Hi ru2569, if the website is well run, this message usually indicates that you are connecting to the site indirectly, through an intermediary.

To get more information, could you click that View Certificate button and see whether Firefox shows a certificate similar to the attached -- a certificate for *.google.com that also covers numerous other Google properties.

If you want to share the certificate you see, there is a way to do that. Click the SEC_ERROR_UNKNOWN_ISSUER error code to open a panel showing an encoded version of the certificate you could copy/paste into a reply here.

I followed you suggestion:

https://www.youtube.com/ Peer’s Certificate issuer is not recognized. HTTP Strict Transport Security: true HTTP Public Key Pinning: true Certificate chain: -----BEGIN CERTIFICATE----- MIIHXDCCBkSgAwIBAgIQUYCCEoCid6mn7dcWVOcEADANBgkqhkiG9w0BAQsFADBv MQswCQYDVQQGEwJVUzELMAkGA1UECBMCQ0ExEjAQBgNVBAcTCUZhaXJmaWVsZDEM MAoGA1UEChMDQVRUMQwwCgYDVQQLEwNDU08xIzAhBgNVBAMTGmNhZmZzc2x2MDEu cHN0LmNzby5hdHQuY29tMB4XDTE5MDYxMTEyNDE1NloXDTE5MDkwMzEyMjEwMFow ZjELMAkGA1UEBhMCVVMxEzARBgNVBAgMCkNhbGlmb3JuaWExFjAUBgNVBAcMDU1v dW50YWluIFZpZXcxEzARBgNVBAoMCkdvb2dsZSBMTEMxFTATBgNVBAMMDCouZ29v Z2xlLmNvbTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABCSIIYgOLtHYbV9oiQOq Pl3MXH0TY35tTAcu73beEfDqCArd8tx15HxzbtMD5CLiiy4gWn/mHnYxWelYc1Pm vnajggTGMIIEwjCCBGoGA1UdEQSCBGEwggRdggwqLmdvb2dsZS5jb22CDSouYW5k cm9pZC5jb22CFiouYXBwZW5naW5lLmdvb2dsZS5jb22CEiouY2xvdWQuZ29vZ2xl LmNvbYIYKi5jcm93ZHNvdXJjZS5nb29nbGUuY29tggYqLmcuY2+CDiouZ2NwLmd2 dDIuY29tghEqLmdjcGNkbi5ndnQxLmNvbYIKKi5nZ3BodC5jboIWKi5nb29nbGUt YW5hbHl0aWNzLmNvbYILKi5nb29nbGUuY2GCCyouZ29vZ2xlLmNsgg4qLmdvb2ds 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Hi ru2569, thank you for the certs. On the first certificate, the issuer is strange:

Name: caffsslv01.pst.cso.att.com Organization: ATT

Hmm, why is AT&T in there??

When I check the next certificate up the chain, its issuer is:

ATT BlueCoat Intermediate CA

I think Blue Coat is a filtering technology/service owned by Symantec which may be integrated into some AT&T services.

If you are using an AT&T network connection, it's possible the service is trying to replace the genuine YouTube page with a message of some kind, but Firefox can't show the message due to a gap in the certificate chain.

How does YouTube work in other browser(s) on your system?

I can access YOUTUBE with no problem using Chrome or Internet Explorer.

The last certificate shows:

• Subject C=US, O=ATT, OU=CSO, CN=ATT BlueCoat Intermediate CA
• Issuer C=US, O=ATT, OU=CSO, CN=ATT Enterprise Internal Root CA

You can check the connection settings.

• Options/Preferences -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

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

You can set this pref to true on the about:config page to import root certificate(s) from the Windows certificate store.

• security.enterprise_roots.enabled = true

See also:

• https://support.mozilla.org/en-US/kb/error-codes-secure-websites