It adds https to the current http URL. For instance, if your current tab is on http://example.com, clicking this add-on icon will take you to the https://example.com. If you're already on a https version, it will take you to the http.
What is Add HTTPS?
Add HTTPS is a Firefox add-on that automatically adds https to the current http URL. It seamlessly switches between http and https versions of websites for improved security and privacy.
Add HTTPS may not be safe to use and it requires some risky permissions. Exercise caution when installing this add-on. Review carefully before installing.
Add HTTPS has earned a fairly good reputation and likely can be trusted.