In my case the Sophos Cloud Protection has blocked the communication between the Browser extension and the app - Sophos protect the app! The support send me a useful link to solve this.
I add the localhost ip 127.0.0.1 to the Web Protection Exceptions in the Sophos Central Management, close all Browser Sessions and the app, restart the app and firefox and when i select the emnpass extension symbol a keycode is shown to fill in this in the app - it works now!
Regards!
Mike