I don’t think you should disable self signed warnings. It would be better to import those than disabling the warning as it is a very important warning.

As for disabling https only mode for certain URLs, I don’t know, and it would be a useful feature. Some of my corporate stuff oddly redirects to HTTPS but just gives a blank screen rather than a connection refused or something. Not sure what it is. Probably something is misconfigured somewhere but it’s not something in my control. I didn’t have time to really inspect it so I just disabled the https only mode for my work laptop.

Install valid certs. Problem solved.

Pretty sure there’s not a per-domain setting for that. If you have HTTPS-Only Mode turned on in the settings it will always try to use HTTPS first and present a warning before switching to HTTP.

If you want to continue using HTTPS you can setup your own CA certificate to sign certificates for your .LAN domain names. All you need to do then is add the CA certificate to your trusted certificates in Firefox and the signed certificate to the device hosting the HTTPS service.

EDIT: TIL there an exclusion feature. Neat. I didn’t see this on Firefox for Android though. https://support.mozilla.org/en-US/kb/https-only-prefs

Install the cert using settings->privacy->certs. Use server option to download and install the cert

For https/hrtp as default either ask google for sertings for prefered protocol, type :80 behind address or specify protocol. I believe they changed it to default to https if none is specified a while back