Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

The System Details list shows you use a rather old Firefox 43.0 version. It is possible that this old version no longer is supported.

Does this also happen with the current Firefox release ?

It was out of date. Updated to the current version and the problem was gone.