[kpremote]

__Edit: I installed the lastest Firefox v67, the problem is gone. It works great.__

Thanks. no it's not in a private browsing window.

I also asked other people to visit the site(mydomainname.com) from their computers and phones, all worked fine.

The hour long struggle left me with painful memories. :) I never had issue like this previously in Firefox or other browsers. On that day, Chrome and IE worked without problem on my computer (Win 7 32bit), so the computer should not be the culprit.

That leaves Firefox v66 standing. I don't know why.