Assigning your Certificate to your email account:
These instructions assume you have previously followed the instructions to save your certificate to a secure location. Thunderbird has it's own certificate store and does not automatically pick up certificates from the Windows certificate store.
Click on the Manage Certificates...
Browse to the location of your PKCS12 certificate and enter any necessary passwords. Once complete the certificate will appear and you will be able to digitally sign e-mails you send out.
Signing an single Email:
Signing an email ensures the recipient knows the email has come from you and informs him / her if it has been tampered with since being signed.
Encrypting a single Email:
Encrypting an email ensures that only the recipient may view the email content and any attachments. Note: in order to encrypt an email for the recipient you must have the recipient's digital certificate, and their digital certificate must be assigned to the relevant entry in your address book
Signing and Encrypting as default.