So you've just purchased your SSL cert, renewed it and installed it or maybe you've had it installed and working fine all the time with all other browsers but you've upgraded to a recent version of Firefox and suddenly get the warning "Error code: sec_error_unknown_issuer" error.
This is terrible since if you bought an SSL cert, you are most likely using it for trust purposes for your business and obviously that message will scare away most potential customers.
The Easy Solution
You need the CA Certificate Bundle which even threw me off because I've always just used the .crt/certificate file issued for the domain and the .key/private key file and things have always worked.
Find the CA Chain file that is also provided by your SSL provider and add it to your configuration. In Apache this means adding a line like this to your Apache Vhost/Control Panel
After that just restart Apache and the error will be gone.
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