Yes... Firefox 73.0.1 has broken login for Asus routers. Both Merlin and Official firmware are affected. I disabled addons, even tried accessing from Private browsing window. It just refuses to login. I don't know how to solve this issue.
I created a new Firefox profile and now it works. Maybe it might be some rogue Nano Adblocker update. Apparently this is a common issue. Quick Google search helped. Should've searched there first.