If you are like me, you login to several sites a day that do not store sensitive information. I am very aware that these sites are not protected by SSL/TLS Certificates. So, while I appreciate their concern for my safety, I don't need the message.
Fortunately, there is a way to disable the message and it's not hard at all.
(1)Open a new browser tab in Firefox. In the location bar, enter "about:config" and hit enter or click the 'go' arrow (at the right of the location bar). See image below.
(2)You will get a message about 'voiding your warranty.' Click the "I accept the risk!" button to continue.
(3)Enter "insec" in the search field. You should see a list of Preference Names that include the letters, insec. Find the item, "security.insecure_field_warning.contentual.enabled" and double-click to change the value from "true" to "false."
Now simply close the browser tab. You will no longer see the 'connection not secure' message. You can follow these instructions to re-enable the message - just make sure the value ends up "true."
Also note that when the browser upgrades, you may need to do this all again.