It seems that the padlock icon (buttom right corner) always refers to the first tab of all open tabs in one Mozilla browser window and ignores the currently active tab, i.e. when the first tab uses a normal http connection and the active tab's page uses an https connection then the padlock does not indicate a secure connection. Plus, when you click on the icon then it shows the page info about the first tab.

Uh-uh. *** This bug has been marked as a duplicate of 101723 ***

vrfy dup