That will only work to let you into a small number of pfSense devices until it trips the bug again. Also it would forget any other HTTPS certificates that were manually marked as trusted.
That db can be managed from inside the settings on Firefox, though it's still a poor workaround for most of us.
If you're on 2.1.5, apply this patch: http://files.pfsense.org/jimp/patches/cert-unique.patch
Afterward, from the shell, run:
pfSsh.php playback generateguicert
Then the GUI will use a certificate that Firefox won't choke on.
If you're on 2.2 already, run the command above from the shell. It's already present. Certs on fresh 2.2 installs are fine.