Choose Account Settings from the Options menu. Under your account heading, click the Security node. On the right, click Select under "Use this certificate to digitally sign messages..." and then select the Secure Email Certificate that you imported. Click OK.
Thunderbird will ask if you want to use this same certificate to encrypt your email as well. Click OK when it asks you this question.
If you want to sign all of your messages by default, click the "Digitally sign messages (by default)" option underneath the Digital Signing section. You can also click "Required" under the Encryption section, but we recommend that you leave the "Never" option selected. This way, you can select which messages to encrypt later on.
Click OK and your certificate is installed and configured.