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 ---- You can try to reset Firefox. *https://support.mozilla.org/kb/reset-firefox-easily-fix-most-problems Reset creates a new profile and tries to import some settings from the old profile leaving the old profile intact. *https://support.mozilla.org/kb/Recovering+important+data+from+an+old+profile *https://support.mozilla.org/kb/Managing+profiles