An error message about XPCOM missing or about platform version usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program folder. You need to do a clean reinstall and remove the Firefox program folder to ensure that all files are replaced. ---- Do a clean reinstall of the current Firefox release and delete the Firefox program folder before installing a fresh copy of the current Firefox release. *download the Firefox installer and save the file to the desktop<br>*https://www.mozilla.org/en-US/firefox/all/#product-desktop-release If possible uninstall your current Firefox version to cleanup the Windows Registry and settings in security software. *do NOT remove "personal data" when you uninstall the current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including personal data in profiles created by other Firefox versions Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer. *(64-bit Firefox) "C:\Program Files\Mozilla Firefox\" *(32-bit Firefox) "C:\Program Files (x86)\Mozilla Firefox\" *it is important to delete the Firefox program folder to remove all the files and make sure there are no problems with files that were leftover after uninstalling Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data when you reinstall or update Firefox, but make sure NOT to remove personal data when you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data. If you keep having problems then you can create a new profile as a quick test to see if your current profile is causing the problem. <hr> *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Reinstall Firefox *https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems