, delete the Firefox installation directory and then {for win}[[How to install Firefox on Windows|reinstall Firefox]]{/for}{for mac}[[How to download and install Firefox on Mac|reinstall Firefox]]{/for}{for linux}[[Install Firefox on Linux|reinstall Firefox]]{/for}.{for win} If Firefox still does not start, see the [[#w_firefox-will-not-start-due-to-security-software|Firefox will not start due to security software]] section below.{/for} {for linux} == 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 on Linux|install Firefox]] if necessary, then modify the shortcut to point to the correct location. {/for} {for win} == 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. # Find the {filepath firefox.exe} program file in the Firefox application folder on your system. # Right-click the icon. # Select {menu Run As Administrator…}. Select '''The following user:'''. From the Username drop-down, select '''Administrator''' or another user that has administrator privileges. Enter the password required to log in to this account. Click {button OK}. # 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. {/for} == 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. {for win} == Firefox will not start due to security software == <!-- https://bugzilla.mozilla.org/show_bug.cgi?id=1590430 --> * 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 {filepath IseGuard64.dll} or {filepath IseGuard32.dll} in your {filepath 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 [https://techdows.com/2019/07/firefox-reportedly-freezing-on-startup-with-avast-or-avg-installed.html this Techdows article]. <!-- https://bugzilla.mozilla.org/show_bug.cgi?id=1567144 --> {for =fx96,=fx97} * Webroot SecureAnywhere software can prevent Firefox from starting or loading any content, as reported in [https://community.webroot.com/webroot-secureanywhere-antivirus-12/webroot-blocking-firefox-97-0-64-bit-350229 this Webroot forum post]. This was fixed in Firefox version 97.0.1 so you should [[Update Firefox to the latest release|update Firefox to the latest version]]. <!-- https://bugzilla.mozilla.org/show_bug.cgi?id=1752466 --> {/for} {/for} == 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. To test to see if this is the problem, use the [[Profile Manager - Create, remove or switch Firefox profiles|Profile Manager]] to create a new profile. If the problem goes away, you can then [[Recovering important data from an old profile|move your bookmarks and other important settings into the new profile]]. {for linux} == 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 [http://www.mozilla.org/firefox/system-requirements.html system requirements]. {/for} <!-- MZ credit --> <br/> <br/> '''''Based on information from [http://kb.mozillazine.org/Browser_will_not_start_up Browser will not start up (mozillaZine KB)]'''''