Why do you care? You know you are not having any risk. Actually by using letsencrypt you are less secure.
The way to do this correctly, is to find out how to import the key generated locally by IPFire into the browser. I remember there was a thread discussing this topic some time ago. If my memory is correct, with chrome is not easy. However with firefox is extremely easy.
Anyhow the point is, why don’t you install a firewall in your brain and simply ignore those messages from your browser? If the problem is that chrome will not allow you to connect, I would consider this one another reason to ditch chrome. If you really must use chrome, try to find that thread. Maybe someone can post a link to it.
Edit: found it: New user, everything works, but can't log in from Chrome - #18 by boycottseattle