Can you start Firefox in Safe Mode by holding down the Shift/Options key? *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes ---- It is possible that your security software (firewall, anti-virus) 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 *https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating ---- 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 28: 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. ---- *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_backup *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall