Smart HTTPS is a useful Firefox add-on that will automatically detect if a website supports secure HTTP (https protocol) and switch to loading over that secure protocol.
Also, for websites that are reliable but don’t offer HTTPS , a whitelist can be maintained by the add-on wherein these websites are listed as an exception.
This is a fairly simple add-on to install and use. Download it from here. No browser restart is needed once it is installed.
Once installed, it will show up as a blue padlock icon.
Browsing websites after installing is the same as before with the auto switching from HTTP to HTTPS happening without any user action.
To whitelist websites that don’t support HTTPS but are trusted, click on the icon and choose Settings.
Scroll down to the whitelist section and add the websites/domains over there. Only add those which are fully trusted and which ideally don’t involve online payment or online financial transactions.
Similarly, a blacklist can be maintained which forces specific sites to only load in HTTPS.