Have you noticed any problems with certificate errors? Often a secure site uses content from multiple servers and even if you make an exception for the top level site, files such as style sheets from related servers may be blocked.

More generally:

When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

"3-bar" menu button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the bank site and logging in again. Does that help?

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All of them.

Did you check for issues caused by the Privacy Badger extension?

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

The problem was indeed the PrivacyBadger extention. Disabling that made the problem go away, which is goodness.

There's probably a way to make an exception for the blocked content in Privacy Badger if you want to keep using it, but I am not familiar with the details myself.

Contact the programs support page.

I found the way to disable it for that website only (by clicking on the RED privacybadger icon.