Starting with version 51, Firefox now displays warnings about insecure login on certain websites.
This warning message is displayed where login details are to be transmitted to a website that is using a plain HTTP connection rather than HTTPS. This is useful as it means that the username and password that is to be transmitted can be potentially compromised because the connection to website isn’t encrypted.
For websites that don’t require transmitting login details, the use of HTTPS connection isn’t essential as no sensitive data is involved.
One way to work around this is to install a useful add-on like HTTPS Everywhere can be used. This add-on forces connection over HTTPS automatically.
It also has an atlas which alphabetically lists websites where the HTTPS Everywhere rules are applicable.
Click on View All Rules from the icon drop-down menu or go here.
If no secure connection is supported by certain websites like forums, news websites and so on but logging in is essential (which don’t involve submitting personal/financial data), it’s better to keep a different username and password with a throwaway email. This is useful when knowing fully well that even if these details are compromised, it won’t have much of an impact.
Happy and safe browsing.