Hello,
Mozilla plans to distrust Symantec certificates in Firefox 63 are now in place in Firefox Nightly as of today.
Here is a blog post summarizing the situation:
https://blog.nightly.mozilla.org/2018/08/14/symantec-distrust-in-firefox-nightly-63/
That means that a lot of sites that haven’t changed their certificate yet will be broken. As a community member, you can contact these sites via their support mechanism and urge them to take action and chance their certificate before Firefox 63 is released in late October.
Cheers
Pascal Chevrel