Web browser doesn't open (XULRunner − Error)

The "Failed to read the configuration file" error is caused by a problem with Firefox program files. The solution is to remove the Firefox browser and then reinstall it:

1. Uninstall Firefox (See the don't know how to uninstall a program / an application for details)
   
   
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 Mozilla Firefox from the official website
   
   

Note:  This solution also applies if you see the message "XULRunner - Error: Platform version is not compatible with minVersion "" (depends on the Firefox's version number), after a software update

This will not remove your passwords, bookmarks or other user data and settings, as they are stored in a different folder.

Use a utility such as Advanced Uninstaller PRO to uninstall programs from your computer to delete all the files (even the installation files) related to them in all the directories.