If your computer or network is protected by a firewall or proxy, make sure

   that Firefox is permitted to access the Web.

If your computer or network is protected by a firewall or proxy, make sure

   that Firefox is permitted to access the Web.

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

See also:

• http://kb.mozillazine.org/Error_loading_websites