Firefox + self-signed certs forces you to add an exception for the site, which makes the cert work and shouts at you again if that cert ever changes, so fulfilling the above. :)
Sure, but that is only helpful if you remember that the site had previously worked before. Otherwise, it's just like your first time visiting it.
Yes it would be nice if the browser prompted more suspicion in those cases when a self-signed cert changes than when one is simply used for the first time. (Theoretically the changing of a regular CA-signed cert shouldn't prompt any suspicion, but I wonder...)