If you receive the SSL error "sec_error_unknown_issuer" when browsing your SSL secured site using Firefox, you have to take a look at your SSL configuration for the specific Apache vhost. Make sure the entry "SSLCertificateChainFile" is set correctly to the location of your chainfile. The following SSL entries should be included in your configuration in order to work properly:
<IfModule mod_ssl.c>
SSLEngine on
SSLCertificateKeyFile /etc/apache2/ssl.key/domain.key
SSLCertificateFile /etc/apache2/ssl.crt/domain.crt
SSLCertificateChainFile /etc/apache2/ssl.bundle/domain.bundle
</IfModule>