If you do not have a S/MIME key-certificate pair installed on your Nitrokey yet or if you did not installed OpenSC, please look at this page first.
You need to have OpenSC installed on your System. Please have a look at the wiki page of the OpenSC project. While GNU/Linux users usually can install OpenSC over the package manager (e.g. sudo apt update && sudo apt install opensc on Ubuntu), macOS and Windows users can download the installation files from the wiki page.
Note: Windows users with 64-bit system (standard) need to install both, the 32-bit and the 64-bit version of OpenSC!
Settings in Thunderbird
Before you can use the Nitrokey in Thunderbird you have to activate S/MIME encryption in the account settings. You can achieve this by clicking on the menu and go to 'Preferences' -> 'Account Settings' and clicking on 'Security' in the account settings window.
Click on "Security Devices" to import the right PCKS11 module. Click on "Load" on the right-hand side. Now give the Module a name (like "OpenSC Module") and click on "Browse" to choose the location of the Module (see below).
On Windows the right file lays under "C:\Windows\System32\opensc-pkcs11.dll".
On macOS and GNU/Linux the file should be in "/lib/pkcs11/opensc-pkcs11.so" or "/usr/lib/pkcs11/opensc-pkcs11.so" or alike. Press "OK" twice and you are back in security section of the account settings. Now you can actually choose a certificate on the upper part of the window. You should get asked for a PIN to unlock your Nitrokey. Please type in your User PIN.
When composing an email you can now choose to encrypt and sign the message.