When you get the warning, click advanced, then scroll down and click "Add exception"

That is the issue the option never comes up just the lock with a slash through it. It is an internal LAN address.

What domain do these intranet pages use because some domains are on the HSTS list ?

Is there a certificate installed or are the pages meant to be visited via http:// ?

These are just http only

Firefox will mark HTTP connections with a padlock with a strike through and shows "Connection is not secure" if hover the padlock icon.

This message is meant as a warning that you could be vulnerable and this shouldn't prevent you from accessing the website.

In a login field you can press the ESC key to close the warning message in the name and password field.

See also:

• https://support.mozilla.org/en-US/kb/insecure-password-warning-firefox
• https://support.mozilla.org/en-US/kb/how-do-i-tell-if-my-connection-is-secure
• https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean