As of today, TLS certificates issued by Symantec are distrusted in Firefox Nightly.

You can learn more about what this change means for websites and our release schedule for that change in our Update on the Distrust of Symantec TLS Certificates post published last July by the Mozilla security team.

The Symantec distrust is already effective in Chrome Canary which means that visitors to a web site with a Symantec certificate which was not replaced now get a warning page:

(left is Chrome Canary, right is Firefox Nightly)

We strongly encourage website operators to replace their distrusted Symantec certificate as soon as possible before this change hits the Firefox 63 release planned for October 23.

If you are a Firefox Nightly user, you can also get involved and help this transition by contacting the support channels of these websites to warn them about this change!

https://blog.nightly.mozilla.org/2018/08/14/symantec-distrust-in-firefox-nightly-63/