Got an annoying error, like this:

WEBSITE-YOU-ARE-ACCESSING has a security policy called HTTP Strict Transport Security (HSTS), which means that Firefox can only connect to it securely. You can’t add an exception to visit this site.

And you really need to access the site, and don't really care that whatever you are sending is encrypted?

Just Ctrl-H to go to history (Command-Shift-H if on MacOS), search for the site, and right click and press Forget About This Site.

This will make your browser completely forget about that site and thus not be able to recall the HSTS header, and let you connect insecurely.

Do not do this if you value your browsing history of that site. It literally says "Forget This Site", act accordingly.

These instrucitons are Firefox and Firefox-based browser specific, but the process is same on other browsers.