Make sure that you allow the updater.exe program in the firewall. If there are problems with updating then best is to download the full version of the Firefox installer. If there are still problems with applying the update then uninstall the currently installed version and delete the Firefox program folder to remove any leftover files. *C:\Program Files\Mozilla Firefox\ *http://kb.mozillazine.org/Installation_directory Download a fresh Firefox copy and save the file to the desktop. *Firefox 15.0.x: http://www.mozilla.org/en-US/firefox/all.html Uninstall your current Firefox version, if possible, 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 will also lose your 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. *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 profile 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. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup