hello, first please make sure that the date & time is set correctly on your system. if this doesn't solve the issue (or it is already set properly), a possible solution depends on different factors:

• what is the error code shown under technical details on the error page?

in case the error code equals sec_error_unknown_issuer, please attempt to add an exception on the bottom of the error page & inspect the certificate (see the screenshot attached for instructions):

• which issuer information does the certificate contain?

thank you!

"This Connection is Untrusted" error message appears - What to do

I attempted to open my bank website. The homepage opened, when I attempted to access my account I received the unsecure message. The certificate is read as follows:

Issued To: CN: secure.whitneybank.com O: DO_NOT_TRUST OU: Created by http://www.fiddler2.com SN: 83:21:32:45:F4:46:2F:9F:40:CB:94:58:84:39:B0:0D Issued By:

CN: DO_NOT_TRUST_FiddlerRoot O: DO_NOT_TRUST OU: Created by http://www.fiddler2.com

Validity Issued on: 3/26/14 Expires on: 3/25/24

Fingerprints

SHA1 fingerprint CC:FE:45:E0:51:B4:DC:5D:31:D2:37:AE:02:0B:8D:9B:D8:AD:89:A2 MD5 fingerprint 61:9A:FB:0A:1F:E0:1F:8B:F4:96:C7:E2:58:DF:50:76

unfortunately the fiddler2 issuer in certificates might be a sign of unwanted software present on your pc that is intercepting secure network traffic. please go into the system control panel and uninstall programs like BrowserSafeguard, BrowserSafe, SafeGuard or other software that sounds suspicious and didn't get installed by you intentionally.
_{reference: https://support.mozilla.org/en-US/questions/982532#answer-520145}

afterwards, run a full scan of your system with different security tools like the free version of malwarebytes & adwcleaner.

Troubleshoot Firefox issues caused by malware