Did you delete the Firefox program folder after uninstalling Firefox? 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.html 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 that "remove personal data" is NOT selected when you uninstall Firefox. If you keep having problems then also create a new profile. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall ---- Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test. *http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/