Is it possible to send S/MIME digitally signed email with Mozilla Thunderbird using qualified digital signing smart card? How?
Yes, if your card reader has a compatible PKCS#11 module.
On Thunderbird, go to Tools → Account Settings, and under your mail account select Security. Click Security Devices and you will see a list all loaded modules and devices. Click Load and select the PKCS#11 module file for your card reader. (On Windows, it's a
Close Security Devices and click View Certificates, and you should see certificates contained in the smartcard. Close the window and use the two Select... buttons to choose encryption and signing certificate(s).