HTTPS-Only Mode breaks HTTP Automatic proxy configuration URLs without the ability to add an exception
If HTTPS-Only Mode is turned on and the Connection Settings has a HTTP URL for the Automatic proxy configuration URL, Firefox does not load the proxy configuration file and all sites that require a proxy will fail to load.
There does not appear to be a way to add an exception in this case. The example given at HTTPS-Only Mode in Firefox does not work because when the configuration URL is navigated to in the address bar, a download prompt appears and there is no lock icon shown.
I believe this is a serious bug in HTTPS-Only Mode because when someone turns it on, it will work because the proxy configuration is already loaded, but when Firefox restarts they will no longer receive their proxy configuration and will lose access to all sites that require a proxy.