Firefox now has HTTPS redirection built-in, so this add-on is NO LONGER NECESSARY. Check your Options to enable it. The setting is at the very bottom under "Privacy & Security", called "HTTPS-Only Mode". The new browser setting does everything that this add-on does, except natively.

I want to thank the Electronic Frontier Foundation for providing this add-on for many years until it was finally supported natively! Your job here is done, but please don't give up the good fight! There are still many more battles to be won in the privacy realm. Keep up the great work.

It's a great extension and warns me when I go to HTTP sites. Recommended!

If a site has both an http and an https version, and this add-on knows about it, it redirects you to the https version always! Also for pages that use both http and https, they make sure everything is routed through https instead!
This should be default browser behaviour IMO

@RandomReviewer1 Yes this does make a difference, with that flag ON Firefox warns you about an unsafe site, with this add-on anysite that CAN be safe WILL be safe. They do 2 very different things

The extension works fine but I have a question:
Does it still do anything if I enable dom.security.https_only_mode in about:config?

Works without issues.