Mozilla reported that during 2018 no longer trust certificates issued by Symantec and its subsidiaries. The process will be carried out gradually until in October all the sites are marked as insecure. The only Symantec's subsidiaries that will remain reliable are those of Google and Apple.
This release is the result of what Google announced earlier this year, stating that they will not trust Symantec certificates since October 2018.
Both organizations decided to distrust this CA after the incident occurred in early 2017, when Google invalidated more than 30,000 certificates issued by Symantec after detecting different security flaws in the validation process.
In addition, we must add the 23,000 private keys DigiCert, owner of Symantec, filtered by Trustico at the end of February 2018, whose objective seems to be to force these users to obtain a reliable CA certificate by both browsers.