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

Thank you. I'm sure this is the problem, as when I look at my firewall log, it shows "denied" for Firefox! (...now to figure out how to to make the change!) Thanks very much for your response. John

You're welcome.

It is usually easiest to remove all existing permission for Firefox.
Then the firewall will ask again for permission the next time you start Firefox.

I went ahead and deinstalled my Nvidia firewall, and am back to using Windows' own firewall. This reply, by the way, is being written from my Firefox browser, version 6.0.2! Thanks again! John