I think you have an incompatible addon with the latest Firefox version. After you open Firefox, can you please go to about:addons and disable all of the addons, or one by one and restart Firefox to see which one it might be?

The other way we can get that working again is to create a new profile for your browser. Profile Manager - Create, remove or switch Firefox profiles <-- please reply with any questions about that article.

Thank you!

What security software do you have?

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/en-US/kb/server-not-found-connection-problem
• https://support.mozilla.org/en-US/kb/Firewalls
• https://support.mozilla.org/en-US/kb/fix-problems-connecting-websites-after-updating