It is possible that your firewall or other security software 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 install 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

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Thanks for the info, I'm using Norton 360, I removed all FF references from the "Smart Firewall" and did a re-install of V50.02. Unfortunately same problem.

Try to boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Try to start Firefox in Safe Mode to see if that works.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

Thanks again but no joy there. My daughter is having the same problem and she also uses Norton 360, there's no update available but I'm wondering if Norton is somehow ruining the party?

Set Norton to allow all and test if it works then. 2nd test get a portable version older then your current version and test if that gives the same problem or not. Then you know if it is update related or Norton related ;)

Some protection programs don't like the update stubs. Download Firefox For All languages And Systems {web link}