If you installed a reliable SSL certificate for your site, and your pages look fine in IE and Chrome, you still may see a message in Firefox that your site is untrusted instead of your page. In this case, check if you installed the root and intermediate certificates, not just the site certificate. Those certificate should’ve come in the package from your SSL provider.
I had 1 root and 2 intermediate certificates from my provider, however I couldn’t find any information how to install them for nginx. Nginx configuration doesn’t have any options for those, compared to Apache server. I tried different ways, and eventually I succeeded.
Copy all your intermediate certificates and the root certificate into the same file. I copied them BACKWARDS – i.e. if you have 2 intermediate certificates, copy the second one first, then the other one below it, then the root certificate below it. You don’t have to save this file. Open your .crt file with the site certificate and add the content of the previously created file below the site certificate. Save it and restart nginx. That’s it.