Here is how to install a senders certificat SMIME : http://kb.mozillazine.org/Installing_an_SMIME_certificate

Thanks for the article - it explained my problem exactly (not being able to install&use my customer's certificate). However, after converting the .crt to a .cer file, and trying to import it into the Authorities list - I get the following message: "This is not a certificate authority certificate, so it can't be imported into the certificate authority list." I converted it by saving it as .cer in several formats. I don't understand why Thunderbird won't let me accept the risk (and the certificate from my customer). thanks very much for your help, Gnospen.