This is usually caused by a problem with a failed update and not all the files were update correctly and you were left with a mixture of files. Best in such a case it to do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release. *Download the full Firefox installer and save the file to the desktop<br>https://www.mozilla.org/en-US/firefox/all/ 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 your current Firefox version, because this will remove all profile folders and you lose personal data like bookmarks and passwords including data in profiles created by other Firefox versions. Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer. *(32 bit Windows) "C:\Program Files\Mozilla Firefox\" *(64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\" *It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling. *http://kb.mozillazine.org/Uninstalling_Firefox Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, 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. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall