Reload web page(s) and bypass the cache. *Press and hold Shift and left-click the Reload button. *Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux) *Press "Cmd + Shift + R" (MAC) Clear the cache and the cookies from sites that cause problems. "Clear the Cache": *Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now" "Remove Cookies" from sites causing problems: *Tools > Options > Privacy > Cookies: "Show Cookies" ----- Start Firefox in <u>[[Safe Mode]]</u> to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes). *Don't make any changes on the Safe mode start window. *https://support.mozilla.com/kb/Safe+Mode ---- A possible cause is security software (firewall) 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 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.com/kb/Server+not+found *https://support.mozilla.com/kb/Firewalls