What version of Firefox are you using? SSL Labs indicates that Firefox 47 is unable to negotiate a cipher suite with your server that’s usable with HTTP/2 (which blacklists some ciphers that your server otherwise supports). I can confirm that it works with Firefox 50, and SSL Labs says it works with Firefox 49 as well (I think TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 was added in that version).
I’d probably try to change the cipher suite configuration and see if you can add any that will make these older Firefox versions work with HTTP/2. Mozilla’s SSL Configuration Generator might help here. Alternatively, I suppose disabling HTTP/2 for now would be an option, if your openssl version doesn’t support any of the ciphers you’d need for full Firefox compatibility (though that’d surprise me).