Looks like the site was banned or removed if they violated Youtube TOS.

https://www.youtube.com/watch?v=S4fXBA2UOr0&feature=youtu.be (1) Почему лояльность сотрудников важна? Эпизод #3 - YouTube

Why is employee loyalty important? Episode # 3

I had no problem with the link.

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

• MOZILLA_PKIX_ERROR_MITM_DETECTED
• uses an invalid security certificate SSL_ERROR_BAD_CERT_DOMAIN
• configured their website improperly

How to troubleshoot the error code "SEC_ERROR_UNKNOWN_ISSUER" on secure websites https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

What site was banned do you mean? Look at the screenshot i've sent: in the address line is youtu.be address (link to YouTube). This is standard YouTube link.It was given to me by YouTube itself as a short link to my video. And FireFox claims: certificate is unknown. I'm sure it means youtu.be certificate is unknown. Am i right? In Chrome or IE there is no problem with opening this link.

No problems here with the youtu.be link.

You can check if there is more detail available about the issuer of the certificate.

• click the "Advanced" button show more detail
• click the blue error text (SEC_ERROR_UNKNOWN_ISSUER) to show the certificate chain
• click "Copy text to clipboard" and paste the base64 certificate chain text in a reply

If clicking the blue error text doesn't provide the certificate chain then try these steps to inspect the certificate.

• open the Servers tab in the Certificate Manager
  • Options/Preferences -> Privacy & Security
    Certificates: View Certificates -> Servers: "Add Exception"
• paste the URL of the website (https://xxx.xxx) in it's Location field

Let Firefox retrieve the certificate -> "Get Certificate"

• click the "View" button and inspect the certificate

You can see detail like the issuer of the certificate and intermediate certificates in the Details tab.

Oleg-1999 said

Links like https://youtu.be/S4fXBA2UOr0 doesn't open. Firefox opens the page with message: Connection doesn't secure. Certificate is unknown...

https://www.youtube.com/watch?v=S4fXBA2UOr0

Worked ok for me in FF 65.0. Did you get version 65.0?

~Pj