Since when did #Firefox start removing the ability to bypass the security checks for websites with invalid TSL-certificates?

Got stuck in a loop where Firefox tried to "upgrade" to https with no way to disable (tried all different about:config settings, nothing worked) and then got stuck on "Warning: Potential Security Risk Ahead" with the usual "Accept the risk, continue"-option gone.

@gustav could it be that the server sets the HTTP Strict Transport Security (HSTS) policy header and in which case the browser might not allow an exception to be added? You could also try in a "private browsing tab" if that helps.
@netmackan it's the local web GUI for OPNSense. And it worked perfectly in Chromium (got the same warning, but could bypass)
@gustav I think you can add exceptions in the HTTPS Only section of this preferences page about:preferences#privacy
@Stanisable yeah, none of those settings do anything