Some of the cheaper or newer SSL suppliers will require this to work properly (otherwise you may be prompted that the cert is invalid when it's not the case but it will certainly scare off your users!).
In the Apache vhost conf for the domain here is what you add:
Here is a full example of an SSL Vhost config in Apache using a CA Certificate file
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.........