I noticed this problem today, after deciding to try my Asus RT-AX86U again to tweak some settings to see if they would fix my smart device issues with that router. Anyway, I changed my bookmark to the router's IP and it's OK. Usually in the past whenever I had an issue with the site, I could just clear cookies and cache for the site, and it would work again. However, not this time.
What's odd though is, firefox still works with tplinkwifi.net for my other router, without issues, and that's not an HTTPS site either. The router.asus.com issue seems recent, as it's been working all along, maybe this is specific to the 104 release of firefox.