Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Please provide a public link (no password) that we can check out. No Personal Information Please !

Thanks for responding. Duolingo is one of my most used sites, so it appears at the top of my Firefox page. I tried clicking it as usual, and I just got 3 dots lighting up sequentially across the page. I waited, but this kept going. I then tried opening the Duolingo site by entering Duolingo.com in the URL space, but I got the same result. This time I let it sit for an hour. Still nothing except the three dots. When I open it on my phone either from the app or from typing in the URL, it opened immediately and showed my progress accurately. I always use this app on my Macbook, not on my phone. I can't think of any further relevant info, but will be glad to send more if you have any further questions.

Further to my earlier response, Duolingo sent me an email today suggesting that I study more. It had a link to their site. The lights on the three dots is still moving on my attempt to reach them. Again, it opens right up on my phone. Thank you again for taking an interest.

Is this a new issue? Did it begin with a recent update? Does it happen in safe mode? https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode The correct url is https://www.duolingo.com/ It opens without issue for me. Enhanced Tracking Protection is on and a finger printer has been blocked. Is it possible that your settings on the Privacy page of Preferences are preventing you from accessing the site?

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean