I've explored this extensively. I have a number of locally hosted webservers, mostly wifi routers and the like. I am unable to access ANY of them via firefox on mac. Safari can access them, and firefox on windows can access them, but firefox on mac can't. Also: chrome on the same mac also fails the same way. Trying to add an ssl cert exception in firefox settings doesn't work, after keying the url and hitting the button it also can't access the cert to add the exception with. This is something that used to work with firefox on mac, but at some point mac changed and is blocking both firefox and chrome. Apparently safari was adjusted to match the change, and firefox/chrome simply need to catch up?