So far we've only been able to duplicate this on Firefox 126.96.36.199, as the "View Mail" button for account management works fine in Firefox 188.8.131.52.
Here's a screenshot of the error:
We're running NE ZCS 4.5.6 and to get this error, all we have to do is highlight an account and click "View Mail". A Firefox "Security Warning" will pop up, and say, "Although this page is encrypted, the information you have entered is to be sent over an unencrypted connection and could easily be read by a third party. Are you sure you want to continue sending this information? [Continue] [Cancel]"
Are we the only ones experiencing this? If I find some magical security setting to disable, I'll be sure to share.