Works as designed, I guess - I had to change the default encryption setting when sending messages in my account settings from "Required" to "Never (do not use encryption)".
If I don't have a certificate the mail is sent without the nasty "no certificate - yadda yadda" error message. If I choose a recipient for whom I DO have a certificate, the plugin shows a dialogue with the title "Certificates found" and asks "Activate message encryption [Yes] [No]". This can be disabled in the add-on settings. Then messages to recipients for whom you do have a certificate would be encrypted transparently (almost, usually you'd have to enter your passphrase first).
Many thanks, waited ages for this - for anyone who's wondering why, see https://bugzilla.mozilla.org/show_bug.cgi?id=135636
This review is for a previous version of the add-on (0.1).