Seems to me they should be able to keep this sort of working by resolving tplinklogin.net to 192.168.0.1 or such on their public DNS and ensuring the router always serves the web page from that IP.
Of course things will break down if a router is ever configured to use a different IP - but then again, the public DNS with local IP might trigger Firefox' heuristics and cause it to fall back to traditional DNS - so thinks may still work.
But the whole concept of using an on-device web page as config UI seems to become problematic, thanks to HTTPS-everywhere, as there appears to be no way to serve HTTPS from a local, unsupervised device that is frictionless and does not open a security vulnerability.
Of course things will break down if a router is ever configured to use a different IP - but then again, the public DNS with local IP might trigger Firefox' heuristics and cause it to fall back to traditional DNS - so thinks may still work.
But the whole concept of using an on-device web page as config UI seems to become problematic, thanks to HTTPS-everywhere, as there appears to be no way to serve HTTPS from a local, unsupervised device that is frictionless and does not open a security vulnerability.