Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files. *Press and hold Shift and left-click the Reload button. *Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux) *Press "Command + Shift + R" (MAC) Clear the cache and the cookies from sites that cause problems. "Clear the Cache": *Tools > Options > Advanced > Network > Cached Web Content: "Clear Now" "Remove Cookies" from sites causing problems: *Tools > Options > Privacy > Cookies: "Show Cookies" *https://support.mozilla.org/kb/how-clear-firefox-cache *https://support.mozilla.org/kb/Deleting+cookies *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes ---- 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