The Firefox installer is a self extracting 7-ZIP archive, so it sounds that there is a problem with this file. Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release. Download a fresh Firefox copy and save the file to the desktop. *Firefox 26: http://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 all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of 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. You may have to disable your anti-virus temporarily if you keep having this problem Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall