Could you disable your add-ons? To do this, hold the Shift key when you start Firefox to get the Safe Mode dialog. Check the box to disable add-ons (just that one box) and click the Make Changes and Restart button. Does Firefox start normally?

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

I started it in safe mode with add of my add-ons disabled and it made no difference.