I signed up with the free email certificate service using internet explorer, as no other browser is supported.
I got my certificate and can use it in outlook express with no problem.
I exported the certificate so I can use it on my notebook, but when I try to import it Thunderbird it asks me for a password. When exporting the certificate I didn’t use a password. So when I give a blank password it tells me the password is wrong.
Please see the knowledgebase articles on exporting/importing an email certificate.