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 Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps. ________________________________________ • http://kb.mozillazine.org/Browser_will_not_start_up • http://kb.mozillazine.org/Error_loading_websites

This is not a "server not found" error. It is "The connection was reset" error. Here is the error"

"The connection was reset

The connection to the server was reset while the page was loading.

   The site could be temporarily unavailable or too busy. Try again in a few moments.
   If you are unable to load any pages, check your computer's network connection.
   If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web."

I tried your suggestion of "let your firewall ask again for permission to get full unrestricted access to internet for Firefox". This did not correct the problem. I'm using Windows Firewall in Windows 7.