A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program. Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process. See: *https://support.mozilla.org/kb/Server+not+found *https://support.mozilla.org/kb/Firewalls ----- If there are problems with updating then best is to download the full version of the Firefox installer. 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 *http://kb.mozillazine.org/Installation_directory 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. 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