Self-signed certificates? Did you mean the certificates auto-generated by Openfire at install, viewable at Server->Server Settings->Server Certificates and available from the 1st time Openfire ran, or did you generate them yourself?
Hmm, I tried your suggestion in changing the purple.ssl.check_certificates, but instabird is taking a mighty long time at "Connecting: initializing stream...", and it still hasn't connected by the time I write this post. Any ideas?