as yet, i have never been unable to accept a site's unrecognized
certificate. [firefox 24.8.0]
There are some certs that are pinned (tor's site is one I remember) and
you can't override those easily. If you have removed the default CAs
and run into this you have to go somewhere esle in the config to unpin
the certs for the special sites. I don't remember all of the details, just
that I needed to do it to be able to visit the tor site again.