Private certificates

Hi. Last year, when I first visited this forum, I got the best answer that anyone ever got to a question that troubled me for a long time (thanks again).
I’ve another, not really important, but annoying problem and searching around in different forums didn’t find how to solve it.
I have a local network with a firewall configuration via https and each time I go to the site, Firefox asks me to confirm that I really want to go there (because of not accepted private certificate). Can someone please tell me what I must do to store a permanent exception for this connection, staying permanent even if I use Firefox with auto-delete everyting at exit? Thank you for any suggestion.