"This Connection is Untrusted" but only on firefox

Please check that the server provides all intermediate certificates (trust chain). A common issue is to forget the intermediate certificates and then get errors on some browsers an no errors on others. This is caused by the browsers caching the intermediate certificates, e.g. if you've visited a site using the same intermediate certificates before, the browser will dutifully use these cached intermediates if the server forgot to server them. But, if the browser never visited such site before the intermediates are not cached and thus the verification will fail.

A good test is to use openssl s_client -connect your.https.server:443 and look at the chain of certificates it provides. Also, https://www.ssllabs.com/ssltest/analyze.html will point out such problems.