that's what SHE said
7 September 2016

So, ever try to permanent store a self-signed SSL certificate in Firefox whilst in private (PR0N!) browsing mode? Really? ARE YOU SURE?!?

Seems you can't do it, anyways. What you CAN do is first load the site in question in non-private mode, accept the cert permanently, THEN reload site in private browsing and you're good to go.