These instructions detail how to install an S/MIME certificate and send secure email messages with Mozilla Thunderbird on Windows PCs.
Testing was done in Thunderbird version 60.5.3 on Windows 10 Enterprise, but Thunderbird has natively supported S/MIME for many years. Please check with Mozilla Support about any issues or specific instructions concerning earlier versions of Thunderbird.
Buy S/MIME Certificates
Download and install the certificate in Thunderbird
- Download a PKCS#12 file with your certificate from your SSL.com account by clicking the link supplied in your Certificate Activation Link email and following the on-screen instructions in your web browser.You will be prompted to create a password before downloading the file. (Keep this password secure – you will need it later.) Make sure to keep track of where you saved your PKCS#12 file, and do not lose it. If you lose your private key, you will be unable to read messages encrypted with your public key.
Note: when downloading your certificate it is possible to choose between the RSA and ECDSA algorithms via the Algorithm drop-down menu. However, ECDSA keys cannot be used for email encryption, so it’s best to leave this set to RSA.






CN=SSL.com
on the Issued by: line. After confirming that the certificate is correct, click OK.


Your S/MIME certificate is now set up and ready to use to sign and encrypt email in Thunderbird.
Sending signed and/or encrypted email messages in Thunderbird
- Begin composing a new message. Under the Security drop-down menu, select Encrypt This Message and/or Digitally Sign This Message. (Note that if you have previously set either or both of these options as the default you can skip this step.)
- When you click Send, your email message will be signed and/or encrypted, as desired. Note that if you do not possess the public key of your recipient, you will receive an error message when attempting to send them encrypted mail.
For more detailed information about S/MIME email, please see our article, Sending Secure Email with S/MIME.
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