Thanks a lot for you work. This plugin is very helpful. But it can confirm the bug report of Dani, the "Digitally sign messages (by default)" setting doesn't work. I'm using v.0.3.0, but not the "Virtual identities" add-on.
This plugin does only change the configuration-dialog, nothing else. Thus the reason for "Digitally sign messages (by default)" not working has to be somewhere else. (For me it's working like a charme.)
Great add-on! I am waiting for something like this for a long time. Don´t you think about na other add-on else? TB doesn´t know to associate a S/MIME certificate with a contact. Sometimes you get a digitally signed mail but you need to encrypt the reply with an other certificate. But TB automatically uses the signing certificate and you cannot simply choose the certificate you need... (http://forums.mozillazine.org/viewtopic.php?f=39&t=770435, http://forums.mozillazine.org/viewtopic.php?f=39&t=727475)