This is the best way to uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall with Current Release Firefox 57.0.1 with a Full Version Installer https://www.mozilla.org/firefox/all/

If issue is still there it maybe in your profile.

Please let us know if this solved your issue or if need further assistance.

A brand new D-Link router using a HTTP connection like http://192.168.0.1. or http://dlinkrouter.local ??

https://blog.mozilla.org/tanvi/2016/01/28/no-more-passwords-over-http-please/ https://blog.mozilla.org/security/2017/01/20/communicating-the-dangers-of-non-secure-http/

Firefox doesn't differentiate between (WWW) website HTTP connections and "local" 192.168.0.# HTTP connections; if it has a Login field Firefox expects it to be HTTPS. Overall, it seems silly to me that a "local" 192 connection is treated the same as a WWW connection is treated; but I'm just a Firefox user and not an IT security person.

This Blog page is a different situation - having to do with all NPAPI Plugins, except for Flash, being deprecated. The only similarity is the HTTPS started one version earlier with Firefox 51. http://www.dlink.cc/knowledgebase/why-is-the-d-link-mydlink-portal-not-working-with-firefox-52-and-above.html 'Firefox 52 has removed all support for Netscape Plugin API (NPAPI). Plugins such as the D-Link mydlink plugin, Silverlight, Java, and Acrobat are no longer supported. mydlink portal uses mydlink plugin to access mydlink devices, and since mydlink plugin no longer works on Firefox 52, Firefox users are stuck at the plugin download page. Please note that customers using Firefox 51 are not affected. Internet Explorer and Safari still support these plugins.

I appreciate both responses but I'll decline to uninstall/reinstall Firefox in this case. (I've done that before when I had problems accessing almost everything.) I'm pretty certain that the issue for the D-Link login is the javascript. The page source shows javascript is used. After searching further, I also found essentially the same information about the problem on java.com, "Java plug-in does not work in Firefox after installing Java" at https://www.java.com/en/download/help/firefox_java.xml. A D-Link support rep said he would forward my concern to their engineering team. Perhaps they'll work out a fix. If not, I'll use IE for the rare times I need to access my router's admin site.