Firefox requires intermediate SSL certificates while some other browsers don’t. I use nginx, and all companies I work with use it too. Its documentation states:
If intermediate certificates should be specified in addition to a primary certificate, they should be specified in the same file in the following order: the primary certificate comes first, then the intermediate certificates.
This means that your certificate should look like this:
-----BEGIN CERTIFICATE-----
your certificate
-----END CERTIFICATE-----
-----BEGIN CERTIFICATE-----
intermediate certificate
-----END CERTIFICATE-----
This blog is about things I encounter while doing web and non-web software development.