Firefox 45.9 connects fine here, but I don't have the credentials. Shouldn't need a certificate and TLS for a regular FTP connection but I guess it could be setup in such a way.
I tested more thoroughly; it puzzled me that other FTP connections worked and this one didn't. And that other people had no problems accessing the FTP server.
When I tried using VPN, I forgot to set the VM to NAT. I've tried that now. Result: When my system is connected via VPN (I tested the USA, Spain, and Germany), everything works: The program finds its update.zip and everything else!
The native FTP client also connects, wget connects as well. Firefox works as well.
If I turn off VPN, no connection is possible. However, Tracert shows that the server is accessible. A test of the Netlabs FTP server shows that neither my router nor my service provider seems to be blocking FTP.
For my virtual machine, VPN is a workaround for now.
However, I can't easily connect devices with a native installation to VPN because the provider's software isn't available for our platform.
Addition: The screenshot shows the situation in the VM without VPN