I ''think'' that refers to the NAS's internal web server not supporting TLS 1.2; Firefox stopped connecting with TLS 1.0 or TLS 1.1 some time ago because they are not considered private any more. But you can reenable them if needed. Unfortunately, Firefox doesn't have a method to only allow insecure protocols on your internal network, the following change will affect your web browsing as well. But maybe you only need it temporarily and then you can change it back? (1) 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.'' (2) In the search box in the page, type or paste '''security.tls.version.min''' and pause while the list is filtered (3) Double-click the preference to display an editing field, and change the value to '''1''' then press Enter or click the blue check mark button to save the change. To later restore the default value of 3, you can click the Reset button (left arching arrow) at the right end of the row. If the device is SO OLD that it doesn't even support TLS 1.0, then you probably need to use Internet Explorer 11 to manage it. (Maybe there's a firmware update available.)

That did the trick, thank you very much!