TLS certificates indicating secure connections are
today’s gurantee that the site you’re visiting is
using secure connections or your transmitted data.
However it seems that not every TLS certificate…

is trustworthy. In July, Mozilla has already announced that Symantec as a CA is no longer trusted, meaning that all certifcates using Symantec as CA are thus untrusful resulting the browser to send an alarm. Symantec itself has sold the TLS certificate business branch to Digicert in 2017.

The reason for Mozilla and also Google (Chrome) or Apple (Safari) to mistrust these certificates by default is that Digicert seems not to follow certain guidelines for issuing secure certificates.

The following Bugzilla entry is listing quite a lot of popular sites that are now affected by the revocation of trustworthiness of the corresponding TLS certificate and it’s CA.

Mozilla however allows a flag to be set in the about:config file to suppress this alarm in total. But, in my humble opinion, that’s not recommended.