Just for the record: I found the dialogue for adding a security exception, but adding one for instawallet.org's certificate didn't show any effect - it was still telling me that the cert had expired and still only gave me the "get me outta here"-button, even after expanding the collapsed paragraph (only one, the "I know..."-one wasn't there).

I then set the system clock of my machine back by two weeks (*before* July 17th), re-opened the site, clicked that button and then set the clock back to the correct date.

This might or might not be an option for others fighting against some stubborn browser (that allows one to add exceptions for completely bogus certs but not for recently expired ones).