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.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

Thank you cor-el for your expedient reply. There were other circumstances that were at play and have since been resolved so that now I am able to get access to your site. Actually, some issues with my Internet Explorer were being carried over to Firefox which then caused the same error issues into Firefox. With the help of my internet provider I have disabled these Explorer issues.