Install a CA Root Certificate on Windows
Installing the Root Certificate you got from this site, this one: ca.ssl.indexnl.com.crt, is easy.
Just double click the certificate, and you get the following dialog boxes: (This is Windows 7, but its the same on Windows 8 and Windows 10)
Click on Install Certificate
Place the Certificate in the Trusted Root Certification Authorities Store
From now on the certificates you generate with this site will be trusted by your computer.....
Firefox keeps its own certificate store, just click the certificate on this page, and Firefox installs it.