Digital Signature Issue in Mozilla ThunderBird

When I try to send a digitally signed message I get a pop-up saying that "Unable to sign message- check that certificates specified in the Mails and Newgsroups Account Settings for this mail account are valid and trusted".

I have install new p12 file into Mozilla Thunder Bird.

Edit → Account Settings → Security → Certificate → View Certificate

Under the “ Your Certificate” tab you need to import the p12 file. Then When I'm try to send emails by digitally signing I got the above error.

Then in the same path open “Authorities” tab and search for the Root CA of the certificate issuer. Point out that Root CA and click “Edit Trust” and make Root CA trusted. Then you will be able to send the email by digitally sign.