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. 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.

   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.

   Find the firefox.exe program file in the Firefox application folder on your system.
   Right-click the icon.
   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.
   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.

You can check if you can start Firefox in Safe Mode by holding down the Shift/Options key.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

It is possible that your security software (firewall, anti-virus) blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls