You might be hit by this if you access Linksys WRT54GL settings page via SSL - I was anyways. Apparently WRT54GL is one of the routers mentioned in the Mozilla support page that "improperly regenerate their security certificates instead of keeping the same one for the life of the device". Nice job, Cisco!
UPDATE: As time goes by, I started to notice that this happens quite often with self-signed certs, and sometimes deleting the certs themselves doesn't even fix it. The important thing is to also delete the associated CA under Authorities tab, as detailed here by sky76.