I use the EFF's Firefox addon called "HTTPS Everywhere". It has a list of websites that have HTTPS enabled, and whenever your browser is directed to the plain-HTTP version, it will go to the HTTPS version instead. https://www.eff.org/https-everywhere A useful (but tbh kinda annoying) companion addon is the HTTPS Finder. It checks to see if the website you're currently browsing also has an HTTPS version, and will add a rule to the HTTPS Everywhere addon. (It also has a "whitelist" of sites that this breaks.) https://addons.mozilla.org/en-US/firefox/addon/https-finder
HTTPS Finder dev here - If you go into settings you can turn off the auto-redirect to HTTPS or the actual drop down alerts (or both).
Then you can go into Preferences > Advanced after some browsing, and you'll see a list of all the good sites found. You can create HTTPS Everywhere rules directly from there, without ever being annoyed during your normal browsing.
I've been using HTTPS Everywhere for a while now, but it's been almost entirely a vanilla install since I got it. The only exception so far has been HN itself, and only because someone put the new rule in a comment. I then had to use Google to find out how to actually install the rule, as it was pretty non-obvious to me.
It seems I've been downvoted, and I think I know why. I can't edit my original comment so a reply will have to do.
My comment was meant to be a sincere thanks for mentioning the HTTPS Finder extension. Reading my comment now, without that clarification, I can understand how it could read as a criticism of the HTTPS Everywhere extension instead. That was not my intention. I merely wanted to express my thanks for making me aware of a complimentary extension which adds useful functionality to an extension I already use.
If the downvote was for another reason, please let me know what was the reason for it so I can try to avoid committing the same error in the future.
