Not perfect, but does the job. The only extension of it's kind I know which allows me to quickly check (even from time to time) whether my connection is not intercepted by e.g. corporate tools. Deserves 5 stars. Thanks.

** Major trust issue alert ** The add-on reports that it now has a compromised back end service API. You may now see an error "This public key does not match the one required by the server" within the add-on about:addons config screen. I have tried this on multiple computers, same catastrophic security error. The public key no longer matches the expected API key. This add-on normally sends all your website URLs to a back end server that the developer hosts free of charge (how "generous"... hmmm). Notwithstanding this suspicious setup which is funded "some how", this error would indicate their server has been compromised. Someone else is now hoovering up all your URLs, maybe the CIA, FBI, MI5, Mossad... who knows.

Also, https://www.ssllabs.com/ssltest/analyze.html?d=checkmyhttps.net suggests the server's certificate chain is incomplete. which is odd.

The Public key you should see in the add-on..

-----BEGIN PUBLIC KEY-----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-----END PUBLIC KEY-----

Good addon, but in its settings, the server reverts to its default value (checkmyhttps.net) each time the browser restarts. So 4 stars only for now.

A good little piece of software. Easy to install, do one thing and doing it well.
About the punycode, look at https://www.xudongz.com/blog/2017/idn-phishing/ and don't forget to tweak "network.IDN_show_punycode" in about:config (set it to true) and do the test again.

The only extension that make possible to detect which web site is hi-jacked by the my company's proxy