Preferences that will not save: How to fix preferences that won't save

and please try a new profile Profile Manager - Create, remove or switch Firefox profiles and then edit the network settings in Options > Advanced > Network > Configure how Firefox connects to the internet.

Looking forward to the reply to further investigate.

It is possible that your security software (firewall, anti-virus) 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

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

Thanks - that's what did the trick.

After Firefox stopped working suddenly Chrome and IE stopped working the next day.

I use BitDefender and tried tweaking the options in the Firewall for access, finally gave up and just deleted all of the existing "Application Rules" and everything started working after that.