.LAN is not an official top level domain. So I assume this is either your home network or work network? In any case your problem has nothing to do with the .LAN doman.

Maybe you have “https everywhere” activated. If so, Firefox will always default to https unless you specify http in the URL. Again, unrelated to .LAN.

For the certificate: what do you mean “when available”? A self signed cert is a self signed cert. There is no “available” or not. You can import the certificate into the Firefox trust store so Firefox will trust that one specific cert but any other self signed cert will cause an error. That is expected and save behaviour (and unrelated to .LAN).