[ihattendorf]

It's because it was issued by StartCom.

https://blog.mozilla.org/security/2016/10/24/distrusting-new...

https://security.googleblog.com/2016/10/distrusting-wosign-a...

[awqrre]

I use Firefox 53 and this certificate appears to be valid in my Browser (green lock), I wonder why...

[icebraining]

Mozilla only distrusted the certificates issued after October 2016; theirs is from September.