Three thoughts: (1) Probably you would know if this was caused by the HTTPS Only feature. This article describes how to make exceptions or turn off that feature: [[HTTPS-Only Mode in Firefox]]. (2) You also can turn off the "HTTPS First" feature using the following steps: <blockquote>(A) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. ''More info on about:config:'' [[Configuration Editor for Firefox]]. ''The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.'' (B) In the search box in the page, type or paste '''https_first''' and pause while the list is filtered (C) If the '''dom.security.https_first''' preference is bolded and has a value of true, double-click it (or click the Toggle button or Reset button at the right end of the row) to restore the default value of false. The HTTPS Only feature is a more manageable approach than this older feature. (D) Double-click the '''dom.security.https_first_pbm''' preference to switch the value from true to false. The HTTPS Only feature is a more manageable approach than this older feature.</blockquote> (3) If any other web app at the same domain/IP address has sent Firefox an HTTP Strict Transport Security header in the past, then this is applied to the other web apps, too. I don't know whether this could be a problem on the loopback, but we can give you some further steps to investigate that.

Is this a local server?