It has an untrusted (by the browser) security certificate used for encryption. As long as you trust it, it's fine. You will just have to put up with the message.
But why are you even trying to use https on your local network? And what is security sensitive about your Fox? It's only telly that's available to anyone.
Doing that only fixes the expiry date.
There were two things FF didn't like before (date and self-signed) - now there is only one (self-signed).
You will always need to add exceptions for that (until FF self-destructs, which given the way that they are going probably won't be all that long).
Yep, unless you're prepared to create your own certificate authority, issues certificates from that and teach your PC to trust it, or play elaborate games with a real Internet domain name, you're just going to have to add the exception. Your browser will still alert you if it changes (although most people will just click add to exceptions again)