Can you remember what the certificate error message said?
The most common causes are security software scanning encrypted connections or malware listening in, replacing legitimate website certificates with their own. In particular, the error code MOZILLA_PKIX_ERROR_MITM_DETECTED indicates that Firefox is able to detect that the connection is intercepted.
How to fix a certificate error in Firefox.
1. On the error page, click or you can add an exception.
2. Click Add Exception. ...
3. Click Get Certificate.
4. Click View to display a window that describes the problem with your z/OSMF site. ...
5. After you have verified the certificate, close the dialog. ...
6. Click Confirm Security Exception to trust the z/OSMF site.